Magnetic resonance imaging (MRI) provides a wonderful capacity for imaging all the tissues involved in rheumatoid arthritis (RA) and osteoarthritis (OA). For rheumatologists, MRI can be useful in both conditions.
MRI does not use ionizing radiation to produce its images, but instead depends on an external magnetic field, radiofrequency pulses, and the actions of hydrogen protons within a given region of interest. On T1-weighted images, bone has high signal intensity (white), while fat-suppressed or short T1 inversion recovery (STIR) images suppress the signal from bone marrow fat so that the only areas with high signal are those with increased water content, such as areas of inflammation. Contrast agents, such as gadolinium, enhance the MR signal in areas of high vascular flow or increased capillary permeability, and such paramagnetic agents are required for optimal detection of synovitis or tenosynovitis.
The majority of MRI scans are done using 1.5 Tesla (T) magnets, although 3T magnets are increasingly available. Usually, increased magnet strength results in improved resolution of images. However, the clinical gains of such increased resolution are offset against clinical issues of feasibility. In the last few years, more low field (<1T) magnets, called extremity (eMRI) machines have been used. These machines provide a much more patient-friendly imaging technique, because they only require insertion of a limb and reduce patient claustrophobia issues. Generally, the image quality is reduced compared with high-field MRI.
Not every patient can have an MRI scan. Metal under the skin can cause artifacts and there is obviously great danger if there are metal fragments in an eye. People with pacemakers should not go near magnetic fields. Occasionally, people may not fit in a large bore MRI machine due to obesity, and sometimes the receiver coil for a joint such as the knee will not fit around a very large knee.
MRI is dramatically increasing our understanding of structural pathology in arthritis.
RA Pathology on MRI
Erosions: The radiographic erosion is the hallmark of RA. The tomographic nature of MRI markedly improves the ability to detect erosions compared with traditional radiographs, even when using eMRI. Initial publications on this imaging modality described the validity of MRI erosions to predict subsequently visualized erosions on radiographs. Computed tomography (CT) is the gold standard for examining cortical bone, and recent studies have shown high correlations between MRI and CT detected erosions in RA, thereby validating that the MR erosions are the equivalent of radiographic erosions, with the increased detection rate reflecting the tomographic advantages of MRI.1,2