Musculoskeletal magnetic resonance imaging (MRI) can be a valuable tool for the diagnosis of a variety of conditions and holds the potential to avoid diagnostic delay that may result in serious morbidity. Unfortunately, MRI remains expensive and is not routinely available at all hours in all emergency departments (ED). In cases where MRI may be diagnostic, the clinician is often confronted with the decision to transfer the patient to another facility or hold the patient pending MRI availability. Even in tertiary care centers, MRI may not be available after normal business hours, except in the direst of emergencies.
Evidence of the diagnostic sensitivity and specificity of MRI for emergency musculoskeletal conditions is limited, and even less evidence exists for the effect of emergency MRI on clinical outcomes. Rigorous methodology would require that MRI be prospectively compared with a high quality gold standard, but few studies achieve this. A few artifacts common on MRI should lead us to question whether all MRI abnormalities represent real pathology. According to a meta-analysis of 19 studies, in one soft tissue application of MRI (screening for breast cancer with magnetic resonance [MR] mammography), MRI is known to have excellent sensitivity but poor specificity ? with 1 false positive test for every 2 true positives.1 It should not be assumed that a positive MRI in other settings automatically indicates the presence of disease. In the case of lumbar spine pathology, studies have shown a 25% rate of abnormal MRI in asymptomatic subjects.2 In asymptomatic patients undergoing wrist MRI, signal intensity in ligaments may be elevated, simulating pathology.3-5
MRI availability and utilization from the emergency department is increasing. A retrospective review of ED utilization at a single academic tertiary care center from 2000 to 2005 found a 390% increase ? mostly due to MRI and MRA examinations of the head. Spinal MRI constituted 29% of total examinations, while lower extremity MRI was performed only 23 times during the study period, constituting 1% of total examinations.8 Whether this increase in ED utilization results in better patient outcomes is unclear. A study from the Mayo Clinic found that 11% of patients with negative x-rays of the hip underwent hip MRI as part of their ED evaluation.9 No data exists on the rate of ED musculoskeletal MRI use nationally. A recent practice survey of radiologists in the United States suggested that MRI scanners are physically present in only 3% of EDs.10
Understanding that MRI is not simply a "radiation-free CT" is important in recognizing why som pathological processes may be better imaged with MRI. MRI uses complex computer algorithms to generate images based on the radio signal generated by protons (hydrogen ions) when they are manipulated by an applied magnetic field. (See Figure 1.) Unlike CT and x-ray, MRI does not use ionizing radiation. Like CT, MRI allows multiplanar two-dimensional (2D) images and three-dimensional (3D) images to be constructed from 2D acquired slices. Gradient echo MRI pulse sequences allow acquisition of a true 3D volume of data that can be reconstructed in any plane. (See Figure 2b.) In contrast, modern multi-slice CT relies on the addition of many fine axial slices to construct a 3D data volume that can then be reconstructed in any plane or displayed in three dimensions. (See Figure 2a.) Consequently, some MRI sequences are not subject to reconstruction artifacts that may occur when CT reconstructions are created.11 CT can overcome this effect by using thin and overlapping slices but at the expense of significant radiation exposure for the patient. Nonetheless, thin-slice CT allows creation of volumes of data that, for clinical purposes, appear to have true 3D characteristics.
Estimated Creatinine Clearance Using Cockcroft-Gault Formula
Creatinine clearance is a more accurate measure of renal dysfunction and risk of gadolinium-related nephrogenic systemic sclerosis than is measured serum creatinine.
eCcr= (140-Age) x Mass (in kilograms) x [0.85 if female, 1.0 if male]
72 x measured serum creatinine (in mg/dl)
When standard doses of gadolinium are administered, the risk of gadolinium-induced NSF is lower than the risk of death from iodinated contrast agents used in CT. However, the American College of Radiology (ACR) recommends informed consent before administration of gadolinium-based contrast for patients with moderate to end-stage kidney disease.15 Hemodialysis patients should receive the lowest possible dose of gadolinium and should receive dialysis as soon after contrast administration as is practical. The ACR recommends hemodialysis within 2 hours after administration of gadolinium contrast for patients with renal failure,15 though this practice is not well-supported by research data. Peritoneal dialysis is relatively ineffective at removing gadolinium, and these patients may be at particularly high risk from gadolinium.15 This is an important consideration because dialysis patients may be at risk for conditions such as epidural abscess, which may be best diagnosed by MRI.16
Probably the single most important emergency application of musculoskeletal MRI is in the evaluation of neurological deficits localizing to the spinal cord, where MRI is usually considered the gold standard diagnostic imaging test. In the case of spinal epidural abscess, delays in diagnosis can have devastating neurological consequences, so early MRI is of particular importance. In a retrospective case-control study of 63 patients with spinal epidural abscess, delay in diagnosis (defined as multiple emergency department visits or admission without a diagnosis and greater than 24 hours until definitive imaging) was common, occurring in 75% of cases. Persistent motor weakness occurred in 45% of patients with diagnostic delay, compared with 13% of those with more rapid diagnosis (odds ratio 5.65, 95% CI, 1.15-27.71, p<0.05).24 The classic triad of fever, spine pain, and neurological abnormalities was present in only 13% of patients, demonstrating that a low threshold for imaging must be applied to avoid misdiagnosis. A well-conducted review of the literature from the New England Journal of Medicine concluded that the single most important predictor of neurological outcome is the patient's neurological status prior to surgery. Therefore, a vigilant effort to diagnose the patient prior to the progression of neurological signs and symptoms is imperative.25 More studies are needed to identify patients at risk for spinal epidural abscess, in order to provide prospective clinical criteria for emergency physicians to guide MRI. MRI examinations demonstrating spinal cord compromise are shown in Figures 6 and 7.
The cost of emergency MRI may vary considerably, depending on the hospital charge and radiologist fee. Additional charges for contrast material may occur ? between $150 and $300 depending on patient weight and materials used. Table 5 lists approximate charges for MRI of the hip, wrist, and spine.82
The length of an MRI examination depends on many factors, including the number and types of sequences acquired, patient positioning, and needs for sedation or pain control. Additional time is needed to reconstruct the images, though fast computers make this process brief. Because of the large number of images generated, the time required by a radiologist to review the images may be considerable. Studies on MRI for hip fracture suggest that limited view sequences may be adequate, and these are relatively rapid to perform. A T1 coronal view of the hip can be acquired in as little as 5 minutes.83 The sensitivity of these limited views has not been well-validated. In general, an MRI of the hip, wrist, or a single region of the spine should be expected to take approximately 45 minutes.