The quality of final MR image is dependent on equipment used, choices made by radiologists and technologists, surrounding environment, several other factors and interplay between all of these variables. But, the most common artifact and one that does not require advanced understanding of MR physics is a simple question: Can a patient hold still during the scan? It is common for new practitioners to think of MRI as a problem solver when clinical diagnosis is not clear. There may be algorithms, pathways, and other guidelines that mention MRI as an option in selecting next imaging modality for a particular situation. Unfortunately, most of such algorithms do not include a box asking “Is patient able to hold still?” Even when they do, the information about amount of time required to obtain each acquisition and the amount of time needed for the entire scan may not be easily available to clinicians. It is way too common for an in-patient to arrive from their hospital room to MR suite and not be able to tolerate even the localizer sequences. Obviously, this results in wasted time on the scanner, and an end result with very little or no diagnostic information. All members of patient’s care team should be cognizant that most MRIs take more than “a few seconds” to acquire. Motion will severely deteriorates or render completely non-diagnostic even the shortest sequences lasting just a minute or two. The majority of common MR scans last at least 15-30 minutes, and some studies may need an hour or more.
There are also artifacts inherent in physics of image acquisition and in biology of imaged tissues. The following is a list of MRI artifacts and brief description of each:
|55 Degree Artifact “Magic Angle” Phenomenon|
|55 degree artifact results from high order of collagen fibers oriented at approximately 55 degrees to the magnetic field. This artifact is the increased signal, which in many cases is a desirable effect. However, when imaging structures of normally low signal intensity such as tendons of the rotator cuff or tendons in the ankle, 55 degree artifact, also known as “magic angle” phenomenon produces high signal, which may be mistaken for pathology in these structures. The artifact is a problem on short TE sequences (T1, PD, some GRE sequences). This artifact disappears on long TE sequences (T2), where high signal intensity in tendons may represent pathology. Magic angle artifact is most troublesome in the joints where tendons demonstrate angulation, such as in the shoulder and ankle.|
|Motion artifact results from patient motion. Keeping the patient calm and comfortable and avoiding long acquisition times are strategies that help reduce this artifact. Respiratory motion artifact can be diminished by placing a patient in prone position when imaging sternum or sternoclavicular joints. Of course, patient has to be able to tolerate such position in the MRI scanner.|
|This artifact is very similar to motion artifact. Physiologic motion of the heart, vessels and CSF results results in pulsation artifact. Ghost images of the vessels or vessel walls are seen along the phase encoding direction. Changing the orientation of the phase encoding may result in better delineation of particular anatomic structure.|
|Susceptibility artifact results from inhomogeneities in the magnetic field. Gradient echo imaging is sensitive to this artifact. Lower flip angles and higher TE exagerate this artifact.|