Magnetic resonance imaging for assessment of vena caval tumor thrombi: a comparative study with venacavography and computerized tomography scanning. We assessed the accuracy of magnetic resonance imaging in demonstrating the presence and extent of vena caval tumor thrombi. The study group included 20 patients with vena caval thrombi from renal cell carcinoma (18), renal pelvic transitional cell carcinoma (1) and adrenal pheochromocytoma (1). Preoperative diagnostic studies included magnetic resonance imaging in all patients, inferior venacavography in 16 and computerized tomography scanning in 15. All patients underwent an operation in which the presence and extent of the vena caval thrombus were confirmed. Magnetic resonance imaging accurately delineated the presence and extent of the thrombus in all 20 patients (100%). Venacavography was accurate in 15 patients (94%) but 8 (50%) required a retrograde and antegrade study. Computerized tomography scanning demonstrated the presence of a tumor thrombus in all 15 patients but accurately delineated the cephalad extent of the thrombus in only 5 (33%). In patients with vena caval tumor thrombi magnetic resonance imaging can provide accurate information regarding the extent of vena caval involvement while avoiding the need for an invasive contrast imaging study. 