Differentiation of necrotizing fasciitis and cellulitis using MR imaging.

OBJECTIVE This study was performed to evaluate the diagnostic value of MR imaging in differentiating necrotizing fasciitis from cellulitis. MATERIALS AND METHODS Spin-echo T1-weighted, T2-weighted, and contrast-enhanced T1-weighted spin-echo sequences were performed in 15 patients with clinically suspected necrotizing fasciitis. In two other patients, only unenhanced imaging was performed. The MR imaging results were correlated with the surgical findings in 11 cases, with autopsy in one case, and with the clinical outcome in five cases. RESULTS Cellulitis was diagnosed when subcutaneous thickening with fluid collections was revealed on T2-weighted images and when subcutaneous tissue or superficial fascia or both showed contrast enhancement. For the diagnosis of necrotizing fasciitis, imaging revealed additional involvement of deep fasciae with fluid collections, thickening, and enhancement after contrast administration. According to these criteria, we found 11 cases of necrotizing fasciitis and six of cellulitis. MR imaging identified all 11 cases of necrotizing fasciitis correctly when compared with the surgical findings. One false-positive case of cellulitis was overstaged and was thought to be necrotizing fasciitis. Contrast-enhanced T1-weighted sequences delineated abscesses and areas of necrosis more clearly than T2-weighted sequences did, but showed no additional lesions. CONCLUSION When no deep fascial involvement is revealed with MR imaging, necrotizing fasciitis can be excluded. However, because its sensitivity exceeds its specificity, MR imaging tends to overestimate the extent of deep fascial involvement. Therefore, the therapeutic regimen should be based on a combination of clinical findings and MR imaging.