MRI has been widely applied to the brain and spinal cord,9 where it provides images that are superior in diagnostic quality to those obtained with CT. Furthermore, this information can be obtained with less risk to patients because CT myelography requires intrathecal contrast agents for a specific diagnosis. Although special intravenous contrast agents are frequently required to improve the sensitivity of MRI, they have been associated with much less toxicity and reactions as compared with the intravenous CT contrast agents.10 Except in the cases of acute intracerebral hemorrhage, skull fracture, and some calcified brain lesions, MRI may completely replace CT of the head. The exact role of MRI versus CT in trauma and degenerative disease of the spine is still evolving. CT visualizes fracture fragment relationships and bone detail more optimally, but MRI visualizes the soft tissues with better resolution. Some spine surgeons still prefer CT myelography to MRI.

MRI has been useful in examining the chest and abdomen (especially the chest wall, mediastinum, liver, spleen, adrenals, and aorta), but has played a lesser role compared with CT, because of respiratory motion and heart pulsation artifacts that degrade anatomic delineation of critical structures. They can be compensated for, to some degree, with electrocardiograph and respiratory gating and associated electronic manipulation, but the methods are cumbersome and difficult to implement with an acutely ill patient. A recent innovation is the introduction of breath-hold MR cholangiography for the noninvasive evaluation of the biliary and pancreatic ducts.11 It has had limited application to cooperative patients with biliary or pancreatic disease not amenable to ultrasound, CT, or other conventional methods. There has also been progress in cardiac MRI, 512 but introduction of these methods into the emergency practice are currently limited due to cost and availability.

MRI has a major role in other areas of the musculoskeletal system6 especially the knee, shoulder, hip, and temporomandibular joints. Although MRI is not indicated for most acute fractures, it may be preferred in the diagnosis of rotator cuff tear of the shoulder, internal derangement of the knee (meniscus, tendon, and ligament tears), tendon or soft tissue injury to any of the small joints, soft tissue injury in the spine, and posttraumatic avascular necrosis of any bone. In addition, carpal tunnel syndrome has been evaluated using MRI. Figure297.-.2 demonstrates a meniscus tear in the knee. Before MRI, arthrography, which involves injection of contrast agents into the joint, was used to detect cartilage injuries. This type of examination is not only painful but carries a small but measurable risk of infection and contrast reaction. MRI of these joints is painless and only requires that the patient be able to hold still for a moderate length of time. The information obtained in the knee and hips exceeds what can be obtained using other methods. In the hips, MRI has proven to be the most reliable method for detecting avascular necrosis.

FIG. 297-2. Meniscal tear: A. Normal medial and lateral menisci as demonstrated on a magnetic resonance proton-density coronal image (partial T2 weighting). The menisci are the dark triangular structures marked with arrowheads. B. Proton-density coronal scan showing a large complex tear involving the posterior horn of the medial meniscus (arrowheads)

In problematic cases, MRI has detected stress fractures and occult fractures in the small bones of the hand. Even though it does not visualize cortex, any break in the medullary cancellous bone can be readily detected.

The sequelae of soft tissue musculoskeletal trauma, such as complete muscular or tendon tears, hemorrhage, and edema, are very easily diagnosed with MRI. Even injuries to the medium-sized nerves and brachial plexus can be demonstrated. —I4

MRI has also been used to study infection in bone and soft tissues, where in many cases it has been superior to modalities such as nuclear medicine and CT. ™161Z However, if the patient has a metallic prostheses in the region of abnormality, rf currents or magnetic field inhomogeneities due to the metal induce artifacts in the MRI scan that reduce the sensitivity (Hg.^Z-S). This is even more of a problem in CT, where x-ray scattering from the prosthesis may completely obliterate the scan. Then, only nuclear medicine studies may be useful, in particular indium 111 tagged to white blood cells.

FIG. 297-3. Magnetic artifacts: A. Lateral radiograph of a foot of a patient with a talar fracture reduced with two cold-worked stainless-steel screws. B. Proton-density-weighted coronal magnetic resonance imaging: ferromagnetic artifacts almost completely obliterate the talar marrow signal, rendering it diagnostically useless. C. Ultra-high-resolution coronal computed tomographic (CT) scan on the same foot yields useful information on the condition of the talar fragments (nonunion and avascular necrosis). This is unusual in that the ferromagnetic properties of the screws are responsible for a disproportionate deleterious effect on the MRI compared with the CT. Note that both show a lateral calcaneal dislocation.

MRI is extremely sensitive and specific in detecting metastatic disease in bone when questions arise after a positive bone scan. MRI is neither practical nor cost effective to use for whole body surveys, but when applied to specific lesions the anatomic information expedites diagnosis.

CT continues to be the modality of choice for suspected head, spine, and abdominal injuries, because it is quick, more widely available, and more compatible with life-support equipment. Although MRI-compatible respirators and pulsed oxymeters are now available, most standard life-support equipment either contains magnetic steel components or sensitive electronics that will not operate properly in the presence of rf or large static or dynamic magnetic fields. MRI is used in an elective setting after a patient has been stabilized and there is time to address the less acute problems.

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