Diagnosis of acute rupture of the quadriceps tendon by magnetic resonance imaging. A case report. Quadriceps tendon rupture occurred in a 56-year-old man receiving anticoagulant therapy. The diagnosis was inconclusive until magnetic resonance imaging (MRI) was used to delineate the rupture site clearly. MRI is a useful adjunct to diagnosis of soft-tissue injuries in cases in which swelling or other soft-tissue abnormalities obscure examination or preclude the use of more routine diagnostic modalities. 