Body Magnetic Resonance Imaging Fast, Efficient, and Comprehensive Bong Soo Kim, MDa, Wirana Angthong, MDb, Yong-Hwan Jeon, MDb, Richard C. Semelka, MDb,* KEYWORDS Abdominal MR imaging Uncooperative patient Motion-resistant MR imaging New MR imaging techniques Artifacts
KEY POINTS Magnetic resonance (MR) imaging of the abdomen should be performed in a fast, efficient, and comprehensive fashion and provide consistent image quality and display of disease processes. Separation of protocols for cooperative and cooperation-challenged patients is necessary because inability of noncooperative patients to hold their breath impairs the image quality on abdominal MR imaging. Motion-resistant protocols including fast imaging and radial acquisition techniques radically improve imaging quality in noncooperative patients.
A major strength of magnetic resonance (MR) imaging is the variety of types of information that it is able to generate. As a result, abdominal MR imaging can provide comprehensive information on organ systems and disease entities. The use of fast scanning techniques provides consistently high image quality and good conspicuity of disease with a decrease of imaging times.1–3 Examination time is critical because the longer MR studies could result in worsening of imaging quality because of motion, which tends to progress through the course of the study from patient exhaustion. The sequence acquisition time in routine MR protocols could generally be too long for high-diagnostic-quality abdominal MR imaging in noncooperative patients such as those who are agitated, sedated, or unconscious. Therefore,
it is necessary to develop separate protocols for noncooperative patients,4–6 distinct from a standard cooperative protocol. To provide appropriate abdominal MR work-up in cooperative and noncooperative patients, this article provides basic information about different sequences and core information about image interpretation.
COOPERATIVE PROTOCOL In practice, it is important to recognize which technique is the most consistent in showing various diseases in order to target lesions in an imaging protocol. Attention to length of examination is critical because longer examinations result in fewer patients who can be examined and a decreased in patient cooperation. The different sequences used should ideally be of short duration and breath held or breathing independent. Another
Disclosure: None of the authors has a conflict of interest. a Department of Radiology, Jeju National University Hospital, Jeju National University School of Medicine, 1753-3, Ara-1-dong, Jeju-si, Jeju-do 690-716, Korea; b Department of Radiology, University of North Carolina Hospitals, University of North Carolina at Chapel Hill, CB 7510, 2001 Old Clinic Building, Chapel Hill, NC 275997510, USA * Corresponding author. E-mail address: [email protected] Radiol Clin N Am - (2014) -–http://dx.doi.org/10.1016/j.rcl.2014.02.007 0033-8389/14/$ – see front matter Ó 2014 Elsevier Inc. All rights reserved.
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INTRODUCTION
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Kim et al consideration is reproducibility of examination protocols, because efficient operation of an MR imaging system requires the use of set protocols, which serves to speed up examinations, render examinations reproducible, and increases use by familiarity with a standard approach. Table 1 lists the parameters of the MR sequences used in cooperative patients on 1.5-T and 3.0-T scanners. For most patients, it is possible to complete the core sequences within 15 minutes.
T1-Weighted Sequences In-phase two-dimensional spoiled gradient echo sequence In-phase imaging has become a routine part of liver MR imaging for investigating disease of the abdomen. This sequence is primarily used to provide the following information: presence of subacute blood or concentrated protein, which both have high signal intensity (Fig. 1); presence of fat as high-signal-intensity tissue (Fig. 2); and abnormally increased fluid content or
fibrous tissue content, which appear as low signal intensity. Out-of-phase two-dimensional spoiled gradient echo sequence Out-of-phase images show a sharply defined black rim around organs with a fat-water interface, and the kidneys are a good organ to look for this. Out-of-phase sequences are helpful for the recognition of diseased tissue in which fat and water protons are present within the same voxel. Signal loss is maximal when fat content approaches 50%. This sequence plays an important role in detecting the presence of fat (steatosis) within the liver and lipid within adrenal masses to characterize them as adenomas, which are shown as loss of signal intensity relative to the in-phase sequence (Fig. 3). In addition, it is important to use in-phase/out-of-phase sequences to show magnetic susceptibility effects from the presence of iron (exogenous or endogenous) or air. Examples include iron storage diseases, such as hemochromatosis or
Table 1 Parameters for cooperative patients used at 1.5 T and 3.0 T on MR imaging scanners Precontrast Sequences T1-weighted 2D-SGE In Phase/Out of Phase Parameter (T)
3.0
1.5
Plane of acquisition
Axial
Axial
Postcontrast Sequences
T2-weighted SS-ETSE 3.0
1.5
Axial, Axial, coronal coronal TR (ms) 169 142 2000 1500 TE (ms) 2.5/1.58 4.4/2.2 95 90 57 70 150 180 Flip angle ( ) Echo-train length — — 179 156 BW/pixel (Hz) 400 490 781 651 Matrix (phase frequency) 204 256 192 256 204 256 192 256 FOV (mm) 400 400 350 350 350 350 400 400 No. of section 28 19 30 20 Section thickness (mm) 8 8 8 8 Intersectional gap (mm) 1.6 1.6 1.6 1.6 No. of signal acquisition 1 1 1 1 Fat suppression None None Fat saturation for axial, none for coronal Respiratory control BH BH BI BI
Dynamic T1-weighted 3D-GREa 3.0
1.5
Axial, coronal 3.07 1.32 13 — 500 192 256 400 400 72 3 0 1 Fat saturation
Axial, coronal 4.3 1.6 10 — 350 160 320 360 360 72 3.5 0 1 Fat saturation
BH
BH
Abbreviations: BH, breath-holding; BI, breathing-independent; BW, bandwidth; FOV, field of view; GRE, gradient echo; Hz, hertz; sat, saturation; SGE, spoiled gradient echo; SS-ETSE, single-shot echo-train spin-echo; TE, echo time; TR, repetition time; 2D, two dimensional; 3D, three dimensional. a Dynamic 3D-GRE sequences were used to acquire all 3 postcontrast phases in 3.0-T and 1.5-T scanners.
Body Magnetic Resonance Imaging
Fig. 1. MR images in a 27-year-old woman with a hemorrhagic corpus luteal cyst. Axial T1-weighted two-dimensional (2D) SGE in-phase (A) and T1-weighted fat-suppressed three-dimensional (3D) GRE (B) images show high signal intensity within an ovarian cystic mass (arrows) in the pelvic cavity, suggesting a hemorrhagic cyst. Hemoperitoneum is also visualized in the pelvic cavity.
hemosiderosis. Susceptibility effects increase with increase in echo time (TE) because of pronounced decay of transverse magnetization, which appears as lower-signal tissue on longer TE sequences (Fig. 4).7,8 Coronal two-dimensional spoiled gradient echo sequence Coronal T1-weighted images are obtained for an overview of the abdomen. Coronal MR images allow an additional perspective that may not clearly be shown on transverse images, and increase the confidence of diagnosis. Coronal images are useful for viewing the aorta, para-aortic lymph nodes, and mesenteric vessels, which display the vascular structures along their longitudinal axes. The coronal views should also be used to inspect the lung bases and the inferior and superior surfaces of the liver for the presence of metastases and other disease processes. Fat-suppressed T1-weighted three-dimensional gradient recalled echo sequence Three-dimensional (3D) gradient recalled echo (GRE) sequences in combination with fatsuppression techniques provide high-quality imaging and very thin sections with adequate
volume coverage in a single breath-hold imaging time. 3D GRE is suitable for dynamic contrast-enhanced MR studies because of its excellent inherent fat suppression and sensitivity to enhancement in tissues after gadoliniumbased contrast agents (GBCAs) are given. Fat-suppression techniques are useful for the diagnosis of diseased tissues composed predominantly of fat, such as ovarian dermoid cyst, angiomyolipoma, lipoma, and myelolipoma (see Fig. 2), which are confidently diagnosed by showing the lesion to show high signal on in-phase images and dark on noncontrast fat-suppressed GRE images.9 Fat-suppression technique improves the delineation of pancreatic borders and the pancreas, which appears homogeneously bright compared with surrounding low-signal-intensity fat and other abdominal organs (see Fig. 2). It is excellent for identifying chronic pancreatitis (less intense than normal high-intensity pancreas) and pancreatic masses such as small ductal adenocarcinoma (Fig. 5) and pancreatic neuroendocrine tumors.10,11 The fat-suppression technique also allows the best visualization of renal corticomedullary differentiation, which is reduced in renal transplant rejection and chronic renal insufficiency.12
Fig. 2. MR images in a 56-year-old woman with an adrenal myelolipoma. Axial T1-weighted 2D SGE in-phase image (A) shows a lesion with high signal intensity (arrows) in left adrenal gland with suppression (arrows) on the T1-weighted fat-suppressed 3D GRE image (B). The normal pancreas is well shown as high signal intensity on axial T1-weighted fat-suppressed 3D GRE image.
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Fig. 3. MR images in a 51-year-old man with an adrenal adenoma. A right adrenal mass (arrow) has substantial signal reduction from the axial T1-weighted 2D SGE in-phase image (A) to the axial T1-weighted 2D SGE out-ofphase image (B).
T2-weighted Sequences Single-shot echo-train spin-echo sequence The important information that T2-weighted images provide includes (1) the presence of abnormal increased fluid content in diseased tissue and fluid-containing tumors (cysts, hamartomas, and hemangiomas), which results in high signal intensity; (2) the presence of chronic fibrotic tissue, which results in low-signal-intensity lesions, and the presence of low-fluid-content lesions, which results in a range of signal intensity from mildly low to mildly high; (3) the presence of iron deposition, which appears as markedly low
signal intensity (see Fig. 4); and (4) the presence of lymph nodes in the porta hepatis, which have high signal intensity compared with liver parenchyma (Fig. 6). Fat suppression should generally be applied for at least one set of T2-weighted images of the liver, and it may also be helpful to perform this in a different plane, to obtain an additional benefit from a second data acquisition. One common circumstance in which fat suppression is useful on T2-weighted sequences is when the patient may have liver metastases and the background liver has steatosis. Without fat suppression, fatty liver has high signal intensity on a
Fig. 4. MR images in a 62-year-old woman with idiopathic hemochromatosis. Axial T1-weighted 2D SGE out-ofphase image (echo time of 2.2 ms) (A) and axial T1-weighted 2D SGE in-phase image (echo time of 4.4 ms) (B) show a marked reduction in signal intensity in the liver, pancreas, and focal areas of the spleen in the image with the longer echo time. There is decreased signal intensity of liver and pancreas (arrow) on coronal T2weighted fat-suppressed SS-ETSE (C).
Body Magnetic Resonance Imaging
Fig. 5. MR images in a 49-year-old man with a small pancreatic cancer. Axial T1-weighted fat-suppressed 3D GRE image (A) shows a small mass arising in the pancreatic head, which invades the common bile duct and is shown as a low-signal-intensity lesion (arrow) with normal pancreas appearing bright. On the axial T1-weighted fat-suppressed 3D GRE arterial dominant phase image (B), a low-signal-intensity tumor (arrow) is identified in the head, clearly demarcated from uniformly well-enhancing normal pancreatic tissue.
standard single-shot echo-train spin-echo (SSETSE) sequence, because fat signal is high with this technique, thereby diminishing contrast with most liver lesions, whereas with fat suppression the liver becomes darker, ensuring optimal contrast with lesions that show mildly intrinsic high T2 signal. Coronal T2-weighted images are also obtained for an overall review of the abdomen. They are particularly useful for viewing the structures of the bowel and biliary system.
An additional advantage is that this sequence is resistance to susceptibility artifact. As a result, the bowel wall can clearly be seen and susceptibility artifact from metallic devices is minimal. In these settings it may be important to not use fat suppression to minimize susceptibility effects. Often a more complex fat-suppression sequence is important to minimize persistent fat signal on this sequence, such as combining fat suppression with inversion recovery.
Fig. 6. MR images in a 45-year-old man with chronic liver disease and an enlarged porta hepatis lymph node. Axial T2-weighted fat-suppressed SS-ETSE image (A) shows the best delineation of a lymph node (arrow) in porta hepatis, compared with axial T1-weighted fat-suppressed 3D GRE arterial dominant phase (B) and early hepatic venous phase images (C). On the axial T2-weighted fat-suppressed SS-ETSE image, lymph nodes have moderately high signal intensity and both liver and background fat are low signal, rendering excellent conspicuity.
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Kim et al Contrast-enhanced Fat-suppressed T1weighted 3D GRE Sequence Hepatic arterial dominant phase The hepatic arterial dominant (capillary) phase (HADP) is the single most important data set when using a nonspecific extracellular GBCA.13 Capillary phase image acquisition is achieved by using a short-duration sequence initiated immediately after gadolinium injection. These images are identified by the presence of contrast in hepatic arteries and portal veins and its absence in the hepatic veins. Greater enhancement of normal pancreas than the liver can be reliably judged on HADP. Hypervascular liver tumors, especially hepatocellular carcinomas, focal nodular hyperplasia (Fig. 7), and hypervascular metastases are well recognized as intensely enhancing lesions on HADP, compared with background liver. HADP images are essential to observe hepatic perfusion abnormalities, such as are present in the full range of inflammatory conditions (Fig. 8). The pancreas shows uniform capillary blush on HADP, which renders it markedly high in signal intensity. In general, pancreatic adenocarcinoma usually appears as a focal hypovascular mass that is readily detected and characterized on HADP (see Fig. 5). In these instances, the tumor is well demarcated from adjacent uninvolved pancreas, which shows greater enhancement. Early hepatic venous phase Images for this phase are acquired 45 to 60 seconds after initiation of the GBCA and are recognized by the presence of contrast in hepatic and portal veins. In this phase, the hepatic parenchyma is maximally enhanced so that hypovascular lesions such as cysts, hypovascular metastases, and scar tissue are most clearly identified as regions of absent or diminished enhancement. This phase is also useful in lesion
characterization of hypervascular tumors, such as hepatocellular carcinoma, which show washout (that is, they become darker than liver) on early hepatic venous phase, and focal nodular hyperplasia, which shows fading (that is, it becomes isointense with liver) (see Fig. 7). This phase also clearly shows patency or thrombosis of portal and hepatic veins. The perfusion abnormalities on HADP such as compromised portal venous flow, acute-on-chronic hepatitis, inflammatory reactions to adjacent infection (eg, acute cholecystitis, liver abscess), and Budd-Chiari syndrome usually disappear on early hepatic venous or interstitial phases (see Fig. 8) and this often forms an important part of the diagnostic criteria for a phenomenon being vascular or inflammatoryvascular in nature. Interstitial phase This phase is acquired 90 seconds to 5 minutes after initiation of the contrast material injection. The interstitial phase shows computed tomography (CT)–like images when performed with fat suppression. Late enhancement features of focal liver lesions are shown, which aid in lesion characterization. These findings assist in establishing a diagnosis of hepatocellular carcinoma by showing washout and delayed capsular enhancement; mass-forming intrahepatic cholangiocarcinoma, which shows progressive enhancement; and hemangioma, which shows coalescence and centripetal progression of enhancing nodules. Enhancement of peritoneal metastasis, inflammatory disease, and pulmonary nodules are well shown at this phase. The coronal interstitial images are acquired after 3 sets of transverse images. These images provide a general survey view of the abdomen from a surgeon’s vantage point. These images are useful for detecting metastases of the musculoskeletal system (especially
Fig. 7. MR images in a 41-year-old woman with a focal nodular hyperplasia. Axial T1-weighted fat-suppressed 3D GRE arterial dominant phase image (A) shows an intensely enhancing mass in the liver with central scar shown by low signal intensity. On early hepatic venous phase image (B), the tumor fades to near isointensity with background parenchyma and a central scar showing delayed enhancement.
Body Magnetic Resonance Imaging
Fig. 8. MR images in a 40-year-old man with multiple liver and bone metastases from pancreatic head cancer. This patient has multiple ringlike enhancing metastases in the liver. On axial T1-weighted fat-suppressed 3D GRE arterial dominant phase imaging (A), sharply demarcated wedge-shaped perilesional enhancements are shown with disappearance on early hepatic venous (B) and interstitial phases (C), suggesting perfusion abnormalities. Coronal T1-weighted fat-suppressed 3D GRE interstitial phase image (D) shows a small enhancing metastasis (arrow) in the right ilium.
thoracolumbar spines, pelvis, and femoral heads) (see Fig. 8),14 peritoneal disease, retroperitoneal disease, and metastases of lung bases. The coronal interstitial images also provide valuable information about bowel and mesenteric abnormalities and may be used to define abnormal enhancement in tumoral and inflamed regions of bowel. Direct coronal imaging (rather than reformatting transverse images) is strongly advised at this phase to substantiate the patency or thrombosis of the portal vein and superior mesenteric veins, which is important for the assessment of liver cirrhosis and pancreatic cancer. Artifacts, including flow artifacts, are a more serious problem with MR than with CT, and an important means for compensating for this is to acquire images in orthogonal planes. If a defect in a vessel represents thrombosis rather than flow, it should be apparent, with the same anticipated location, from one plane to the next. Flow artifacts show different morphologies from one plane to the next.
MOTION-RESISTANT PROTOCOLS Motion-resistant protocols achieve improved image quality in settings of noncooperation by either
acquiring the critical data for image creation during only a small fraction of the time for free breathing, rendering this technique insensitive to the patient’s motion (single-shot approaches) or by modifying the data acquisition to minimize effects of motion (eg, radial acquisition, multiple data averages). To facilitate patient throughput at our center, the primary individual determining whether a motion-resistant protocol should be performed is the scanning technologist, who decides whether the patient is following commands or whether they have difficulty breathing holding, and, if they are having difficulty, then the scanning technologist implements the motionresistant protocol. To perform an adequate MR examination in noncooperative patients, it is useful to obtain the following sequences: T1-weighted dualecho magnetization-prepared rapid-acquisition gradient echo (MP-RAGE), T1-weighted MPRAGE before and after gadolinium, T1-weighted water excitation MP-RAGE (WE-MP-RAGE) after gadolinium, T1-weighted radial GRE sequence before and after gadolinium, and T1-weighted SS-ETSE. Table 2 provides the MR imaging parameters for motion-resistant protocols on 1.5-T and 3.0-T scanners.
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Table 2 Parameters for motion-resistant protocol used on 1.5-T and 3.0-T MR imaging scanners Precontrast Sequences T1-weighted 2D MP-RAGE In Phase/ Out of Phase
Postcontrast Sequences T1-weighted MPRAGE
T2-weighted SS-ETSE
Parameter (T)
3.0
1.5
3.0
1.5
Plane of acquisition
Axial
Axial
TR (ms) TE (ms) TI (ms) Flip angle ( ) Echo-train length BW/pixel (Hz) Matrix (phase frequency) FOV (mm) No. of sections Section thickness (mm) Intersectional gap (mm) No. of signal acquisition Fat suppression
1800 2.3/3.6 1200 20 — 190/180 256 189 380 380 40 5 1.0 1 None
1540 4.1/2.3 900 15 — 180/150 256 156 380 380 38 6 1.2 2 None
Respiratory control
BI
BI
Axial, Axial, coronal coronal 2000 1500 95 90 — — 150 180 179 156 781 651 204 256 192 256 350 350 400 400 30 20 8 8 1.6 1.6 1 1 Fat sat for axial, none for coronal BI BI
T1-weighted WE-MPRAGE
T1-weighted Radial 3D-GRE
3.0
1.5
3.0
1.5
3.0
1.5
Axial
Axial
2000 2.5 1200 20 — 600 180 256 400 400 28 8 1.6 — None
2000 1.7 700 15 — 180 180 256 350 350 20 8 1.6 — None
Axial, coronal 2000 2.8 1200 20 — 600 180 256 400 400 28 8 1.6 — Fat sat
Axial, coronal 2000 6.2 700 15 — 180 128 192 350 350 20 8 1.6 — Fat sat
Axial, coronal 3.8 1.6 — 10 — 600 380 380 380 380 72 3.0 0 1 Fat sat
Axial, coronal 3.8 1.6 — 10 — 350 380 380 380 380 80 3.0 0 1 Fat sat
BI
BI
BI
BI
FB
FB
Abbreviations: FB, free breathing; MP-RAGE, magnetization-prepared rapid-acquisition gradient echo; sat, saturation; TI, inversion time; WE-MP-RAGE, water excitation magnetization-prepared rapid-acquisition gradient echo.
Body Magnetic Resonance Imaging T1-weighted Sequences Two-dimensional MP-RAGE sequence MP-RAGE, for example turbo fast low-angle shot (FLASH), is a single shot–type sequence that operates as a slice-by-slice single-shot technique and can generate T1-weighted images that are resistant to deterioration from respiratory motion (Figs. 9 and 10). This sequence can be used to obtain motion-free and moderate-quality images with acquisition times as short as 1 second. It has the intrinsic issue that signal/noise ratio (SNR) and contrast/noise ratio (CNR) are lower than in regular gradient echo images.15 An inversion pulse leads to the longitudinal magnetization that creates the T1 contrast. The sequence acquires data using a very short repetition time (TR) and low flip-angle excitation pulses to reduce acquisition time and maintain the prepared magnetization.
Standard in-phase/out-of-phase GRE imaging are performed as multislice GRE acquisitions that require patients to suspend respiration, because sequences are generally 10 to 20 seconds in duration. New implementations of MP-RAGE in-phase/ out-of-phase images are able to show the presence of fat, which is necessary to evaluate the liver and also adrenal masses in patients who cannot cooperate with standard in-phase/out-of-phase GRE imaging, such as in the elderly, the severely debilitated, and young children (see Fig. 10; Fig. 11).16–18 Image quality and lesion conspicuity of MP-RAGE in-phase/out-of-phase imaging are also comparable with standard in-phase/out-ofphase GRE imaging.18 Two-dimensional WE-MP-RAGE Spatial-spectral selective water excitation is a newer fat-attenuation technique that selectively
Fig. 9. MR images in a 10-year-old boy. Standard axial T1-weighted 2D SGE in-phase (A) and out-of-phase (B) images shows motion artifacts caused by respiration. In this patient, who could not suspend respiration, the study was switched to a motion-resistant protocol because unacceptable image quality was expected. The motionresistant protocol included axial T1-weighted 2D MP-RAGE in-phase (C), axial T1-weighted 2D MP-RAGE out-of-phase (D), axial T1-weighted radial 3D GRE (E), axial T2-weighted fat-suppressed SS-ETSE (F), contrastenhanced axial T1-weighted 2D MP-RAGE (G), contrast-enhanced axial T1-weighted 2D-WE-MP-RAGE (H), and contrast-enhanced axial T1-weighted radial 3D GRE images (I). This protocol shows substantially reduced artifacts and motion-free high-quality images in this difficult-to-scan child.
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Fig. 10. MR images in a 64-year-old man with liver cirrhosis. There is a reticular pattern fibrosis through the liver, which has low signal intensity on T1-weighted images. Axial 2D SGE in-phase (A) and axial out-of-phase (B) images show motion artifacts caused by breathing and parallel imaging artifacts. No motion artifacts are present on axial 2D MP-RAGE in-phase (C) and out-of-phase (D) images. The fine pattern of fibrosis is particularly well shown on the axial 2D MP-RAGE out-of-phase image.
Fig. 11. MR images in a 48-year-old woman with liver steatosis and liver metastasis. The liver shows a reduction of signal intensity on the axial 2D SGE out-of-phase image (B) compared with the axial 2D SGE in-phase image (A). This reduction of signal intensity is also visualized on axial 2D MP-RAGE out-of-phase imaging (D), comparing with axial 2D MP-RAGE in-phase imaging (C). A small liver metastasis (arrow) is appreciated in S8 of the liver, better depicted in axial 2D MP-RAGE out-of-phase (D) than in axial 2D SGE out-of-phase (B) imaging.
Body Magnetic Resonance Imaging excites water protons leaving protons in fat unperturbed, rather than exciting all protons and then spoiling the signal from fat protons. This is a faster fat-attenuating scheme than conventional exciting-spoiling fat suppression, and this speed is necessary for use on a rapid gradient technique such as MP-RAGE. WE-MP-RAGE can provide fat-attenuated contrast-enhanced T1-weighted images (Fig. 12). WE-MP-RAGE at 3 T can achieve better image quality and fat attenuation than at 1.5 T because of the intrinsic properties of the higher field strength, such as higher SNR and CNR, and increased chemical frequency shift between fat and water that reduces the duration of WE-radiofrequency pulses and data acquisition time.4 This sequence, similar to fat-suppressed
spoiled gradient echo (SGE), could be used to detect abnormally increased fluid content or fibrous tissue content that appears as low signal intensity, and presence of subacute blood or concentrated protein, which are both high signal intensity. 3D radial GRE sequence Standard T1-weighted GRE sequences acquire k-space data on a horizontal-filling line-by-line basis, which is a scheme that is sensitive to motion artifacts. Radial acquisition technique (projection reconstruction) has higher sampling density for central k-space (which contributes the bulk of the signal in images), because it acquires data in a radial spoke-wheel fashion, and undersamples
Fig. 12. MR images in a 25-year-old man with Crohn disease and abscess involving liver and psoas muscle. Multiple abscesses in liver are clearly shown in a noncooperative patient on axial T2-weighted fat-suppressed SS-ETSE (A), contrast-enhanced axial 2D MP-RAGE (B), contrast-enhanced axial 2D WE-MP-RAGE (C), and contrastenhanced axial 3D radial GE (D) images. The abscess in psoas muscle (arrow) is well visualized on contrastenhanced axial 2D WE-MP-RAGE (E), and contrast-enhanced axial 3D radial GE (F) images. Mild pixel graininess is present on contrast-enhanced axial 2D MP-RAGE (B) and contrast-enhanced axial 2D WE-MP-RAGE (C, E) images. Axial 3D radial GRE images (D, F) show clear definition of abdominal structure and lesion conspicuity by using a higher matrix and the thinner section slice in this sequence.
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Fig. 13. MR images in a 68-year-old man with an oncocytoma in the left kidney. MR images on precontrast T1weighted fat-suppressed 3D GRE (A) and T1-weighted fat-suppressed 3D GRE at arterial dominant phase (C) show blurred resolution of the tumor and pancreatic margin caused by motion. Better sharpness of tumor and pancreatic edge and clearer depiction of intrahepatic vessels are shown on precontrast radial GRE and interstitial phase radial GRE images (B, D).
the margins of k-space, with the net effect that images are less sensitive to motion artifacts from phase errors.19–24 In clinical practice, freebreathing fat-suppressed 3D radial GRE sequence can provide excellent motion-controlled images with high spatial resolution in noncooperative patients, especially sedated children (see Fig. 12; Fig. 13).5 3D radial GRE sequence can generate
streak artifacts, which are not seen with conventional rectilinear 3D GRE sequence, but these artifacts are usually of a mild degree and the images are of good diagnostic quality despite their presence.5 They are most apparent in large patients, and are virtually absent in children (Fig. 14). The lesser presence of streak artifacts in pediatric patients relates to their smaller abdominal transverse
Fig. 14. MR images in a 2-year-old boy with a neuroblastoma in the right adrenal gland. Precontrast T1-weighted fat-suppressed axial 3D radial GRE image (A) shows a bulky, heterogenous, solid mass with hemorrhagic component. The mass extends across the midline adjacent to the aorta at the level of the renal arteries and displaces the inferior vena cava and pancreatic head on contrast-enhanced T1-weighted fat-suppressed axial 3D radial GRE imaging (B). Radial 3D GRE imaging shows high image quality with minimal motion artifacts in this difficult-to-scan child.
Body Magnetic Resonance Imaging
Fig. 15. MR images in a 51-year-old man with 2 small hemangiomas in the liver. Gadolinium-enhanced 3D GRE at 3 T (A) shows more increased conspicuity of hemangiomas (arrows) than at 1.5 T (B) because of higher SNR and CNR at 3.0 T compared with 1.5 T. MR images show more characteristics of hemangiomas (peripheral globular enhancement) at 3.0 T than at 1.5 T. Higher SNR at 3.0 T improves visualization of portal veins and hepatic veins beyond those at 1.5 T.
dimensions compared with adults. In the near future, the development of compressed sensing, parallel technique and radial sampling in radial 3D GRE may make it possible to achieve image acquisition at high temporal resolution using a smaller number of radial spokes, which will allow dynamic gadolinium-enhanced imaging, including in the hepatic arterial phase, in clinical practice.25
T2-weighted Sequences SS-ETSE sequence The SS-ETSE sequence is a breathing-independent technique that is useful both in standard and motion-resistant protocols. This technique acquires the full image after a single excitation pulse. A variety of strategies compensate for the entire k-space not being acquired over this short time period, and what is described as half-acquisition is a common approach that samples approximately 60% of k-space in the horizontal filling approach, and uses the added 10% to correct for the undersampling by filling in the remaining 40% of k-space. Images are obtained in less than a second with virtually no motion artifact even during free breathing (see Fig. 12).15 This sequence is discussed in more detail earlier.
HIGH-RESOLUTION IMAGES Parallel MR Imaging Parallel MR imaging with suitable phased-array coils can be used in combination with most MR sequences to reduce scan time. Parallel imaging is often used to increase spatial resolution, decrease data acquisition time, and most often a combination of both. This technique makes high temporal resolution possible in noncooperative patients, as well as higher spatial resolution (useful in the pelvis), reduced effective interecho spacing (eg, less image blurring and image distortion in
SS-ETSE), and reduced specific absorption rate caused by shorter echo trains at 3 T.
3.0-T MR Imaging 3-T MR imaging can provide higher SNR and CNR than 1.5-T MR imaging, which leads to improved image resolution.26,27 The higher SNR combined with higher in-plane and throughplane spatial resolution at 3 T improves lesion conspicuity on gadolinium-enhanced 3D GRE sequences (Fig. 15). 3-T MR images have many advantages compared with 1.5-T images, including use in small children because of the higher SNR on MP-RAGE, small-volume disease in general, and concurrent vascular imaging with tissue imaging (visualization of arteries especially but also venous structures) using the tissue-imaging sequence of 3D-GRE alone.
SUMMARY To achieve routine use in clinical practice, abdominal MR imaging must consistently provide both superior and important information on organ systems and disease entities. The suppression of, or compensation for, motion artifacts is the most important determinant of diagnostic efficacy in upper abdominal MR imaging. This suppression or compensation can largely be achieved by the use of separate protocols for cooperative and noncooperative patients. The clinical MR study should be performed in a fast, efficient, and comprehensive fashion, should focus on the benefit to the patient, and should emphasize clinically essential strategies.
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MR imaging with breath-hold MR imaging. Radiology 1996;200(2):459–64. Semelka RC, Balci NC, Op de Beeck B, et al. Evaluation of a 10-minute comprehensive MR imaging examination of the upper abdomen. Radiology 1999;211(1):189–95. Semelka RC, Kelekis NL, Thomasson D, et al. HASTE MR imaging: description of technique and preliminary results in the abdomen. J Magn Reson Imaging 1996;6(4):698–9. Altun E, Semelka RC, Dale BM, et al. Water excitation MPRAGE: an alternative sequence for postcontrast imaging of the abdomen in noncooperative patients at 1.5 Tesla and 3.0 Tesla MRI. J Magn Reson Imaging 2008;27(5):1146–54. Azevedo RM, de Campos RO, Ramalho M, et al. Free-breathing 3D T1-weighted gradient-echo sequence with radial data sampling in abdominal MRI: preliminary observations. AJR Am J Roentgenol 2011;197(3):650–7. Sharma P, Martin DR, Dale BM, et al. Diagnostic approach to protocoling and interpreting MR studies of the abdomen and pelvis. In: Semelka RC, editor. Abdominal-pelvic MRI. Hoboken (NJ): WileyBlackwell; 2010. p. 1–43 Chapter 1. Merkle EM, Nelson RC. Dual gradient-echo in-phase and opposed-phase hepatic MR imaging: a useful tool for evaluating more than fatty infiltration or fatty sparing. Radiographics 2006;26(5):1409–18. Kierans AS, Leonardou P, Shaikh F, et al. Body MR imaging: sequences we use and why. Appl Radiol 2009;38(5):7–12. Delfaut EM, Beltran J, Johnson G, et al. Fat suppression in MR imaging: techniques and pitfalls. Radiographics 1999;19(2):373–82. Gallix BP, Bret PM, Atri M, et al. Comparison of qualitative and quantitative measurements on unenhanced T1-weighted fat saturation MR images in predicting pancreatic pathology. J Magn Reson Imaging 2005;21(5):583–9. Pamuklar E, Semelka RC. MR imaging of the pancreas. Magn Reson Imaging Clin N Am 2005; 13(2):313–30. Kettritz U, Semelka RC, Brown ED, et al. MR findings in diffuse renal parenchymal disease. J Magn Reson Imaging 1996;6(1):136–44. Semelka RC, Helmberger TK. Contrast agents for MR imaging of the liver. Radiology 2001;218(1):27–38. Northam M, de Campos RO, Ramalho M, et al. Bone metastases: evaluation of acuity of lesions using dynamic gadolinium-chelate enhancement, preliminary results. J Magn Reson Imaging 2011;34(1):120–7. Semelka RC, Martin DR, Balci NC. Magnetic resonance imaging of the liver: how I do it. J Gastroenterol Hepatol 2006;21(4):632–7. Ramalho M, Heredia V, de Campos RO, et al. Inphase and out-of-phase gradient-echo imaging in
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abdominal studies: intra-individual comparison of three different techniques. Acta Radiol 2012;53(4): 441–9. Ferreira A, Ramalho M, de Campos RO, et al. Comparison of T1-weighted in- and out-of-phase single shot magnetization-prepared gradient-recalledecho with three-dimensional gradient-recalled-echo at 3.0 Tesla: preliminary observations in abdominal studies. J Magn Reson Imaging 2012;35(5):1187–95. Heredia V, Ramalho M, de Campos RO, et al. Comparison of a single shot T1-weighted in- and out-ofphase magnetization prepared gradient recalled echo with a standard two-dimensional gradient recalled echo: preliminary findings. J Magn Reson Imaging 2011;33(6):1482–90. Chandarana H, Block TK, Rosenkrantz AB, et al. Freebreathing radial 3D fat-suppressed T1-weighted gradient echo sequence: a viable alternative for contrast-enhanced liver imaging in patients unable to suspend respiration. Invest Radiol 2011;46(10): 648–53. Rasche V, de Boer RW, Holz D, et al. Continuous radial data acquisition for dynamic MRI. Magn Reson Med 1995;34(5):754–61. Song HK, Dougherty L. Dynamic MRI with projection reconstruction and KWIC processing for simultaneous high spatial and temporal resolution. Magn Reson Med 2004;52(4):815–24. Spuentrup E, Katoh M, Buecker A, et al. Freebreathing 3D steady-state free precession coronary MR angiography with radial k-space sampling: comparison with Cartesian k-space sampling and Cartesian gradient-echo coronary MR angiography–pilot study. Radiology 2004;231(2):581–6. Kim KW, Lee JM, Jeon YS, et al. Free-breathing dynamic contrast-enhanced MRI of the abdomen and chest using a radial gradient echo sequence with K-space weighted image contrast (KWIC). Eur Radiol 2012;23(5):1352–60. Bamrungchart S, Tantaway EM, Midia EC, et al. Free breathing three-dimensional gradient echosequence with radial data sampling (radial 3DGRE) examination of the pancreas: comparison with standard 3D-GRE volumetric interpolated breathhold examination (VIBE). J Magn Reson Imaging 2013;38(6):1572–7. Chandarana H, Feng L, Block TK, et al. Free-breathing contrast-enhanced multiphase MRI of the liver using a combination of compressed sensing, parallel imaging, and golden-angle radial sampling. Invest Radiol 2013;48(1):10–6. Chang KJ, Kamel IR, Macura KJ, et al. 3.0-T MR imaging of the abdomen: comparison with 1.5 T. Radiographics 2008;28(7):1983–98. Erturk SM, Alberich-Bayarri A, Herrmann KA, et al. Use of 3.0-T MR imaging for evaluation of the abdomen. Radiographics 2009;29(6):1547–63.
KEY POINTS Magnetic resonance (MR) imaging of the abdomen should be performed in a fast, efficient, and comprehensive fashion and provide consistent image quality and display of disease processes. Separation of protocols for cooperative and cooperation-challenged patients is necessary because inability of noncooperative patients to hold their breath impairs the image quality on abdominal MR imaging. Motion-resistant protocols including fast imaging and radial acquisition techniques radically improve imaging quality in noncooperative patients.
A major strength of magnetic resonance (MR) imaging is the variety of types of information that it is able to generate. As a result, abdominal MR imaging can provide comprehensive information on organ systems and disease entities. The use of fast scanning techniques provides consistently high image quality and good conspicuity of disease with a decrease of imaging times.1–3 Examination time is critical because the longer MR studies could result in worsening of imaging quality because of motion, which tends to progress through the course of the study from patient exhaustion. The sequence acquisition time in routine MR protocols could generally be too long for high-diagnostic-quality abdominal MR imaging in noncooperative patients such as those who are agitated, sedated, or unconscious. Therefore,
it is necessary to develop separate protocols for noncooperative patients,4–6 distinct from a standard cooperative protocol. To provide appropriate abdominal MR work-up in cooperative and noncooperative patients, this article provides basic information about different sequences and core information about image interpretation.
COOPERATIVE PROTOCOL In practice, it is important to recognize which technique is the most consistent in showing various diseases in order to target lesions in an imaging protocol. Attention to length of examination is critical because longer examinations result in fewer patients who can be examined and a decreased in patient cooperation. The different sequences used should ideally be of short duration and breath held or breathing independent. Another
Disclosure: None of the authors has a conflict of interest. a Department of Radiology, Jeju National University Hospital, Jeju National University School of Medicine, 1753-3, Ara-1-dong, Jeju-si, Jeju-do 690-716, Korea; b Department of Radiology, University of North Carolina Hospitals, University of North Carolina at Chapel Hill, CB 7510, 2001 Old Clinic Building, Chapel Hill, NC 275997510, USA * Corresponding author. E-mail address: [email protected] Radiol Clin N Am - (2014) -–http://dx.doi.org/10.1016/j.rcl.2014.02.007 0033-8389/14/$ – see front matter Ó 2014 Elsevier Inc. All rights reserved.
radiologic.theclinics.com
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Kim et al consideration is reproducibility of examination protocols, because efficient operation of an MR imaging system requires the use of set protocols, which serves to speed up examinations, render examinations reproducible, and increases use by familiarity with a standard approach. Table 1 lists the parameters of the MR sequences used in cooperative patients on 1.5-T and 3.0-T scanners. For most patients, it is possible to complete the core sequences within 15 minutes.
T1-Weighted Sequences In-phase two-dimensional spoiled gradient echo sequence In-phase imaging has become a routine part of liver MR imaging for investigating disease of the abdomen. This sequence is primarily used to provide the following information: presence of subacute blood or concentrated protein, which both have high signal intensity (Fig. 1); presence of fat as high-signal-intensity tissue (Fig. 2); and abnormally increased fluid content or
fibrous tissue content, which appear as low signal intensity. Out-of-phase two-dimensional spoiled gradient echo sequence Out-of-phase images show a sharply defined black rim around organs with a fat-water interface, and the kidneys are a good organ to look for this. Out-of-phase sequences are helpful for the recognition of diseased tissue in which fat and water protons are present within the same voxel. Signal loss is maximal when fat content approaches 50%. This sequence plays an important role in detecting the presence of fat (steatosis) within the liver and lipid within adrenal masses to characterize them as adenomas, which are shown as loss of signal intensity relative to the in-phase sequence (Fig. 3). In addition, it is important to use in-phase/out-of-phase sequences to show magnetic susceptibility effects from the presence of iron (exogenous or endogenous) or air. Examples include iron storage diseases, such as hemochromatosis or
Table 1 Parameters for cooperative patients used at 1.5 T and 3.0 T on MR imaging scanners Precontrast Sequences T1-weighted 2D-SGE In Phase/Out of Phase Parameter (T)
3.0
1.5
Plane of acquisition
Axial
Axial
Postcontrast Sequences
T2-weighted SS-ETSE 3.0
1.5
Axial, Axial, coronal coronal TR (ms) 169 142 2000 1500 TE (ms) 2.5/1.58 4.4/2.2 95 90 57 70 150 180 Flip angle ( ) Echo-train length — — 179 156 BW/pixel (Hz) 400 490 781 651 Matrix (phase frequency) 204 256 192 256 204 256 192 256 FOV (mm) 400 400 350 350 350 350 400 400 No. of section 28 19 30 20 Section thickness (mm) 8 8 8 8 Intersectional gap (mm) 1.6 1.6 1.6 1.6 No. of signal acquisition 1 1 1 1 Fat suppression None None Fat saturation for axial, none for coronal Respiratory control BH BH BI BI
Dynamic T1-weighted 3D-GREa 3.0
1.5
Axial, coronal 3.07 1.32 13 — 500 192 256 400 400 72 3 0 1 Fat saturation
Axial, coronal 4.3 1.6 10 — 350 160 320 360 360 72 3.5 0 1 Fat saturation
BH
BH
Abbreviations: BH, breath-holding; BI, breathing-independent; BW, bandwidth; FOV, field of view; GRE, gradient echo; Hz, hertz; sat, saturation; SGE, spoiled gradient echo; SS-ETSE, single-shot echo-train spin-echo; TE, echo time; TR, repetition time; 2D, two dimensional; 3D, three dimensional. a Dynamic 3D-GRE sequences were used to acquire all 3 postcontrast phases in 3.0-T and 1.5-T scanners.
Body Magnetic Resonance Imaging
Fig. 1. MR images in a 27-year-old woman with a hemorrhagic corpus luteal cyst. Axial T1-weighted two-dimensional (2D) SGE in-phase (A) and T1-weighted fat-suppressed three-dimensional (3D) GRE (B) images show high signal intensity within an ovarian cystic mass (arrows) in the pelvic cavity, suggesting a hemorrhagic cyst. Hemoperitoneum is also visualized in the pelvic cavity.
hemosiderosis. Susceptibility effects increase with increase in echo time (TE) because of pronounced decay of transverse magnetization, which appears as lower-signal tissue on longer TE sequences (Fig. 4).7,8 Coronal two-dimensional spoiled gradient echo sequence Coronal T1-weighted images are obtained for an overview of the abdomen. Coronal MR images allow an additional perspective that may not clearly be shown on transverse images, and increase the confidence of diagnosis. Coronal images are useful for viewing the aorta, para-aortic lymph nodes, and mesenteric vessels, which display the vascular structures along their longitudinal axes. The coronal views should also be used to inspect the lung bases and the inferior and superior surfaces of the liver for the presence of metastases and other disease processes. Fat-suppressed T1-weighted three-dimensional gradient recalled echo sequence Three-dimensional (3D) gradient recalled echo (GRE) sequences in combination with fatsuppression techniques provide high-quality imaging and very thin sections with adequate
volume coverage in a single breath-hold imaging time. 3D GRE is suitable for dynamic contrast-enhanced MR studies because of its excellent inherent fat suppression and sensitivity to enhancement in tissues after gadoliniumbased contrast agents (GBCAs) are given. Fat-suppression techniques are useful for the diagnosis of diseased tissues composed predominantly of fat, such as ovarian dermoid cyst, angiomyolipoma, lipoma, and myelolipoma (see Fig. 2), which are confidently diagnosed by showing the lesion to show high signal on in-phase images and dark on noncontrast fat-suppressed GRE images.9 Fat-suppression technique improves the delineation of pancreatic borders and the pancreas, which appears homogeneously bright compared with surrounding low-signal-intensity fat and other abdominal organs (see Fig. 2). It is excellent for identifying chronic pancreatitis (less intense than normal high-intensity pancreas) and pancreatic masses such as small ductal adenocarcinoma (Fig. 5) and pancreatic neuroendocrine tumors.10,11 The fat-suppression technique also allows the best visualization of renal corticomedullary differentiation, which is reduced in renal transplant rejection and chronic renal insufficiency.12
Fig. 2. MR images in a 56-year-old woman with an adrenal myelolipoma. Axial T1-weighted 2D SGE in-phase image (A) shows a lesion with high signal intensity (arrows) in left adrenal gland with suppression (arrows) on the T1-weighted fat-suppressed 3D GRE image (B). The normal pancreas is well shown as high signal intensity on axial T1-weighted fat-suppressed 3D GRE image.
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Fig. 3. MR images in a 51-year-old man with an adrenal adenoma. A right adrenal mass (arrow) has substantial signal reduction from the axial T1-weighted 2D SGE in-phase image (A) to the axial T1-weighted 2D SGE out-ofphase image (B).
T2-weighted Sequences Single-shot echo-train spin-echo sequence The important information that T2-weighted images provide includes (1) the presence of abnormal increased fluid content in diseased tissue and fluid-containing tumors (cysts, hamartomas, and hemangiomas), which results in high signal intensity; (2) the presence of chronic fibrotic tissue, which results in low-signal-intensity lesions, and the presence of low-fluid-content lesions, which results in a range of signal intensity from mildly low to mildly high; (3) the presence of iron deposition, which appears as markedly low
signal intensity (see Fig. 4); and (4) the presence of lymph nodes in the porta hepatis, which have high signal intensity compared with liver parenchyma (Fig. 6). Fat suppression should generally be applied for at least one set of T2-weighted images of the liver, and it may also be helpful to perform this in a different plane, to obtain an additional benefit from a second data acquisition. One common circumstance in which fat suppression is useful on T2-weighted sequences is when the patient may have liver metastases and the background liver has steatosis. Without fat suppression, fatty liver has high signal intensity on a
Fig. 4. MR images in a 62-year-old woman with idiopathic hemochromatosis. Axial T1-weighted 2D SGE out-ofphase image (echo time of 2.2 ms) (A) and axial T1-weighted 2D SGE in-phase image (echo time of 4.4 ms) (B) show a marked reduction in signal intensity in the liver, pancreas, and focal areas of the spleen in the image with the longer echo time. There is decreased signal intensity of liver and pancreas (arrow) on coronal T2weighted fat-suppressed SS-ETSE (C).
Body Magnetic Resonance Imaging
Fig. 5. MR images in a 49-year-old man with a small pancreatic cancer. Axial T1-weighted fat-suppressed 3D GRE image (A) shows a small mass arising in the pancreatic head, which invades the common bile duct and is shown as a low-signal-intensity lesion (arrow) with normal pancreas appearing bright. On the axial T1-weighted fat-suppressed 3D GRE arterial dominant phase image (B), a low-signal-intensity tumor (arrow) is identified in the head, clearly demarcated from uniformly well-enhancing normal pancreatic tissue.
standard single-shot echo-train spin-echo (SSETSE) sequence, because fat signal is high with this technique, thereby diminishing contrast with most liver lesions, whereas with fat suppression the liver becomes darker, ensuring optimal contrast with lesions that show mildly intrinsic high T2 signal. Coronal T2-weighted images are also obtained for an overall review of the abdomen. They are particularly useful for viewing the structures of the bowel and biliary system.
An additional advantage is that this sequence is resistance to susceptibility artifact. As a result, the bowel wall can clearly be seen and susceptibility artifact from metallic devices is minimal. In these settings it may be important to not use fat suppression to minimize susceptibility effects. Often a more complex fat-suppression sequence is important to minimize persistent fat signal on this sequence, such as combining fat suppression with inversion recovery.
Fig. 6. MR images in a 45-year-old man with chronic liver disease and an enlarged porta hepatis lymph node. Axial T2-weighted fat-suppressed SS-ETSE image (A) shows the best delineation of a lymph node (arrow) in porta hepatis, compared with axial T1-weighted fat-suppressed 3D GRE arterial dominant phase (B) and early hepatic venous phase images (C). On the axial T2-weighted fat-suppressed SS-ETSE image, lymph nodes have moderately high signal intensity and both liver and background fat are low signal, rendering excellent conspicuity.
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Kim et al Contrast-enhanced Fat-suppressed T1weighted 3D GRE Sequence Hepatic arterial dominant phase The hepatic arterial dominant (capillary) phase (HADP) is the single most important data set when using a nonspecific extracellular GBCA.13 Capillary phase image acquisition is achieved by using a short-duration sequence initiated immediately after gadolinium injection. These images are identified by the presence of contrast in hepatic arteries and portal veins and its absence in the hepatic veins. Greater enhancement of normal pancreas than the liver can be reliably judged on HADP. Hypervascular liver tumors, especially hepatocellular carcinomas, focal nodular hyperplasia (Fig. 7), and hypervascular metastases are well recognized as intensely enhancing lesions on HADP, compared with background liver. HADP images are essential to observe hepatic perfusion abnormalities, such as are present in the full range of inflammatory conditions (Fig. 8). The pancreas shows uniform capillary blush on HADP, which renders it markedly high in signal intensity. In general, pancreatic adenocarcinoma usually appears as a focal hypovascular mass that is readily detected and characterized on HADP (see Fig. 5). In these instances, the tumor is well demarcated from adjacent uninvolved pancreas, which shows greater enhancement. Early hepatic venous phase Images for this phase are acquired 45 to 60 seconds after initiation of the GBCA and are recognized by the presence of contrast in hepatic and portal veins. In this phase, the hepatic parenchyma is maximally enhanced so that hypovascular lesions such as cysts, hypovascular metastases, and scar tissue are most clearly identified as regions of absent or diminished enhancement. This phase is also useful in lesion
characterization of hypervascular tumors, such as hepatocellular carcinoma, which show washout (that is, they become darker than liver) on early hepatic venous phase, and focal nodular hyperplasia, which shows fading (that is, it becomes isointense with liver) (see Fig. 7). This phase also clearly shows patency or thrombosis of portal and hepatic veins. The perfusion abnormalities on HADP such as compromised portal venous flow, acute-on-chronic hepatitis, inflammatory reactions to adjacent infection (eg, acute cholecystitis, liver abscess), and Budd-Chiari syndrome usually disappear on early hepatic venous or interstitial phases (see Fig. 8) and this often forms an important part of the diagnostic criteria for a phenomenon being vascular or inflammatoryvascular in nature. Interstitial phase This phase is acquired 90 seconds to 5 minutes after initiation of the contrast material injection. The interstitial phase shows computed tomography (CT)–like images when performed with fat suppression. Late enhancement features of focal liver lesions are shown, which aid in lesion characterization. These findings assist in establishing a diagnosis of hepatocellular carcinoma by showing washout and delayed capsular enhancement; mass-forming intrahepatic cholangiocarcinoma, which shows progressive enhancement; and hemangioma, which shows coalescence and centripetal progression of enhancing nodules. Enhancement of peritoneal metastasis, inflammatory disease, and pulmonary nodules are well shown at this phase. The coronal interstitial images are acquired after 3 sets of transverse images. These images provide a general survey view of the abdomen from a surgeon’s vantage point. These images are useful for detecting metastases of the musculoskeletal system (especially
Fig. 7. MR images in a 41-year-old woman with a focal nodular hyperplasia. Axial T1-weighted fat-suppressed 3D GRE arterial dominant phase image (A) shows an intensely enhancing mass in the liver with central scar shown by low signal intensity. On early hepatic venous phase image (B), the tumor fades to near isointensity with background parenchyma and a central scar showing delayed enhancement.
Body Magnetic Resonance Imaging
Fig. 8. MR images in a 40-year-old man with multiple liver and bone metastases from pancreatic head cancer. This patient has multiple ringlike enhancing metastases in the liver. On axial T1-weighted fat-suppressed 3D GRE arterial dominant phase imaging (A), sharply demarcated wedge-shaped perilesional enhancements are shown with disappearance on early hepatic venous (B) and interstitial phases (C), suggesting perfusion abnormalities. Coronal T1-weighted fat-suppressed 3D GRE interstitial phase image (D) shows a small enhancing metastasis (arrow) in the right ilium.
thoracolumbar spines, pelvis, and femoral heads) (see Fig. 8),14 peritoneal disease, retroperitoneal disease, and metastases of lung bases. The coronal interstitial images also provide valuable information about bowel and mesenteric abnormalities and may be used to define abnormal enhancement in tumoral and inflamed regions of bowel. Direct coronal imaging (rather than reformatting transverse images) is strongly advised at this phase to substantiate the patency or thrombosis of the portal vein and superior mesenteric veins, which is important for the assessment of liver cirrhosis and pancreatic cancer. Artifacts, including flow artifacts, are a more serious problem with MR than with CT, and an important means for compensating for this is to acquire images in orthogonal planes. If a defect in a vessel represents thrombosis rather than flow, it should be apparent, with the same anticipated location, from one plane to the next. Flow artifacts show different morphologies from one plane to the next.
MOTION-RESISTANT PROTOCOLS Motion-resistant protocols achieve improved image quality in settings of noncooperation by either
acquiring the critical data for image creation during only a small fraction of the time for free breathing, rendering this technique insensitive to the patient’s motion (single-shot approaches) or by modifying the data acquisition to minimize effects of motion (eg, radial acquisition, multiple data averages). To facilitate patient throughput at our center, the primary individual determining whether a motion-resistant protocol should be performed is the scanning technologist, who decides whether the patient is following commands or whether they have difficulty breathing holding, and, if they are having difficulty, then the scanning technologist implements the motionresistant protocol. To perform an adequate MR examination in noncooperative patients, it is useful to obtain the following sequences: T1-weighted dualecho magnetization-prepared rapid-acquisition gradient echo (MP-RAGE), T1-weighted MPRAGE before and after gadolinium, T1-weighted water excitation MP-RAGE (WE-MP-RAGE) after gadolinium, T1-weighted radial GRE sequence before and after gadolinium, and T1-weighted SS-ETSE. Table 2 provides the MR imaging parameters for motion-resistant protocols on 1.5-T and 3.0-T scanners.
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Table 2 Parameters for motion-resistant protocol used on 1.5-T and 3.0-T MR imaging scanners Precontrast Sequences T1-weighted 2D MP-RAGE In Phase/ Out of Phase
Postcontrast Sequences T1-weighted MPRAGE
T2-weighted SS-ETSE
Parameter (T)
3.0
1.5
3.0
1.5
Plane of acquisition
Axial
Axial
TR (ms) TE (ms) TI (ms) Flip angle ( ) Echo-train length BW/pixel (Hz) Matrix (phase frequency) FOV (mm) No. of sections Section thickness (mm) Intersectional gap (mm) No. of signal acquisition Fat suppression
1800 2.3/3.6 1200 20 — 190/180 256 189 380 380 40 5 1.0 1 None
1540 4.1/2.3 900 15 — 180/150 256 156 380 380 38 6 1.2 2 None
Respiratory control
BI
BI
Axial, Axial, coronal coronal 2000 1500 95 90 — — 150 180 179 156 781 651 204 256 192 256 350 350 400 400 30 20 8 8 1.6 1.6 1 1 Fat sat for axial, none for coronal BI BI
T1-weighted WE-MPRAGE
T1-weighted Radial 3D-GRE
3.0
1.5
3.0
1.5
3.0
1.5
Axial
Axial
2000 2.5 1200 20 — 600 180 256 400 400 28 8 1.6 — None
2000 1.7 700 15 — 180 180 256 350 350 20 8 1.6 — None
Axial, coronal 2000 2.8 1200 20 — 600 180 256 400 400 28 8 1.6 — Fat sat
Axial, coronal 2000 6.2 700 15 — 180 128 192 350 350 20 8 1.6 — Fat sat
Axial, coronal 3.8 1.6 — 10 — 600 380 380 380 380 72 3.0 0 1 Fat sat
Axial, coronal 3.8 1.6 — 10 — 350 380 380 380 380 80 3.0 0 1 Fat sat
BI
BI
BI
BI
FB
FB
Abbreviations: FB, free breathing; MP-RAGE, magnetization-prepared rapid-acquisition gradient echo; sat, saturation; TI, inversion time; WE-MP-RAGE, water excitation magnetization-prepared rapid-acquisition gradient echo.
Body Magnetic Resonance Imaging T1-weighted Sequences Two-dimensional MP-RAGE sequence MP-RAGE, for example turbo fast low-angle shot (FLASH), is a single shot–type sequence that operates as a slice-by-slice single-shot technique and can generate T1-weighted images that are resistant to deterioration from respiratory motion (Figs. 9 and 10). This sequence can be used to obtain motion-free and moderate-quality images with acquisition times as short as 1 second. It has the intrinsic issue that signal/noise ratio (SNR) and contrast/noise ratio (CNR) are lower than in regular gradient echo images.15 An inversion pulse leads to the longitudinal magnetization that creates the T1 contrast. The sequence acquires data using a very short repetition time (TR) and low flip-angle excitation pulses to reduce acquisition time and maintain the prepared magnetization.
Standard in-phase/out-of-phase GRE imaging are performed as multislice GRE acquisitions that require patients to suspend respiration, because sequences are generally 10 to 20 seconds in duration. New implementations of MP-RAGE in-phase/ out-of-phase images are able to show the presence of fat, which is necessary to evaluate the liver and also adrenal masses in patients who cannot cooperate with standard in-phase/out-of-phase GRE imaging, such as in the elderly, the severely debilitated, and young children (see Fig. 10; Fig. 11).16–18 Image quality and lesion conspicuity of MP-RAGE in-phase/out-of-phase imaging are also comparable with standard in-phase/out-ofphase GRE imaging.18 Two-dimensional WE-MP-RAGE Spatial-spectral selective water excitation is a newer fat-attenuation technique that selectively
Fig. 9. MR images in a 10-year-old boy. Standard axial T1-weighted 2D SGE in-phase (A) and out-of-phase (B) images shows motion artifacts caused by respiration. In this patient, who could not suspend respiration, the study was switched to a motion-resistant protocol because unacceptable image quality was expected. The motionresistant protocol included axial T1-weighted 2D MP-RAGE in-phase (C), axial T1-weighted 2D MP-RAGE out-of-phase (D), axial T1-weighted radial 3D GRE (E), axial T2-weighted fat-suppressed SS-ETSE (F), contrastenhanced axial T1-weighted 2D MP-RAGE (G), contrast-enhanced axial T1-weighted 2D-WE-MP-RAGE (H), and contrast-enhanced axial T1-weighted radial 3D GRE images (I). This protocol shows substantially reduced artifacts and motion-free high-quality images in this difficult-to-scan child.
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Fig. 10. MR images in a 64-year-old man with liver cirrhosis. There is a reticular pattern fibrosis through the liver, which has low signal intensity on T1-weighted images. Axial 2D SGE in-phase (A) and axial out-of-phase (B) images show motion artifacts caused by breathing and parallel imaging artifacts. No motion artifacts are present on axial 2D MP-RAGE in-phase (C) and out-of-phase (D) images. The fine pattern of fibrosis is particularly well shown on the axial 2D MP-RAGE out-of-phase image.
Fig. 11. MR images in a 48-year-old woman with liver steatosis and liver metastasis. The liver shows a reduction of signal intensity on the axial 2D SGE out-of-phase image (B) compared with the axial 2D SGE in-phase image (A). This reduction of signal intensity is also visualized on axial 2D MP-RAGE out-of-phase imaging (D), comparing with axial 2D MP-RAGE in-phase imaging (C). A small liver metastasis (arrow) is appreciated in S8 of the liver, better depicted in axial 2D MP-RAGE out-of-phase (D) than in axial 2D SGE out-of-phase (B) imaging.
Body Magnetic Resonance Imaging excites water protons leaving protons in fat unperturbed, rather than exciting all protons and then spoiling the signal from fat protons. This is a faster fat-attenuating scheme than conventional exciting-spoiling fat suppression, and this speed is necessary for use on a rapid gradient technique such as MP-RAGE. WE-MP-RAGE can provide fat-attenuated contrast-enhanced T1-weighted images (Fig. 12). WE-MP-RAGE at 3 T can achieve better image quality and fat attenuation than at 1.5 T because of the intrinsic properties of the higher field strength, such as higher SNR and CNR, and increased chemical frequency shift between fat and water that reduces the duration of WE-radiofrequency pulses and data acquisition time.4 This sequence, similar to fat-suppressed
spoiled gradient echo (SGE), could be used to detect abnormally increased fluid content or fibrous tissue content that appears as low signal intensity, and presence of subacute blood or concentrated protein, which are both high signal intensity. 3D radial GRE sequence Standard T1-weighted GRE sequences acquire k-space data on a horizontal-filling line-by-line basis, which is a scheme that is sensitive to motion artifacts. Radial acquisition technique (projection reconstruction) has higher sampling density for central k-space (which contributes the bulk of the signal in images), because it acquires data in a radial spoke-wheel fashion, and undersamples
Fig. 12. MR images in a 25-year-old man with Crohn disease and abscess involving liver and psoas muscle. Multiple abscesses in liver are clearly shown in a noncooperative patient on axial T2-weighted fat-suppressed SS-ETSE (A), contrast-enhanced axial 2D MP-RAGE (B), contrast-enhanced axial 2D WE-MP-RAGE (C), and contrastenhanced axial 3D radial GE (D) images. The abscess in psoas muscle (arrow) is well visualized on contrastenhanced axial 2D WE-MP-RAGE (E), and contrast-enhanced axial 3D radial GE (F) images. Mild pixel graininess is present on contrast-enhanced axial 2D MP-RAGE (B) and contrast-enhanced axial 2D WE-MP-RAGE (C, E) images. Axial 3D radial GRE images (D, F) show clear definition of abdominal structure and lesion conspicuity by using a higher matrix and the thinner section slice in this sequence.
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Fig. 13. MR images in a 68-year-old man with an oncocytoma in the left kidney. MR images on precontrast T1weighted fat-suppressed 3D GRE (A) and T1-weighted fat-suppressed 3D GRE at arterial dominant phase (C) show blurred resolution of the tumor and pancreatic margin caused by motion. Better sharpness of tumor and pancreatic edge and clearer depiction of intrahepatic vessels are shown on precontrast radial GRE and interstitial phase radial GRE images (B, D).
the margins of k-space, with the net effect that images are less sensitive to motion artifacts from phase errors.19–24 In clinical practice, freebreathing fat-suppressed 3D radial GRE sequence can provide excellent motion-controlled images with high spatial resolution in noncooperative patients, especially sedated children (see Fig. 12; Fig. 13).5 3D radial GRE sequence can generate
streak artifacts, which are not seen with conventional rectilinear 3D GRE sequence, but these artifacts are usually of a mild degree and the images are of good diagnostic quality despite their presence.5 They are most apparent in large patients, and are virtually absent in children (Fig. 14). The lesser presence of streak artifacts in pediatric patients relates to their smaller abdominal transverse
Fig. 14. MR images in a 2-year-old boy with a neuroblastoma in the right adrenal gland. Precontrast T1-weighted fat-suppressed axial 3D radial GRE image (A) shows a bulky, heterogenous, solid mass with hemorrhagic component. The mass extends across the midline adjacent to the aorta at the level of the renal arteries and displaces the inferior vena cava and pancreatic head on contrast-enhanced T1-weighted fat-suppressed axial 3D radial GRE imaging (B). Radial 3D GRE imaging shows high image quality with minimal motion artifacts in this difficult-to-scan child.
Body Magnetic Resonance Imaging
Fig. 15. MR images in a 51-year-old man with 2 small hemangiomas in the liver. Gadolinium-enhanced 3D GRE at 3 T (A) shows more increased conspicuity of hemangiomas (arrows) than at 1.5 T (B) because of higher SNR and CNR at 3.0 T compared with 1.5 T. MR images show more characteristics of hemangiomas (peripheral globular enhancement) at 3.0 T than at 1.5 T. Higher SNR at 3.0 T improves visualization of portal veins and hepatic veins beyond those at 1.5 T.
dimensions compared with adults. In the near future, the development of compressed sensing, parallel technique and radial sampling in radial 3D GRE may make it possible to achieve image acquisition at high temporal resolution using a smaller number of radial spokes, which will allow dynamic gadolinium-enhanced imaging, including in the hepatic arterial phase, in clinical practice.25
T2-weighted Sequences SS-ETSE sequence The SS-ETSE sequence is a breathing-independent technique that is useful both in standard and motion-resistant protocols. This technique acquires the full image after a single excitation pulse. A variety of strategies compensate for the entire k-space not being acquired over this short time period, and what is described as half-acquisition is a common approach that samples approximately 60% of k-space in the horizontal filling approach, and uses the added 10% to correct for the undersampling by filling in the remaining 40% of k-space. Images are obtained in less than a second with virtually no motion artifact even during free breathing (see Fig. 12).15 This sequence is discussed in more detail earlier.
HIGH-RESOLUTION IMAGES Parallel MR Imaging Parallel MR imaging with suitable phased-array coils can be used in combination with most MR sequences to reduce scan time. Parallel imaging is often used to increase spatial resolution, decrease data acquisition time, and most often a combination of both. This technique makes high temporal resolution possible in noncooperative patients, as well as higher spatial resolution (useful in the pelvis), reduced effective interecho spacing (eg, less image blurring and image distortion in
SS-ETSE), and reduced specific absorption rate caused by shorter echo trains at 3 T.
3.0-T MR Imaging 3-T MR imaging can provide higher SNR and CNR than 1.5-T MR imaging, which leads to improved image resolution.26,27 The higher SNR combined with higher in-plane and throughplane spatial resolution at 3 T improves lesion conspicuity on gadolinium-enhanced 3D GRE sequences (Fig. 15). 3-T MR images have many advantages compared with 1.5-T images, including use in small children because of the higher SNR on MP-RAGE, small-volume disease in general, and concurrent vascular imaging with tissue imaging (visualization of arteries especially but also venous structures) using the tissue-imaging sequence of 3D-GRE alone.
SUMMARY To achieve routine use in clinical practice, abdominal MR imaging must consistently provide both superior and important information on organ systems and disease entities. The suppression of, or compensation for, motion artifacts is the most important determinant of diagnostic efficacy in upper abdominal MR imaging. This suppression or compensation can largely be achieved by the use of separate protocols for cooperative and noncooperative patients. The clinical MR study should be performed in a fast, efficient, and comprehensive fashion, should focus on the benefit to the patient, and should emphasize clinically essential strategies.
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