Fundamentals Behind the 10 Most Common Magnetic Resonance Imaging Artifacts with Correction Strategies and 10 High-Yield Points

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1 Fundamentals Behind the 10 Most Common Magnetic Resonance Imaging Artifacts with Correction Strategies and 10 High-Yield Points Award: Magna Cum Laude Poster No.: C-1248 Congress: ECR 2011 Type: Educational Exhibit Authors: R. Javan, J. R. O'Rear, J. E. Machin; Memphis, TN/US Keywords: MR DOI: /ecr2011/C-1248 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 82

2 Learning objectives The purpose of this exhibit is to describe the most common Magnetic Resonance Imaging (MRI) artifacts under 10 categories, geared towards enhancing fundamental radiologic physics knowledge, along with strategies for reducing these artifacts when possible. A set of top 10 take-home high-yield points are provided in the end. Some images are obtained from our institution's PACS, some from peer-reviewed journals with proper permission obtained and some from educational web tutorials. The content is categorized in the following fashion; 1. Magnetism (a. Magnetic Susceptibility, b. Main Magnetic Field Inhomogeneity, c. Anatomic Distortion or Bending due to Gradient Nonlinearity, d. Standing Wave) 2. Resonance Frequency (a. Chemical Shift, b."the 2nd Kind" or Black Boundary) 3. Motion (a. Voluntary Patient Motion, b. Cardiac and Respiratory Cycle, c. Blood and CSF Flow, d. Flow Enhancement or Entry Slice) 4. Voxel (a. Partial-Volume or Averaging, b. Cross-Talk or Cross-Excitation or Slice-overlap) 5. K-Space (a. Spike or Herringbone, b. Zero Fill-In or at times called Zebra, c. N/2 Ghost and Segmental K-Space) 6. Sampling (a. Wraparound or Aliasing, b. Truncation or Gibbs Ringing) 7. Coil (a. Inadequate Shimming, b. Improper Distance From the Skin, c. Phased Array Coil Malfunction, d. Central Point or DC Offset and Quadrature Ghost) 8. Radiofrequency (a. RF Noise or Zipper, b. RF Overflow) 9. Moire Fringes or Zebra 10. Magic Angle Page 2 of 82

3 Background 1. MAGNETISM a. Magnetic Susceptibility: This results from differences in the field at interfaces of different susceptibilities seen greatest surrounding ferromagnetic objects inside diamagnetic materials; these gradients cause dephasing and frequency shifts of surrounding tissues creating bright and dark areas (Fig. 1 on page 18) with spatial distortion; greatest with long echo times and gradient echo sequences. It can be seen due to dental fillings (Fig. 2 on page 18), deposited hemosiderin or even mascara on eyelids (Fig. 3 on page 19). This effect can be minimized by a large receiver bandwidth and a decreased echo time. Fast spin-echo acquisitions with a high bandwidth typically work well. The MARS technique is used in MSK radiology for periprosthetic evaluation by RF increase, narrow section thickness, tilted view angle, and increased read gradient. Fig.: Magnetic Susceptibility seen as high intensity areas with echo-planar imaging technique in diffusion weighted images, due to abrupt change at interfaces of different susceptibilities. References: Radiology, Baptist Memorial Hospital - Memphis/US b. Main Magnetic Field Inhomogeneity: There are inherent small main magnetic field non-uniformities, which are essentially imperfections. Magnetic shimming is done to reduce this effect. (See Coils) c. Anatomic Distortion or Bending: This occurse due to gradient non-linearity especially seen at the edges of the gradient (Fig. 4 on page 19, 5 on page 20). These may be fixed by algorithms provided by the vendor. Page 3 of 82

4 d. Standing Wave: Occurrs with 3T magnets and is often incorrectly called a "dielectric resonance" effect. Strong signal variations across an image can be seen, especially brightening or dark "holes" in regions away from the receive coil. These are more pronounced in obese patients with a distended abdomen, during pregnancy (Fig. 6 on page 21) or patients with ascites. Therefore, ultrahigh-field MR should not be performed in patients with large ascites or during pregnancy. Fig.: Severe standing wave artifact in pregnant woman during fetal ultrahigh-fieldstrength MRI at 3.0 T shows marked signal loss (long arrows). Short thin arrows mark placenta; thick arrows mark fetal torso. References: Merkle EM, Dale BM. "Abdominal MRI at 3.0 T: The Basics Revisited". AJR 2006; 186: RESONANCE FREQUENCY Page 4 of 82

5 a. Chemical Shift: Fat containing structures are shifted in the frequency direction from their true positions, due to differences in resonance frequencies. In the spine, this causes one end plate to appear thicker than the opposite one; in the abdomen and orbits, this causes a black border at one fat-water interface and a bright border at the opposite side (Fig. 7 on page 22, 8 on page 23). It is greater at high main magnetic fields and low gradient strengths. Correction may be performed by using a fat suppression technique or use a wide receiver bandwidth (e.g., +32 KHz) b."the 2nd Kind" or Black Boundary: If a pixel is shared by tissues (e.g. fat and water) with different resonant frequencies, the selection of certain TEs picks a point at which they are out of phase and thus their spins are cancelled (Fig. 9 on page 24, 10 on page 25). Therefore, avoid multiples of the TEs where they cancel each other. This artifact at times is used for diagnostic purposes, such as in adrenal or liver imaging for tissue characterization. Fig.: Chemical Shift of "the 2nd kind" seen as black line along the interface of fat and water. Page 5 of 82

6 References: Radiology, Baptist Memorial Hospital - Memphis/US 3. MOTION a. Voluntary Patient Motion: Ghost and blur appearance is seen (Fig. 11a on page 26). Coil may also move. It is very important to educate patient. Sedation may be necessary, especially in children. Fig.: Motion artifacts - on the left image, ghost lines seen anterior to the abdominal wall due to breathing; on the right image, bright circles anterior to the aorta are seen representing blood pulsation artifacts. References: Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: b. Cardiac and Respiratory Cycle: These tend to be more prominent in the phase direction; they can be reduced with presaturation pulses outside of FOV, swapping PEG and FEG, using Propeller acquisition (Fig. 12 on page 27) (oversampling k-space center), gradient moment nulling to rephase moving spins, and gated techniques (Fig. 13 on page 28). Page 6 of 82

7 Fig.: Placement of the navigator section for respiratory motion compensation. Left image with aqua overlay shows the navigator section from which the displacement information is obtained to determine the diaphragmatic position. Graph shows diagphragmatic movement, indicated by the white wave and green line. The yellow boxes represent the best time to image ("window of opportunity"). References: Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: c. Blood and CSF Flow: Blood pulsation artifact may be seen in aorta (Fig. 11b on page 26), internal carotid artery or any vascular structure with pulsatile flow. In CSF this artifact can mimic intradural lesion and may be reduced by flow compensation and presaturation. Page 7 of 82

8 Fig.: CSF flow artifact secondary to unsaturated protons moving randomly, seen as dark signal areas, which may be mistaken as intradural lesions, such as AVMs. References: Radiology, Baptist Memorial Hospital - Memphis/US d. Flow Enhancement or Entry Slice: Normally, flow voids (Fig. 14 on page 29) are seen in vessels. But this artifact may mimic vessel occlusion and manifest as a bright signal in the first slice that the vessel enters (usually seen on several slices and fades with distance). It can be reduced with use of gradient echo flow technique and presaturation. 4. VOXEL a. Partial-Volume or Averaging: Caused by voxel containing two different tissues and therefore a signal average of both tissues (Fig. 15 on page 29). This can lead to Page 8 of 82

9 diagnostic misinterpretations. It can be corrected by obtaining thinner slices, which can compromise SNR. Fig.: Partial volume averaging artifact, left image obtained at 10mm thickness does not show the VII and VIII cranial nerves while the left image does, obtained at 3 mm. References: Radiology, Baptist Memorial Hospital - Memphis/US b. Cross-Talk or Cross-Excitation or Slice-overlap: Occurs with multi-angle multi-slice acquisition when there's overlap such that images include areas where the spins have already been saturated resulting in a band of signal loss horizontally and usually greatest posteriorly - as long as the saturated area is posterior to the spinal canal there is no harm. This artifact looks like a big black stripe in the image (Fig. 16 on page 30). This is corrected by continuous parallel imaging. Page 9 of 82

10 Fig.: Cross-talk artifact seen in the lumbar spine due to overlapping intersection of imaging slices, producing band-like areas of signal void posterior to the fifth lumbar vertebral body. This artifact is eliminated by repeating sagittal and axial images using parallel contiguous slices References: Peh WCG, Chan JHM. "Artifacts in musculoskeletal magnetic resonance imaging: identification and correction." Skeletal Radiology 2001; 30: K-SPACE a. Spike or Herringbone: Bad data points in k-space result in band artifacts (Fig. 17 on page 31). Spike noise usually occurs because of loose electrical connections that produce arcs or because of the breakdown of interconnections in an RF coil, and it is more evident with the use of high-duty-cycle sequences. This artifact can be used advantagously for the purpose of tagging suring the cardiac cycle for assessing cardiac motion. Page 10 of 82

11 Fig.: Spike artifact. Bad data points in k-space (arrow on right) result in band artifacts on the MR image on left. References: Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: b. Zero Fill-In or at times called Zebra: This happens due to missing data or data that is set to zero in k-space by the scanner. Abrupt change from signal to no signal can give a zebra like artifact (Fig. 18 on page 31). c. N/2 Ghost and Segmental K-Space: These are ghosts caused by phase errors as discontinuities in k-space (Fig. 19 on page 32). Fast spin-echo sequences are also susceptible to segmented k-space artifacts. Usually requires a call to service. 6. SAMPLING a. Wraparound or Aliasing: Occurs when FOV is smaller than the body part imaged (Fig. 20 on page 33, 21 on page 34, 25 on page 37). It can be corrected by using a larger FOV and oversampling. In the phase (x) direction it can be corrected with a higher number of phase encoding steps; in the frequency direction (y) can be fixed by sampling the signal twice as fast. One can also apply spatial presaturation to undesired tissue or swap frequency/phase-encoding directions. Page 11 of 82

12 Fig.: Aliasing artifact on left, which was fixed by a larger FOV. References: Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: b. Truncation or Gibbs Ringing: bright and dark edges at the lateral sides making giving the appearance of a ringing bell (Fig. 22 on page 36, 23 on page 35, 24 on page 35). It happens at abrupt intensity interfaces and can mimic a syrinx in the cord. Occurs more with fewer encoding steps (128 x 256 vs 256 x 256). Correct by increasing matrix size along the phase-encoding direction. 7. COIL a. Inadequate Shimming: Poor shimming causes inhomogeneous fat saturation and as a result the fat appears bright on one side and dark on another (Fig. 25 on page 37). Active (using better coils) or passive (removing steel from magnet poles) correction stategies may be applied. Page 12 of 82

13 Fig.: Inadequate shimming seen as asymmetric bright intensity in breast (open arrow), zebra artifact (arrowhead) and aliasing artifact (arrows). References: Harvey JA, et al. "Breast MR Imaging Artifacts: How to Recognize and Fix Them." RadioGraphics 2007; 27:S131-S145. b. Improper Distance From the Skin: If skin touches the coil (Fig. 26 on page 38) or is too far, or if coil does not fit the body part abnormal signal can be created. c. Phased Array Coil Malfunction: One coil of a phased array multi-coil is out of phase with the other coils. This results in bands of phase addition and cancellation (Fig. 27 on page 39, 28 on page 40). Call service to have this issue corrected. d. Central Point or DC Offset in coil and Quadrature Ghost: A bright dot may be created in the center of an image (Fig. 29 on page 41) after Fourier transformation by offset of the DC voltage in the receiver. This can be corrected by recalibration. Keep a constant temperature for the equipment. Quadratue Ghost occurs due to unbalanced gain in the two channels of a quadrature coil. Combining Page 13 of 82

14 two signals of different intensity causes some frequencies to become less than zero causing 180 degree "ghost" (Fig. 30 on page 42) Fig.: Quadrature ghost artifact seen as a ghost 180 degrees flipped. References: Hornack JP, Accessed Jan 20, RADIOFREQUENCY a. RF Noise or Zipper: Leakage of electromagnetic energy into the magnet room, e.g. by equipment brought into the room leading to RF interference. It appears as a region of increased noise with a width of 1 or 2 pixels that extends in the frequency direction (Fig. Page 14 of 82

15 31 on page 43, 32 on page 44). Correct by turning off other equipment brought into the room. Otherwise, the Faraday cage (shield) may have been compromised. Fig.: Zipper artifact (RF leakage artifact) seen as bright dots along a horizontal line. References: Harvey JA, et al. "Breast MR Imaging Artifacts: How to Recognize and Fix Them." RadioGraphics 2007; 27:S131-S145. b. RF Overflow: This occurs due to non-uniform washed out appearance secondary to a signal that is too intense to be accurately digitized by the analog to digital converter (Fig. 33 on page 44). This may be reduced with adjusting the gain or using autoprescanning. 9. MOIRE FRINGES OR ZEBRA Moire fringes are an interference pattern most commonly seen when performing gradient echo images. Because of lack of perfect homogeneity of the main magnetic field from one side of the body to the other, aliasing of one side of the body to the other results in superimposition of signals of different phases that alternatively add and cancel. This causes the banding appearance and is similar to the effect of looking though two screen windows, which creates psychedelic curly zebra like effect (Fig. 34 on page 45, 25 on page 37). Usually can be corrected by avoiding the combination of wraparound and poor shimming. Page 15 of 82

16 Fig.: Zebra artifact seen at the edges of the image, with the appearance of psychedelic bright and dark lines, as if looking through two screen windows. References: Radiology, Baptist Memorial Hospital - Memphis/US 10. MAGIC ANGLE Joints whose tendons and ligaments are oriented 55 degrees to the main field have dipolar interactions that become zero yielding a 100 x increase in T2 times and a bright signal (Fig. 35 on page 46) (seen in rotator cuff and patellar tendon). Normally water signal from tendon collage has a very short T2. Be aware during interpretation. To correct, repeat sequence after patient repositioning, or can use a TE more than 37 ms. Page 16 of 82

17 Fig.: Magic angle phenomenon. Sagittal spin-echo T1-weighted image of the knee shows an area of increased signal intensity (arrows) in the upper posterior cruciate ligament, angled at 55 to the static magnetic field, producing increased artifactual signal. References: Peh WCG, Chan JHM. "Artifacts in musculoskeletal magnetic resonance imaging: identification and correction." Skeletal Radiology 2001; 30: Page 17 of 82

18 Images for this section: Fig. 0: Magnetic Susceptibility seen as high intensity areas with echo-planar imaging technique in diffusion weighted images, due to abrupt change at interfaces of different susceptibilities. Radiology, Baptist Memorial Hospital - Memphis/US Fig. 0: Magnetic Susceptibility due to metallic implant. Page 18 of 82

19 Radiology, Baptist Memorial Hospital - Memphis/US Fig. 0: Magnetic susceptibility in eyelids due to mascara. Ballinger R, Accessed Jan 20, 2011.i Page 19 of 82

20 Fig. 0: Spin-echo image obtained with a large field of view shows the result of gradient geometric distortion on left. Right image obtained with a vendor-supplied correction algorithm shows correction. Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Page 20 of 82

21 Fig. 0: Geometric distortion, with decreasing linearity (solid line) as the distance from the magnet isocenter increases. The red dotted line shows the desired linear gradient profile. Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Page 21 of 82

22 Fig. 0: Severe standing wave artifact in pregnant woman during fetal ultrahigh-fieldstrength MRI at 3.0 T shows marked signal loss (long arrows). Short thin arrows mark placenta; thick arrows mark fetal torso. Merkle EM, Dale BM. "Abdominal MRI at 3.0 T: The Basics Revisited". AJR 2006; 186: Page 22 of 82

23 Fig. 0: Chemical shift artifact at echo-planar imaging, left image shows severe chemical shift artifact from insufficient fat suppression; right image obtained with fat saturation shows minimization of the chemical shift artifact or off-resonance effect. Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Page 23 of 82

24 Fig. 0: Chemical shift artifact, a black border at one fat-water interface and a bright border at the opposite side. Merkle EM, Dale BM. "Abdominal MRI at 3.0 T: The Basics Revisited". AJR 2006; 186: Page 24 of 82

25 Fig. 0: Chemical Shift of "the 2nd kind" seen as black line along the interface of fat and water. Radiology, Baptist Memorial Hospital - Memphis/US Page 25 of 82

26 Fig. 0: Chemical Shift of "the 2nd kind" as dark outline seen in breast at the interface of fat and water. Harvey JA, et al. "Breast MR Imaging Artifacts: How to Recognize and Fix Them." RadioGraphics 2007; 27:S131-S145. Page 26 of 82

27 Fig. 0: Motion artifacts - on the left image, ghost lines seen anterior to the abdominal wall due to breathing; on the right image, bright circles anterior to the aorta are seen representing blood pulsation artifacts. Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Page 27 of 82

28 Fig. 0: Pattern of k-space in PROPELLER technique in reducing motion artifact by oversampling the center of the k-space. Radiology, Baptist Memorial Hospital - Memphis/US Page 28 of 82

29 Fig. 0: Placement of the navigator section for respiratory motion compensation. Left image with aqua overlay shows the navigator section from which the displacement information is obtained to determine the diaphragmatic position. Graph shows diagphragmatic movement, indicated by the white wave and green line. The yellow boxes represent the best time to image ("window of opportunity"). Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Fig. 0: Flow voids seen in vessels of the vertebrobasilar system. Radiology, Baptist Memorial Hospital - Memphis/US Page 29 of 82

30 Fig. 0: Partial volume averaging artifact, left image obtained at 10mm thickness does not show the VII and VIII cranial nerves while the left image does, obtained at 3 mm. Radiology, Baptist Memorial Hospital - Memphis/US Page 30 of 82

31 Fig. 0: Cross-talk artifact seen in the lumbar spine due to overlapping intersection of imaging slices, producing band-like areas of signal void posterior to the fifth lumbar vertebral body. This artifact is eliminated by repeating sagittal and axial images using parallel contiguous slices Peh WCG, Chan JHM. "Artifacts in musculoskeletal magnetic resonance imaging: identification and correction." Skeletal Radiology 2001; 30: Fig. 0: Spike artifact. Bad data points in k-space (arrow on right) result in band artifacts on the MR image on left. Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Page 31 of 82

32 Fig. 0: K-space array missing or set to zero by the scanner, causing abrupt change from signal to no signal results in artifacts in the images such as zebra stripes and other anomalies. Ballinger R, Accessed Jan 20, 2011 Page 32 of 82

33 Fig. 0: N/2 ghost, a typical result of phase error (difference between odd- and evennumbered echoes), seen as two ghost outlines. Radiology, Baptist Memorial Hospital - Memphis/US Page 33 of 82

34 Fig. 0: Aliasing seen in the upper and lower parts of the image. Radiology, Baptist Memorial Hospital - Memphis/US Page 34 of 82

35 Fig. 0: Aliasing artifact on left, which was fixed by a larger FOV. Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Fig. 0: Gibbs ringing artifact seen on left image as curvilinear bright dotted line due to undersampling, which is seen on a 128 x 128 matrix and is then alleviated by a 256 x 256 matrix. Ballinger R, Accessed Jan 20, Page 35 of 82

36 Fig. 0: Truncation artifact. Sagittal fat-suppressed fast spin- echo T2-weighted image of the cervical spine shows a band of increased signal intensity within the spinal cord. This mimics a syrinx and is due to insufficient phase-encoding steps in the anterior-posterior direction. Peh WCG, Chan JHM. "Artifacts in musculoskeletal magnetic resonance imaging: identification and correction." Skeletal Radiology 2001; 30: Page 36 of 82

37 Fig. 0: Gibbs ring artifact. (a) Axial image obtained with a low spatial resolution (128 x 128) in a cylinder shows a Gibbs ring artifact at the edges of the cylinder. (b) Image obtained with a higher spatial resolution (256 x 256) shows minimization of the artifact. The dotted line indicates the desired object profile, and the red line indicates the object profile with two different resolution parameters. Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Page 37 of 82

38 Fig. 0: Inadequate shimming seen as asymmetric bright intensity in breast (open arrow), zebra artifact (arrowhead) and aliasing artifact (arrows). Harvey JA, et al. "Breast MR Imaging Artifacts: How to Recognize and Fix Them." RadioGraphics 2007; 27:S131-S145. Page 38 of 82

39 Fig. 0: Coil too close to the skin causing bright signal. Harvey JA, et al. "Breast MR Imaging Artifacts: How to Recognize and Fix Them." RadioGraphics 2007; 27:S131-S145. Page 39 of 82

40 Fig. 0: Phased array coil malfunction artifact in pelvis and chest. Ballinger R, Accessed Jan 20, 2011 Page 40 of 82

41 Fig. 0: Phased array coil malfunction. Radiology, Baptist Memorial Hospital - Memphis/US Page 41 of 82

42 Fig. 0: Central point artifact as a bright intensity in the center of the image. Ballinger R, Accessed Jan 20, Page 42 of 82

43 Fig. 0: Quadrature ghost artifact seen as a ghost 180 degrees flipped. Hornack JP, Accessed Jan 20, Page 43 of 82

44 Fig. 0: Zipper artifact (RF leakage artifact) seen as bright dots along a horizontal line. Harvey JA, et al. "Breast MR Imaging Artifacts: How to Recognize and Fix Them." RadioGraphics 2007; 27:S131-S145. Fig. 0: Zipper artifact (RF leakage artifact). Images show constant-frequency artifacts (arrows) produced by RF leakage from electronic components brought into the magnet room. Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Page 44 of 82

45 Fig. 0: RF overflow artifact seen as washed-out nonuniform appearance. Ballinger R, Accessed Jan 20, Page 45 of 82

46 Fig. 0: Zebra artifact seen at the edges of the image, with the appearance of psychedelic bright and dark lines, as if looking through two screen windows. Radiology, Baptist Memorial Hospital - Memphis/US Page 46 of 82

47 Fig. 0: Magic angle phenomenon. Sagittal spin-echo T1-weighted image of the knee shows an area of increased signal intensity (arrows) in the upper posterior cruciate ligament, angled at 55 to the static magnetic field, producing increased artifactual signal. Peh WCG, Chan JHM. "Artifacts in musculoskeletal magnetic resonance imaging: identification and correction." Skeletal Radiology 2001; 30: Page 47 of 82

48 Imaging findings OR Procedure details In this section, focus is on imaging appearance of the artifacts only. The sidebar can serve as a pictorial essay and review of the fundamentals mentioned earlier. 1. MAGNETISM a. Magnetic Susceptibility: Intensity change at interface of abruptly different susceptibilities (Fig. 1 on page 51) or loss of signal when metal is present (Fig. 2 on page 51). b. Main Magnetic Field Inhomogeneity: Imperfections in the expected overall homogenous appearance (See coils). c. Anatomic Distortion or Bending: Curvy or skewed appearance to the image near the edges (Fig. 3 on page 52, 4 on page 52). d. Standing Wave: Large dark area in the center of image (Fig. 5 on page 53). 2. RESONANCE FREQUENCY a. Chemical Shift: Bright or dark areas created along fat-water interface in frequency direction as if a shifted second overlying image is present due to misregistration of voxels containing fat (Fig. 6 on page 54, 7 on page 55). b."the 2nd Kind" or Black Boundary: Black outline along all fat containing interfaces (Fig. 8 on page 56, 9 on page 57). 3. MOTION a. Voluntary Patient Motion: Random ghosts in phase direction. b. Cardiac and Respiratory Cycle: Multiple sequential ghosts (Fig. 10 on page 58, 11 on page 59). c. Blood and CSF Flow: Multiple ghosts in the phase direction along a line passing through a vessel with pulsatile flow (Fig. 10 on page 58), and inhomogenous dark signal within the CSF space (Fig. 12 on page 59). d. Flow Enhancement or Entry Slice: Bright signal inside a vessel at the first slice, where normally there should be a flow void (Fig. 13 on page 60) present. Page 48 of 82

49 4. VOXEL a. Partial-Volume or Averaging: Loss of small detail due to averaging of the brightness of adjacent voxels, "smudging" the detail (Fig. 14 on page 61). b. Cross-Talk or Cross-Excitation or Slice-overlap: Large horizontal dark line (Fig. 15 on page 61). 5. K-SPACE a. Spike or Herringbone: Alternating parallel bright and dark ghost lines overlying the entire image (Fig. 16 on page 62). b. Zero Fill-In or at times called Zebra: distortions or zebra-like lines in anatomic interfaces (Fig. 17 on page 63). c. N/2 Ghost and Segmental K-Space: Ghosts of the image recurring in evenly distributed intervals (Fig. 18 on page 63). 6. SAMPLING a. Wraparound or Aliasing: Appearance of anatomy outside the field of view in the other side of the image (Fig. 19 on page 64, 20 on page 65). b. Truncation or Gibbs Ringing: Curvilinear bright dotted lines at the edges of the structure imaged (Fig. 21 on page 66, 22 on page 66, 23 on page 67). False appearance of syrinx in spinal cord is an example. 7. COIL a. Inadequate Shimming: Abnormal bright signal diffusely present in one side of the image or randomly distorted/wavy appearance to image (Fig. 24 on page 68). b. Improper Distance From the Skin: Abnormal signal near the surface of the body part where the distance from coil is too close or too far (Fig. 25 on page 69). c. Phased Array Coil Malfunction: Alternating bright and dark circular or curvilinear lines occupying part of image (Fig. 26 on page 69, 27 on page 70). Page 49 of 82

50 d. Central Point or DC Offset in coil and Quadrature Ghost: Bright dot or tiny circle in the very center of image (Fig. 28 on page 71); quadrature artifact is a ghost of the image flipped 180 degrees (Fig. 29 on page 73, 30 on page 72). 8. RADIOFREQUENCY a. RF Noise or Zipper: Bright dots scattered along a line (Fig. 31 on page 75, 32 on page 74). b. RF Overflow: Washed out appearance of image (Fig. 33 on page 75). 9. MOIRE FRINGES OR ZEBRA: Psychedelic bright and dark intensity along edges as if looking through two window screens (Fig. 34 on page 76). 10. MAGIC ANGLE: Bright signal when tendon is at 55 degrees with main magnetic field direction (Fig. 35 on page 77). Page 50 of 82

51 Images for this section: Fig. 0: Magnetic Susceptibility seen as high intensity areas with echo-planar imaging technique in diffusion weighted images, due to abrupt change at interfaces of different susceptibilities. Radiology, Baptist Memorial Hospital - Memphis/US Fig. 0: Magnetic Susceptibility due to metallic implant. Page 51 of 82

52 Radiology, Baptist Memorial Hospital - Memphis/US Fig. 0: Spin-echo image obtained with a large field of view shows the result of gradient geometric distortion on left. Right image obtained with a vendor-supplied correction algorithm shows correction. Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Page 52 of 82

53 Fig. 0: Geometric distortion, with decreasing linearity (solid line) as the distance from the magnet isocenter increases. The red dotted line shows the desired linear gradient profile. Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Page 53 of 82

54 Fig. 0: Severe standing wave artifact in pregnant woman during fetal ultrahigh-fieldstrength MRI at 3.0 T shows marked signal loss (long arrows). Short thin arrows mark placenta; thick arrows mark fetal torso. Merkle EM, Dale BM. "Abdominal MRI at 3.0 T: The Basics Revisited". AJR 2006; 186: Page 54 of 82

55 Fig. 0: Chemical shift artifact at echo-planar imaging, left image shows severe chemical shift artifact from insufficient fat suppression; right image obtained with fat saturation shows minimization of the chemical shift artifact or off-resonance effect. Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Page 55 of 82

56 Fig. 0: Chemical shift artifact, a black border at one fat-water interface and a bright border at the opposite side. Merkle EM, Dale BM. "Abdominal MRI at 3.0 T: The Basics Revisited". AJR 2006; 186: Page 56 of 82

57 Fig. 0: Chemical Shift of "the 2nd kind" seen as black line along the interface of fat and water. Radiology, Baptist Memorial Hospital - Memphis/US Page 57 of 82

58 Fig. 0: Chemical Shift of "the 2nd kind" as dark outline seen in breast at the interface of fat and water. Harvey JA, et al. "Breast MR Imaging Artifacts: How to Recognize and Fix Them." RadioGraphics 2007; 27:S131-S145. Page 58 of 82

59 Fig. 0: Motion artifacts - on the left image, ghost lines seen anterior to the abdominal wall due to breathing; on the right image, bright circles anterior to the aorta are seen representing blood pulsation artifacts. Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Fig. 0: Placement of the navigator section for respiratory motion compensation. Left image with aqua overlay shows the navigator section from which the displacement information is obtained to determine the diaphragmatic position. Graph shows diagphragmatic movement, indicated by the white wave and green line. The yellow boxes represent the best time to image ("window of opportunity"). Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Page 59 of 82

60 Fig. 0: CSF flow artifact secondary to unsaturated protons moving randomly, seen as dark signal areas, which may be mistaken as intradural lesions, such as AVMs. Radiology, Baptist Memorial Hospital - Memphis/US Page 60 of 82

61 Fig. 0: Flow voids seen in vessels of the vertebrobasilar system. Radiology, Baptist Memorial Hospital - Memphis/US Fig. 0: Partial volume averaging artifact, left image obtained at 10mm thickness does not show the VII and VIII cranial nerves while the left image does, obtained at 3 mm. Radiology, Baptist Memorial Hospital - Memphis/US Page 61 of 82

62 Fig. 0: Cross-talk artifact seen in the lumbar spine due to overlapping intersection of imaging slices, producing band-like areas of signal void posterior to the fifth lumbar vertebral body. This artifact is eliminated by repeating sagittal and axial images using parallel contiguous slices Peh WCG, Chan JHM. "Artifacts in musculoskeletal magnetic resonance imaging: identification and correction." Skeletal Radiology 2001; 30: Page 62 of 82

63 Fig. 0: Spike artifact. Bad data points in k-space (arrow on right) result in band artifacts on the MR image on left. Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Fig. 0: K-space array missing or set to zero by the scanner, causing abrupt change from signal to no signal results in artifacts in the images such as zebra stripes and other anomalies. Ballinger R, Accessed Jan 20, 2011 Page 63 of 82

64 Fig. 0: N/2 ghost, a typical result of phase error (difference between odd- and evennumbered echoes), seen as two ghost outlines. Radiology, Baptist Memorial Hospital - Memphis/US Page 64 of 82

65 Fig. 0: Aliasing seen in the upper and lower parts of the image. Radiology, Baptist Memorial Hospital - Memphis/US Page 65 of 82

66 Fig. 0: Aliasing artifact on left, which was fixed by a larger FOV. Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Fig. 0: Truncation artifact. Sagittal fat-suppressed fast spin- echo T2-weighted image of the cervical spine shows a band of increased signal intensity within the spinal cord. This mimics a syrinx and is due to insufficient phase-encoding steps in the anterior-posterior direction. Peh WCG, Chan JHM. "Artifacts in musculoskeletal magnetic resonance imaging: identification and correction." Skeletal Radiology 2001; 30: Page 66 of 82

67 Fig. 0: Gibbs ring artifact. (a) Axial image obtained with a low spatial resolution (128 x 128) in a cylinder shows a Gibbs ring artifact at the edges of the cylinder. (b) Image obtained with a higher spatial resolution (256 x 256) shows minimization of the artifact. The dotted line indicates the desired object profile, and the red line indicates the object profile with two different resolution parameters. Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Page 67 of 82

68 Fig. 0: Gibbs ringing artifact seen on left image as curvilinear bright dotted line due to undersampling, which is seen on a 128 x 128 matrix and is then alleviated by a 256 x 256 matrix. Ballinger R, Accessed Jan 20, Page 68 of 82

69 Fig. 0: Inadequate shimming seen as asymmetric bright intensity in breast (open arrow), zebra artifact (arrowhead) and aliasing artifact (arrows). Harvey JA, et al. "Breast MR Imaging Artifacts: How to Recognize and Fix Them." RadioGraphics 2007; 27:S131-S145. Fig. 0: Coil too close to the skin causing bright signal. Harvey JA, et al. "Breast MR Imaging Artifacts: How to Recognize and Fix Them." RadioGraphics 2007; 27:S131-S145. Page 69 of 82

70 Fig. 0: Phased array coil malfunction artifact in pelvis and chest. Ballinger R, Accessed Jan 20, 2011 Page 70 of 82

71 Fig. 0: Phased array coil malfunction. Radiology, Baptist Memorial Hospital - Memphis/US Page 71 of 82

72 Fig. 0: Central point artifact as a bright intensity in the center of the image. Ballinger R, Accessed Jan 20, Page 72 of 82

73 Fig. 0: Quadrature ghost artifact, a flipped 180 degree ghost. Radiology, Baptist Memorial Hospital - Memphis/US Page 73 of 82

74 Fig. 0: Quadrature ghost artifact seen as a ghost 180 degrees flipped. Hornack JP, Accessed Jan 20, Page 74 of 82

75 Fig. 0: Zipper artifact (RF leakage artifact). Images show constant-frequency artifacts (arrows) produced by RF leakage from electronic components brought into the magnet room. Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Fig. 0: Zipper artifact (RF leakage artifact) seen as bright dots along a horizontal line. Harvey JA, et al. "Breast MR Imaging Artifacts: How to Recognize and Fix Them." RadioGraphics 2007; 27:S131-S145. Page 75 of 82

76 Fig. 0: RF overflow artifact seen as washed-out nonuniform appearance. Ballinger R, Accessed Jan 20, Page 76 of 82

77 Fig. 0: Zebra artifact seen at the edges of the image, with the appearance of psychedelic bright and dark lines, as if looking through two screen windows. Radiology, Baptist Memorial Hospital - Memphis/US Page 77 of 82

78 Fig. 0: Magic angle phenomenon. Sagittal spin-echo T1-weighted image of the knee shows an area of increased signal intensity (arrows) in the upper posterior cruciate ligament, angled at 55 to the static magnetic field, producing increased artifactual signal. Peh WCG, Chan JHM. "Artifacts in musculoskeletal magnetic resonance imaging: identification and correction." Skeletal Radiology 2001; 30: Page 78 of 82

79 Conclusion Every radiologist must become comfortable with the most commonly occurring artifacts on MRI to be able to distingush between actual pathology and artifacts. It is also important to know how to reduce or alleviate these when they occur. Top 10 High-Yield Points 1. Magnetic susceptibility artifacts can be minimized by using a lower magnetic field strength, smaller voxel, a large receiver bandwidth or a decreased echo time. Fast spin-echo acquisitions with a high bandwidth typically work well. MARS technique is used in MSK radiology for periprosthetic evaluation. 2. Avoid standing wave artifact by not using ultrahigh-field 3T magnets in large patients, patients with ascites or during pregnancy. 3. Chemical shift artifact reduction may be performed by using lower magnetic field strength, using a fat suppression technique, decreasing voxel size, or use a wide receiver bandwidth (e.g., +32 KHz). For the 2nd kind, avoid multiples of the TEs where they cancel each other. Clinical utility of the 2nd kind chemical shift artifact is in tissue characterization in adrenal imaging or detection of focal fat in liver. 4. Difference between first and second kind chemical shift artifacts: Type 1 is seen in the frequency-encoding direction and only concerns field strengths higher than 1 T. Type 2 can be found at any field strength but requires GE sequences with particular TEs. 5. Reducing motion artifacts: Random motion (educating patient, sedation in children); Respiratory and cardiac motion (presaturation pulses outside of FOV, swapping PEG and FEG, using PROPELLER acquisition, gradient moment nulling, gated techniques); Blood and CSF flow (flow compensation and presaturation); Peristalsis (Glucagon/buscopan administration); Entry slice (use of gradient echo flow technique and presaturation). 6. In imaging of the spinal cord, one must not mistake CSF flow artifact for intradural AVMs and also not to mistake a truncation or Gibbs ringing artifact as a syrinx. 7. Spike artifacts are usually due to loose connection and require a service call. 8. Wraparound or aliasing artifact can be alleviated by a larger FOV, oversampling, applying spatial presaturation to undesired tissue or swap frequency/phase-encoding directions. 9. Reduce truncation or Gibbs ringing by increasing matrix size along the phase-encoding direction. 10. When a zipper artifact is seen, a disturbance in radiofrequency is caused likely by presence of equipment in the room or due to compromise of the Faraday cage. Summary Table Page 79 of 82

80 1. MAGNETISM a. Magnetic Susceptibility b. Main Magnetic Field Inhomogen. c. Anatomic Distortion/Bending d. Standing Wave 2. RESONANCE FREQUENCY a. Chemical Shift b. "The 2nd Kind"/Black Boundary 3. MOTION a. Voluntary Patient Motion b. Cardiac and Respiratory Cycle c. Blood and CSF Flow d. Flow Enhancement/Entry Slice 4. VOXEL a. Partial-Volume or Averaging b. Cross-Talk/Cross-Excitation/Sliceoverlap 5. K-SPACE a. Spike or Herringbone b. Zero Fill-In (at times Zebra) c. N/2 Ghost/Segmental K-Space 6. SAMPLING a. Wraparound or Aliasing b. Truncation or Gibbs Ringing 7. COIL a. Inadequate Shimming b. Improper Distance From Skin c. Phased Array Coil Malfunction d. Central Point/DC Offset in coil and Quadrature Ghost 8. RADIOFREQUENCY a. RF Noise or Zipper b. RF Overflow 9. MOIRE FRINGES OR ZEBRA 10. MAGIC ANGLE Page 80 of 82

81 Personal Information Contact Information: Ramin Javan, MD PGY3 Radiology Resident Baptist Memorial Hospital 6019 Walnut Grove Rd Memphis, TN (901) Page 81 of 82

82 References Ballinger R, Accessed Jan 20, Bushberg, JT, Seibert, JA, Leidholdt Jr, EM, & Boone, JM (2001). The Essential Physics of Medical Imaging (2nd Ed). Sacramento, CA. Harvey JA, et al. "Breast MR Imaging Artifacts: How to Recognize and Fix Them." RadioGraphics 2007; 27:S131-S145. Hornack JP, Accessed Jan 20, Merkle EM, Dale BM. "Abdominal MRI at 3.0 T: The Basics Revisited". AJR 2006; 186: Peh WCG, Chan JHM. "Artifacts in musculoskeletal magnetic resonance imaging: identification and correction." Skeletal Radiology 2001; 30: Smith TB, Nayak KS. "MRI artifacts and correction strategies" Imaging Med. 2010; 2:4, Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26: Page 82 of 82