MRI ARTEFACTS

Chemical Shift Artifacts The chemical shift artifact is commonly noticed in the spine at the vertebral body end plates, in the abdomen, and in the orbits where fat and other tissues form borders. In the frequency direction, the MRI scanner uses the frequency of the signal to indicate spatial position. Since water in organs and muscle resonate at a different frequency than fat, the MRI scanner mistakes the frequency difference as a spatial (positional) difference. As a result, fat containing structures are shifted in the frequency direction from their true positions. 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 border. This artifact is shown in the following axial image of a kidney where the bright border along the top of the kidney and the dark border along the bottom of the kidney represent the artifact. This artifact is greater at higher field strengths and lesser at higher gradient strengths. Practically about the only way to eliminate this artifact is to use a fat suppression technique.

Aliasing or "Wrap-around " Aliasing or wrap-around is a common artifact that occurs when the field of view (FOV) is smaller than the body part being imaged. The part of the body that lies beyond the edge of the FOV is projected on to the other side of the image. This can be corrected, if necessary, by oversampling the data. In the frequency direction, this is accomplished by sampling the signal twice as fast. In the phase direction, the number of phase-encoding steps must be increased with a longer study as a result. The following axial images of the brain demonstrate this artifact. the first image shows wrap-around of the back of the head on to the front of the head, where the phase-encoded direction is anterior-posterior.The second image has the phase and frequency directions reversed resulting in absence of the aliasing artifact. Oversampling was used in the frequency direction to eliminate the aliasing.

Black Boundary Artifact The Black Boundary Artifact is an artificially created black line located at fat-water interfaces such as muscle-fat interfaces. This results in a sharp delineation of the muscle-fat boundary that is sometimes visually appealing but not an anatomical structure. The following is a coronal image through the upper body with an echo time of 7ms. A black line is seen surrounding the muscles of the shoulder girdle as well as around the liver. This artifact can occur for a couple of reasons. The most common reason I have found is a result of selecting an echo time (TE) in which the fat and water spins (located in the same pixel at an interface) are out of phase, cancelling each other. At 1.5 T, the 3.5 PPM difference in frequency between water and saturated fat results in cancellation of spins at 4.5 ms multiples, starting at about 2.3 ms; for example at 6.8ms, 11.3ms, and 15.9 ms. To avoid this artifact, TE's close to 4.5ms, 9ms, 13.6ms,... should be chosen.

Gibbs or Truncation Artifacts Gibbs or truncation artifacts are bright or dark lines that are seen parallel and adjacent to borders of abrupt intensity change, as when going from bright CSF to dark spinal cord on a T2-weighted image. In the spinal cord, this artifact can simulate a small syrinx to the unaware. It is also seen in other locations as at the brain/calvarium interface. This artifact is related to the finite number of encoding steps used by the Fourier transform to reconstruct an image. The more encoding steps, the less intense and narrower the artifacts. The first axial image is a phantom containing water, surrounded by air. The image was encoded 128 times in the horizontal direction and 256 times in the vertical direction. Note the prominent light and dark line along the sides that fade as they approach the top and bottom of the phantom. The second image was encoded 256 times in both directions. Minimal artifact is seen uniformly around the periphery of the phantom

Zipper Artifacts There are various causes for zipper artifacts in images. Most of them are related to hardware or software problems beyond the radiologist immediate control. The zipper artifacts that can be controlled easily are those due to RF entering the scanning room when the door is open during acquisition of images. RF from some radio transmitters will cause zipper artifacts that are oriented perpendicular to the frequency axis of your image. Frequently there is more than one artifact line on an image from this cause. Other equipment and software problems can cause zippers in either axis. Clicking the icon will show an axial MRI of the head in a patient. The scanner room door was left open during the acquisition causing the zipper artifacts shown.

Phase-encoded Motion Artifacts Phase-encoded motion artifacts appear as bright noise or repeating densities oriented in the phase direction, occurring as the results of motion during acquisition of a sequence. These artifacts may be seen from arterial pulsations, swallowing, breathing, peristalsis, and physical movement of a patient. They 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. Phase-encoded artifacts can be reduced by various techniques depending on their cause and location. Arterial pulsation artifacts can be reduced by spatial presaturation pulses prior to entry of the vessel into the slices. Spatial presaturation can also reduce some swallowing and breathing artifacts. Surface coil localization can reduce artifacts generated at a distance from the area of interest. Pulse sequences can be shortened, and respiratory and/or cardiac or peripheral gaiting techniques may also help. The following axial image of the head shows a phase-encoded motion artifact running transversely across the back of the head (posterior fossa) as a result of venous flow in the transverse sinuses

Entry Slice Phenomenon Entry slice phenomenon occurs when unsaturated spins in blood first enter into a slice or slices. It is characterized by bright signal in a blood vessel (artery or vein) at the first slice that the vessel enters. Usually the signal is seen on more than one slice, fading with distance. This artifact has been confused with thrombosis with disastrous results. The characteristic location and if necessary, the use of gradient echo flow techniques can be used to differentiate entry slice artifacts from occlusions.

Slice-overlap Artifacts The slice-overlap artifact is a name I've given to the loss of signal seen in an image from a multi-angle, multi-slice acquisition, as is obtained commonly in the lumbar spine. If the slices obtained at different disk spaces are not parallel, then the slices may overlap. If two levels are done at the same time, e.g., L4-5 and L5-S1, then the level acquired second will include spins that have already been saturated. This causes a band of signal loss crossing horizontally in your image, usually worst posteriorly.The dark horizontal bands in the bottom of the following axial image through the lumbar spine demonstrates this artifact. As long as the saturated area stays posterior to the spinal canal it causes no harm.

Magic Angle Effects Magic angle effects are seen most frequently in tendons and ligaments that are oriented at about a 55 degree angle to the main magnetic field. Signal from water molecules associates with the tendon collagen fibers is not normally seen because of dipolar interactions that result in very short T2 Times. At an angle of about 55 degrees to the main magnetic field, the dipolar interactions become zero, resulting in an increase of the T2 Times about 100 fold. This results in signal being visible in tedons with ordinary pulse sequences. A bright signal from this artifact is commonly seen in the rotator cuff and occasionally in the patellar tendon and elsewhere. The following image shows increase signal in the distal patellar tendon from this magic angle effect.

Moire Fringes Moire fringes are an interference pattern most commonly seen when doing gradient echo images with the body coil as shown in the figure. 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.

RF Overflow Artifacts RF overflow artifacts cause a nonuniform, washed-out appearance to an image as shown in the following axial image of a head. This artifact occurs when the signal received by the scanner from the patient is too intense to be accurately digitized by the analog-to-digital converter. Autoprescanning usually adjusts the receiver gain to prevent this from occurring but if the artifact still occurs, the receiver gain can be decreased manually.

Central Point Artifact The central point artifact is a focal dot of increased signal in the center of an image. It is caused by a constant offset of the DC voltage in the receiver. After fourier transformation, this constant offset gives the bright dot in the center of the image as shown in the diagram below. he following axial MRI image of the head shows a central point artifact projecting in the pons (bright dot in the middle of the image).

Susceptibility Artifacts Susceptibility artifacts occur as the result of microscopic gradients or variations in the magnetic field strength that occurs near the interfaces of substance of different magnetic susceptibility. Large susceptibility artifacts are commonly seen surrounding ferromagnetic objects inside of diamagnetic materials (such as the human body). These gradients cause dephasing of spins and frequency shifts of the surrounding tissues. The net result are bright and dark areas with spatial distortion of surrounding anatomy. These artifacts are worst with long echo times and with gradient echo sequences. Clicking the icon will show an axial MRI of the head in a patient with mascara on her eyelids. Susceptibility artifacts from the mascara obscure the front half of the globes.

Zero-Fill Artifacts Occasionally, data in the K-space array will be missing or will be set to zero by the scanner as shown in the figure below. The abrupt change from signal to no signal at all results in artifacts in the images such as zebra stripes and other anomalies. The following coronal image of the shoulder shows an example of a zero-fill artifact.