MRI Artifacts: Understanding Causes and Solutions
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Understanding Artifacts in MRI
Magnetic Resonance Imaging (MRI) stands as a cornerstone in modern medical diagnostics, providing detailed and non-invasive views of the human body. However, like any advanced technology, MRI is not without its challenges. One of the intriguing aspects that often perplex both rad techs and radiologists alike is the presence of artifacts in MRI images. These artifacts, unintended anomalies or distortions in the images, can arise from various sources and impact the diagnostic quality of the scans. In this article, we will explore a variety of MRI artifacts, their origins, types, and strategies to reduce their effects.
The MRI artifacts that will be discussed in this article are the following:
- Motion or ghosting
- Non-uniform/heterogeneous fat suppression
- Aliasing/wrap-around artifacts
- Non-uniform/heterogeneous signal within anatomy
- Susceptibility
- Chemical shift
- Geometric distortion
- RF leak (zipper artifact)
- Mis-registration of subtracted images
- Truncation/ringing/ Gibbs artifacts
Motion or Ghosting Artifact
Perhaps one of the most common artifacts in MRI is the motion artifact. This occurs when a patient is moving during a scan. There are several factors that could be uncomfortable for patients during an MRI scan, such as prone positions, superman positions, pain, obesity, breathing instructions, claustrophobia, and others. These factors affect the stability and comfort of patients, triggering them to move or adjust their position. This type of movement is categorized as a “voluntary” movement caused by the patient; and may be solved through proper patient education on the effect of motion on the image and time, as well as providing proper instructions and ensuring the comfort of the patient at the beginning of the examination. In more extreme cases, a sedative may be administered by a physician prior to the exam to help maintain the stability of the patient.
Whereas “involuntary” movement is caused by the natural motion of biological processes within the body, such as respiration, and cardiovascular pulsation, peristaltic movement of bowels. Some anatomy is situated near the chest (such as the heart or breast), which makes them more susceptible to respiratory and cardiac artifacts. Another example is the uterus (which is situated anteriorly to the colon), making it more susceptible to peristaltic movement. The involuntary motion from the respiration and cardiovascular pulsation could be reduced by different approaches, depending on the cause of the artifact: changing phase encoding direction, increasing NSA, and placing a rest slab over the area causing the motion, cardiac and respiratory gating, respiratory compensation, instructions for respiration, closing the eyes, relaxing the muscles and others…
Figure 1 a. b. and c. are breast MRIs of a patient with breast implants. MRI was indicated as a regular screening for the integrity of the implants, and to rule out silicone leakage after the patient has complained of breast pain. In the axial images, the cardiac pulsation artifact was very apparent, sequences like T2-weighted, T1-weighted, and STIR. In some slices, the artifact was obscuring some of the anatomy of the breast and part of the lymph nodes. Note that his artifact is common in breast MRI and is seen in many patients, however, this patient in particular was quite thin and barely had fat or muscle in the chest that could act as a barrier between the chest and the breasts, making the heart extremely close, almost attached to the breasts, thus appearing more prominent as an artifact.
Figure 1: Case of breast MRI of a patient with silicone implants. Cardiac pulsation in this case is causing a ghosting artifact, obscuring some of the anatomy of the breast and lymph nodes.
- Axial T2 weighted image
- Axial STIR
- Axial T2-weighted image
By changing the phase direction from the routine “right-left” direction to “anterior-posterior” we noticed the significant difference in the two images (Figure 2 a. and b.), the artifact was completely gone, and a small shadow of the heart pulsation appeared in the medial part of the left breast; not affecting the anatomy as it had done before.
Figure 2: Comparison of axial STIR between phase direction “right-left” and “anterior-posterior” to avoid ghosting artifacts caused by cardiac pulsation.
- Axial STIR with phase direction “right-left” (or RL)
- Axial STIR with phase direction “anterior-posterior” (AP)
In rare but possible cases, motion artifacts could be due to an unstable installation of gradients from the manufacturer, causing the MR table to have a slight motion during the scan.
Non-uniform/Heterogeneous Fat Suppression
Another common artifact is the non-uniform fat suppression, or heterogeneous fat suppression, which by the name we can understand immediately the appearance. It is an uneven darkening or suppression of the fat signal in different portions of the anatomy being imaged instead of a homogenous fat signal suppressed over the entire field of view. This could be due to a heterogeneous magnetic field or heterogeneous radiofrequency field, caused by the presence of metal or metallic deposits (metallic deposits could come from creams or deodorants), the RF door being opened, interference of external radio frequency signal, or other technical malfunctions… Reducing this artifact is quite simple, it can be through simple steps taken by the technician that ensure the homogeneity of the magnetic field, by always checking for metal within the patient’s garment or within the MR room itself, and making sure that the RF door is shut well. As well as observing the repetition of this artifact in several patients, the MR manufacturer’s maintenance team should be contacted for technical malfunctions.
Aliasing/Wrap-Around Artifacts
Aliasing or wrap-around artifact is the folding of the image around onto itself. It is caused by the detection of signals from the tissues outside of the selected field of view. For example in the case of cervical spine MRI, the signal may be from the shoulders, chin, or brain, depending on the anatomy found in the phase-encoding direction of the sequence performed. The signal then wraps back into the displayed field of view on the opposite side of the image. This artifact is easily eliminated by increasing the field of view or applying phase oversampling. In the example in Figure 3, the cervical spine was superimposed by the anatomy of the brain, this is due to a small FOV and lack of oversampling in the feet-head direction.
Figure 3. Aliasing artifact is detected through the overlapping anatomy of the brain on the cervical spine.
Non-uniform/Heterogeneous Signal
A nonuniform signal, or a heterogeneous appearance within the anatomy appears as black circles, a portion of the anatomy is canceled, or an error in the scan message that appears on the monitor before scanning. It is caused by radiofrequency heterogeneity, receiver coil non-uniformity, non-functioning coil elements, and other patient-related causes such as improper patient position or the presence of a metal in the magnet. This could also be solved by double-checking for metals, correcting of patient’s position, and checking for technical malfunctions with the manufacturer. The presence of metallic objects within or on the body (figure 4), such as implants, dental work, or jewelry, can cause distortions in the magnetic field, leading to artifacts. Specialized sequences like metal artifact reduction techniques (MARS) are designed to minimize these distortions and improve image quality.
Figure 4. Metallic implants in the knee cause a heterogeneous signal, where a portion of the anatomy is erased.
Magnetic Susceptibility Artifact
Susceptibility artifact is the localized or geometric field distortion and non-uniformity, such as bright or dark areas. This is due to the difference in magnetic susceptibilities of different adjacent tissues especially at air-tissue, or around metal implant interface. They can be minimized by using shorter TE values and by using fast spin-echo instead of gradient-echo sequences; increasing gradient strength for a given field of view and avoiding narrow bandwidth techniques. Thinner slices also help as do the use of parallel imaging techniques.
Figure 5. Magnetic susceptibility artifacts pointed by the arrows, cause local signal loss.
Chemical Shift Artifact
Chemical shift artifact appears as a thin intense band or line of bright (or high signal) or dark (or low signal). It is caused by the spatial displacement of water and fat due to their difference in resonance frequencies. This artifact occurs along the frequency encoding direction at fat/water soft tissue interfaces. It is commonly seen in breast MRIs in the cases of implants. Narrow bandwidth techniques should be avoided, for reducing the bandwidth per pixel accentuates this artifact. In the image seen here, the displacement of fat relative to water produces a bright band where water and fat overlap, and a dark band appears where water and fat are shifted apart.
Figure 6. The coronal abdomen shows a dark band around the kidney pointed by the red arrows.
Geometric Distortion Artifact
Geometric distortion is the misrepresentation of the anatomical structure’s size and orientation in a field of view, such as the elongation or appearance of a black shape covering a portion of the anatomy, it is most commonly seen in the diffusion-weighted sequence. This is due to off-center scanning, or the presence of metal in the imaged FOV. The technician can fix this artifact by repositioning the patient well and adjusting the center, as well as checking for the presence of a metal.
RF Leak or Zipper Artifact
RF leak or zipper artifact appears as a linear hyperintense or a variable of intensity lines, that are parallel to the phase encoding directions. This artifact is caused by a leakage or penetration of RF signals into the scan room, which could be originating from outside or within the scan room; such as electronic equipment or lightbulbs. It is caused by damage in the seal of the RF door or the RF shielding.
Mis-registration of subtracted images
Misregistration of subtracted images. This artifact or error is caused by the movement of the patient between the T1 pre-contrast images, and the T1 post-contrast images, causing an incomplete subtraction of background tissue signals in the subtracted images. It is better to repeat this sequence because it could blur the borders of a mass making them more suspicious or make a mass disappear on a reformatted image.
Truncation/ringing/Gibbs artifacts
Truncation, Gibbs, or ringing artifact appears as periodic or several parallel lines or ringing adjacent to borders or tissue discontinuities. This could be observed in the phase-encoding direction or the frequency-encoding direction. This is due to the selection of a small matrix in one or both in-plane directions, occurring most commonly in the phase-encoding direction. Ringing artifact is reduced, but not eliminated completely through increasing the number of phase encoding steps, and decreasing the size of the field of view. As you can see here in Figure 7, the artifact is displayed as rings of high and low signal intensity. The artifact is the result of a phase shift across the image, likely due in this case to both poor shimming and image wrap of signal-producing tissues.
Figure 7. Coronal T1 Brain MRI showing a band of high and low signal intensities.
While MRI artifacts pose challenges in the quest for clear and accurate diagnostic imaging, advancements in technology and imaging techniques continue to push the boundaries of what is possible. Understanding the origins of artifacts and implementing strategies to mitigate their effects not only enhances the diagnostic capabilities of MRI but also ensures a more comprehensive and reliable assessment of the human body’s intricate structures. As technology evolves, the future promises even greater refinement in imaging quality and a deeper understanding of the intricacies of MRI artifacts.
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