Magnetic Resonance Imaging
Last updated: October 9, 2014
Synonyms: Nuclear magnetic resonance.
Description: MRI is an imaging method that uses a magnetic field to detect changes in the spin orientation of hydrogen nuclei, or protons, located in living tissues. Patients are placed within a magnetic coil and then exposed to pulsed radiofrequency waves that cause the protons in tissues to change spin orientation. After the pulse, the magnet measures the time required for protons to return to baseline, which is translated into a signal used to characterize tissue composition.
Methods: Most studies involve acquisition of two or more types of images, the most common of which are designated T1- and T2-weighted. These images use different repetition times and sample after different echo times. The T1- weighted image usually provides greater anatomic detail, but the T2-weighted image is more likely to show common tissue pathology such as inflammation, which is associated with increased free water content.
Confounding Factors: Patients with indwelling magnetic metal cannot be evaluated; this includes those with pacemakers, insulin pumps, sutures or artificial components in the eye, wire sutures in brain tissues, and rods or screws in bones. Most joint replacement components in current use are made of non-magnetic materials, so these patients can be safely scanned. Wire sutures in areas such as the abdomen also do not present problems.
Adverse Effects: No radiation exposure is involved, and, at present, there are no known health risks associated with exposure to the magnetic field. A minority of patients may experience claustrophobia or other discomfort. At times, these reactions require stopping the examination; in other cases, use of sedative agents is required to complete the scan.
Indications: Unlike standard radiographs, MRI can define soft tissues in great detail, so joint structures such as ligaments, tendons, synovium, and articular cartilage can be evaluated. Examples of joint abnormalities that can be delineated with MRI include meniscal tears, ligamentous damage, tendon rupture, Baker cysts, pigmented villonodular synovitis, and simple effusions. MRI has largely replaced invasive radiologic procedures (e.g., arthrography) in evaluation of rotator cuff and shoulder disorders. Inflammation in muscles can be detected in patients with myositis and related syndromes. MRI may be useful in evaluating the integrity of bone and nerve tissues in the spine.
Cost: $1,000–2,000. Expense is significantly greater than per site other techniques. MRI should therefore be used selectively.
BIBLIOGRAPHY
Ghozlan R, Vacher H. Where is imaging going in rheumatology? Baillieres Best Pract Res Clin Rheumatol 2000;14:617–633. PMID: 11092792
Santiago Restrepo C, Gimenez CR, McCarthy K. Imaging of osteomyelitis and musculoskeletal soft tissue infections: current concepts. Rheum Dis Clin North Am 2003;29:89–109. PMID: 12635502
