MR diagnosis of internal derangements of the knee: effect of field strength on efficacy.

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Abstract
The purpose of the study was to test the hypothesis that diagnoses of internal derangements of the knee based on findings on MR images obtained at 0.5 T are as accurate as those based on findings on MR images obtained at 1.5 T. The MR findings in 118 consecutive patients who had MR imaging of the knee at 0.5 T and subsequent arthroscopy were reviewed retrospectively. Ninety-two patients had arthroscopically proved tears of the medial meniscus, lateral meniscus, and/or anterior cruciate ligament. Sensitivity, specificity, and accuracy for MR evaluation of the medial and lateral menisci and anterior cruciate ligament were determined by using arthroscopy as the gold standard. The results were compared with similar data from 11 recently published studies in which MR images obtained at 1.5 T were used. To account for ambiguity in MR interpretation, the MR findings in this study were reanalyzed by using a five-point scale of diagnostic certainty, and receiver-operating-characteristic curves were constructed for each of the three anatomic structures examined. For tears of the medial meniscus, the sensitivity, specificity, and accuracy of MR at 0.5 T were 93%, 90%, and 92%, respectively. For tears of the lateral meniscus, the sensitivity was 81%, the specificity was 97%, and the accuracy was 93%. The sensitivity, specificity, and accuracy for detecting complete tears of the anterior cruciate ligament were 100%, 97%, and 97%, respectively. No clinically significant field strength-dependent differences were found. The results suggest that, allowing for necessary discrepancies in imaging protocol, magnetic field strength is not a significant determinant of diagnostic reliability of MR assessments of internal derangement of the knee.

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