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Artifacts in Musculoskeletal MR Imaging Dinesh R. Singh, MBBS, MMed, DNB, FRCR1 Michael S.M. Chin, DCR1 Wilfred C.G. Peh, MBBS, MD, FRCP(Glasg), FRCP(Edin), FRCR1
Alexandra Health, Singapore, Republic of Singapore Semin Musculoskelet Radiol 2014;18:12–22.
Abstract
Keywords
► chemical shift artifact ► magnetic resonance imaging ► motion artifact ► protocol-error artifacts ► susceptibility artifact
Address for correspondence Wilfred C.G. Peh, MBBS, MD, FRCP(Glasg), FRCP(Edin), FRCR, Department of Diagnostic Radiology, Khoo Teck Puat Hospital, Alexandra Health, 90 Yishun Central, Singapore 768828, Republic of Singapore (e-mail: [email protected]).
MR imaging has become an important diagnostic tool in the evaluation of a vast number of pathologies and is of foremost importance in the evaluation of spine, joints, and soft tissue structures of the musculoskeletal system. MR imaging is susceptible to various artifacts that may affect the image quality or even simulate pathologies. Some of these artifacts have gained special importance with the use of higher field strength magnets and with the increasing need for MR imaging in postoperative patients, especially those with previous joint replacements or metallic implants. Artifacts may arise from patient motion or could be due to periodic motion, such as vascular and cardiac pulsation. Artifacts could also arise from various protocol errors including saturation, wraparound, truncation, shading, partial volume averaging, and radiofrequency interference artifacts. Susceptibility artifact occurs at interfaces with different magnetic susceptibilities and is of special importance with increasing use of metallic joint replacement prostheses. Magic angle phenomenon is a special type of artifact that occurs in musculoskeletal MR imaging. It is essential to recognize these artifacts and to correct them because they may produce pitfalls in image interpretation.
MR imaging is one of the most important diagnostic tools in evaluating the abnormalities of the musculoskeletal system. It is routinely used in the evaluation of the spine, various joints, and soft tissue structures. However, MR imaging of the musculoskeletal system is subject to several artifacts. Many of these artifacts are more prominent with increasing use of higher field strength machines, especially with 3-T MR imaging.1 These artifacts may arise due to a number of factors, some physical and some technical, and affect the image quality or even simulate pathologies. Artifacts may arise due to motion or may be due to protocol errors, for example, saturation, wraparound, truncation, shading, radiofrequency, and partial volume averaging artifacts. Motion artifacts may be related to patient motion or may result from periodic motion such as cardiac or vascular pulsation. Respiratory motion and bowel peristalsis also contribute to these motion artifacts. Cerebrospinal fluid (CSF) pulsation artifacts are also
Issue Theme Variants and Pitfalls in Musculoskeletal Imaging; Guest Editor, Wilfred C.G. Peh, MBBS, MD, FRCP (Glasg), FRCP(Edin), FRCR
commonly encountered in spine imaging and can be misinterpreted as sinister abnormalities of the spine. The magic angle phenomenon needs particular mention because it is a special type of artifact seen in musculoskeletal imaging and may cause misinterpretation of the signal abnormalities if not identified and corrected. Susceptibility artifacts are of practical importance with increasing number of joint replacement procedures and subsequent MR imaging. The susceptibility effects are directly proportional to the magnetic field strength and are more prominent on newer 3-T machines.1,2 Although these artifacts have been reduced with the increasing use of titanium alloys in modern implants, they still remain of concern because these cause image distortion close to the metallic hardware.3 It is essential to be aware of these artifacts, to avoid any possible misinterpretation, and to improve the image quality. This in turn may have a significant impact on overall patient care. We describe the
Copyright © 2014 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0034-1365831. ISSN 1089-7860.
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1 Department of Diagnostic Radiology, Khoo Teck Puat Hospital,
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Fig. 1 Patient motion artifact in MR imaging of the shoulder, resulting in poor quality (a) sagittal and (b) axial images.
various MR artifacts encountered in musculoskeletal imaging and their identification, and we suggest ways to correct them, if possible.
MR Artifacts MR imaging artifacts arise from several complex interactions involving the main magnet, the gradient coils, the radiofrequency (RF) transmitter and receiver, and the reconstruction algorithm used.4 They are related to a variety of factors, some patient related, some hardware related; some might be protocol related.
Motion Artifacts Motion artifacts are the most common artifacts in the imaging of the musculoskeletal system and are easily recognized. Standard image reconstruction techniques ideally require a perfectly stationary patient, which rarely happens. Patient motion (►Fig. 1) and motion due to various physiologic processes often result in artifacts.5 These artifacts occur because the phase gradient cannot predictably encode the radio waves arising from moving structures.6 The type of motion, moving object speed, and magnetic field strength are
Fig. 2 Extensive breathing artifacts degrading the MR image quality.
the factors controlling motion artifacts, and they are stronger on higher field strengths.7 Motion artifacts may arise from random patient movement, swallowing, peristalsis, and breathing (►Fig. 2). Ghosting and smearing are the common artifacts arising from voluntary or involuntary patient motion.8 Reassuring the patient, using sequences with shorter acquisition time, using an appropriate coil, performing the scan under sedation, using soft pads between the inner surface of coil and the patient’s skin, and the use of immobilizing devices like Velcro straps are some of the ways of reducing random motion artifacts. With advances in MR imaging, there has been significant improvement in the imaging time, which is possible with use of higher magnetic field strengths, development of stronger gradients, use of multichannel coils, and newer image acquisition techniques.8 Some of these techniques include multisection imaging,8 single-shot single-section imaging,9 and parallel imaging.8 All these techniques are useful in reducing motion artifacts arising from random motion. Multisection imaging involves the simultaneous imaging of multiple interleaved sections. This reduces the scan time but is more susceptible to patient motion. As a result, this imaging technique is less favorable in a patient who is motion prone. Singleshot single-section imaging is very useful in a motion-prone patient because these are fast sequences, reducing the acquisition times. These sequences acquire the images by filling only half of the k-space, thereby reducing acquisition times.8 Parallel imaging also reduces the acquisition time and thereby limits motion artifacts. There is reduced acquisition of dataset in the phase-encoding direction, and signals from multiple coil arrays are combined. This technique uses a multicoil multichannel technology, with individual coils receiving signal from a distinct region from the field of scanning, with a known efficiency.8 Special reconstruction algorithms can be used for image reconstruction in parallel imaging. Some of these include generalized auto calibrating partially parallel acquisition (GRAPPA), sensitivity encoding (SENSE), integrated parallel acquisition techniques (IPAT), and partially parallel imaging with localized sensitivity (PILS). Seminars in Musculoskeletal Radiology
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Singh et al. blood. Continuous motion can also appear as ghosting in pulsatile flow and as increased signal on gradient-echo images due to inflowing blood.6 Identifying the cause of the artifact and applying necessary modifications to the scanning protocol can reduce periodic motion artifacts. Some of these modifications include application of flow compensation, use of out-of-phase saturation pulses, increasing the number of acquired signals, and switching the directions of the phase and frequency-encoding gradients. Patient immobilization or performing the scan under sedation may also help reduce these artifacts. CSF pulsation (►Fig. 4) is another source of motion artifact. This results in ghost images in the spinal canal in the phaseencoding direction and can be misinterpreted as intradural lesions or abnormal vascular flow voids. In some cases, absence of this artifact has been found useful as an indicator of cord compression.11 These artifacts are difficult to eliminate using the conventional methods12 and can be corrected by using gradient-echo sequences.6
Protocol-Error Artifacts Fig. 3 Sagittal fat-suppressed proton-density-weighted MR image of the knee shows popliteal artery pulsation producing ghost images (arrows) in the phase-encoding direction, obscuring the posterior horn of the meniscus.
Motion can also be repetitive, for example, due to cardiac or vascular pulsations (►Fig. 3). This type of periodic motion results in ghost images along the phase-encoding direction.4,6 The amplitude and speed of motion determine the brightness of this ghosting artifact.10 Techniques such as cardiac gating, use of navigator pulse, use of sequences with short acquisition times, or even respiratory gating are being used to minimize these artifacts. These artifacts are especially problematic in cardiac and abdominal MR imaging. Continuous motion, like that of flowing blood, can also cause motion artifacts. This artifact is best seen on T2-weighted images and can appear as signal voids in continuous flowing
Protocol-error artifacts arise from poor planning or suboptimal selection of imaging parameters. The various protocolerror artifacts include saturation, wraparound, truncation, shading, partial volume averaging, and RF interference. An experienced operator should be able to prevent these artifacts from occurring. Sufficient basic knowledge of MR imaging principles and adequate training are therefore a key aspect of the solution.
Saturation Artifacts There is signal loss in saturation artifacts due to overlapping intersection of the imaging slices.6 This happens when imaging slices intersect due to different obliquities, resulting in repeated RF excitation of the overlapping central tissues. There is simultaneous excitation of the overlapping adjacent slice during RF excitation of the first slice. This results in saturation of the overlapped slice and reduces its signal output. Saturation artifact is most often seen in axial imaging of the lumbar spine (►Fig. 5) and in imaging of the meniscus.
Fig. 4 (a) Sagittal fast spin-echo T2-weighted MR image of the thoracic spine shows cerebrospinal fluid pulsation artifact mimicking an intradural lesion (arrows). There is also focal tethering of the spinal cord. (b) This artifact is also noted on the axial fat-suppressed fast spin-echo T2-weighted MR image but is eliminated on the (c) gradient-echo T2-weighted MR image taken at the same level. Seminars in Musculoskeletal Radiology
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Fig. 5 (a, b) Saturation artifacts are seen as dark bands traversing axial T1-weighted MR images of the lumbar spine.
Placing the intersection zone away from the region of interest or imaging in nonintersecting parallel slices can eliminate this artifact. This artifact can also be reduced by the use of gradient-echo sequences because this sequence flips the tissue magnetization only by a small angle, enabling easy recovery.
Wraparound Artifacts Wraparound artifacts (►Fig. 6) are also known as aliasing artifacts. This is a commonly encountered artifact, and arises with use of a small field of view (FOV) insufficient to encompass the tissues being imaged. This results in superimposition of the phase encoded signals from outside and within the FOV and resultant wraparound of a structure to the opposite side of the image. Wraparound artifacts are always encountered in the phase-encoding direction because the system normally oversamples the frequency-encoding direction.4 These artifacts can often obscure underlying abnormalities, and hence it is important to overcome them. Using a larger FOV can reduce wraparound or aliasing artifacts; however, this results in reduction of the image quality. The image quality is unchanged, however, with the use of oversampling techniques. Saturation pulses can also be
used because these can exclude the undesirable signal from the structures outside the field of scanning. Use of a rectangular FOV and switching the frequency and phase-encoding directions also reduce these artifacts.6 Some MR units have the “no phase wrap” technique, which eliminates the wraparound artifact by doubling the FOV in the phase-encoding direction and subsequently cropping the image to a square format.6,7
Truncation Artifacts Truncation artifacts are also known as the Gibbs phenomenon13 or ringing. These artifacts are best seen at tissue interfaces with an abrupt change in intensity of the MR signal and occur with undersampling of several phase-encoding steps of high spatial resolution.6 The artifact is seen as dark or bright lines that appear parallel to the margin of the area showing an abrupt change in signal intensity.5 Truncation artifact occurs with insufficient acquisition of samples in the phase-encoding direction or the readout direction.5,7 Truncation artifacts commonly occur at fat–muscle and spinal cord–CSF interfaces and can result in an area of false high signal intensity within the cord (►Fig. 7), which can be misinterpreted as a syrinx14,15 or an atrophic cord.16
Fig. 6 Wraparound artifacts seen in two different patients on (a) axial T2-weighted MR image of the lumbar spine and (b) axial T1-weighted MR image of the cervical spine. Seminars in Musculoskeletal Radiology
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Fig. 7 Truncation artifacts seen as linear areas of hyperintense signal within the spinal cord (arrows), mimicking a syrinx, on sagittal (a) fast spinecho fat-suppressed T2-weighted and (b) gradient-echo T2 -weighted MR images of the cervical spine of two different patients.
Truncation artifact may also mimic a disc abnormality17 or even a meniscal tear in the knee.18 These may also produce false abnormalities in the cartilage.19,20 Using at least 192 phase-encoding steps, using a smaller FOV, and increasing the matrix size along the phase-encoding direction (►Fig. 8), without changing the FOV, can reduce truncation artifacts. Switching the phase- and frequency-encoding directions or the use of special reconstruction filters, like the Hanning filter,1 also reduce truncation artifacts.6
Shading Artifacts Shading artifacts are also known as intensity gradient artifacts. Nonuniformity of the RF field results in these artifacts.
This is especially seen with the use of surface coils.5 The strength of RF at a location determines the signal intensity of that voxel.21 These artifacts result in loss of brightness, variable image contrast, and deterioration of the image quality (►Fig. 9), due to decreasing RF signal in structures located further away from the coil. Shading artifacts can be corrected by the use of a larger surface coil or an enclosing coil.6 These can also be corrected by the use of RF pulses that do not depend on the field homogeneity. Operators should make the effort to change the coil instead of persisting with the originally used coil when these artifacts become apparent. Surface coil intensity correction can also be applied.5 In spine imaging, it is possible to correct for nonuniformity of
Fig. 8 Correction of truncation artifact seen within the spinal cord (arrows) on (a) initial sagittal T2-weighted MR image of the cervical spine and (b) by increasing the matrix size along the phase-encoding direction on the repeat sagittal T2-weighted MR image. Seminars in Musculoskeletal Radiology
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Fig. 9 Shading artifact causing decrease in brightness, contrast, and image quality across the upper and right side of image on this axial T1weighted MR image of the sacrum.
the receiver coil by dividing the patient image by a uniform phantom image.6
Partial Volume Averaging Artifacts Partial volume averaging artifacts occur when a single voxel includes signals from tissues of different MR properties. The final signal represents an average of the different tissue signals. Partial volume averaging artifacts typically occur when the voxel size is large in relation to the size of the structures being imaged. These artifacts are often problematic in the assessment of small structures, for example, mimicking a radial tear of the meniscus or defect of the glenoid labrum. These can also result in distortion in size of structures. Using a smaller FOV and using thinner slices are ways to reduce partial volume averaging artifacts.6
Radiofrequency Interference Artifacts/Zipper Artifacts The zipper artifact is commonly seen due to leakage of electromagnetic waves in the scan room.4 External electromagnetic waves can be detected by the receiver coil and
Chemical Shift Artifacts The resonance frequencies of protons are different in fat and in water; the difference is 224 Hz at 1.5 T.6 This difference is directly proportional to the magnetic field strength and even higher on 3-T imaging.5 Chemical shift on 3 T is estimated to be double that on 1.5 T.1 Although signals from different chemical structures originate from the same spatial position, they may occupy different positions in the image,6 producing the chemical shift artifact. This artifact is seen in the frequency-encoding direction as high signal areas where fat and water overlap and low signal where they separate (►Fig. 11).22 This phenomenon of chemical shift has been made use of in several applications, especially popular in the present age with the increasing use of 3-T machines. Chemical shift property is used in MR spectroscopy to differentiate the constituting molecules in a tissue.1 In spine imaging, chemical shift artifacts can be observed at the vertebral end plates, the ligamentum flavum, epidural fat, lipomatous tumors, and also in cystic structures surrounded by fatty tissue, such as synovial cysts and ganglion cysts.23,24 This results in superimposition of fat signal on the adjacent tissues with resultant loss of anatomical detail. In-phase and opposed-phase imaging sequences have been developed on the principles of chemical shift and are routinely used in abdominal imaging to diagnose hepatic steatosis. Some of the ways to suppress these artifacts include switching the frequency- and phase-
Fig. 10 Radiofrequency interference artifacts are seen as zipper-like linear bands on axial (a) fast spin-echo T1-weighted and (b) gradient-echo T2 -weighted MR images of the cervical spine. Seminars in Musculoskeletal Radiology
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interfere with image processing or recording. This results in image distortion and is seen as a region of increased noise with linear bands of variable signal intensity, perpendicular to the frequency-encoding direction (►Fig. 10).6 RF interference can arise from various sources that include radio broadcasting stations, fluorescent lights, electronic devices, static discharge, and imager hardware malfunction.6 Removing the source of the external electromagnetic waves eliminates the artifact. Ensuring that the door to the MR suite is tightly sealed also helps prevent these artifacts because this prevents entry of RF waves from external sources. In the scenario of persisting zipper artifacts, one has to consider the possibility of RF shield compromise.
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Fig. 12 Axial fast spin-echo T2-weighted MR image shows susceptibility artifacts, particularly at the posterior calcaneum, resulting in an area of severe signal loss in the region of metallic implant.
Fig. 11 Creation of chemical shift artifacts that are seen as a dark band at areas of cerebrospinal fluid (CSF)-fat separation (arrows) and as a bright band at areas of CSF-fat overlap when frequency encoding is switched in the (a) right-left and (b) anterior-posterior directions. (c) Increasing the bandwidth corrects the artifacts on these axial T2-weighted MR images of the lumbar spine.
encoding gradient directions, using fat suppression techniques, or increasing the receiver bandwidth (►Fig. 11).6
Susceptibility Artifacts Magnetic susceptibility is a measure of the extent of magnetization of an object placed in an external magnetic field. This susceptibility is variable for different tissues and directly proportional to the magnetic field strength. The effects of Seminars in Musculoskeletal Radiology
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magnetic susceptibility variations are estimated to be twice as large on 3 T compared with 1.5 T.1 Hence 3-T MR imaging has more pronounced susceptibility artifacts than on 1.5-T imaging. Susceptibility artifacts arise due to the inhomogeneity of the local magnetic field at the interface of structures with differing magnetic susceptibilities. This produces spatial misregistration as a result of distortion of the magnetic field.6 These artifacts are severe in the regions of ferromagnetic materials and occur in the presence of joint prostheses, metallic implants, dental prostheses, surgical clips, or metallic foreign bodies (►Fig. 12). These artifacts are also severe in gradient-echo sequences and may result in a blooming effect with apparent enlargement of bone and the adjacent soft tissue appearing smaller.6,25,26 Blooming arising from metal implants can cause loss of signal, failure of fat suppression, distortion of the anatomy, and signal pileup.27 Blooming artifacts from fine metallic debris can mimic spinal stenosis or hypertrophic bone formation because both bone and metal are hypointense in signal.28–30 Magnetic susceptibility artifacts cause a shift in the resonance frequency of lipid and water, and they reduce the efficacy of fat suppression. There may also be a false impression of a water-suppressed image. In routine MR imaging of the cervical spine, fat suppression artifacts are commonly seen in the neck region (►Fig. 13).31,32 Intra-articular air foci may cause susceptibility artifacts and can be misinterpreted as a lesion.33,34 Various dental prostheses may also cause susceptibility artifacts, thereby degrading MR image quality.35 There is an increasing need for MR imaging in the postoperative spine,36 and it is important to reduce these artifacts to ensure good image quality. These artifacts often also cause significant degradation of the image quality in patients with metallic implants (►Fig. 14). Susceptibility artifacts depend
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Fig. 13 Susceptibility artifact causing incomplete fat suppression and water suppression. (a) Sagittal fast spin-echo fat-suppressed T2-weighted MR image of the cervical spine shows incomplete fat suppression of the lower cervical and upper thoracic subcutaneous fat and vertebral marrow fat signals. There is also suppression of the cerebrospinal fluid (CSF) at the cervicothoracic area due to spatial misregistration resulting from inhomogeneity of the local magnetic field. (b) Repeat sagittal short tau inversion recovery MR image shows the contents of the spinal canal, including the CSF, as well as homogeneous fat suppression of the vertebral marrow signal and near-complete fat suppression of the subcutaneous tissue.
on the type of metal used in manufacturing of the implant, with titanium alloy causing less severe artifacts compared with stainless steel.37 Several techniques are available to reduce the effects of susceptibility artifacts. If an external metallic object or a metallic clothing part causes the artifact, removing it corrects the artifact. However, these artifacts more often arise due to anatomical or fixed structures (e.g., metal implants or dental prostheses). In such situations, adjusting the scanning parameters reduces the artifact. The effects of magnetic susceptibility can be reduced by using fast spin-echo sequences with short echo times (►Fig. 15) or by increasing the receiver bandwidth. Use of small FOV, high-resolution matrix, and high gradient strength has been found to reduce susceptibility artifacts.37 In spinal imaging post-instrumentation, orienting the frequency-encoding gradient along the long axis of pedicle screws has helped reduced these artifacts.38 Special three-dimensional (3D) MR imaging techniques have been developed for reduc-
tion of metal-induced susceptibility artifacts, especially in the imaging of joints after insertion of metallic prostheses. These include slice encoding for metal artifact correction (SEMAC) 39,40 and multi-acquisition variable-resonance image combination (MAVRIC).39,41 These techniques not only reduce metal-induced susceptibility artifacts, but they also enable visualization of the interface between the bone and metal. Both these techniques can give images similar to those acquired using standard T1-, T2-, proton density (PD), and short tau inversion recovery sequences.39 SEMAC makes use of a 3D spin-echo acquisition that resolves through-slice distortion caused by the metal. The in-plane distortion is resolved by using a compensation gradient. MAVRIC, in contrast, uses a standard 3D read-out and excites limited spectral bandwidths.39 Iterative decomposition of water and fat with the echo asymmetry and least squares estimation (IDEAL) technique has also been used to reduce susceptibility artifacts arising in postoperative spine imaging.36,42 IDEAL
Fig. 14 Contrast-enhanced (a) axial and (b) coronal fat-suppressed T1-weighted MR images of the pelvis show susceptibility artifacts and failure of fat-suppression caused by metallic hip implants. Seminars in Musculoskeletal Radiology
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Fig. 15 (a) Lateral radiograph of the cervical spine shows a metallic implant at the C5–C6 intervertebral disc level. (b) Axial gradient-echo MR image shows extensive susceptibility artifacts. Axial fast spin-echo (c) T1-weighted and (d) T2-weighed MR images show significant improvement in the image quality due to marked reduction of susceptibility artifacts.
significantly reduces the peripheral rim of high signal intensity around the metal and thereby improves visualization of the thecal sac and paraspinal soft tissue. This technique provides uniform fat saturation, can be used with gadolinium contrast, and has a very high signal-to-noise ratio. These newer techniques ensure good soft tissue resolution and to an extent overcome the inherent limitation of MR imaging around metal.
Magic Angle Phenomenon The magic angle phenomenon occurs in sequences with short echo time. These include T1- and PD-weighted sequences. Musculoskeletal tissues such as the menisci,
tendons, and hyaline cartilage may be affected by the magic angle phenomenon.6 When placed in an external magnetic field, these structures behave in an anisotropic manner and produce little or no signal when imaged using conventional MR imaging.6 The magic angle phenomenon occurs when fibers are oriented at an angle of 55 degrees to the static magnetic field. At this angle, the spin-spin interactions from the static local field are nullified, resulting in T2 decay being controlled only by the dynamic local field. As a result, the T2 relaxation time rises while the T2 decay is not as rapid, causing anisotropic structures such as normal hyaline cartilage, tendons, and menisci to appear bright at short TE (echo time) values.6,43,44
Fig. 16 (a) The magic angle phenomenon is seen as an area of spurious high signal (arrows) in the Achilles tendon on the sagittal fast spin-echo T1weighted MR image of the ankle. (b) The artifact is not seen on the sagittal fat-suppressed T2-weighted MR image because this sequence uses a TE value > 37 milliseconds. Seminars in Musculoskeletal Radiology
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Conclusion MR imaging can be susceptible to several artifacts. The artifacts related to protocol errors can be corrected by modifications to the scanning technique. Other artifacts, such as random and periodic motion artifacts, require the use of sequences with shorter acquisition times, use of newer MR imaging techniques and reconstruction algorithms, along with the use of cardiac gating, navigator pulse, or respiratory gating. Some artifacts such as chemical shift and susceptibility are more prominent with increasing use of 3-T imaging because these are directly related to the field strength of the magnet. The increasing need of MR imaging in the postoperative spine and post–metallic implant joint evaluation necessitates eliminating susceptibility artifacts. Identifying and correcting these artifacts is essential, to improve the overall image quality and to avoid possible misinterpretation of these artifacts.
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These artifacts have been identified in MR imaging of various regions, such as the shoulder, wrist, knee, and ankle (►Fig. 16). The magic angle phenomenon has been known to simulate degeneration or tears in various ankle tendons,45 supraspinatus tendon, Achilles tendon,46 posterior cruciate ligament, glenoid labrum, articular cartilage,47,48 or even the menisci. The magic angle phenomenon has also been described as a known artifact on MR neurography on 3-T imaging.49,50 MR neurography has been used in the evaluation of brachial plexus nerve roots, ulnar nerve at elbow, sciatic nerve, and also in assessment of the median nerve, in suspected cases of carpal tunnel syndrome.50 These artifacts can give rise to areas of increased signal intensity in the peripheral nerves, which can falsely mimic underlying disease. The artifact can be corrected by closely comparing abnormalities with those on T2-weighted images, repeating the scan with a TE value of 37 milliseconds,43 and by repositioning the patient.51,52 Repositioning the patient changes the angle of orientation of the structure to the static magnetic field. The signal abnormality, if arising from the magic angle phenomenon, is eliminated on repeat imaging.
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28 Peterman SB, Hoffman JC Jr, Malko JA. Magnetic resonance artifact
40 Lu W, Pauly KB, Gold GE, Pauly JM, Hargreaves BA. SEMAC: Slice
in the postoperative cervical spine. A potential pitfall. Spine 1991; 16(7):721–725 Levitt M, Benjamin V, Kricheff II. Potential misinterpretation of cervical spondylosis with cord compression caused by metallic artifacts in magnetic resonance imaging of the postoperative spine. Neurosurgery 1990;27(1):126–129; discussion 129–130 Salazar JL, Misra M, Bloom D, Dobben G. MRI artifacts following anterior cervical diskectomy. Surg Neurol 1997;48(1):23–29 Anzai Y, Lufkin RB, Jabour BA, Hanafee WN. Fat-suppression failure artifacts simulating pathology on frequency-selective fat-suppression MR images of the head and neck. AJNR Am J Neuroradiol 1992; 13(3):879–884 Delfaut EM, Beltran J, Johnson G, Rousseau J, Marchandise X, Cotten A. Fat suppression in MR imaging: techniques and pitfalls. Radiographics 1999;19(2):373–382 Patten RM. Vacuum phenomenon: a potential pitfall in the interpretation of gradient-recalled-echo MR images of the shoulder. AJR Am J Roentgenol 1994;162(6):1383–1386 Gückel C, Nidecker A. The rope ladder: an uncommon artifact and potential pitfall in MR arthrography of the shoulder. AJR Am J Roentgenol 1997;168(4):947–950 Eggers G, Rieker M, Kress B, Fiebach J, Dickhaus H, Hassfeld S. Artefacts in magnetic resonance imaging caused by dental material. MAGMA 2005;18(2):103–111 Cha JG, Jin W, Lee MH, et al. Reducing metallic artifacts in postoperative spinal imaging: usefulness of IDEAL contrast-enhanced T1- and T2-weighted MR imaging—phantom and clinical studies. Radiology 2011;259(3):885–893 Lee MJ, Kim S, Lee SA, et al. Overcoming artifacts from metallic orthopedic implants at high-field-strength MR imaging and multidetector CT. Radiographics 2007;27(3):791–803 Frazzini VI, Kagetsu NJ, Johnson CE, Destian S. Internally stabilized spine: optimal choice of frequency-encoding gradient direction during MR imaging minimizes susceptibility artifact from titanium vertebral body screws. Radiology 1997;204(1):268–272 Chen CA, Chen W, Goodman SB, et al. New MR imaging methods for metallic implants in the knee: artifact correction and clinical impact. J Magn Reson Imaging 2011;33(5):1121–1127
encoding for metal artifact correction in MRI. Magn Reson Med 2009;62(1):66–76 Koch KM, Lorbiecki JE, Hinks RS, King KF. A multispectral threedimensional acquisition technique for imaging near metal implants. Magn Reson Med 2009;61(2):381–390 Reeder SB, Pineda AR, Wen Z, et al. Iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL): application with fast spin-echo imaging. Magn Reson Med 2005;54(3):636–644 Peh WCG, Chan JHM. The magic angle phenomenon in tendons: effect of varying the MR echo time. Br J Radiol 1998;71(841):31–36 Erickson SJ, Cox IH, Hyde JS, Carrera GF, Strandt JA, Estkowski LD. Effect of tendon orientation on MR imaging signal intensity: a manifestation of the “magic angle” phenomenon. Radiology 1991; 181(2):389–392 Mengiardi B, Pfirrmann CW, Schöttle PB, et al. Magic angle effect in MR imaging of ankle tendons: influence of foot positioning on prevalence and site in asymptomatic subjects and cadaveric tendons. Eur Radiol 2006;16(10):2197–2206 Du J, Pak BC, Znamirowski R, et al. Magic angle effect in magnetic resonance imaging of the Achilles tendon and enthesis. Magn Reson Imaging 2009;27(4):557–564 Disler DG, Recht MP, McCauley TR. MR imaging of articular cartilage. Skeletal Radiol 2000;29(7):367–377 Xia Y. Magic-angle effect in magnetic resonance imaging of articular cartilage: a review. Invest Radiol 2000;35(10):602–621 Kästel T, Heiland S, Bäumer P, Bartsch AJ, Bendszus M, Pham M. Magic angle effect: a relevant artifact in MR neurography at 3T? AJNR Am J Neuroradiol 2011;32(5):821–827 Chappell KE, Robson MD, Stonebridge-Foster A, et al. Magic angle effects in MR neurography. AJNR Am J Neuroradiol 2004;25(3): 431–440 Hayes CW, Parellada JA. The magic angle effect in musculoskeletal MR imaging. Top Magn Reson Imaging 1996;8(1):51–56 Timins ME, Erickson SJ, Estkowski LD, et al. Increased signal in the normal supraspinatus tendon on MR imaging: diagnostic pitfall caused by the magic-angle effect. AJR Am J Roentgenol 1995; 165(1):109–114
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Artifacts in Musculoskeletal MR Imaging Dinesh R. Singh, MBBS, MMed, DNB, FRCR1 Michael S.M. Chin, DCR1 Wilfred C.G. Peh, MBBS, MD, FRCP(Glasg), FRCP(Edin), FRCR1
Alexandra Health, Singapore, Republic of Singapore Semin Musculoskelet Radiol 2014;18:12–22.
Abstract
Keywords
► chemical shift artifact ► magnetic resonance imaging ► motion artifact ► protocol-error artifacts ► susceptibility artifact
Address for correspondence Wilfred C.G. Peh, MBBS, MD, FRCP(Glasg), FRCP(Edin), FRCR, Department of Diagnostic Radiology, Khoo Teck Puat Hospital, Alexandra Health, 90 Yishun Central, Singapore 768828, Republic of Singapore (e-mail: [email protected]).
MR imaging has become an important diagnostic tool in the evaluation of a vast number of pathologies and is of foremost importance in the evaluation of spine, joints, and soft tissue structures of the musculoskeletal system. MR imaging is susceptible to various artifacts that may affect the image quality or even simulate pathologies. Some of these artifacts have gained special importance with the use of higher field strength magnets and with the increasing need for MR imaging in postoperative patients, especially those with previous joint replacements or metallic implants. Artifacts may arise from patient motion or could be due to periodic motion, such as vascular and cardiac pulsation. Artifacts could also arise from various protocol errors including saturation, wraparound, truncation, shading, partial volume averaging, and radiofrequency interference artifacts. Susceptibility artifact occurs at interfaces with different magnetic susceptibilities and is of special importance with increasing use of metallic joint replacement prostheses. Magic angle phenomenon is a special type of artifact that occurs in musculoskeletal MR imaging. It is essential to recognize these artifacts and to correct them because they may produce pitfalls in image interpretation.
MR imaging is one of the most important diagnostic tools in evaluating the abnormalities of the musculoskeletal system. It is routinely used in the evaluation of the spine, various joints, and soft tissue structures. However, MR imaging of the musculoskeletal system is subject to several artifacts. Many of these artifacts are more prominent with increasing use of higher field strength machines, especially with 3-T MR imaging.1 These artifacts may arise due to a number of factors, some physical and some technical, and affect the image quality or even simulate pathologies. Artifacts may arise due to motion or may be due to protocol errors, for example, saturation, wraparound, truncation, shading, radiofrequency, and partial volume averaging artifacts. Motion artifacts may be related to patient motion or may result from periodic motion such as cardiac or vascular pulsation. Respiratory motion and bowel peristalsis also contribute to these motion artifacts. Cerebrospinal fluid (CSF) pulsation artifacts are also
Issue Theme Variants and Pitfalls in Musculoskeletal Imaging; Guest Editor, Wilfred C.G. Peh, MBBS, MD, FRCP (Glasg), FRCP(Edin), FRCR
commonly encountered in spine imaging and can be misinterpreted as sinister abnormalities of the spine. The magic angle phenomenon needs particular mention because it is a special type of artifact seen in musculoskeletal imaging and may cause misinterpretation of the signal abnormalities if not identified and corrected. Susceptibility artifacts are of practical importance with increasing number of joint replacement procedures and subsequent MR imaging. The susceptibility effects are directly proportional to the magnetic field strength and are more prominent on newer 3-T machines.1,2 Although these artifacts have been reduced with the increasing use of titanium alloys in modern implants, they still remain of concern because these cause image distortion close to the metallic hardware.3 It is essential to be aware of these artifacts, to avoid any possible misinterpretation, and to improve the image quality. This in turn may have a significant impact on overall patient care. We describe the
Copyright © 2014 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0034-1365831. ISSN 1089-7860.
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1 Department of Diagnostic Radiology, Khoo Teck Puat Hospital,
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Fig. 1 Patient motion artifact in MR imaging of the shoulder, resulting in poor quality (a) sagittal and (b) axial images.
various MR artifacts encountered in musculoskeletal imaging and their identification, and we suggest ways to correct them, if possible.
MR Artifacts MR imaging artifacts arise from several complex interactions involving the main magnet, the gradient coils, the radiofrequency (RF) transmitter and receiver, and the reconstruction algorithm used.4 They are related to a variety of factors, some patient related, some hardware related; some might be protocol related.
Motion Artifacts Motion artifacts are the most common artifacts in the imaging of the musculoskeletal system and are easily recognized. Standard image reconstruction techniques ideally require a perfectly stationary patient, which rarely happens. Patient motion (►Fig. 1) and motion due to various physiologic processes often result in artifacts.5 These artifacts occur because the phase gradient cannot predictably encode the radio waves arising from moving structures.6 The type of motion, moving object speed, and magnetic field strength are
Fig. 2 Extensive breathing artifacts degrading the MR image quality.
the factors controlling motion artifacts, and they are stronger on higher field strengths.7 Motion artifacts may arise from random patient movement, swallowing, peristalsis, and breathing (►Fig. 2). Ghosting and smearing are the common artifacts arising from voluntary or involuntary patient motion.8 Reassuring the patient, using sequences with shorter acquisition time, using an appropriate coil, performing the scan under sedation, using soft pads between the inner surface of coil and the patient’s skin, and the use of immobilizing devices like Velcro straps are some of the ways of reducing random motion artifacts. With advances in MR imaging, there has been significant improvement in the imaging time, which is possible with use of higher magnetic field strengths, development of stronger gradients, use of multichannel coils, and newer image acquisition techniques.8 Some of these techniques include multisection imaging,8 single-shot single-section imaging,9 and parallel imaging.8 All these techniques are useful in reducing motion artifacts arising from random motion. Multisection imaging involves the simultaneous imaging of multiple interleaved sections. This reduces the scan time but is more susceptible to patient motion. As a result, this imaging technique is less favorable in a patient who is motion prone. Singleshot single-section imaging is very useful in a motion-prone patient because these are fast sequences, reducing the acquisition times. These sequences acquire the images by filling only half of the k-space, thereby reducing acquisition times.8 Parallel imaging also reduces the acquisition time and thereby limits motion artifacts. There is reduced acquisition of dataset in the phase-encoding direction, and signals from multiple coil arrays are combined. This technique uses a multicoil multichannel technology, with individual coils receiving signal from a distinct region from the field of scanning, with a known efficiency.8 Special reconstruction algorithms can be used for image reconstruction in parallel imaging. Some of these include generalized auto calibrating partially parallel acquisition (GRAPPA), sensitivity encoding (SENSE), integrated parallel acquisition techniques (IPAT), and partially parallel imaging with localized sensitivity (PILS). Seminars in Musculoskeletal Radiology
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Singh et al. blood. Continuous motion can also appear as ghosting in pulsatile flow and as increased signal on gradient-echo images due to inflowing blood.6 Identifying the cause of the artifact and applying necessary modifications to the scanning protocol can reduce periodic motion artifacts. Some of these modifications include application of flow compensation, use of out-of-phase saturation pulses, increasing the number of acquired signals, and switching the directions of the phase and frequency-encoding gradients. Patient immobilization or performing the scan under sedation may also help reduce these artifacts. CSF pulsation (►Fig. 4) is another source of motion artifact. This results in ghost images in the spinal canal in the phaseencoding direction and can be misinterpreted as intradural lesions or abnormal vascular flow voids. In some cases, absence of this artifact has been found useful as an indicator of cord compression.11 These artifacts are difficult to eliminate using the conventional methods12 and can be corrected by using gradient-echo sequences.6
Protocol-Error Artifacts Fig. 3 Sagittal fat-suppressed proton-density-weighted MR image of the knee shows popliteal artery pulsation producing ghost images (arrows) in the phase-encoding direction, obscuring the posterior horn of the meniscus.
Motion can also be repetitive, for example, due to cardiac or vascular pulsations (►Fig. 3). This type of periodic motion results in ghost images along the phase-encoding direction.4,6 The amplitude and speed of motion determine the brightness of this ghosting artifact.10 Techniques such as cardiac gating, use of navigator pulse, use of sequences with short acquisition times, or even respiratory gating are being used to minimize these artifacts. These artifacts are especially problematic in cardiac and abdominal MR imaging. Continuous motion, like that of flowing blood, can also cause motion artifacts. This artifact is best seen on T2-weighted images and can appear as signal voids in continuous flowing
Protocol-error artifacts arise from poor planning or suboptimal selection of imaging parameters. The various protocolerror artifacts include saturation, wraparound, truncation, shading, partial volume averaging, and RF interference. An experienced operator should be able to prevent these artifacts from occurring. Sufficient basic knowledge of MR imaging principles and adequate training are therefore a key aspect of the solution.
Saturation Artifacts There is signal loss in saturation artifacts due to overlapping intersection of the imaging slices.6 This happens when imaging slices intersect due to different obliquities, resulting in repeated RF excitation of the overlapping central tissues. There is simultaneous excitation of the overlapping adjacent slice during RF excitation of the first slice. This results in saturation of the overlapped slice and reduces its signal output. Saturation artifact is most often seen in axial imaging of the lumbar spine (►Fig. 5) and in imaging of the meniscus.
Fig. 4 (a) Sagittal fast spin-echo T2-weighted MR image of the thoracic spine shows cerebrospinal fluid pulsation artifact mimicking an intradural lesion (arrows). There is also focal tethering of the spinal cord. (b) This artifact is also noted on the axial fat-suppressed fast spin-echo T2-weighted MR image but is eliminated on the (c) gradient-echo T2-weighted MR image taken at the same level. Seminars in Musculoskeletal Radiology
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Fig. 5 (a, b) Saturation artifacts are seen as dark bands traversing axial T1-weighted MR images of the lumbar spine.
Placing the intersection zone away from the region of interest or imaging in nonintersecting parallel slices can eliminate this artifact. This artifact can also be reduced by the use of gradient-echo sequences because this sequence flips the tissue magnetization only by a small angle, enabling easy recovery.
Wraparound Artifacts Wraparound artifacts (►Fig. 6) are also known as aliasing artifacts. This is a commonly encountered artifact, and arises with use of a small field of view (FOV) insufficient to encompass the tissues being imaged. This results in superimposition of the phase encoded signals from outside and within the FOV and resultant wraparound of a structure to the opposite side of the image. Wraparound artifacts are always encountered in the phase-encoding direction because the system normally oversamples the frequency-encoding direction.4 These artifacts can often obscure underlying abnormalities, and hence it is important to overcome them. Using a larger FOV can reduce wraparound or aliasing artifacts; however, this results in reduction of the image quality. The image quality is unchanged, however, with the use of oversampling techniques. Saturation pulses can also be
used because these can exclude the undesirable signal from the structures outside the field of scanning. Use of a rectangular FOV and switching the frequency and phase-encoding directions also reduce these artifacts.6 Some MR units have the “no phase wrap” technique, which eliminates the wraparound artifact by doubling the FOV in the phase-encoding direction and subsequently cropping the image to a square format.6,7
Truncation Artifacts Truncation artifacts are also known as the Gibbs phenomenon13 or ringing. These artifacts are best seen at tissue interfaces with an abrupt change in intensity of the MR signal and occur with undersampling of several phase-encoding steps of high spatial resolution.6 The artifact is seen as dark or bright lines that appear parallel to the margin of the area showing an abrupt change in signal intensity.5 Truncation artifact occurs with insufficient acquisition of samples in the phase-encoding direction or the readout direction.5,7 Truncation artifacts commonly occur at fat–muscle and spinal cord–CSF interfaces and can result in an area of false high signal intensity within the cord (►Fig. 7), which can be misinterpreted as a syrinx14,15 or an atrophic cord.16
Fig. 6 Wraparound artifacts seen in two different patients on (a) axial T2-weighted MR image of the lumbar spine and (b) axial T1-weighted MR image of the cervical spine. Seminars in Musculoskeletal Radiology
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Fig. 7 Truncation artifacts seen as linear areas of hyperintense signal within the spinal cord (arrows), mimicking a syrinx, on sagittal (a) fast spinecho fat-suppressed T2-weighted and (b) gradient-echo T2 -weighted MR images of the cervical spine of two different patients.
Truncation artifact may also mimic a disc abnormality17 or even a meniscal tear in the knee.18 These may also produce false abnormalities in the cartilage.19,20 Using at least 192 phase-encoding steps, using a smaller FOV, and increasing the matrix size along the phase-encoding direction (►Fig. 8), without changing the FOV, can reduce truncation artifacts. Switching the phase- and frequency-encoding directions or the use of special reconstruction filters, like the Hanning filter,1 also reduce truncation artifacts.6
Shading Artifacts Shading artifacts are also known as intensity gradient artifacts. Nonuniformity of the RF field results in these artifacts.
This is especially seen with the use of surface coils.5 The strength of RF at a location determines the signal intensity of that voxel.21 These artifacts result in loss of brightness, variable image contrast, and deterioration of the image quality (►Fig. 9), due to decreasing RF signal in structures located further away from the coil. Shading artifacts can be corrected by the use of a larger surface coil or an enclosing coil.6 These can also be corrected by the use of RF pulses that do not depend on the field homogeneity. Operators should make the effort to change the coil instead of persisting with the originally used coil when these artifacts become apparent. Surface coil intensity correction can also be applied.5 In spine imaging, it is possible to correct for nonuniformity of
Fig. 8 Correction of truncation artifact seen within the spinal cord (arrows) on (a) initial sagittal T2-weighted MR image of the cervical spine and (b) by increasing the matrix size along the phase-encoding direction on the repeat sagittal T2-weighted MR image. Seminars in Musculoskeletal Radiology
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Fig. 9 Shading artifact causing decrease in brightness, contrast, and image quality across the upper and right side of image on this axial T1weighted MR image of the sacrum.
the receiver coil by dividing the patient image by a uniform phantom image.6
Partial Volume Averaging Artifacts Partial volume averaging artifacts occur when a single voxel includes signals from tissues of different MR properties. The final signal represents an average of the different tissue signals. Partial volume averaging artifacts typically occur when the voxel size is large in relation to the size of the structures being imaged. These artifacts are often problematic in the assessment of small structures, for example, mimicking a radial tear of the meniscus or defect of the glenoid labrum. These can also result in distortion in size of structures. Using a smaller FOV and using thinner slices are ways to reduce partial volume averaging artifacts.6
Radiofrequency Interference Artifacts/Zipper Artifacts The zipper artifact is commonly seen due to leakage of electromagnetic waves in the scan room.4 External electromagnetic waves can be detected by the receiver coil and
Chemical Shift Artifacts The resonance frequencies of protons are different in fat and in water; the difference is 224 Hz at 1.5 T.6 This difference is directly proportional to the magnetic field strength and even higher on 3-T imaging.5 Chemical shift on 3 T is estimated to be double that on 1.5 T.1 Although signals from different chemical structures originate from the same spatial position, they may occupy different positions in the image,6 producing the chemical shift artifact. This artifact is seen in the frequency-encoding direction as high signal areas where fat and water overlap and low signal where they separate (►Fig. 11).22 This phenomenon of chemical shift has been made use of in several applications, especially popular in the present age with the increasing use of 3-T machines. Chemical shift property is used in MR spectroscopy to differentiate the constituting molecules in a tissue.1 In spine imaging, chemical shift artifacts can be observed at the vertebral end plates, the ligamentum flavum, epidural fat, lipomatous tumors, and also in cystic structures surrounded by fatty tissue, such as synovial cysts and ganglion cysts.23,24 This results in superimposition of fat signal on the adjacent tissues with resultant loss of anatomical detail. In-phase and opposed-phase imaging sequences have been developed on the principles of chemical shift and are routinely used in abdominal imaging to diagnose hepatic steatosis. Some of the ways to suppress these artifacts include switching the frequency- and phase-
Fig. 10 Radiofrequency interference artifacts are seen as zipper-like linear bands on axial (a) fast spin-echo T1-weighted and (b) gradient-echo T2 -weighted MR images of the cervical spine. Seminars in Musculoskeletal Radiology
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interfere with image processing or recording. This results in image distortion and is seen as a region of increased noise with linear bands of variable signal intensity, perpendicular to the frequency-encoding direction (►Fig. 10).6 RF interference can arise from various sources that include radio broadcasting stations, fluorescent lights, electronic devices, static discharge, and imager hardware malfunction.6 Removing the source of the external electromagnetic waves eliminates the artifact. Ensuring that the door to the MR suite is tightly sealed also helps prevent these artifacts because this prevents entry of RF waves from external sources. In the scenario of persisting zipper artifacts, one has to consider the possibility of RF shield compromise.
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Fig. 12 Axial fast spin-echo T2-weighted MR image shows susceptibility artifacts, particularly at the posterior calcaneum, resulting in an area of severe signal loss in the region of metallic implant.
Fig. 11 Creation of chemical shift artifacts that are seen as a dark band at areas of cerebrospinal fluid (CSF)-fat separation (arrows) and as a bright band at areas of CSF-fat overlap when frequency encoding is switched in the (a) right-left and (b) anterior-posterior directions. (c) Increasing the bandwidth corrects the artifacts on these axial T2-weighted MR images of the lumbar spine.
encoding gradient directions, using fat suppression techniques, or increasing the receiver bandwidth (►Fig. 11).6
Susceptibility Artifacts Magnetic susceptibility is a measure of the extent of magnetization of an object placed in an external magnetic field. This susceptibility is variable for different tissues and directly proportional to the magnetic field strength. The effects of Seminars in Musculoskeletal Radiology
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magnetic susceptibility variations are estimated to be twice as large on 3 T compared with 1.5 T.1 Hence 3-T MR imaging has more pronounced susceptibility artifacts than on 1.5-T imaging. Susceptibility artifacts arise due to the inhomogeneity of the local magnetic field at the interface of structures with differing magnetic susceptibilities. This produces spatial misregistration as a result of distortion of the magnetic field.6 These artifacts are severe in the regions of ferromagnetic materials and occur in the presence of joint prostheses, metallic implants, dental prostheses, surgical clips, or metallic foreign bodies (►Fig. 12). These artifacts are also severe in gradient-echo sequences and may result in a blooming effect with apparent enlargement of bone and the adjacent soft tissue appearing smaller.6,25,26 Blooming arising from metal implants can cause loss of signal, failure of fat suppression, distortion of the anatomy, and signal pileup.27 Blooming artifacts from fine metallic debris can mimic spinal stenosis or hypertrophic bone formation because both bone and metal are hypointense in signal.28–30 Magnetic susceptibility artifacts cause a shift in the resonance frequency of lipid and water, and they reduce the efficacy of fat suppression. There may also be a false impression of a water-suppressed image. In routine MR imaging of the cervical spine, fat suppression artifacts are commonly seen in the neck region (►Fig. 13).31,32 Intra-articular air foci may cause susceptibility artifacts and can be misinterpreted as a lesion.33,34 Various dental prostheses may also cause susceptibility artifacts, thereby degrading MR image quality.35 There is an increasing need for MR imaging in the postoperative spine,36 and it is important to reduce these artifacts to ensure good image quality. These artifacts often also cause significant degradation of the image quality in patients with metallic implants (►Fig. 14). Susceptibility artifacts depend
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Fig. 13 Susceptibility artifact causing incomplete fat suppression and water suppression. (a) Sagittal fast spin-echo fat-suppressed T2-weighted MR image of the cervical spine shows incomplete fat suppression of the lower cervical and upper thoracic subcutaneous fat and vertebral marrow fat signals. There is also suppression of the cerebrospinal fluid (CSF) at the cervicothoracic area due to spatial misregistration resulting from inhomogeneity of the local magnetic field. (b) Repeat sagittal short tau inversion recovery MR image shows the contents of the spinal canal, including the CSF, as well as homogeneous fat suppression of the vertebral marrow signal and near-complete fat suppression of the subcutaneous tissue.
on the type of metal used in manufacturing of the implant, with titanium alloy causing less severe artifacts compared with stainless steel.37 Several techniques are available to reduce the effects of susceptibility artifacts. If an external metallic object or a metallic clothing part causes the artifact, removing it corrects the artifact. However, these artifacts more often arise due to anatomical or fixed structures (e.g., metal implants or dental prostheses). In such situations, adjusting the scanning parameters reduces the artifact. The effects of magnetic susceptibility can be reduced by using fast spin-echo sequences with short echo times (►Fig. 15) or by increasing the receiver bandwidth. Use of small FOV, high-resolution matrix, and high gradient strength has been found to reduce susceptibility artifacts.37 In spinal imaging post-instrumentation, orienting the frequency-encoding gradient along the long axis of pedicle screws has helped reduced these artifacts.38 Special three-dimensional (3D) MR imaging techniques have been developed for reduc-
tion of metal-induced susceptibility artifacts, especially in the imaging of joints after insertion of metallic prostheses. These include slice encoding for metal artifact correction (SEMAC) 39,40 and multi-acquisition variable-resonance image combination (MAVRIC).39,41 These techniques not only reduce metal-induced susceptibility artifacts, but they also enable visualization of the interface between the bone and metal. Both these techniques can give images similar to those acquired using standard T1-, T2-, proton density (PD), and short tau inversion recovery sequences.39 SEMAC makes use of a 3D spin-echo acquisition that resolves through-slice distortion caused by the metal. The in-plane distortion is resolved by using a compensation gradient. MAVRIC, in contrast, uses a standard 3D read-out and excites limited spectral bandwidths.39 Iterative decomposition of water and fat with the echo asymmetry and least squares estimation (IDEAL) technique has also been used to reduce susceptibility artifacts arising in postoperative spine imaging.36,42 IDEAL
Fig. 14 Contrast-enhanced (a) axial and (b) coronal fat-suppressed T1-weighted MR images of the pelvis show susceptibility artifacts and failure of fat-suppression caused by metallic hip implants. Seminars in Musculoskeletal Radiology
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Fig. 15 (a) Lateral radiograph of the cervical spine shows a metallic implant at the C5–C6 intervertebral disc level. (b) Axial gradient-echo MR image shows extensive susceptibility artifacts. Axial fast spin-echo (c) T1-weighted and (d) T2-weighed MR images show significant improvement in the image quality due to marked reduction of susceptibility artifacts.
significantly reduces the peripheral rim of high signal intensity around the metal and thereby improves visualization of the thecal sac and paraspinal soft tissue. This technique provides uniform fat saturation, can be used with gadolinium contrast, and has a very high signal-to-noise ratio. These newer techniques ensure good soft tissue resolution and to an extent overcome the inherent limitation of MR imaging around metal.
Magic Angle Phenomenon The magic angle phenomenon occurs in sequences with short echo time. These include T1- and PD-weighted sequences. Musculoskeletal tissues such as the menisci,
tendons, and hyaline cartilage may be affected by the magic angle phenomenon.6 When placed in an external magnetic field, these structures behave in an anisotropic manner and produce little or no signal when imaged using conventional MR imaging.6 The magic angle phenomenon occurs when fibers are oriented at an angle of 55 degrees to the static magnetic field. At this angle, the spin-spin interactions from the static local field are nullified, resulting in T2 decay being controlled only by the dynamic local field. As a result, the T2 relaxation time rises while the T2 decay is not as rapid, causing anisotropic structures such as normal hyaline cartilage, tendons, and menisci to appear bright at short TE (echo time) values.6,43,44
Fig. 16 (a) The magic angle phenomenon is seen as an area of spurious high signal (arrows) in the Achilles tendon on the sagittal fast spin-echo T1weighted MR image of the ankle. (b) The artifact is not seen on the sagittal fat-suppressed T2-weighted MR image because this sequence uses a TE value > 37 milliseconds. Seminars in Musculoskeletal Radiology
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Conclusion MR imaging can be susceptible to several artifacts. The artifacts related to protocol errors can be corrected by modifications to the scanning technique. Other artifacts, such as random and periodic motion artifacts, require the use of sequences with shorter acquisition times, use of newer MR imaging techniques and reconstruction algorithms, along with the use of cardiac gating, navigator pulse, or respiratory gating. Some artifacts such as chemical shift and susceptibility are more prominent with increasing use of 3-T imaging because these are directly related to the field strength of the magnet. The increasing need of MR imaging in the postoperative spine and post–metallic implant joint evaluation necessitates eliminating susceptibility artifacts. Identifying and correcting these artifacts is essential, to improve the overall image quality and to avoid possible misinterpretation of these artifacts.
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These artifacts have been identified in MR imaging of various regions, such as the shoulder, wrist, knee, and ankle (►Fig. 16). The magic angle phenomenon has been known to simulate degeneration or tears in various ankle tendons,45 supraspinatus tendon, Achilles tendon,46 posterior cruciate ligament, glenoid labrum, articular cartilage,47,48 or even the menisci. The magic angle phenomenon has also been described as a known artifact on MR neurography on 3-T imaging.49,50 MR neurography has been used in the evaluation of brachial plexus nerve roots, ulnar nerve at elbow, sciatic nerve, and also in assessment of the median nerve, in suspected cases of carpal tunnel syndrome.50 These artifacts can give rise to areas of increased signal intensity in the peripheral nerves, which can falsely mimic underlying disease. The artifact can be corrected by closely comparing abnormalities with those on T2-weighted images, repeating the scan with a TE value of 37 milliseconds,43 and by repositioning the patient.51,52 Repositioning the patient changes the angle of orientation of the structure to the static magnetic field. The signal abnormality, if arising from the magic angle phenomenon, is eliminated on repeat imaging.
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encoding for metal artifact correction in MRI. Magn Reson Med 2009;62(1):66–76 Koch KM, Lorbiecki JE, Hinks RS, King KF. A multispectral threedimensional acquisition technique for imaging near metal implants. Magn Reson Med 2009;61(2):381–390 Reeder SB, Pineda AR, Wen Z, et al. Iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL): application with fast spin-echo imaging. Magn Reson Med 2005;54(3):636–644 Peh WCG, Chan JHM. The magic angle phenomenon in tendons: effect of varying the MR echo time. Br J Radiol 1998;71(841):31–36 Erickson SJ, Cox IH, Hyde JS, Carrera GF, Strandt JA, Estkowski LD. Effect of tendon orientation on MR imaging signal intensity: a manifestation of the “magic angle” phenomenon. Radiology 1991; 181(2):389–392 Mengiardi B, Pfirrmann CW, Schöttle PB, et al. Magic angle effect in MR imaging of ankle tendons: influence of foot positioning on prevalence and site in asymptomatic subjects and cadaveric tendons. Eur Radiol 2006;16(10):2197–2206 Du J, Pak BC, Znamirowski R, et al. Magic angle effect in magnetic resonance imaging of the Achilles tendon and enthesis. Magn Reson Imaging 2009;27(4):557–564 Disler DG, Recht MP, McCauley TR. MR imaging of articular cartilage. Skeletal Radiol 2000;29(7):367–377 Xia Y. Magic-angle effect in magnetic resonance imaging of articular cartilage: a review. Invest Radiol 2000;35(10):602–621 Kästel T, Heiland S, Bäumer P, Bartsch AJ, Bendszus M, Pham M. Magic angle effect: a relevant artifact in MR neurography at 3T? AJNR Am J Neuroradiol 2011;32(5):821–827 Chappell KE, Robson MD, Stonebridge-Foster A, et al. Magic angle effects in MR neurography. AJNR Am J Neuroradiol 2004;25(3): 431–440 Hayes CW, Parellada JA. The magic angle effect in musculoskeletal MR imaging. Top Magn Reson Imaging 1996;8(1):51–56 Timins ME, Erickson SJ, Estkowski LD, et al. Increased signal in the normal supraspinatus tendon on MR imaging: diagnostic pitfall caused by the magic-angle effect. AJR Am J Roentgenol 1995; 165(1):109–114
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