Comparison of CT and MRI for diagnosis of suspected scaphoid fractures

There is no consensus on the optimum imaging method to use to confirm the diagnosis of true scaphoid fractures among patients with suspected scaphoid fractures. This study tested the null hypothesis that computed tomography (CT) and magnetic resonance imaging (MRI) have the same diagnostic performance characteristics for the diagnosis of scaphoid fractures.

Thirty-four consecutive patients with a suspected scaphoid fracture (tenderness of the scaphoid and normal radiographic findings after a fall on the outstretched hand) underwent CT and MRI within ten days after a wrist injury. The reference standard for a true fracture of the scaphoid was six-week follow-up radiographs in four views. A panel including surgeons and radiologists came to a consensus diagnosis for each type of imaging. The images were considered in a randomly ordered, blinded fashion, independent of the other types of imaging. We calculated sensitivity, specificity, and accuracy as well as positive and negative predictive values.

The reference standard revealed six true fractures of the scaphoid (prevalence, 18%). CT demonstrated a fracture in five patients (15%), with one false-positive, two false-negative, and four true-positive results. MRI demonstrated a fracture in seven patients (21%), with three false-positive, two false-negative, and four true-positive results. The sensitivity, specificity, and accuracy were 67%, 96%, and 91%, respectively, for CT and 67%, 89%, and 85%, respectively, for MRI. According to the McNemar test for paired binary data, these differences were not significant. The positive predictive value with use of the Bayes formula was 0.76 for CT and 0.54 for MRI. The negative predictive value was 0.94 for CT and 0.93 for MRI.

CT and MRI had comparable diagnostic characteristics. Both were better at excluding scaphoid fractures than they were at confirming them, and both were subject to false-positive and false-negative interpretations. The best reference standard is debatable, but it is now unclear whether or not bone edema on MRI and small unicortical lines on CT represent a true fracture.