ª Springer Science+Business Media New York 2014
Abdominal Imaging
Abdom Imaging (2014) DOI: 10.1007/s00261-014-0108-y
MR imaging of the pelvis: a guide to incidental musculoskeletal findings for abdominal radiologists Kara Gaetke-Udager, Gandikota Girish, Ravi K. Kaza, Jon Jacobson, David Fessell, Yoav Morag, David Jamadar Department of Radiology, University of Michigan Health System, 1500 E Medical Center Drive, TC 2910, Ann Arbor, MI 48109, USA
Abstract Occasionally patients who undergo magnetic resonance imaging for presumed pelvic disease demonstrate unexpected musculoskeletal imaging findings in the imaged field. Such incidental findings can be challenging to the abdominal radiologist, who may not be familiar with their appearance or know the appropriate diagnostic considerations. Findings can include both normal and abnormal bone marrow, osseous abnormalities such as Paget’s disease, avascular necrosis, osteomyelitis, stress and insufficiency fractures, and athletic pubalgia, benign neoplasms such as enchondroma and bone island, malignant processes such as metastasis and chondrosarcoma, soft tissue processes such as abscess, nerve-related tumors, and chordoma, joint- and bursal-related processes such as sacroiliitis, iliopsoas bursitis, greater trochanteric pain syndrome, and labral tears, and iatrogenic processes such as bone graft or bone biopsy. Though not all-encompassing, this essay will help abdominal radiologists to identify and describe this variety of pelvic musculoskeletal conditions, understand key radiologic findings, and synthesize a differential diagnosis when appropriate. Key words: Pelvic MRI—Incidental findings—Musculoskeletal MRI—Abdominal radiologists
Patients who undergo MRI to evaluate for pelvic disease can also have a variety of musculoskeletal imaging findings. Such findings can range from incidental to symptomatic and can include pathologic processes as fracture, infection, inflammation, and neoplasm. Correspondence to: Gandikota Girish; email: [email protected]
Though not all-inclusive, the purpose of this pictorial essay is to review incidental musculoskeletal imaging findings that can be encountered by the abdominal radiologist and to aid in developing a differential diagnosis.
Bone marrow Bone marrow consists of trabecular bone, red (hematopoietic) marrow, and yellow (fatty) marrow. The amount and distribution of red and yellow marrow change with age [1] undergoing a process called marrow conversion. With increasing age the red marrow present at birth is replaced by yellow marrow in a predictable pattern, starting with the long-bone diaphyses, and then extending to involve the epiphyses, distal metaphyses, and proximal metaphyses, sequentially. Most marrow conversion is typically completed by the age of 25 and is symmetric; marked asymmetry should raise suspicion for a pathologic process [1–3]. Residual red marrow is seen in the proximal femoral metaphysis [1]. Reconversion, a reverse process involving the transformation of yellow to red marrow, is seen in states of increased hematopoiesis. Reconversion of marrow is seen in cases of sickle cell anemia, thalassemia, high altitudes, and high-level athletes. This process is also symmetric. Focal areas of fatty marrow in the midst of normal red marrow are of no clinical significance [1]. The most useful MRI sequence for marrow evaluation is the T1-weighted spin-echo sequence, although T2 and short tau inversion recovery (STIR) sequences are also helpful; the proton density sequence is not a substitute for the T1-weighted sequence. Normal yellow marrow follows the signal of fat in T1- and T2-weighted imaging, STIR, and fat-saturated sequences [4] (Fig. 1). Normal red marrow shows intermediate signal intensity
K. Gaetke-Udager et al.: MR imaging of the pelvis
on T1- and T2-weighted images and is isointense to muscle on STIR and fat-saturated images (Fig. 1). Red marrow should be lower in signal intensity than yellow marrow on T1-weighted images and hyperintense to yellow marrow on T2-weighted images [5]. These differences are due to the relative fat content of marrow; normal red marrow contains some fat (40 %), but overall the fat content of yellow marrow is much higher (80 %) [6]. Yellow marrow does not lose signal on out-of-phase images compared to in-phase images because of its highfat content (Fig. 2) [6]. Red marrow contains equal parts
of fat and water, and therefore, its signal drops a great deal on out-of-phase images compared to in-phase images (Fig. 2). There can be an apparent demarcation between normal yellow marrow and reconverted marrow in the proximal femoral metadiaphyses, sometimes referred to as a ‘‘pseudolesion’’ (Fig. 3) [7]. This sharp demarcation is due to the interface between yellow and reconverted red marrow, and can mimic a focal lesion. Familiarity with this entity and its bilateral symmetric appearance should prevent misdiagnosis.
Marrow pathology Imaging findings in various marrow pathologies are usually nonspecific and only serve to raise suspicion for a disease process [4, 8]. Indicators of abnormal marrow include an abnormal distribution or asymmetry of the normal marrow pattern or abnormal marrow signal (Fig. 4). Pathologic processes infiltrate and replace the normal fat content of marrow, and therefore, show lower signal than muscle or intervertebral disks on the T1-weighted images and higher signal than fat on the T2-weighted images. Neoplasm does not typically contain microscopic lipid and, therefore, should not lose signal on out-of-phase images, while red marrow and benign lesions such as fracture will lose signal because of the presence of microscopic fat [5].
Marrow proliferative disorders Marrow proliferative disorders are generally diffuse processes arising from overproduction of cells originating in the bone marrow. Examples include myelofibrosis, polycythemia vera, myelodysplastic syndrome, leukemia, multiple myeloma, and amyloidosis; multiple myeloma may also exist in a focal form. Marrow can sometimes appear normal despite the presence of a disease process, as the marrow fat cells might not yet have been replaced. As the process continues, the pathologic areas become lower in signal intensity than muscle on T1-weighted images and generally higher in signal than muscle on T2weighted images [9].
Marrow replacement disorders Fig. 1. 41-year-old man. Normal red marrow is intermediate in signal on coronal T1-weighted (A, between white arrowheads) and T2-weighted images and isointense to muscle on coronal STIR (B, white arrow) and fat-saturated images. Normal yellow marrow is high in signal on coronal T1weighted (A, black circle) and T2-weighted images, similar to subcutaneous fat (black arrow) and low in signal on coronal STIR (B, white circle) sequences.
Marrow replacement disorders arise from infiltration of cells other than bone marrow and present as focal or multi-focal lesions. Examples include primary bone tumors, osteomyelitis, lymphoma, and metastases (Fig. 5). Focal areas of red marrow can be difficult to distinguish from metastases, although red marrow will lose signal on in- and out-of-phase imaging, while metastases will not
K. Gaetke-Udager et al.: MR imaging of the pelvis
Fig. 2. 35-year-old woman. Normal yellow marrow on the coronal T1-weighted in-phase image (A, white arrow), does not lose signal on the out-of-phase image (B, white arrow). Normal red marrow seen on the coronal in-phase image
(A, black arrow) loses signal on the out-of-phase image (b, black arrow). Failure of red marrow to lose signal indicates pathology.
[9]. Diffuse marrow replacement can be difficult to identify, for example with increased iron deposition due to sickle cell anemia, thalassemia, chronic blood infusions (Fig. 6), or AIDS. Normal yellow marrow signal should always resemble that of subcutaneous fat; any deviation from this suggests pathology.
phase, in which osteoblasts predominate [11]. The etiology is unknown, although both hereditary and viral causes have been suggested [12]. The disorder can be asymptomatic, or patients can present with deformity and pain. Patients are at increased risk for development of bone neoplasms, usually osteosarcoma; approximately 1 % of patients will have malignant transformation [11]. Cortical bone and trabeculae are thickened in Paget’s disease (Fig. 7), which is better appreciated on radiographs than on MRI. Indeed, if MR imaging findings suggest the presence of Paget’s disease, correlation with radiographs is essential. MR images show the thickened cortex as a thick rim of low-signal intensity, which is distinct from the higher signal intensity marrow on both T1-weighted and T2-weighted images. In the presence of active bone remodeling or periosteal formation, the cortex can have areas of increased signal on T2-weighted images or post-contrast enhancement. Medullary appearance on MRI is variable, depending on the phase
Bone pathology Paget’s disease Paget’s disease was once the second most common metabolic disorder of bone, but both the incidence and prevalence have been decreasing markedly in recent years [10]. The process is seen mostly in older individuals with no gender predilection. Paget’s disease results from a combination of abnormal bone formation and resorption; three pathologic stages have been described, including the lytic phase, in which osteoclasts predominate; the mixed phase, in which osteoblasts attempt repair superimposed on the resorption; and the blastic
K. Gaetke-Udager et al.: MR imaging of the pelvis
Fig. 3. 41-year-old man with femoral metadiaphyseal ‘‘pseudolesion’’. On the coronal STIR image (A), the apparent dermarction (arrow, showing the right pseudolesion) is seen between normal and reconverted marrow bilaterally. The T1weighted image (B) also shows the bilateral demarcation (arrow showing the right pseudolesion).
of disease. In most cases, normal fatty marrow signal is maintained on all pulse sequences. The lytic or early mixed phase can show heterogeneous signal on both T1and T2-weighted images, whereas the late blastic phase can show low signal in the medullary space on all pulse sequences [11]. Sarcomatous degeneration typically shows a soft tissue mass and change in the underling bone marrow signal, with low to intermediate signal on T1-weighted imaging and either high or low signal on T2-weighted images [13].
Avascular necrosis (AVN) ‘‘AVN’’ is a term usually used when referring to osteonecrosis involving subchondral surfaces in the epiphysis,
Fig. 4. 58-year-old man with renal cell carcinoma status post nephrectomy. Axial T2-weighted fat-saturated image shows an asymmetric, hyperintense focus in the right pubic ramus that was overlooked on a post-nephrectomy exam (A, arrow). 4 months later, the osseous lesion had become large and destructive with a soft tissue component seen on the axial T2-weighted fat-saturated image (B, arrow) and was subsequently found to be a renal cell carcinoma metastasis.
whereas ‘‘bone infarct’’ is a term typically used to refer to osteonecrosis in the metadiaphysis of long bones; we will refer to the process as ‘‘AVN’’ in the pelvis. Histologically, the terms represent the same phenomenon. The main etiologies of AVN in the hip and pelvis are trauma, steroids, alcoholism, and radiation therapy; higher doses of steroids (>40 mg per day) show a stronger association with development of AVN. All age groups are affected, and Legg-Calve-Perthes disease is included in this category. AVN is bilateral in many cases and can be acute or chronic. The etiology is likely a combination of factors including vascular damage, metabolic processes, and mechanical stressors. Areas of avascular necrosis show peripheral linear, bright signal on T2-weighted images with an outer, low signal line (the ‘‘double-line’’ sign) (Fig. 8). In the femoral heads, the linear signal abnormality is typically in the 10 to 2 o’clock position. Histopathologically, these
K. Gaetke-Udager et al.: MR imaging of the pelvis
Fig. 5. 63-year-old woman with metastatic breast cancer. Marrow signal in the femoral necks is low on the coronal T1weighted image (A, arrows) and high on the coronal STIR image (B, arrows) due to replacement by metastases. Metastatic lesions are seen throughout the pelvic bones and femurs.
lines represent an area of tissue repair. A half-moon shaped subchondral area of bone marrow edema and/or joint effusion may be present, which indicate active disease. Large lesions that involve more than one-third to one-half of the epiphysis can progress to collapse [14]. Occasionally, when the classic serpiginous rim is not prominent, AVN can be difficult to differentiate from a chondroid lesion.
Osteomyelitis Osteomyelitis can occur by hematogenous spread, contiguous spread (e.g., adjacent abscess), or direct inoculation from a skin ulcer, trauma, or a surgical procedure. The process occurs frequently in diabetic patients and debilitated patients with pressure ulcers. MRI is the preferred modality in evaluation for osteomyelitis, especially acute osteomyelitis, and intravenous contrast is indicated whenever possible to help evaluate for abscess
Fig. 6. 23-year-old woman with secondary hemosiderosis due to numerous blood transfusions for sickle cell anemia. Marrow signal is abnormally low on coronal T1-weighted image (A, white arrowheads) and coronal T2 fat-saturated image (B, white arrows); it should follow the signal of subcutaneous fat, as normal yellow marrow does due to its fat content.
and soft tissue tracts. MRI sensitivity and specificity are currently reported as over 90 % [15]. High-T2 signal in bone marrow without low-T1 signal is a potential pitfall, as this finding could represent reactive bone marrow changes and not osteomyelitis. For this reason, STIR sequences are highly sensitive but less specific than spin-echo sequences for acute osteomyelitis. Surrounding fluid collections/abscesses with cortical breakthrough and post-contrast enhancement are also very suggestive (Fig. 9). In chronic osteomyelitis, tissue planes and intraosseous edema are more sharply demarcated. A peripheral ‘‘rim sign’’ of low T1 and T2 signal may be seen due to devascularized fibrotic or necrotic tissue [12]. One of the pitfalls of MRI is that osteomyelitis often has an aggressive appearance with marrow replacement, which can be confused with malignancy.
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Fig. 8. 43-year-old man with metastatic lung cancer and bilateral femoral head AVN. There are geographic areas with peripheral linear signal abnormality in the femoral heads; the linear signal abnormality is low signal on the coronal T1weighted image (A) and high signal on the coronal fat-saturated T2-weighted image (B, arrows), also known as the ‘‘double line’’ sign. There is no evidence of articular margin collapse.
Fractures Stress fractures
Fig. 7. 88-year-old man with Paget’s disease. There is marked cortical and trabecular thickening of the pelvis with bone expansion, seen best in the iliac bones on the coronal T1-weighted images (A, arrows). These findings are also wellseen on the axial CT image (B, arrows) and pelvis radiograph (C, arrows).
Stress fractures result from overuse without adequate time for osseous adaptation, which can lead to accumulation of microfractures that exceed the remodeling capacity of bone [16]. These fractures occur in normal bone and are often seen in athletes. Stress fractures can be occult on radiographs at the time of presentation, and therefore, MRI is crucial in diagnosing a stress fracture. Common locations are the medial or superolateral part of the femoral neck (Fig. 10), the pubic rami, and the sacrum. The diagnosis may be suspected clinically if the patient has pain in one of these areas caused by a specific activity. Imaging findings of stress fracture on MRI include low-bone marrow signal on T1-weighted images and high-marrow signal on T2weighted images, representing edema. A fracture line must be visible to designate the injury as a stress fracture; this appears as linear low signal on both
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Fig. 9. 44-year-old man with T4 paraplegia and pressure ulcer on the left hip. MR coronal (A) and axial (B) T1-weighted images show a skin defect (A, short arrow) with complete
marrow replacement in the proximal femur (A, long arrow) and ischial tuberosity (B, arrow) consistent with osteomyelitis.
T1-weighted and T2-weighted images. Without the presence of a fracture line, the process should be classified as a stress reaction; the presence of marrow edema and stress reaction is important to recognize and diagnose, as it might alter athletic training and activity. Bone marrow edema should resolve within 6 months of the initial imaging study; if not, a re-injury should be suspected [17]. Osteoid osteoma, particularly in the femoral cortex of a younger person, may be difficult to differentiate from a stress fracture given the cortical thickening, edema, and periosteal reaction that can be seen with this lesion. Osteoid osteoma often has a lucent central area with a nidus, and patients typically have night pain relieved by aspirin.
pubic rami, the sacrum, and the supra-acetabular region. Bisphosphonate use has been associated with subtrochanteric femoral shaft fractures [18]. MR imaging appearance is the same as for stress fractures, with bone marrow edema and a low-signal fracture line on T1-weighted images (Fig. 11). Some studies have reported patients with insufficiency fractures, whose MRI findings simulate AVN of the femoral head; in these cases, the clinical history is paramount in differentiating the two. Patients with insufficiency fractures are typically older, osteopenic, have acute onset of symptoms, have a normal opposite hip, and do not have risk factors for osteonecrosis [19].
Insufficiency fractures
Athletic pubalgia
Insufficiency fractures occur with normal load on abnormal ‘‘weak’’ bone; these fractures are frequently seen in older, osteopenic women who present with hip pain. Classic locations include the superior and inferior
Athletic pubalgia is a syndrome usually characterized by nonspecific, chronic groin pain, although some patients may report sudden pain after twisting or hyperextension. The injury is common in sports that involve twisting at
K. Gaetke-Udager et al.: MR imaging of the pelvis
Fig. 10. 23-year-old female long-distance runner with left hip pain. There is focal, linear low signal in the region of the lesser trochanter on the coronal T1-weighted image (A, arrow) and a
central low-signal line with surrounding high signal on the coronal fat-saturated T2-weighted image (B, arrow) consistent with a stress fracture with bone edema.
the waist and sudden directional change, such as soccer, football, and ice hockey. MRI shows subchondral bone marrow edema surrounding the pubic symphysis (Fig. 12), which is known as osteitis pubis, a mechanical process causing osseous changes [4]. Degenerative change of the pubic symphysis can also occur, and evaluation for subchondral sclerosis and cysts should be made; these findings may be better demonstrated with radiography. Rectus abdominis insertional injury and/or thigh adductor tendon injury can also occur, which manifest as high signal in these muscles and tendons on T2-weighted MR images. Atrophy of the muscles on the injured side suggests a chronic injury [20]. Athletic pubalgia can mimic clinically a number of other causes of pelvic pain, including stress fracture, labral tear, and inguinal hernia. MRI can distinguish these etiologies.
cartilage; the lesions also have a distinctive MRI appearance. The tumor contains multiple hyaline cartilage lobules which are high in signal intensity on T2weighted images and are separated by thin, low signal, fibrous septa (Fig. 13). The internal calcified cartilage seen on radiographs can appear as low-signal foci on both T1- and T2-weighted MRI. Post-contrast images can show enhancing rings and arcs, likely due to blood vessels in the fibrous septa. While enchondromas are much less common in the pelvis than in long bones, they are likely to be encountered in the proximal femurs. Knowledge of imaging findings is important to distinguish these benign tumors from chondrosarcomas. Imaging findings suggesting chondrosarcoma include endosteal scalloping greater than two-thirds the depth of the cortex, cortical destruction, a soft tissue mass, and edema in the bone marrow or adjacent soft tissues on STIR images [5, 21]. Nevertheless, it is difficult and sometimes impossible to distinguish enchondroma from low-grade chondrosarcoma on imaging, and biopsy might be necessary. Occasionally a chondroid lesion can be confused with a bone infarct, as well. Chondroid lesions should have
Benign bone tumors Enchondroma Enchondromas are benign tumors with a classic stippled appearance on radiography due to their calcified
K. Gaetke-Udager et al.: MR imaging of the pelvis
Fig. 12. 22-year-old long-distance runner with athletic pubalgia. A Coronal fat-saturated T2-weighted image shows high signal (arrows) in the bilateral pubic rami extending to the pubic symphysis. B Radiograph of a different patient with osteitis pubis shows sclerosis and irregularity in the bilateral pubic bodies (arrows). Fig. 11. 72-year-old woman with insufficiency fracture of the left sacrum. The fracture appears as low signal on the coronal T1-weighted image (A, arrow) and high signal on the coronal fat-saturated T2-weighted image (B, arrow). Axial CT image on the same patient shows increased sclerosis in the same location corresponding to healing. The location in the sacral ala and corresponding history of pain support the diagnosis.
lobulations and a stippled appearance with central enhancement, whereas a bone infarct will appear as a geographic area, usually with central signal characteris-
tics of fat surrounded by a defined border with the ‘‘double line sign’’ discussed previously.
Osteochondroma Osteochondroma is a cartilage-covered, osseous protuberance, usually pointing away from the nearby joint. [22] On radiography, the lesion either appears pedunculated with a thin pedicle directed away from the growth plate (Fig. 14), or less commonly, a sessile growth with a
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Fig. 13. 50-year-old woman with duodenal adenocarcinoma and right hip pain with incidental enchondroma. The lesion is of low-signal on the coronal T1-weighted image (A, arrow) and high signal on the axial fat-saturated T2-weighted image
(B, arrow) with eccentric positioning and no surrounding edema, endosteal scalloping, or cortical disruption. The radiograph shows a sclerotic, stippled appearance.
broad base. This lesion can be distinguished by the continuity of the cortex and medullary portion of the lesion with the adjacent bone. Patients are typically
young (less than 30) and present with a painless mass. Symptoms can be due either to compression of adjacent structures, such as a nerve, or to inflammatory changes
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Fig. 14. 18-year-old woman with osteochondroma of the right iliac bone. Osseous protuberance arising from the right iliac crest (arrow), which shows contiguity with the medullary
space (arrowhead) on the coronal T1-weighted image (A). The coronal T2-weighted image (B) shows a high-signal cartilage cap (arrowhead).
involving the adventitial bursa covering the cartilaginous cap. In the absence of trauma, nerve compression, or bursitis, the onset of pain in a previously asymptomatic osteochondroma could indicate malignant transformation. The extent of cartilage cap calcification should be closely evaluated: a small, well-defined cap is most likely benign, whereas a large, poorly defined cap, containing irregular or incomplete calcification, must be considered as a possible malignancy. A cartilage cap more than 2 cm thick or an increase in the thickness of the cartilage cap after puberty should raise concern for malignant change [22, 23]. The patterns of calcification are variable and may be irregular; thus, discerning benign from a malignantly transformed lesion may be difficult.
lesions. Lesions arise from the cortex, are usually less than 1.5 cm in size, and have a classically described lucent appearance on CT and radiography with a central sclerotic nidus. MRI shows central high signal on T2-weighted images and post-contrast enhancement, but the central nidus is better seen on CT. There is usually significant surrounding edema on MRI [25].
Osteoid osteoma Osteoid osteomas are benign tumors most commonly seen in patients less than 30 years old (Fig. 15). These lesions are relatively rare in the pelvis but are common in the proximal femurs (20 % of all osteoid osteomas) [24]. Radiologic diagnosis is important because radiofrequency ablation has been successful in treating these
Enostosis (bone island) An enostosis or so-called ‘‘bone island’’ is an area of compact bone in the medullary space surrounded by trabecular bone. These lesions are common and rarely reach sizes greater than 1 cm, except in the pelvis where they can measure up to 2.5 cm (known as a ‘‘giant bone island’’). These benign lesions have a sclerotic appearance with a spiculated margin blended with surrounding trabeculae that is diagnostic on radiography and can also be seen on CT. On MRI, enostoses are very low in signal on all sequences including T2-weighted images, similar to the bone cortex (Fig. 16). These lesions can
K. Gaetke-Udager et al.: MR imaging of the pelvis
occasionally present with large size or exhibit interval growth [26], which can be misleading. Bone scan is often helpful, as an enostosis usually shows no significant radiotracer uptake, but rarely the lesion will show faint uptake [27]. In this situation, attention to the imaging features on radiographs, CT, and MRI with correlation of clinical history should allow for an accurate diagnosis.
Malignant bone tumors Metastases Metastases are the most common malignant osseous lesions in the pelvis [28] and should be included in the differential diagnosis of aggressive-appearing lesions. A patient history of neoplasm, especially of the breast, prostate, or lung, should raise suspicion. Many of the imaging features associated with metastases are nonspecific (see Fig. 3). Metastases usually present as focal lesions with low signal on T1weighted images; surrounding bone marrow edema and cortical destruction are signs of aggressive lesions. Normal red marrow can sometimes simulate the appearance of a metastasis; one or more areas of high signal on T1weighted images within a lesion of low-signal intensity (the so-called ‘‘bull’s eye’’ sign) can indicate an area of red marrow with internal focal fat. On the other hand, a rim of high-T2 signal around an area of low-T1 signal (the ‘‘halo’’ sign) is a reliable indicator of metastasis, especially when there is more than one lesion with a rim of high-T2 signal [29]. Single metastatic lesions can mimic aggressive lesions such as primary bone tumors and infection, especially when large.
Chondrosarcoma
Fig. 15. 21-year-old man with osteoid osteoma of the proximal femur. A Axial T2-weighted, fat-saturated image shows a focus of high signal in the lateral femoral cortex (arrow) with edema in adjacent medullary cavity (arrowhead). B Coronal T2-weighted, fat-saturated image demonstrates the focal high signal (white arrow) with a tiny low-signal nidus (white arrowhead) and surrounding marrow edema (black arrowhead).
Chondrosarcoma occurs most often during the fourth– sixth decades of life [30]. Ninety percent of chondrosarcomas are low grade, despite the fact that most are large (>5 cm) at presentation. In general, chondrosarcomas of the pelvis tend to be primary rather than arising from an existing lesion, and most are central and metaphyseal in location. Most chondrosarcomas exhibit chondroid matrix. Like enchondroma, MRI features of low-grade chondrosarcoma include multiple discrete lobules with high-T2 signal and post-contrast enhancement, but highgrade tumors will have heterogeneous high signal on T2weighted images (Fig. 17). Mineralized chondroid matrix has low-signal intensity on all MRI sequences; low-grade tumors have less chondroid matrix than high-grade tumors [26]. Other differentiating factors between low- and high-grade tumors include more marked endosteal
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Fig. 16. 34-year-old with enostosis. A Axial T1-weighted image shows a small, oval focus of low signal (arrow); this is also low in signal (arrow) on the axial T2-weighted, fat-saturated image (B). There is no surrounding edema.
Fig. 17. 55-year-old man with right hip pain. The right acetabulum shows slight acetabular expansion with low signal on the coronal T1-weighted image (A, arrow) and heterogeneous
high-signal on the coronal fat-saturated T2-weighted image (B, arrow); there was also heterogeneous post-contrast enhancement. Biopsy showed low-grade chondrosarcoma.
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scalloping, osseous expansion, and cortical breakthrough, with the presence of a soft tissue mass in highgrade tumors.
Chordoma Chordoma is a midline tumor that arises from remnants of the embryonic notochord. Although rare overall, the tumor makes up about 50 % of primary sacral malignant neoplasms [31]. The tumor is comprised of abundant mucous separated by fibrous bands, and its most striking imaging feature is relatively high signal on T1-weighted images (Fig. 18). The tumor also has high signal on T2weighted images with multiple isointense septations [32]. Chordomas can be locally aggressive, invading the sacrum and surrounding pelvic muscles. Differential diagnosis includes giant cell tumor and teratoma, and tissue sampling is often necessary to make the diagnosis. Although not often in the midline, a pelvic nerve sheath tumor might have a similar appearance to a small chordoma.
Soft tissue pathology Psoas abscess Psoas (or iliopsoas) abscess can be seen in patients with immunocompromised status, diverticulitis, and
Fig. 18. 69-year-old man with low back and pelvic pain. Sagittal T2-weighted fat-saturated image (A) shows a multilobulated mass in the midline pelvis with mostly high signal and numerous low-signal septations (black arrows). Axial
inflammatory bowel disease, among others. It can present with the classic triad of fever, flank pain, and limping or limited hip flexion, although this triad is seen in less than one-half of affected patients [33]. Spinal osteomyelitis, including spinal tuberculosis infection, can cause a psoas abscess by contiguous spread from the vertebra to the muscle [34]. Findings are typical of abscess elsewhere in the body, with a rim-enhancing fluid collection seen on post-contrast images (Fig. 19). A thorough search for the underlying cause should be made, and imaging can play a key role in distinguishing primary abscess versus secondary abscess due to gastrointestinal, genitourinary, or other infection. In addition, psoas hematoma or other complex fluid collection, such as a postoperative seroma or urinoma, can have similar MRI appearances to an abscess. Hematoma could be distinguished by its high signal on T1-weighted images in the acute and subacute phases; in the chronic phase, T1 signal would be low.
Nerve-related tumors Given the large number of exiting nerves in the pelvis, numerous types of nerve-related entities can occur. Peripheral nerve sheath tumors, either schwannoma or localized neurofibroma, can occur anywhere along a peripheral nerve, and these two histologically different
T1-weighted image (B) shows the mass (white arrow) with characteristic high signal with internal septations. Image guided biopsy confirmed the diagnosis of chordoma.
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Fig. 19. 57-year-old woman with fever, thigh pain, and a soft tissue abscess. MR imaging shows a fluid collection displacing the iliopsoas muscle (arrow) on the axial fat-saturated T2weighted image (A). The axial T1 fat-saturated post-contrast
image (B) shows peripheral enhancement (arrow) of the collection, compatible with abscess culture positive for Staphylococcus.
tumors share many, if not all, imaging features. The tumors are usually less than 5 cm in diameter and appear on MRI as slightly greater signal than muscle on T1weighted images and high signal on T2-weighted images. The ‘‘target sign’’ is sometimes present, in which the center of the tumor is low in signal on T2-weighted images with an outer rim of high signal [35]. Small tumors enhance intensely after gadolinium administration, while larger tumors might have peripheral or more heterogeneous enhancement. The tumor sometimes appears to have a ‘‘tail’’ on either side, representing the entering and exiting nerve. Plexiform neurofibromas are one of the major criteria for diagnosis of neurofibromatosis type 1 and can present in childhood before the appearance of cutaneous lesions; risk for malignant transformation is 8–12 % [36]. These tumors have similar signal characteristics compared with schwannomas but expand and distort a large segment of the nerve, often with a ‘‘bag of worms’’ appearance with multiple lower signal internal septations (Fig. 20). These tumors can grow to massive sizes and involve an entire limb [37]. Imaging signs of malignant degeneration include large size, ill-defined margins, and peripheral enhancement with internal necrosis. PET-CT has a high sensitivity (100 % in one study) for malignant nerve sheath tumors in neurofibromatosis patients [38].
Bone-associated/joint pathologies Sacroiliitis The sacroiliac (SI) joints consist of ligamentous and synovial components. A number of inflammatory conditions can affect the SI joints as part of a systemic or local inflammatory process. Ankylosing spondylitis and inflammatory bowel disease (Fig. 21) typically cause bilateral, fairly symmetric SI joint involvement, osteoarthritis can cause symmetric or asymmetric changes, and psoriatic arthritis and reactive arthritis manifest as asymmetric SI joint involvement. While radiography can help evaluate the SI joints for widening, ankylosis, and erosions, MRI can help evaluate both osseous and soft tissue inflammation, which may not be visualized initially on radiographs. MRI in sacroiliitis can show increased signal in the joint on T2-weighted images representing joint effusion, as well as increased signal on T1-weighted images due to bone erosion and fat infiltration, which can occur early in the disease course. STIR sequences are very sensitive in the detection of bone marrow edema, which is frequently seen with inflammation [39]. Infection of the SI joint is the main differential diagnosis and must be excluded in cases of unilateral disease.
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Fig. 20. 24-year-old man with plexiform neurofibroma with malignant degeneration. Axial MRI of the pelvis shows a large, complex mass (long arrows) extending through the greater sciatic foramen with mostly high signal but with internal lower signal septations on the T2 fat-saturated image (A) and peripheral post-contrast enhancement on the T1 fatsaturated post-gadolinium image (B). Numerous smaller masses (small arrows) are present.
Fig. 21. 35-year-old man with ulcerative colitis and bilateral sacroiliitis. The MR coronal STIR image (A) shows bilateral subchondral edema (arrows) around the SI joints. The coronal CT image (B) shows SI joint erosions (arrows), most prominent on the left. Asymmetric erosions can be seen despite the underlying bilateral process.
Iliopsoas bursitis The iliopsoas bursa, the largest bursa in the body, lies between the iliopsoas tendon and the lesser trochanter, extending upward into the iliac fossa beneath the iliacus muscle; the bursa passes anterior to the acetabulum and hip joint. Normally the bursa contains no fluid, although in some patients, the bursa communicates with the hip joint and can fill with fluid in the presence of a hip effusion. Iliopsoas bursitis is an inflammation of the bursa caused by repetitive use, trauma, or systemic inflammatory disease. Infection is rare [40]. MRI shows the bursa filled with fluid (increased signal on T2-weighted images) and may show post-contrast enhancement of the bursal sac (Fig. 22). Differentiation can be made from paralabral cysts based on the location; fluid does not extend to the labral margin, as it does in a paralabral cyst [7].
Greater trochanter pain syndrome Lateral hip pain over the region of the greater trochanter is a challenging clinical diagnosis due to the complexity of structures in that region. Gluteus medius and/or minimus tendinosis, iliotibial tract pathology, and greater trochanteric bursitis, among other etiologies, can cause lateral hip pain, and all of these can be seen incidentally on pelvic imaging. MRI findings of gluteus medius and minimus tendinosis include thickening of the gluteus medius and minimus tendons with high signal in and surrounding the tendons on T2weighted images (Fig. 23). Calcific tendinosis of the gluteal tendons can also occur, in which the calcium deposits in the tendon cause significant edema (Fig. 23). This process can be exquisitely painful and can present acutely.
K. Gaetke-Udager et al.: MR imaging of the pelvis
at baseline, if there is frank fluid associated with the bursa, and especially if the patient’s pain correlates with the location of the fluid, bursitis is likely the etiology of the pain [7].
Paralabral cysts MRI can demonstrate paralabral cysts, which are of high signal on T2-weighted imaging, located adjacent to the labrum. Paralabral cysts indicate the presence of a labral tear (Fig. 24), which might be occult on non-arthrogram MRI. These cysts are usually lobulated structures located in the anterior or anterosuperior labrum, as these are the most common sites of labral tears, and can have a neck extending toward the acetabular rim. The amount of fluid in the cyst is often disproportionate to the amount of joint fluid [7]. Paralabral cysts can cause anterior hip or groin pain. MRI fluid-sensitive sequences can show labral tears, specifically when there is joint effusion, but are unreliable for diagnosis, and magnetic resonance arthrography is the diagnostic test of choice [42]. The labrum is made of fibrocartilage, low in signal on all sequences, and usually triangular shaped, but it is known to have variation in shape and size. The presence of irregular labral edges, a fluid-filled cleft within the labrum, a paralabral cyst, and/or chondral abnormality favor a labral tear over a normal variant sulcus. A sulcus is variant of normal anatomy in which a gap is present where the labrum meets the acetabulum, and this can mimic a labral tear; both most often occur anteriorly [43]. Fig. 22. 66-year-old woman with intractable right hip pain. There is fluid within the right iliopsoas bursa, which is low signal on the coronal T1-weighted image (A, arrow) and high signal on the coronal fat-saturated T2-weighted image (B, arrow).
While greater trochanteric bursitis has often been thought to be one of the primary culprits causing pain over the greater trochanter, a recent study showed that greater trochanteric pain is most often a combination of pathology involving the gluteus tendons and iliotibial tract, with bursitis present only in a minority of patients [41]. In greater trochanteric bursitis, the bursa becomes fluid-filled and inflamed due to repetitive trauma or adjacent pathology in the gluteus muscle insertions. MRI shows a fluid-filled bursa with post-contrast enhancement of the bursal wall, similar to the findings in iliopsoas bursitis. Although many obese patients have nonspecific edema in the region of the trochanteric bursa
Iatrogenic Bone graft Bone grafts are used to promote healing, provide stability, or both, and they may be used to bridge largebone defects, to aid healing in cases of nonunion, or to bridge joints for arthrodesis. The iliac crest is the most common donor site and is an excellent source of cancellous bone. Complications at the donor site are rare, but fracture and bleeding can occur; less commonly, patients may develop hernias through the donor site, in which fat or other structures can push through the osseous defect [29]. MRI immediately after graft removal from the donor site will show a focal defect with surrounding bone marrow edema and possibly blood products (Fig. 25). Sclerosis at the margins of the defect indicates healing, and sometimes bony excrescences occur
K. Gaetke-Udager et al.: MR imaging of the pelvis
K. Gaetke-Udager et al.: MR imaging of the pelvis
53-year-old woman with gluteus medius tendinosis. Axial (A) and coronal (B) fat-saturated T2-weighted images show thickening and high signal surrounding the gluteus medius tendon (A, short arrows) and fluid tracking along the tendon (B, long arrow) at its attachment to the greater trochanter. 44 year-old man with gluteus medius calcific tendinosis. Axial T2-weighted image (C) shows thickening and irregularity of the tendon at its greater trochanter insertion (arrow) with marked surrounding edema (arrowhead). D Coronal STIR image also shows high T2 signal within the gluteus medius tendon (arrow). Radiograph (E) shows the subtle calcifications (arrow) within the tendon.
b Fig. 23.
imaging and high in signal on T2-weighted imaging (Fig. 26). Complications include hemorrhage in the acute setting, which will appear as adjacent blood products, or infection in the subacute to chronic phase, which will appear as spread of the low signal on T1-weighted images and high signal on T2-weighted images present in the bone beyond the linear tract.
Conclusion
The sequela of recent bone biopsy is recognizable on MRI. The biopsy needle generally creates a linear tract extending from the bone entry point to the biopsied lesion; this tract will appear low in signal on T1-weighted
Abdominal radiologists interpreting MRI of the pelvis may not be familiar with the many forms of musculoskeletal pathology in this region. This essay illustrates and describes both normal and common pathologic appearances of skeletal findings that can be encountered in a pelvic MRI study. In most cases, there is a differential diagnosis to consider based on the imaging appearance. An awareness of the expected imaging findings and relevant clinical history can help distinguish lesions, although some skeletal findings will require further investigation.
Fig. 24. A 41-year-old man with right hip pain and paralabral cyst. Coronal STIR image shows lobulated high signal (arrow) near the superior labrum; this suggests the presence of a labral tear. B 25 year-old woman with paralabral cyst. Sagittal
fat-saturated PD image shows lobulated high signal (arrow) along the anterosuperior labrum, suggesting labral tear. This patient also has subchondral cystic change in the femoral head.
and cause pain; these are better evaluated with radiography or CT.
Bone biopsy
K. Gaetke-Udager et al.: MR imaging of the pelvis
Fig. 25. 57-year-old woman status post bone graft harvest in the left iliac bone. Low signal is seen on the axial T2weighted image (A, arrow) at the harvest site. Heterogeneous
increased and decreased signal (B, arrow) is seen on an adjacent axial T2-weighted image representing blood products from the procedure.
Fig. 26. 51-year-old woman with chronic lymphocytic leukemia status post recent bone marrow biopsy in the left iliac bone. The arrow on the axial T1-weighted image (A) shows
the linear low T1 signal from the biopsy tract; the arrow on the axial fat-saturated T2-weighted image shows the corresponding high signal in the tract consistent with edema.
K. Gaetke-Udager et al.: MR imaging of the pelvis
Acknowledgments. The authors wish to acknowledge Olaf Magerkuth, MD, for his assistance in preparing this manuscript. Disclosures. We would like to make the following disclosures for author Jon Jacobson, Consultant: BioClinica, Book Royalties: Elsevier, Grant: American Institute of Ultrasound in Medicine, Research Equipment: Harvest Technologies
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20. Mullens FE, Zoga AC, Morrison WB, Meyers WC (2012) Review of MRI technique and imaging findings in athletic pubalgia and the ‘‘sports hernia’’. Eur J Radiol 81(12):3780–3792 21. Choi BB, Jee WH, Sunwoo HJ, et al. (2013) MR differentiation of low-grade chondrosarcoma from enchondroma. Clin Imaging 37(3):542–547 22. Brien EW, Mirra JM, Luck JV Jr (1999) Benign and malignant cartilage tumors of bone and joint: their anatomic and theoretical basis with an emphasis on radiology, pathology and clinical biology. II. Juxtacortical cartilage tumors. Skeletal Radiol 28(1):1–20 23. Girish G, Finlay K, Morag Y, et al. (2012) Imaging review of skeletal tumors of the pelvis—part I: benign tumors of the pelvis. Sci World J 2012:290930 24. Bloem JL, Reidsma II (2012) Bone and soft tissue tumors of hip and pelvis. Eur J Radiol 81(12):3793–3801 25. Schaefer MP, Smith J (2003) The diagnostic and therapeutic challenge of femoral head osteoid osteoma presenting as thigh pain: a case report. Arch Phys Med Rehabil 84(6):904–905 26. Manaster J, May DA, Disler DG (2013) Musculoskeletal Imaging: The Requisites, 4th edn. Philadelphia: Saunders 27. Greenspan A (1995) Bone island (enostosis): current concept–a review. Skeletal Radiol 24(2):111–115 28. Coleman RE (2006) Clinical features of metastatic bone disease and risk of skeletal morbidity. Clin Cancer Res 12(20 Pt 2):6243s–6249s 29. Resnick D, Kransdorf M (2004) Bone and Joint Imaging, 3rd edn. Philadelphia: Saunders 30. Anderson SE, Steinbach LS, Schlicht S, et al. (2007) Magnetic resonance imaging of bone tumors and joints. Top Magn Reson Imaging 18(6):457–465 31. Gerber S, Ollivier L, Leclere J, et al. (2008) Imaging of sacral tumours. Skeletal Radiol 37(4):277–289 32. Si MJ, Wang CS, Ding XY, et al. (2013) Differentiation of primary chordoma, giant cell tumor and schwannoma of the sacrum by CT and MRI. Eur J Radiol 82(12):2309–2315 33. Tonolini M, Campari A, Bianco R (2012) Common and unusual diseases involving the iliopsoas muscle compartment: spectrum of cross-sectional imaging findings. Abdom Imaging 37(1):118–139 34. O’Keeffe AB, Terris J, Hormbrey P (2012) A sinister cause of back pain in a young man. BMJ 344:e3015 35. Woertler K (2010) Tumors and tumor-like lesions of peripheral nerves. Semin Musculoskelet Radiol 14(5):547–558 36. Weiss SW, Goldblum JR, Enzinger FM (2001) Soft Tissue Tumors, 4th edn. St. Louis: Mosby 37. Ros PR, Eshaghi N (1991) Plexiform neurofibroma of the pelvis: CT and MRI findings. Magn Reson Imaging 9(3):463–465 38. Derlin T, Tornquist K, Munster S, et al. (2013) Comparative effectiveness of 18F-FDG PET/CT versus whole-body MRI for detection of malignant peripheral nerve sheath tumors in neurofibromatosis type 1. Clin Nucl Med 38(1):e19–e25 39. Guglielmi G, Scalzo G, Cascavilla A, et al. (2009) Imaging of the sacroiliac joint involvement in seronegative spondylarthropathies. Clin Rheumatol 28(9):1007–1019 40. Wunderbaldinger P, Bremer C, Schellenberger E, et al. (2002) Imaging features of iliopsoas bursitis. Eur Radiol 12(2):409–415 41. Long SS, Surrey DE, Nazarian LN (2013) Sonography of greater trochanteric pain syndrome and the rarity of primary bursitis. AJR 201(5):1083–1086 42. Groh MM, Herrera J (2009) A comprehensive review of hip labral tears. Curr Rev Musculoskelet Med 2(2):105–117 43. Nguyen MS, Kheyfits V, Giordano BD, Dieudonne G, Monu JU (2013) Hip anatomic variants that may mimic abnormalities at MRI: labral variants. AJR 201(3):W394–W400
Abdominal Imaging
Abdom Imaging (2014) DOI: 10.1007/s00261-014-0108-y
MR imaging of the pelvis: a guide to incidental musculoskeletal findings for abdominal radiologists Kara Gaetke-Udager, Gandikota Girish, Ravi K. Kaza, Jon Jacobson, David Fessell, Yoav Morag, David Jamadar Department of Radiology, University of Michigan Health System, 1500 E Medical Center Drive, TC 2910, Ann Arbor, MI 48109, USA
Abstract Occasionally patients who undergo magnetic resonance imaging for presumed pelvic disease demonstrate unexpected musculoskeletal imaging findings in the imaged field. Such incidental findings can be challenging to the abdominal radiologist, who may not be familiar with their appearance or know the appropriate diagnostic considerations. Findings can include both normal and abnormal bone marrow, osseous abnormalities such as Paget’s disease, avascular necrosis, osteomyelitis, stress and insufficiency fractures, and athletic pubalgia, benign neoplasms such as enchondroma and bone island, malignant processes such as metastasis and chondrosarcoma, soft tissue processes such as abscess, nerve-related tumors, and chordoma, joint- and bursal-related processes such as sacroiliitis, iliopsoas bursitis, greater trochanteric pain syndrome, and labral tears, and iatrogenic processes such as bone graft or bone biopsy. Though not all-encompassing, this essay will help abdominal radiologists to identify and describe this variety of pelvic musculoskeletal conditions, understand key radiologic findings, and synthesize a differential diagnosis when appropriate. Key words: Pelvic MRI—Incidental findings—Musculoskeletal MRI—Abdominal radiologists
Patients who undergo MRI to evaluate for pelvic disease can also have a variety of musculoskeletal imaging findings. Such findings can range from incidental to symptomatic and can include pathologic processes as fracture, infection, inflammation, and neoplasm. Correspondence to: Gandikota Girish; email: [email protected]
Though not all-inclusive, the purpose of this pictorial essay is to review incidental musculoskeletal imaging findings that can be encountered by the abdominal radiologist and to aid in developing a differential diagnosis.
Bone marrow Bone marrow consists of trabecular bone, red (hematopoietic) marrow, and yellow (fatty) marrow. The amount and distribution of red and yellow marrow change with age [1] undergoing a process called marrow conversion. With increasing age the red marrow present at birth is replaced by yellow marrow in a predictable pattern, starting with the long-bone diaphyses, and then extending to involve the epiphyses, distal metaphyses, and proximal metaphyses, sequentially. Most marrow conversion is typically completed by the age of 25 and is symmetric; marked asymmetry should raise suspicion for a pathologic process [1–3]. Residual red marrow is seen in the proximal femoral metaphysis [1]. Reconversion, a reverse process involving the transformation of yellow to red marrow, is seen in states of increased hematopoiesis. Reconversion of marrow is seen in cases of sickle cell anemia, thalassemia, high altitudes, and high-level athletes. This process is also symmetric. Focal areas of fatty marrow in the midst of normal red marrow are of no clinical significance [1]. The most useful MRI sequence for marrow evaluation is the T1-weighted spin-echo sequence, although T2 and short tau inversion recovery (STIR) sequences are also helpful; the proton density sequence is not a substitute for the T1-weighted sequence. Normal yellow marrow follows the signal of fat in T1- and T2-weighted imaging, STIR, and fat-saturated sequences [4] (Fig. 1). Normal red marrow shows intermediate signal intensity
K. Gaetke-Udager et al.: MR imaging of the pelvis
on T1- and T2-weighted images and is isointense to muscle on STIR and fat-saturated images (Fig. 1). Red marrow should be lower in signal intensity than yellow marrow on T1-weighted images and hyperintense to yellow marrow on T2-weighted images [5]. These differences are due to the relative fat content of marrow; normal red marrow contains some fat (40 %), but overall the fat content of yellow marrow is much higher (80 %) [6]. Yellow marrow does not lose signal on out-of-phase images compared to in-phase images because of its highfat content (Fig. 2) [6]. Red marrow contains equal parts
of fat and water, and therefore, its signal drops a great deal on out-of-phase images compared to in-phase images (Fig. 2). There can be an apparent demarcation between normal yellow marrow and reconverted marrow in the proximal femoral metadiaphyses, sometimes referred to as a ‘‘pseudolesion’’ (Fig. 3) [7]. This sharp demarcation is due to the interface between yellow and reconverted red marrow, and can mimic a focal lesion. Familiarity with this entity and its bilateral symmetric appearance should prevent misdiagnosis.
Marrow pathology Imaging findings in various marrow pathologies are usually nonspecific and only serve to raise suspicion for a disease process [4, 8]. Indicators of abnormal marrow include an abnormal distribution or asymmetry of the normal marrow pattern or abnormal marrow signal (Fig. 4). Pathologic processes infiltrate and replace the normal fat content of marrow, and therefore, show lower signal than muscle or intervertebral disks on the T1-weighted images and higher signal than fat on the T2-weighted images. Neoplasm does not typically contain microscopic lipid and, therefore, should not lose signal on out-of-phase images, while red marrow and benign lesions such as fracture will lose signal because of the presence of microscopic fat [5].
Marrow proliferative disorders Marrow proliferative disorders are generally diffuse processes arising from overproduction of cells originating in the bone marrow. Examples include myelofibrosis, polycythemia vera, myelodysplastic syndrome, leukemia, multiple myeloma, and amyloidosis; multiple myeloma may also exist in a focal form. Marrow can sometimes appear normal despite the presence of a disease process, as the marrow fat cells might not yet have been replaced. As the process continues, the pathologic areas become lower in signal intensity than muscle on T1-weighted images and generally higher in signal than muscle on T2weighted images [9].
Marrow replacement disorders Fig. 1. 41-year-old man. Normal red marrow is intermediate in signal on coronal T1-weighted (A, between white arrowheads) and T2-weighted images and isointense to muscle on coronal STIR (B, white arrow) and fat-saturated images. Normal yellow marrow is high in signal on coronal T1weighted (A, black circle) and T2-weighted images, similar to subcutaneous fat (black arrow) and low in signal on coronal STIR (B, white circle) sequences.
Marrow replacement disorders arise from infiltration of cells other than bone marrow and present as focal or multi-focal lesions. Examples include primary bone tumors, osteomyelitis, lymphoma, and metastases (Fig. 5). Focal areas of red marrow can be difficult to distinguish from metastases, although red marrow will lose signal on in- and out-of-phase imaging, while metastases will not
K. Gaetke-Udager et al.: MR imaging of the pelvis
Fig. 2. 35-year-old woman. Normal yellow marrow on the coronal T1-weighted in-phase image (A, white arrow), does not lose signal on the out-of-phase image (B, white arrow). Normal red marrow seen on the coronal in-phase image
(A, black arrow) loses signal on the out-of-phase image (b, black arrow). Failure of red marrow to lose signal indicates pathology.
[9]. Diffuse marrow replacement can be difficult to identify, for example with increased iron deposition due to sickle cell anemia, thalassemia, chronic blood infusions (Fig. 6), or AIDS. Normal yellow marrow signal should always resemble that of subcutaneous fat; any deviation from this suggests pathology.
phase, in which osteoblasts predominate [11]. The etiology is unknown, although both hereditary and viral causes have been suggested [12]. The disorder can be asymptomatic, or patients can present with deformity and pain. Patients are at increased risk for development of bone neoplasms, usually osteosarcoma; approximately 1 % of patients will have malignant transformation [11]. Cortical bone and trabeculae are thickened in Paget’s disease (Fig. 7), which is better appreciated on radiographs than on MRI. Indeed, if MR imaging findings suggest the presence of Paget’s disease, correlation with radiographs is essential. MR images show the thickened cortex as a thick rim of low-signal intensity, which is distinct from the higher signal intensity marrow on both T1-weighted and T2-weighted images. In the presence of active bone remodeling or periosteal formation, the cortex can have areas of increased signal on T2-weighted images or post-contrast enhancement. Medullary appearance on MRI is variable, depending on the phase
Bone pathology Paget’s disease Paget’s disease was once the second most common metabolic disorder of bone, but both the incidence and prevalence have been decreasing markedly in recent years [10]. The process is seen mostly in older individuals with no gender predilection. Paget’s disease results from a combination of abnormal bone formation and resorption; three pathologic stages have been described, including the lytic phase, in which osteoclasts predominate; the mixed phase, in which osteoblasts attempt repair superimposed on the resorption; and the blastic
K. Gaetke-Udager et al.: MR imaging of the pelvis
Fig. 3. 41-year-old man with femoral metadiaphyseal ‘‘pseudolesion’’. On the coronal STIR image (A), the apparent dermarction (arrow, showing the right pseudolesion) is seen between normal and reconverted marrow bilaterally. The T1weighted image (B) also shows the bilateral demarcation (arrow showing the right pseudolesion).
of disease. In most cases, normal fatty marrow signal is maintained on all pulse sequences. The lytic or early mixed phase can show heterogeneous signal on both T1and T2-weighted images, whereas the late blastic phase can show low signal in the medullary space on all pulse sequences [11]. Sarcomatous degeneration typically shows a soft tissue mass and change in the underling bone marrow signal, with low to intermediate signal on T1-weighted imaging and either high or low signal on T2-weighted images [13].
Avascular necrosis (AVN) ‘‘AVN’’ is a term usually used when referring to osteonecrosis involving subchondral surfaces in the epiphysis,
Fig. 4. 58-year-old man with renal cell carcinoma status post nephrectomy. Axial T2-weighted fat-saturated image shows an asymmetric, hyperintense focus in the right pubic ramus that was overlooked on a post-nephrectomy exam (A, arrow). 4 months later, the osseous lesion had become large and destructive with a soft tissue component seen on the axial T2-weighted fat-saturated image (B, arrow) and was subsequently found to be a renal cell carcinoma metastasis.
whereas ‘‘bone infarct’’ is a term typically used to refer to osteonecrosis in the metadiaphysis of long bones; we will refer to the process as ‘‘AVN’’ in the pelvis. Histologically, the terms represent the same phenomenon. The main etiologies of AVN in the hip and pelvis are trauma, steroids, alcoholism, and radiation therapy; higher doses of steroids (>40 mg per day) show a stronger association with development of AVN. All age groups are affected, and Legg-Calve-Perthes disease is included in this category. AVN is bilateral in many cases and can be acute or chronic. The etiology is likely a combination of factors including vascular damage, metabolic processes, and mechanical stressors. Areas of avascular necrosis show peripheral linear, bright signal on T2-weighted images with an outer, low signal line (the ‘‘double-line’’ sign) (Fig. 8). In the femoral heads, the linear signal abnormality is typically in the 10 to 2 o’clock position. Histopathologically, these
K. Gaetke-Udager et al.: MR imaging of the pelvis
Fig. 5. 63-year-old woman with metastatic breast cancer. Marrow signal in the femoral necks is low on the coronal T1weighted image (A, arrows) and high on the coronal STIR image (B, arrows) due to replacement by metastases. Metastatic lesions are seen throughout the pelvic bones and femurs.
lines represent an area of tissue repair. A half-moon shaped subchondral area of bone marrow edema and/or joint effusion may be present, which indicate active disease. Large lesions that involve more than one-third to one-half of the epiphysis can progress to collapse [14]. Occasionally, when the classic serpiginous rim is not prominent, AVN can be difficult to differentiate from a chondroid lesion.
Osteomyelitis Osteomyelitis can occur by hematogenous spread, contiguous spread (e.g., adjacent abscess), or direct inoculation from a skin ulcer, trauma, or a surgical procedure. The process occurs frequently in diabetic patients and debilitated patients with pressure ulcers. MRI is the preferred modality in evaluation for osteomyelitis, especially acute osteomyelitis, and intravenous contrast is indicated whenever possible to help evaluate for abscess
Fig. 6. 23-year-old woman with secondary hemosiderosis due to numerous blood transfusions for sickle cell anemia. Marrow signal is abnormally low on coronal T1-weighted image (A, white arrowheads) and coronal T2 fat-saturated image (B, white arrows); it should follow the signal of subcutaneous fat, as normal yellow marrow does due to its fat content.
and soft tissue tracts. MRI sensitivity and specificity are currently reported as over 90 % [15]. High-T2 signal in bone marrow without low-T1 signal is a potential pitfall, as this finding could represent reactive bone marrow changes and not osteomyelitis. For this reason, STIR sequences are highly sensitive but less specific than spin-echo sequences for acute osteomyelitis. Surrounding fluid collections/abscesses with cortical breakthrough and post-contrast enhancement are also very suggestive (Fig. 9). In chronic osteomyelitis, tissue planes and intraosseous edema are more sharply demarcated. A peripheral ‘‘rim sign’’ of low T1 and T2 signal may be seen due to devascularized fibrotic or necrotic tissue [12]. One of the pitfalls of MRI is that osteomyelitis often has an aggressive appearance with marrow replacement, which can be confused with malignancy.
K. Gaetke-Udager et al.: MR imaging of the pelvis
Fig. 8. 43-year-old man with metastatic lung cancer and bilateral femoral head AVN. There are geographic areas with peripheral linear signal abnormality in the femoral heads; the linear signal abnormality is low signal on the coronal T1weighted image (A) and high signal on the coronal fat-saturated T2-weighted image (B, arrows), also known as the ‘‘double line’’ sign. There is no evidence of articular margin collapse.
Fractures Stress fractures
Fig. 7. 88-year-old man with Paget’s disease. There is marked cortical and trabecular thickening of the pelvis with bone expansion, seen best in the iliac bones on the coronal T1-weighted images (A, arrows). These findings are also wellseen on the axial CT image (B, arrows) and pelvis radiograph (C, arrows).
Stress fractures result from overuse without adequate time for osseous adaptation, which can lead to accumulation of microfractures that exceed the remodeling capacity of bone [16]. These fractures occur in normal bone and are often seen in athletes. Stress fractures can be occult on radiographs at the time of presentation, and therefore, MRI is crucial in diagnosing a stress fracture. Common locations are the medial or superolateral part of the femoral neck (Fig. 10), the pubic rami, and the sacrum. The diagnosis may be suspected clinically if the patient has pain in one of these areas caused by a specific activity. Imaging findings of stress fracture on MRI include low-bone marrow signal on T1-weighted images and high-marrow signal on T2weighted images, representing edema. A fracture line must be visible to designate the injury as a stress fracture; this appears as linear low signal on both
K. Gaetke-Udager et al.: MR imaging of the pelvis
Fig. 9. 44-year-old man with T4 paraplegia and pressure ulcer on the left hip. MR coronal (A) and axial (B) T1-weighted images show a skin defect (A, short arrow) with complete
marrow replacement in the proximal femur (A, long arrow) and ischial tuberosity (B, arrow) consistent with osteomyelitis.
T1-weighted and T2-weighted images. Without the presence of a fracture line, the process should be classified as a stress reaction; the presence of marrow edema and stress reaction is important to recognize and diagnose, as it might alter athletic training and activity. Bone marrow edema should resolve within 6 months of the initial imaging study; if not, a re-injury should be suspected [17]. Osteoid osteoma, particularly in the femoral cortex of a younger person, may be difficult to differentiate from a stress fracture given the cortical thickening, edema, and periosteal reaction that can be seen with this lesion. Osteoid osteoma often has a lucent central area with a nidus, and patients typically have night pain relieved by aspirin.
pubic rami, the sacrum, and the supra-acetabular region. Bisphosphonate use has been associated with subtrochanteric femoral shaft fractures [18]. MR imaging appearance is the same as for stress fractures, with bone marrow edema and a low-signal fracture line on T1-weighted images (Fig. 11). Some studies have reported patients with insufficiency fractures, whose MRI findings simulate AVN of the femoral head; in these cases, the clinical history is paramount in differentiating the two. Patients with insufficiency fractures are typically older, osteopenic, have acute onset of symptoms, have a normal opposite hip, and do not have risk factors for osteonecrosis [19].
Insufficiency fractures
Athletic pubalgia
Insufficiency fractures occur with normal load on abnormal ‘‘weak’’ bone; these fractures are frequently seen in older, osteopenic women who present with hip pain. Classic locations include the superior and inferior
Athletic pubalgia is a syndrome usually characterized by nonspecific, chronic groin pain, although some patients may report sudden pain after twisting or hyperextension. The injury is common in sports that involve twisting at
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Fig. 10. 23-year-old female long-distance runner with left hip pain. There is focal, linear low signal in the region of the lesser trochanter on the coronal T1-weighted image (A, arrow) and a
central low-signal line with surrounding high signal on the coronal fat-saturated T2-weighted image (B, arrow) consistent with a stress fracture with bone edema.
the waist and sudden directional change, such as soccer, football, and ice hockey. MRI shows subchondral bone marrow edema surrounding the pubic symphysis (Fig. 12), which is known as osteitis pubis, a mechanical process causing osseous changes [4]. Degenerative change of the pubic symphysis can also occur, and evaluation for subchondral sclerosis and cysts should be made; these findings may be better demonstrated with radiography. Rectus abdominis insertional injury and/or thigh adductor tendon injury can also occur, which manifest as high signal in these muscles and tendons on T2-weighted MR images. Atrophy of the muscles on the injured side suggests a chronic injury [20]. Athletic pubalgia can mimic clinically a number of other causes of pelvic pain, including stress fracture, labral tear, and inguinal hernia. MRI can distinguish these etiologies.
cartilage; the lesions also have a distinctive MRI appearance. The tumor contains multiple hyaline cartilage lobules which are high in signal intensity on T2weighted images and are separated by thin, low signal, fibrous septa (Fig. 13). The internal calcified cartilage seen on radiographs can appear as low-signal foci on both T1- and T2-weighted MRI. Post-contrast images can show enhancing rings and arcs, likely due to blood vessels in the fibrous septa. While enchondromas are much less common in the pelvis than in long bones, they are likely to be encountered in the proximal femurs. Knowledge of imaging findings is important to distinguish these benign tumors from chondrosarcomas. Imaging findings suggesting chondrosarcoma include endosteal scalloping greater than two-thirds the depth of the cortex, cortical destruction, a soft tissue mass, and edema in the bone marrow or adjacent soft tissues on STIR images [5, 21]. Nevertheless, it is difficult and sometimes impossible to distinguish enchondroma from low-grade chondrosarcoma on imaging, and biopsy might be necessary. Occasionally a chondroid lesion can be confused with a bone infarct, as well. Chondroid lesions should have
Benign bone tumors Enchondroma Enchondromas are benign tumors with a classic stippled appearance on radiography due to their calcified
K. Gaetke-Udager et al.: MR imaging of the pelvis
Fig. 12. 22-year-old long-distance runner with athletic pubalgia. A Coronal fat-saturated T2-weighted image shows high signal (arrows) in the bilateral pubic rami extending to the pubic symphysis. B Radiograph of a different patient with osteitis pubis shows sclerosis and irregularity in the bilateral pubic bodies (arrows). Fig. 11. 72-year-old woman with insufficiency fracture of the left sacrum. The fracture appears as low signal on the coronal T1-weighted image (A, arrow) and high signal on the coronal fat-saturated T2-weighted image (B, arrow). Axial CT image on the same patient shows increased sclerosis in the same location corresponding to healing. The location in the sacral ala and corresponding history of pain support the diagnosis.
lobulations and a stippled appearance with central enhancement, whereas a bone infarct will appear as a geographic area, usually with central signal characteris-
tics of fat surrounded by a defined border with the ‘‘double line sign’’ discussed previously.
Osteochondroma Osteochondroma is a cartilage-covered, osseous protuberance, usually pointing away from the nearby joint. [22] On radiography, the lesion either appears pedunculated with a thin pedicle directed away from the growth plate (Fig. 14), or less commonly, a sessile growth with a
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Fig. 13. 50-year-old woman with duodenal adenocarcinoma and right hip pain with incidental enchondroma. The lesion is of low-signal on the coronal T1-weighted image (A, arrow) and high signal on the axial fat-saturated T2-weighted image
(B, arrow) with eccentric positioning and no surrounding edema, endosteal scalloping, or cortical disruption. The radiograph shows a sclerotic, stippled appearance.
broad base. This lesion can be distinguished by the continuity of the cortex and medullary portion of the lesion with the adjacent bone. Patients are typically
young (less than 30) and present with a painless mass. Symptoms can be due either to compression of adjacent structures, such as a nerve, or to inflammatory changes
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Fig. 14. 18-year-old woman with osteochondroma of the right iliac bone. Osseous protuberance arising from the right iliac crest (arrow), which shows contiguity with the medullary
space (arrowhead) on the coronal T1-weighted image (A). The coronal T2-weighted image (B) shows a high-signal cartilage cap (arrowhead).
involving the adventitial bursa covering the cartilaginous cap. In the absence of trauma, nerve compression, or bursitis, the onset of pain in a previously asymptomatic osteochondroma could indicate malignant transformation. The extent of cartilage cap calcification should be closely evaluated: a small, well-defined cap is most likely benign, whereas a large, poorly defined cap, containing irregular or incomplete calcification, must be considered as a possible malignancy. A cartilage cap more than 2 cm thick or an increase in the thickness of the cartilage cap after puberty should raise concern for malignant change [22, 23]. The patterns of calcification are variable and may be irregular; thus, discerning benign from a malignantly transformed lesion may be difficult.
lesions. Lesions arise from the cortex, are usually less than 1.5 cm in size, and have a classically described lucent appearance on CT and radiography with a central sclerotic nidus. MRI shows central high signal on T2-weighted images and post-contrast enhancement, but the central nidus is better seen on CT. There is usually significant surrounding edema on MRI [25].
Osteoid osteoma Osteoid osteomas are benign tumors most commonly seen in patients less than 30 years old (Fig. 15). These lesions are relatively rare in the pelvis but are common in the proximal femurs (20 % of all osteoid osteomas) [24]. Radiologic diagnosis is important because radiofrequency ablation has been successful in treating these
Enostosis (bone island) An enostosis or so-called ‘‘bone island’’ is an area of compact bone in the medullary space surrounded by trabecular bone. These lesions are common and rarely reach sizes greater than 1 cm, except in the pelvis where they can measure up to 2.5 cm (known as a ‘‘giant bone island’’). These benign lesions have a sclerotic appearance with a spiculated margin blended with surrounding trabeculae that is diagnostic on radiography and can also be seen on CT. On MRI, enostoses are very low in signal on all sequences including T2-weighted images, similar to the bone cortex (Fig. 16). These lesions can
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occasionally present with large size or exhibit interval growth [26], which can be misleading. Bone scan is often helpful, as an enostosis usually shows no significant radiotracer uptake, but rarely the lesion will show faint uptake [27]. In this situation, attention to the imaging features on radiographs, CT, and MRI with correlation of clinical history should allow for an accurate diagnosis.
Malignant bone tumors Metastases Metastases are the most common malignant osseous lesions in the pelvis [28] and should be included in the differential diagnosis of aggressive-appearing lesions. A patient history of neoplasm, especially of the breast, prostate, or lung, should raise suspicion. Many of the imaging features associated with metastases are nonspecific (see Fig. 3). Metastases usually present as focal lesions with low signal on T1weighted images; surrounding bone marrow edema and cortical destruction are signs of aggressive lesions. Normal red marrow can sometimes simulate the appearance of a metastasis; one or more areas of high signal on T1weighted images within a lesion of low-signal intensity (the so-called ‘‘bull’s eye’’ sign) can indicate an area of red marrow with internal focal fat. On the other hand, a rim of high-T2 signal around an area of low-T1 signal (the ‘‘halo’’ sign) is a reliable indicator of metastasis, especially when there is more than one lesion with a rim of high-T2 signal [29]. Single metastatic lesions can mimic aggressive lesions such as primary bone tumors and infection, especially when large.
Chondrosarcoma
Fig. 15. 21-year-old man with osteoid osteoma of the proximal femur. A Axial T2-weighted, fat-saturated image shows a focus of high signal in the lateral femoral cortex (arrow) with edema in adjacent medullary cavity (arrowhead). B Coronal T2-weighted, fat-saturated image demonstrates the focal high signal (white arrow) with a tiny low-signal nidus (white arrowhead) and surrounding marrow edema (black arrowhead).
Chondrosarcoma occurs most often during the fourth– sixth decades of life [30]. Ninety percent of chondrosarcomas are low grade, despite the fact that most are large (>5 cm) at presentation. In general, chondrosarcomas of the pelvis tend to be primary rather than arising from an existing lesion, and most are central and metaphyseal in location. Most chondrosarcomas exhibit chondroid matrix. Like enchondroma, MRI features of low-grade chondrosarcoma include multiple discrete lobules with high-T2 signal and post-contrast enhancement, but highgrade tumors will have heterogeneous high signal on T2weighted images (Fig. 17). Mineralized chondroid matrix has low-signal intensity on all MRI sequences; low-grade tumors have less chondroid matrix than high-grade tumors [26]. Other differentiating factors between low- and high-grade tumors include more marked endosteal
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Fig. 16. 34-year-old with enostosis. A Axial T1-weighted image shows a small, oval focus of low signal (arrow); this is also low in signal (arrow) on the axial T2-weighted, fat-saturated image (B). There is no surrounding edema.
Fig. 17. 55-year-old man with right hip pain. The right acetabulum shows slight acetabular expansion with low signal on the coronal T1-weighted image (A, arrow) and heterogeneous
high-signal on the coronal fat-saturated T2-weighted image (B, arrow); there was also heterogeneous post-contrast enhancement. Biopsy showed low-grade chondrosarcoma.
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scalloping, osseous expansion, and cortical breakthrough, with the presence of a soft tissue mass in highgrade tumors.
Chordoma Chordoma is a midline tumor that arises from remnants of the embryonic notochord. Although rare overall, the tumor makes up about 50 % of primary sacral malignant neoplasms [31]. The tumor is comprised of abundant mucous separated by fibrous bands, and its most striking imaging feature is relatively high signal on T1-weighted images (Fig. 18). The tumor also has high signal on T2weighted images with multiple isointense septations [32]. Chordomas can be locally aggressive, invading the sacrum and surrounding pelvic muscles. Differential diagnosis includes giant cell tumor and teratoma, and tissue sampling is often necessary to make the diagnosis. Although not often in the midline, a pelvic nerve sheath tumor might have a similar appearance to a small chordoma.
Soft tissue pathology Psoas abscess Psoas (or iliopsoas) abscess can be seen in patients with immunocompromised status, diverticulitis, and
Fig. 18. 69-year-old man with low back and pelvic pain. Sagittal T2-weighted fat-saturated image (A) shows a multilobulated mass in the midline pelvis with mostly high signal and numerous low-signal septations (black arrows). Axial
inflammatory bowel disease, among others. It can present with the classic triad of fever, flank pain, and limping or limited hip flexion, although this triad is seen in less than one-half of affected patients [33]. Spinal osteomyelitis, including spinal tuberculosis infection, can cause a psoas abscess by contiguous spread from the vertebra to the muscle [34]. Findings are typical of abscess elsewhere in the body, with a rim-enhancing fluid collection seen on post-contrast images (Fig. 19). A thorough search for the underlying cause should be made, and imaging can play a key role in distinguishing primary abscess versus secondary abscess due to gastrointestinal, genitourinary, or other infection. In addition, psoas hematoma or other complex fluid collection, such as a postoperative seroma or urinoma, can have similar MRI appearances to an abscess. Hematoma could be distinguished by its high signal on T1-weighted images in the acute and subacute phases; in the chronic phase, T1 signal would be low.
Nerve-related tumors Given the large number of exiting nerves in the pelvis, numerous types of nerve-related entities can occur. Peripheral nerve sheath tumors, either schwannoma or localized neurofibroma, can occur anywhere along a peripheral nerve, and these two histologically different
T1-weighted image (B) shows the mass (white arrow) with characteristic high signal with internal septations. Image guided biopsy confirmed the diagnosis of chordoma.
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Fig. 19. 57-year-old woman with fever, thigh pain, and a soft tissue abscess. MR imaging shows a fluid collection displacing the iliopsoas muscle (arrow) on the axial fat-saturated T2weighted image (A). The axial T1 fat-saturated post-contrast
image (B) shows peripheral enhancement (arrow) of the collection, compatible with abscess culture positive for Staphylococcus.
tumors share many, if not all, imaging features. The tumors are usually less than 5 cm in diameter and appear on MRI as slightly greater signal than muscle on T1weighted images and high signal on T2-weighted images. The ‘‘target sign’’ is sometimes present, in which the center of the tumor is low in signal on T2-weighted images with an outer rim of high signal [35]. Small tumors enhance intensely after gadolinium administration, while larger tumors might have peripheral or more heterogeneous enhancement. The tumor sometimes appears to have a ‘‘tail’’ on either side, representing the entering and exiting nerve. Plexiform neurofibromas are one of the major criteria for diagnosis of neurofibromatosis type 1 and can present in childhood before the appearance of cutaneous lesions; risk for malignant transformation is 8–12 % [36]. These tumors have similar signal characteristics compared with schwannomas but expand and distort a large segment of the nerve, often with a ‘‘bag of worms’’ appearance with multiple lower signal internal septations (Fig. 20). These tumors can grow to massive sizes and involve an entire limb [37]. Imaging signs of malignant degeneration include large size, ill-defined margins, and peripheral enhancement with internal necrosis. PET-CT has a high sensitivity (100 % in one study) for malignant nerve sheath tumors in neurofibromatosis patients [38].
Bone-associated/joint pathologies Sacroiliitis The sacroiliac (SI) joints consist of ligamentous and synovial components. A number of inflammatory conditions can affect the SI joints as part of a systemic or local inflammatory process. Ankylosing spondylitis and inflammatory bowel disease (Fig. 21) typically cause bilateral, fairly symmetric SI joint involvement, osteoarthritis can cause symmetric or asymmetric changes, and psoriatic arthritis and reactive arthritis manifest as asymmetric SI joint involvement. While radiography can help evaluate the SI joints for widening, ankylosis, and erosions, MRI can help evaluate both osseous and soft tissue inflammation, which may not be visualized initially on radiographs. MRI in sacroiliitis can show increased signal in the joint on T2-weighted images representing joint effusion, as well as increased signal on T1-weighted images due to bone erosion and fat infiltration, which can occur early in the disease course. STIR sequences are very sensitive in the detection of bone marrow edema, which is frequently seen with inflammation [39]. Infection of the SI joint is the main differential diagnosis and must be excluded in cases of unilateral disease.
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Fig. 20. 24-year-old man with plexiform neurofibroma with malignant degeneration. Axial MRI of the pelvis shows a large, complex mass (long arrows) extending through the greater sciatic foramen with mostly high signal but with internal lower signal septations on the T2 fat-saturated image (A) and peripheral post-contrast enhancement on the T1 fatsaturated post-gadolinium image (B). Numerous smaller masses (small arrows) are present.
Fig. 21. 35-year-old man with ulcerative colitis and bilateral sacroiliitis. The MR coronal STIR image (A) shows bilateral subchondral edema (arrows) around the SI joints. The coronal CT image (B) shows SI joint erosions (arrows), most prominent on the left. Asymmetric erosions can be seen despite the underlying bilateral process.
Iliopsoas bursitis The iliopsoas bursa, the largest bursa in the body, lies between the iliopsoas tendon and the lesser trochanter, extending upward into the iliac fossa beneath the iliacus muscle; the bursa passes anterior to the acetabulum and hip joint. Normally the bursa contains no fluid, although in some patients, the bursa communicates with the hip joint and can fill with fluid in the presence of a hip effusion. Iliopsoas bursitis is an inflammation of the bursa caused by repetitive use, trauma, or systemic inflammatory disease. Infection is rare [40]. MRI shows the bursa filled with fluid (increased signal on T2-weighted images) and may show post-contrast enhancement of the bursal sac (Fig. 22). Differentiation can be made from paralabral cysts based on the location; fluid does not extend to the labral margin, as it does in a paralabral cyst [7].
Greater trochanter pain syndrome Lateral hip pain over the region of the greater trochanter is a challenging clinical diagnosis due to the complexity of structures in that region. Gluteus medius and/or minimus tendinosis, iliotibial tract pathology, and greater trochanteric bursitis, among other etiologies, can cause lateral hip pain, and all of these can be seen incidentally on pelvic imaging. MRI findings of gluteus medius and minimus tendinosis include thickening of the gluteus medius and minimus tendons with high signal in and surrounding the tendons on T2weighted images (Fig. 23). Calcific tendinosis of the gluteal tendons can also occur, in which the calcium deposits in the tendon cause significant edema (Fig. 23). This process can be exquisitely painful and can present acutely.
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at baseline, if there is frank fluid associated with the bursa, and especially if the patient’s pain correlates with the location of the fluid, bursitis is likely the etiology of the pain [7].
Paralabral cysts MRI can demonstrate paralabral cysts, which are of high signal on T2-weighted imaging, located adjacent to the labrum. Paralabral cysts indicate the presence of a labral tear (Fig. 24), which might be occult on non-arthrogram MRI. These cysts are usually lobulated structures located in the anterior or anterosuperior labrum, as these are the most common sites of labral tears, and can have a neck extending toward the acetabular rim. The amount of fluid in the cyst is often disproportionate to the amount of joint fluid [7]. Paralabral cysts can cause anterior hip or groin pain. MRI fluid-sensitive sequences can show labral tears, specifically when there is joint effusion, but are unreliable for diagnosis, and magnetic resonance arthrography is the diagnostic test of choice [42]. The labrum is made of fibrocartilage, low in signal on all sequences, and usually triangular shaped, but it is known to have variation in shape and size. The presence of irregular labral edges, a fluid-filled cleft within the labrum, a paralabral cyst, and/or chondral abnormality favor a labral tear over a normal variant sulcus. A sulcus is variant of normal anatomy in which a gap is present where the labrum meets the acetabulum, and this can mimic a labral tear; both most often occur anteriorly [43]. Fig. 22. 66-year-old woman with intractable right hip pain. There is fluid within the right iliopsoas bursa, which is low signal on the coronal T1-weighted image (A, arrow) and high signal on the coronal fat-saturated T2-weighted image (B, arrow).
While greater trochanteric bursitis has often been thought to be one of the primary culprits causing pain over the greater trochanter, a recent study showed that greater trochanteric pain is most often a combination of pathology involving the gluteus tendons and iliotibial tract, with bursitis present only in a minority of patients [41]. In greater trochanteric bursitis, the bursa becomes fluid-filled and inflamed due to repetitive trauma or adjacent pathology in the gluteus muscle insertions. MRI shows a fluid-filled bursa with post-contrast enhancement of the bursal wall, similar to the findings in iliopsoas bursitis. Although many obese patients have nonspecific edema in the region of the trochanteric bursa
Iatrogenic Bone graft Bone grafts are used to promote healing, provide stability, or both, and they may be used to bridge largebone defects, to aid healing in cases of nonunion, or to bridge joints for arthrodesis. The iliac crest is the most common donor site and is an excellent source of cancellous bone. Complications at the donor site are rare, but fracture and bleeding can occur; less commonly, patients may develop hernias through the donor site, in which fat or other structures can push through the osseous defect [29]. MRI immediately after graft removal from the donor site will show a focal defect with surrounding bone marrow edema and possibly blood products (Fig. 25). Sclerosis at the margins of the defect indicates healing, and sometimes bony excrescences occur
K. Gaetke-Udager et al.: MR imaging of the pelvis
K. Gaetke-Udager et al.: MR imaging of the pelvis
53-year-old woman with gluteus medius tendinosis. Axial (A) and coronal (B) fat-saturated T2-weighted images show thickening and high signal surrounding the gluteus medius tendon (A, short arrows) and fluid tracking along the tendon (B, long arrow) at its attachment to the greater trochanter. 44 year-old man with gluteus medius calcific tendinosis. Axial T2-weighted image (C) shows thickening and irregularity of the tendon at its greater trochanter insertion (arrow) with marked surrounding edema (arrowhead). D Coronal STIR image also shows high T2 signal within the gluteus medius tendon (arrow). Radiograph (E) shows the subtle calcifications (arrow) within the tendon.
b Fig. 23.
imaging and high in signal on T2-weighted imaging (Fig. 26). Complications include hemorrhage in the acute setting, which will appear as adjacent blood products, or infection in the subacute to chronic phase, which will appear as spread of the low signal on T1-weighted images and high signal on T2-weighted images present in the bone beyond the linear tract.
Conclusion
The sequela of recent bone biopsy is recognizable on MRI. The biopsy needle generally creates a linear tract extending from the bone entry point to the biopsied lesion; this tract will appear low in signal on T1-weighted
Abdominal radiologists interpreting MRI of the pelvis may not be familiar with the many forms of musculoskeletal pathology in this region. This essay illustrates and describes both normal and common pathologic appearances of skeletal findings that can be encountered in a pelvic MRI study. In most cases, there is a differential diagnosis to consider based on the imaging appearance. An awareness of the expected imaging findings and relevant clinical history can help distinguish lesions, although some skeletal findings will require further investigation.
Fig. 24. A 41-year-old man with right hip pain and paralabral cyst. Coronal STIR image shows lobulated high signal (arrow) near the superior labrum; this suggests the presence of a labral tear. B 25 year-old woman with paralabral cyst. Sagittal
fat-saturated PD image shows lobulated high signal (arrow) along the anterosuperior labrum, suggesting labral tear. This patient also has subchondral cystic change in the femoral head.
and cause pain; these are better evaluated with radiography or CT.
Bone biopsy
K. Gaetke-Udager et al.: MR imaging of the pelvis
Fig. 25. 57-year-old woman status post bone graft harvest in the left iliac bone. Low signal is seen on the axial T2weighted image (A, arrow) at the harvest site. Heterogeneous
increased and decreased signal (B, arrow) is seen on an adjacent axial T2-weighted image representing blood products from the procedure.
Fig. 26. 51-year-old woman with chronic lymphocytic leukemia status post recent bone marrow biopsy in the left iliac bone. The arrow on the axial T1-weighted image (A) shows
the linear low T1 signal from the biopsy tract; the arrow on the axial fat-saturated T2-weighted image shows the corresponding high signal in the tract consistent with edema.
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Acknowledgments. The authors wish to acknowledge Olaf Magerkuth, MD, for his assistance in preparing this manuscript. Disclosures. We would like to make the following disclosures for author Jon Jacobson, Consultant: BioClinica, Book Royalties: Elsevier, Grant: American Institute of Ultrasound in Medicine, Research Equipment: Harvest Technologies
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