At present, there are two areas where MRI is the procedure of choice in the acute setting: (1) evaluation of suspected spinal cord compression from any cause and (2) radiographically occult femoral intertrochanteric and neck fractures. In both cases, the unique ability to form images in axial, coronal, or sagittal planes gives MRI a distinct advantage. Another major factor is the superb contrast resolution of MRI that facilitates detection of spinal cord injury or fracture through cancellous bone of the hip. Figure 297-4 demonstrates an example of cervical cord compression resulting from a traumatically herniated disc. Figure 297-5 is an example of an occult femoral intertrochanteric fracture best demonstrated on MRI. Small studies1 l9 have confirmed higher sensitivity and specificity for MRI as compared with radionuclide bone scanning, tomography, and CT in the detection of occult fractures, especially in femoral head and neck fractures.
FIG. 297-4. Cervical spinal cord compression: T1-weighted sagittal magnetic resonance imaging demonstrates moderate spinal cord compression by an acute traumatically herniated disc at the C2-3 level (arrow). The patient was in a motor vehicle accident and also suffered associated bilateral C2 pedicle fractures.
FIG. 297-5. Occult hip fracture: A. Anteroposterior radiograph of a 55-year-old patient on steroids who had right hip pain after a fall. No fracture is evident. B. Tl-weighted coronal magnetic resonance scan of the same hip clearly demonstrates a non-displaced intertrochanteric femoral shaft fracture ( arrows).
Another potential area for MRI evaluation in the acute setting is aortic dissection. MRI is superior to contrast-enhanced CT and possibly transesophageal ultrasound in delineating the intimal aortic flap. Unfortunately, many of these patients are unstable hemodynamically and are agitated, requiring life support and sedation. Thus, few, if any, good candidates for MRI. As more MRI-compatible life-support and monitoring equipment becomes available, this situation will change.
Finally, a second potential application is in pediatric fractures 20 when there may be significant injury to unossified cartilage around open growth plates. Fractures through cartilage are not seen on plane films but are easily identified on MRI.
One innovation—the development of low and very low magnetic field imaging systems—may have some impact on emergency departments. Most of the hazards previously delineated apply to high-field systems. In low-field systems where the magnetic flux density is less in magnitude and more restricted in spatial extent, it is easier to accommodate life-support equipment. The design of these units allows more access to the patient and reduces the chances of interference with the proper operation of the life-support electronics. Thus, installation of a low-field MRI scanner in the emergency department becomes more feasible. However, because of theoretical considerations, signal to noise is much less (i.e., less signal) at low field. Therefore, there may be trade-off in diagnostic quality of the scans. Some signal can be recovered with optimal design of the software and hardware, but this remains a controversial area. There may be a place for low-field MRI in the emergent setting in the evaluation of subacute intracerebral hemorrhage and brain edema.
MR angiography, which (except for innocuous intravenous contrast) is noninvasive, has been evolving slowly and improving steadily. 52 22 It may eventually be the method of choice in the emergent evaluation of suspected subarachnoid hemorrhage or in leaking aortic aneurysms. An example of a normal noncontrast Mr angiogram is shown in Fig.297,-6.
FIG. 297-6. Normal magnetic resonance angiogram of the circle of Willis in the brain. Summation image of maximum-intensity projection data from computer reconstructions after a three-dimensional time-of-flight acquisition in the axial plane. The examination required about 9 min, and no intravenous contrast was necessary.
Recent developments in echo planar and diffusion imaging make it possible to diagnose cytotoxic cerebral edema almost immediately after an acute ischemic event, even earlier than with CT or conventional MRI.23 This has obvious implications with the advent of more aggressive lytic therapy aimed at early salvage of brain tissue after strokes.
MRI continues to evolve with more potential applications to emergency medicine in addition to its role in spinal cord compression, radiographically occult fractures, and acute aortic dissection. The new areas are diffusion imaging (to detect early strokes), noninvasive cerebral and body MRI angiography, and in the future cardiac and pulmonary MRI angiography. Taking advantage of its compatibility with life support equipment, further development of a low magnetic field system with improved signal to noise might accelerate introduction of the above applications into emergency medicine practice.
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