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Appendix C: Artifacts

motion artifacts
 
      Motion affects all fetal MR examinations due to the combination of maternal breathing or whole body motion, maternal bowel peristalsis and arterial pulsations, and fetal motion. Since images are obtained in 300-400 msec, SSFSE imaging allows for diagnostic quality images despite motion.
 
· Bulk Motion: (Fig. 1). Motion of the entire object during the imaging sequence generally results in a blurring of the entire image, with ghost images in the phase encoding direction. Movement of a small portion of the imaged object results in a blurring of that small portion of the object across the image. Bulk motion artifacts can be distinguished from Gibbs or truncation artifacts because they extend across the entire FOV, unlike truncation artifacts that diminish quickly away from the boundary causing them. If bulk motion is present, remind the patient to keep still. In general, breathholding is not needed during sub-second imaging sequences, but if the patient is moving during imaging a breathhold could be helpful.
 
figure 1
 
Fig. 1. Bulk movement. Note the blur of the entire image (soft tissues as well as fetus) due to bulk movement of the patient during the scan.
 
· Fluid Motion: This artifact is characterized by a signal void occurring in fluid. Fluid motion artifact occurs when spins excited by a slice-selective radio frequency (RF) pulse change position with respect to the slice and/or spatial encoding gradients before their signal is recorded. Motion artifact can be seen in amniotic fluid (Fig 2-4), and in other fetal fluid collections such as cerebrospinal fluid (Fig 2) and fetal urine (Fig 5). Since fetal imaging is typically performed with single shot sequences, only the slice that was obtained during the motion is affected. As long as the fetus is not continuously moving, then typically only one or two slices are degraded by motion during a typical sequence acquisition. If the affected slices are not in the region of interest, then the sequence does not need to be repeated. A pitfall in the assumption that dark fluid on SSFSE imaging is due to motion is shown in Fig. 6, where the low signal is due to blood products.
 
figure 2a

figure 2b

 
Fig. 2. Fluid motion. Two adjacent images from the same sequence. In (A), fluid motion has caused the amniotic fluid (arrow) and the fluid around the spinal cord to lose signal. In (B) the amniotic fluid motion is not visualized (arrow), and the amniotic fluid and cerebrospinal fluid around the spinal cord is of high signal intensity.
 
figure 3
 
Fig. 3. Fluid motion around a fetus at 20 weeks gestation. At times, fluid motion will be visualized as low signal in theamniotic fluid rimmed by a bright "layer" (arrows). This brightness is generally due to a lack of motion at the periphery of the fluid space, however the high signal intensity may also be due to subcutaneous fat (adjacent to the fetus) or fluid in the subamniotic space.
 
figure 4
 
Fig. 4. Fluid motion. Sagittal view of a fetus at 26 week gestation. The fetus was exhaling from the nose during image acquisition causing the fluid immediately anterior to the face to lose signal, imitating a mass.
 
figure 5
 
Fig. 5. Fluid motion in the bladder. transverse view of the bladder in fetus at 22 weeks gestation. Note loss of signal (arrow) due to jet of urine entering the bladder.
 
figure 6a figure 6b
 
Fig. 6. Pitfall of fluid motion. A. Sagittal T2-weighted image shows dark fluid above the internal os (arrow). B. Sagittal T1-weighted image (TR/TE 88.2/1.5) shows this same area to have heterogenous slightly increased signal, consistent with marginal subchorionic hematoma (arrow). Also note the adnexal cyst (C).
 
· Repeat visualization of structure or nonvisualization of a structure: If the fetus moves during the sequence, and the movement is in plane with imaging, it is possible that a portion of the anatomy will be seen more than once (i.e., a leg or arm appears in two places in the same sequence, Fig. 7). More commonly, an extremity will move out of the image plane during sequence acquisition and will not be visualized.
 
figure 7a figure 7b
 
Fig. 7. Two sequential images of the fetal hand at 32 weeks gestation. Moving extremities can cause either a structure to be visualized twice, or not visualized at all. In this case the same hand is seen twice: open with fingers extended in (A), and is in a more relaxed position in (B).
 
Aliasing (Wrap-around)
 
      This artifact can be identified when anatomic structures that extend outside the FOV in the phase encode direction appear to "wrap around" into the opposite side of the image. Depending on the anatomy and the placement of the FOV, the "wrapped" anatomy may overlie and obscure other anatomy. This artifact occurs because the tissue outside the FOV is not correctly phase encoded. Any excited tissue outside the FOV still gives signal during readout, but tissue outside the FOV will have acquired a phase identical to a position inside the FOV. Since this spatial position is determined from the phase of the signal emitted by the tissue, all signal with the same phase will be displayed in the same position inside the FOV. The most straightforward method for eliminating this artifact is to increase the FOV so that it contains all maternal anatomy. This will result in either reduced in-plane resolution or increased scan times. For fetal imaging, it is best to use the smallest FOV that permits imaging of the region of interest (Fig 8).
 
figure 8
 
Fig. 8. A fetus with tuberous sclerosis at 34 weeks gestation. The oblique image plane with respect to maternal anatomy (patient imaged in lateral decubitus position) gives bright wrap-around artifact. This artifact does not overlie the area of interest in the fetal brain, as the subependymal tubers (arrows) are well visualized.
 
RF Interference
 
      This artifact is characterized by isolated lines or broad bands of lines in the phase encode direction being obscured by "zipper" artifacts (Fig. 9). Often, a single area of very high SNR is visualized (Fig. 10). This artifact occurs when unwanted RF signals from outside the magnet are picked up during data reception. Most causes of RF contamination are beyond immediate control. Things to do if you see this artifact:
  • Ensure scanner room door is closed when scanning.
  • Shut off extraneous equipment inside the scanner room.
  • Ensure no wires from other medical equipment are entering the scanner room from the outside.
  • Call service engineer.
figure 9
 
Fig. 9. RF interference. Coronal image of a fetus with neural tube defect at 19 weeks gestation. Note the angular appearance of the mildly dilated cerebral ventricles, characteristic of a neural tube defect. The image quality is diffusely decreased by lines running through the image.
 
figure 10a figure 10b
 
Fig. 10. RF interference. Image of the fetal head at 22 weeks gestation (A) and twins at 24 weeks gestation (B). In both of these images a very bright area is seen which does not correspond to any anatomic structure. In (A), the artifact is in the fetal brain and could interfere with diagnosis; in (B) the artifact is in the maternal soft tissues and is not important to making a diagnosis. Note that in (B) the entire image is distorted by lines of alternating increased and decreased signal intensity.
 
Susceptibility Artifact
 
      This artifact is characterized by localized distortions of the geometry or intensity of the image. Inhomogeneities in the main magnetic field (Bo) cause these distortions. Spatial distortion results from long-range field gradients, where Bo varies over scales that span many voxels. These changes in Bo cause the spins in different voxels to have slightly different precession frequencies. Since spatial position is encoded by the precessional frequency of the spins, these alterations in frequency can make the signal from spins in one location appear to come from a different position, resulting in geometric distortions of the image. Susceptibility artifact is rare with SSFSE imaging, but it can occur (Fig. 11). Things to do if you see this artifact:
  • Perform shimming to improve the Bo homogeneity.
  • Use shorter TE sequences.
  • Increase readout bandwidth
figure 11
 
Fig. 11. Susceptibility artifact. Coronal image through the uterus at 20 weeks gestational age shows multiple geographic areas of increased and decreased signal in the amniotic fluid. The pattern of these alternating lines suggests susceptibility artifact.
 
Gibbs ringing artifact
 
      The Gibbs ringing artifact (also called a truncation artifact) is typified by alternating bright and dark lines running parallel to a sharp signal interface that diminish quickly away from the boundary causing them (Fig. 12). Gibbs ringing occurs when the echo has not decayed to zero at the edges of the acquisition window, so it is most often seen in images when a small acquisition matrix is used. As long as this artifact is recognized and not confused with a real structure, it typically does not limit fetal imaging. This artifact can be reduced by increasing the resolution of the image or by applying a filter to the reconstructed image. Both of these strategies require tradeoffs. Increasing resolution will require either longer imaging times or reduced image SNR, while filtering will reduce the resolution of the reconstructed image.
 
figure 12
 
Fig. 12. Gibbs ringing artifact. A fetus with bilateral cleft lip and palate and a pseudomass in the midline face at18 weeks gestation Ripples of high and low signal intensity (arrows) radiating away from the fetal-amniotic fluid interface are due to Gibbs artifact.
 
Partial volume artifact
 
      As in all tomographic imaging, if only a portion of an anatomic region is in the slice, partial volume artifact can occur (Fig 13). What is different in obstetric imaging is that this artifact can include structures outside of the fetus, for example, in the placenta (Fig. 14).
figure 13
 
Fig. 13. Partial volume artifact in fetus at 26 weeks gestational age. This image has the liver and lung in a single slice, creating the appearance of a lung mass (arrow) adjacent to the heart (H). Evaluating adjacent images (not shown) makes this artifact obvious.
 
figure 14
 
Fig. 14. Partial volume artifact in a fetus at 19 weeks gestation. This image shows the fetal hand adjacent to the placenta. A prominent vein (arrow) in the placenta looks like a hyperextended thumb.