Body MR Imaging: Artifacts, k-Space, and Solutions - PubMed (original) (raw)

Review

. 2015 Sep-Oct;35(5):1439-60.

doi: 10.1148/rg.2015140289. Epub 2015 Jul 24.

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Review

Body MR Imaging: Artifacts, k-Space, and Solutions

Susie Y Huang et al. Radiographics. 2015 Sep-Oct.

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Abstract

Body magnetic resonance (MR) imaging is challenging because of the complex interaction of multiple factors, including motion arising from respiration and bowel peristalsis, susceptibility effects secondary to bowel gas, and the need to cover a large field of view. The combination of these factors makes body MR imaging more prone to artifacts, compared with imaging of other anatomic regions. Understanding the basic MR physics underlying artifacts is crucial to recognizing the trade-offs involved in mitigating artifacts and improving image quality. Artifacts can be classified into three main groups: (a) artifacts related to magnetic field imperfections, including the static magnetic field, the radiofrequency (RF) field, and gradient fields; (b) artifacts related to motion; and (c) artifacts arising from methods used to sample the MR signal. Static magnetic field homogeneity is essential for many MR techniques, such as fat saturation and balanced steady-state free precession. Susceptibility effects become more pronounced at higher field strengths and can be ameliorated by using spin-echo sequences when possible, increasing the receiver bandwidth, and aligning the phase-encoding gradient with the strongest susceptibility gradients, among other strategies. Nonuniformities in the RF transmit field, including dielectric effects, can be minimized by applying dielectric pads or imaging at lower field strength. Motion artifacts can be overcome through respiratory synchronization, alternative k-space sampling schemes, and parallel imaging. Aliasing and truncation artifacts derive from limitations in digital sampling of the MR signal and can be rectified by adjusting the sampling parameters. Understanding the causes of artifacts and their possible solutions will enable practitioners of body MR imaging to meet the challenges of novel pulse sequence design, parallel imaging, and increasing field strength.

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Figures

Figure 1a

Figure 1a

Schematic diagrams show the effect of susceptibility gradients on spin precession. (a) In the absence of susceptibility gradients, spin dephasing is governed by T2 relaxation. (b) The presence of susceptibility gradients accelerates spin dephasing, which leads to signal loss near boundaries between materials of differing susceptibilities.

Figure 1b

Figure 1b

Schematic diagrams show the effect of susceptibility gradients on spin precession. (a) In the absence of susceptibility gradients, spin dephasing is governed by T2 relaxation. (b) The presence of susceptibility gradients accelerates spin dephasing, which leads to signal loss near boundaries between materials of differing susceptibilities.

Figure 2a

Figure 2a

Susceptibility artifact in a 65-year-old man who had undergone endovascular repair of an abdominal aortic aneurysm. (a, b) Coronal balanced steady-state free precession (SSFP) (a) and axial gradient-echo (b) MR images of the abdomen show marked signal loss (arrow in b) caused by spin dephasing near the metallic aortic stent-graft. (c) Axial T2-weighted MR image of the abdomen was acquired with a fast spin-echo sequence. Both the balanced SSFP sequence used in a, which has T2* and T1 weighting, and the gradient-echo sequence used in b are more sensitive to susceptibility artifacts than the fast spin-echo sequence used in c. Arrow = location of signal loss in b. (d) Kidney, ureter, bladder posteroanterior radiograph shows the metallic components of the aortic stent-graft.

Figure 2b

Figure 2b

Susceptibility artifact in a 65-year-old man who had undergone endovascular repair of an abdominal aortic aneurysm. (a, b) Coronal balanced steady-state free precession (SSFP) (a) and axial gradient-echo (b) MR images of the abdomen show marked signal loss (arrow in b) caused by spin dephasing near the metallic aortic stent-graft. (c) Axial T2-weighted MR image of the abdomen was acquired with a fast spin-echo sequence. Both the balanced SSFP sequence used in a, which has T2* and T1 weighting, and the gradient-echo sequence used in b are more sensitive to susceptibility artifacts than the fast spin-echo sequence used in c. Arrow = location of signal loss in b. (d) Kidney, ureter, bladder posteroanterior radiograph shows the metallic components of the aortic stent-graft.

Figure 2c

Figure 2c

Susceptibility artifact in a 65-year-old man who had undergone endovascular repair of an abdominal aortic aneurysm. (a, b) Coronal balanced steady-state free precession (SSFP) (a) and axial gradient-echo (b) MR images of the abdomen show marked signal loss (arrow in b) caused by spin dephasing near the metallic aortic stent-graft. (c) Axial T2-weighted MR image of the abdomen was acquired with a fast spin-echo sequence. Both the balanced SSFP sequence used in a, which has T2* and T1 weighting, and the gradient-echo sequence used in b are more sensitive to susceptibility artifacts than the fast spin-echo sequence used in c. Arrow = location of signal loss in b. (d) Kidney, ureter, bladder posteroanterior radiograph shows the metallic components of the aortic stent-graft.

Figure 2d

Figure 2d

Susceptibility artifact in a 65-year-old man who had undergone endovascular repair of an abdominal aortic aneurysm. (a, b) Coronal balanced steady-state free precession (SSFP) (a) and axial gradient-echo (b) MR images of the abdomen show marked signal loss (arrow in b) caused by spin dephasing near the metallic aortic stent-graft. (c) Axial T2-weighted MR image of the abdomen was acquired with a fast spin-echo sequence. Both the balanced SSFP sequence used in a, which has T2* and T1 weighting, and the gradient-echo sequence used in b are more sensitive to susceptibility artifacts than the fast spin-echo sequence used in c. Arrow = location of signal loss in b. (d) Kidney, ureter, bladder posteroanterior radiograph shows the metallic components of the aortic stent-graft.

Figure 3

Figure 3

Sampling bandwidth and B0 artifact. B0 field sensitivity depends on the time duration of the MR signal sampling. The longer the duration of sampling, the more time is allowed for the magnetization to evolve. In the presence of B0 inhomogeneities caused by the static field, by tissue susceptibility, or even by chemical shift, a “low” sampling bandwidth will produce greater artifacts than a “high” sampling bandwidth. On the other hand, low bandwidth sampling will have less noise and a better signal-to-noise ratio (SNR).

Figure 4a

Figure 4a

Banding artifacts. (a) Axial balanced SSFP MR image of the liver of a patient who had undergone endovascular repair of an abdominal aortic aneurysm shows a banding artifact (arrow) in the upper part of the abdomen. (b) Coronal balanced SSFP MR image of the upper part of the abdomen of a different patient shows banding artifacts (arrow) near the edges of the FOV. (c) Coronal T2-weighted MR image of the upper part of the abdomen of the same patient as in b acquired with the half-Fourier RARE sequence does not show the banding artifacts depicted in b.

Figure 4b

Figure 4b

Banding artifacts. (a) Axial balanced SSFP MR image of the liver of a patient who had undergone endovascular repair of an abdominal aortic aneurysm shows a banding artifact (arrow) in the upper part of the abdomen. (b) Coronal balanced SSFP MR image of the upper part of the abdomen of a different patient shows banding artifacts (arrow) near the edges of the FOV. (c) Coronal T2-weighted MR image of the upper part of the abdomen of the same patient as in b acquired with the half-Fourier RARE sequence does not show the banding artifacts depicted in b.

Figure 4c

Figure 4c

Banding artifacts. (a) Axial balanced SSFP MR image of the liver of a patient who had undergone endovascular repair of an abdominal aortic aneurysm shows a banding artifact (arrow) in the upper part of the abdomen. (b) Coronal balanced SSFP MR image of the upper part of the abdomen of a different patient shows banding artifacts (arrow) near the edges of the FOV. (c) Coronal T2-weighted MR image of the upper part of the abdomen of the same patient as in b acquired with the half-Fourier RARE sequence does not show the banding artifacts depicted in b.

Figure 5

Figure 5

Segmented echo-planar imaging. A drawback to single-shot echo-planar imaging (EPI) is its sensitivity to B0 inhomogeneities that lead to image distortions. One way to mitigate this sensitivity is to perform multishot or segmented echo-planar imaging. Segmentation can be done either in the kx direction or the ky direction.

Figure 6a

Figure 6a

Echo-planar imaging artifacts at diffusion-weighted MR imaging. (a) Coronal whole-body contrast-enhanced MR image shows the B0 field (green dashed rectangle) and the excitation slab (red rectangle) for echo-planar diffusion-weighted imaging. (b) Axial diffusion-weighted MR image of the upper part of the abdomen shows the effect of B0 inhomogeneity over the lungs, which, in combination with poor fat suppression and eddy current effects, leads to noticeable phase-encoding artifact. (c) Axial diffusion-weighted MR image of the upper part of the abdomen obtained with improved shimming and with fat suppression shows decreased image distortion and better image quality.

Figure 6b

Figure 6b

Echo-planar imaging artifacts at diffusion-weighted MR imaging. (a) Coronal whole-body contrast-enhanced MR image shows the B0 field (green dashed rectangle) and the excitation slab (red rectangle) for echo-planar diffusion-weighted imaging. (b) Axial diffusion-weighted MR image of the upper part of the abdomen shows the effect of B0 inhomogeneity over the lungs, which, in combination with poor fat suppression and eddy current effects, leads to noticeable phase-encoding artifact. (c) Axial diffusion-weighted MR image of the upper part of the abdomen obtained with improved shimming and with fat suppression shows decreased image distortion and better image quality.

Figure 6c

Figure 6c

Echo-planar imaging artifacts at diffusion-weighted MR imaging. (a) Coronal whole-body contrast-enhanced MR image shows the B0 field (green dashed rectangle) and the excitation slab (red rectangle) for echo-planar diffusion-weighted imaging. (b) Axial diffusion-weighted MR image of the upper part of the abdomen shows the effect of B0 inhomogeneity over the lungs, which, in combination with poor fat suppression and eddy current effects, leads to noticeable phase-encoding artifact. (c) Axial diffusion-weighted MR image of the upper part of the abdomen obtained with improved shimming and with fat suppression shows decreased image distortion and better image quality.

Figure 7a

Figure 7a

B1 inhomogeneity (standing waves) at 3-T MR imaging of a patient with cirrhosis and ascites. Coronal T2-weighted (a) and axial out-of-phase T1-weighted (b) MR images show a signal void in the center of the images because the wavelength of the RF transmission field is on the same order of magnitude as the dimension of the patient. The resulting variations in the RF transmission field produce focal areas of decreased signal intensity.

Figure 7b

Figure 7b

B1 inhomogeneity (standing waves) at 3-T MR imaging of a patient with cirrhosis and ascites. Coronal T2-weighted (a) and axial out-of-phase T1-weighted (b) MR images show a signal void in the center of the images because the wavelength of the RF transmission field is on the same order of magnitude as the dimension of the patient. The resulting variations in the RF transmission field produce focal areas of decreased signal intensity.

Figure 8

Figure 8

Schematic diagram showing the origin of chemical shift artifacts. Fat and water protons resonate at different frequencies within the static magnetic field because of differences in the chemical environment. In the frequency-encoding direction (x-axis), the precession frequency difference between a fat proton (white square) and a water proton (black square) is translated directly into differences in physical position. No chemical shift artifact occurs in the phase-encoding direction (y-axis) for the fat and water protons (gray squares) with conventional pulse sequences. These differences are more pronounced at higher field strengths because the frequency difference is directly proportional to the field strength. MRI = MR imaging, NMR = nuclear magnetic resonance imaging.

Figure 9

Figure 9

Type I chemical shift artifact. Coronal MR image of the retroperitoneum obtained at 3 T shows that spatial misregistration (arrows) caused by chemical shift differences between water and fat occurs along the frequency-encoding (readout) direction (craniocaudal direction) along the interface of the psoas muscle and the retroperitoneal fat.

Figure 10a

Figure 10a

Type II chemical shift artifact. Axial dual-echo out-of-phase (echo time, 2.2 msec) (a) and in-phase (echo time, 4.4 msec) (b) gradient-echo MR images of the abdomen showing fat and water were obtained at 1.5 T. The out-of-phase image shows the characteristic black border outlining the fat-water interfaces that is due to the differences in chemical shift between fat and water in the same voxel.

Figure 10b

Figure 10b

Type II chemical shift artifact. Axial dual-echo out-of-phase (echo time, 2.2 msec) (a) and in-phase (echo time, 4.4 msec) (b) gradient-echo MR images of the abdomen showing fat and water were obtained at 1.5 T. The out-of-phase image shows the characteristic black border outlining the fat-water interfaces that is due to the differences in chemical shift between fat and water in the same voxel.

Figure 11a

Figure 11a

Poor fat suppression. (a, b) Sagittal (a) and axial (b) MR images show that B0 inhomogeneity leads to poor fat suppression in the breasts at the edges of the FOV. (c) Axial short inversion time inversion-recovery (STIR) MR image shows improved fat suppression with STIR.

Figure 11b

Figure 11b

Poor fat suppression. (a, b) Sagittal (a) and axial (b) MR images show that B0 inhomogeneity leads to poor fat suppression in the breasts at the edges of the FOV. (c) Axial short inversion time inversion-recovery (STIR) MR image shows improved fat suppression with STIR.

Figure 11c

Figure 11c

Poor fat suppression. (a, b) Sagittal (a) and axial (b) MR images show that B0 inhomogeneity leads to poor fat suppression in the breasts at the edges of the FOV. (c) Axial short inversion time inversion-recovery (STIR) MR image shows improved fat suppression with STIR.

Figure 12a

Figure 12a

RF interference artifacts. Axial MR images show absence of RF interference artifacts (a) and presence of RF interference artifacts (b). RF noise arising from devices external to the patient or from improper shielding of the MR imaging room results in a characteristic line or band with the appearance of a zipper (hence the alternative term zipper artifact). Removing the source of RF interference will eliminate the artifacts.

Figure 12b

Figure 12b

RF interference artifacts. Axial MR images show absence of RF interference artifacts (a) and presence of RF interference artifacts (b). RF noise arising from devices external to the patient or from improper shielding of the MR imaging room results in a characteristic line or band with the appearance of a zipper (hence the alternative term zipper artifact). Removing the source of RF interference will eliminate the artifacts.

Figure 13a

Figure 13a

Motion artifact related to ascites in a 65-year-old man who was undergoing routine surveillance for hepatocellular carcinoma. Coronal (a) and axial (b) T2-weighted MR images show flow-related signal loss caused by motion within the ascites. Flow-related artifact within ascites can be confused with particulate material and peritonitis. The axial image also shows motion artifacts related to respiration, which manifest as periodic ghosts that are oriented parallel to the anterior abdominal wall.

Figure 13b

Figure 13b

Motion artifact related to ascites in a 65-year-old man who was undergoing routine surveillance for hepatocellular carcinoma. Coronal (a) and axial (b) T2-weighted MR images show flow-related signal loss caused by motion within the ascites. Flow-related artifact within ascites can be confused with particulate material and peritonitis. The axial image also shows motion artifacts related to respiration, which manifest as periodic ghosts that are oriented parallel to the anterior abdominal wall.

Figure 14a

Figure 14a

Respiratory motion compensation with a navigator sequence. (a, b) Coronal gradient-echo MR images obtained without (a) and with (b) respiratory synchronization by using a navigator sequence. (c) Coronal MR image shows the result of using the navigator sequence; the green rectangle demonstrates placement of an additional pulse to excite magnetization over the diaphragm, which allows a one-dimensional real-time tracing of diaphragmatic excursion. From this information, imaging can be timed prospectively or reconstructed retrospectively to minimize motion related to respiration.

Figure 14b

Figure 14b

Respiratory motion compensation with a navigator sequence. (a, b) Coronal gradient-echo MR images obtained without (a) and with (b) respiratory synchronization by using a navigator sequence. (c) Coronal MR image shows the result of using the navigator sequence; the green rectangle demonstrates placement of an additional pulse to excite magnetization over the diaphragm, which allows a one-dimensional real-time tracing of diaphragmatic excursion. From this information, imaging can be timed prospectively or reconstructed retrospectively to minimize motion related to respiration.

Figure 14c

Figure 14c

Respiratory motion compensation with a navigator sequence. (a, b) Coronal gradient-echo MR images obtained without (a) and with (b) respiratory synchronization by using a navigator sequence. (c) Coronal MR image shows the result of using the navigator sequence; the green rectangle demonstrates placement of an additional pulse to excite magnetization over the diaphragm, which allows a one-dimensional real-time tracing of diaphragmatic excursion. From this information, imaging can be timed prospectively or reconstructed retrospectively to minimize motion related to respiration.

Figure 15

Figure 15

Radial k-space acquisition to minimize motion artifact. Top row: With conventional rectilinear sampling, signal perturbations caused by bulk motion produce ghosts in the phase-encoding direction on the axial MR image (top right). Bottom row: When radial sampling is used, ghosting no longer is generated in a particular direction on the axial MR image (bottom right).

Figure 16

Figure 16

Periodically rotated overlapping parallel lines with enhanced reconstruction and spiral k-space acquisition for mitigating motion artifact. Top row: The periodically rotated overlapping parallel lines with enhanced reconstruction techniques (PROPELLER or BLADE) use a combination of rectilinear and radial trajectories in each blade to correct for motion artifacts on the axial MR image (top right). Bottom row: Spiral k-space trajectories are also robust to motion because of their faster acquisition compared with conventional rectilinear sampling strategies.

Figure 17

Figure 17

Parallel imaging artifact. Axial MR image shows that the central band of noise artifact is exacerbated by poor coil geometry factors and high acceleration.

Figure 18a

Figure 18a

Parallel imaging artifact caused by a missing multichannel receive coil in a 47-year-old woman undergoing evaluation for uterine fibroids. (a) Coronal T2-weighted MR image shows placement of the parallel imaging receive coil array over the patient. The large light-gray circle represents B0, and multichannel receive coils (small dark-gray circles) overlie the pelvis. One of the lower coil elements is out (black circle). (b) Axial T2-weighted MR image shows marked signal loss (arrow) in the left anterior portion of the pelvis.

Figure 18b

Figure 18b

Parallel imaging artifact caused by a missing multichannel receive coil in a 47-year-old woman undergoing evaluation for uterine fibroids. (a) Coronal T2-weighted MR image shows placement of the parallel imaging receive coil array over the patient. The large light-gray circle represents B0, and multichannel receive coils (small dark-gray circles) overlie the pelvis. One of the lower coil elements is out (black circle). (b) Axial T2-weighted MR image shows marked signal loss (arrow) in the left anterior portion of the pelvis.

Figure 19

Figure 19

Aliasing and undersampling of k-space. Top row: Aliasing, or wraparound artifact, occurs when the FOV of encoding is smaller than the imaged anatomic structures. The finer the sampling in k-space (eg, smaller Δky), the larger the FOV dimension of the image. Bottom row: When k-space is sampled only with every other line, the Δky is doubled and the FOV is halved, leading to aliasing caused by the undersampling. Parallel acquisition techniques such as GRAPPA or SENSE are used to synthesize the missing lines from the undersampling in k-space and therefore remove the aliasing.

Figure 20a

Figure 20a

Aliasing artifact. (a) Axial T2-weighted MR image of the abdomen shows an aliasing artifact, or wraparound artifact, of the arms in the expected location of the flanks (arrow). (b) Coronal T1-weighted MR image shows the imaged FOV (red rectangle). The aliasing artifact could be mitigated with oversampling in the phase-encoding direction (green dashed rectangle).

Figure 20b

Figure 20b

Aliasing artifact. (a) Axial T2-weighted MR image of the abdomen shows an aliasing artifact, or wraparound artifact, of the arms in the expected location of the flanks (arrow). (b) Coronal T1-weighted MR image shows the imaged FOV (red rectangle). The aliasing artifact could be mitigated with oversampling in the phase-encoding direction (green dashed rectangle).

Figure 21a

Figure 21a

Aliasing artifact along the section direction in a three-dimensional acquisition. (a) Axial T2-weighted MR image acquired with a three-dimensional fast spin-echo sequence (SPACE [Sampling Perfection with Application optimized Contrasts using different flip angle Evolutions]; Siemens Healthcare) shows an artifact from the perineum (arrows) overlying the upper portion of the pelvis. (b) Repeat axial T2-weighted fast spin-echo (SPACE) MR image with oversampling in the section direction shows amelioration of the artifact at the cost of increased imaging time. (c) Coronal T2-weighted MR image shows the imaged FOV (red rectangle) and the oversampled volume (green dashed rectangle).

Figure 21b

Figure 21b

Aliasing artifact along the section direction in a three-dimensional acquisition. (a) Axial T2-weighted MR image acquired with a three-dimensional fast spin-echo sequence (SPACE [Sampling Perfection with Application optimized Contrasts using different flip angle Evolutions]; Siemens Healthcare) shows an artifact from the perineum (arrows) overlying the upper portion of the pelvis. (b) Repeat axial T2-weighted fast spin-echo (SPACE) MR image with oversampling in the section direction shows amelioration of the artifact at the cost of increased imaging time. (c) Coronal T2-weighted MR image shows the imaged FOV (red rectangle) and the oversampled volume (green dashed rectangle).

Figure 21c

Figure 21c

Aliasing artifact along the section direction in a three-dimensional acquisition. (a) Axial T2-weighted MR image acquired with a three-dimensional fast spin-echo sequence (SPACE [Sampling Perfection with Application optimized Contrasts using different flip angle Evolutions]; Siemens Healthcare) shows an artifact from the perineum (arrows) overlying the upper portion of the pelvis. (b) Repeat axial T2-weighted fast spin-echo (SPACE) MR image with oversampling in the section direction shows amelioration of the artifact at the cost of increased imaging time. (c) Coronal T2-weighted MR image shows the imaged FOV (red rectangle) and the oversampled volume (green dashed rectangle).

Figure 22

Figure 22

Truncation (Gibbs ringing) artifact. Truncation artifact, or Gibbs ringing, is caused by truncation of the signal in k-space that is due to the fact that the MR signal is finitely sampled. As a consequence, high-spatial-frequency information is lost, and the approximation errors associated with the Fourier transform lead to a ringing effect at boundaries in the image. From an imaging perspective, truncation artifact increases as the spatial resolution of encoding is decreased.

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