MULTIPHASIC RENAL CT - COMPARISON OF RENAL MASS ENHANCEMENT DURING THE CORTICOMEDULLARY AND NEPHROGRAPHIC PHASES

PURPOSE: To evaluate thin-section computed tomography (CT) performed d uring the corticomedullary and nephrographic phases of contrast materi al enhancement in the characterization of renal masses. MATERIALS AND METHODS: A prospective study of 30 patients was undertaken with CT to characterize 31 ''indeterminate'' renal masses. In all patients, 5-mm- thick, contiguous, high-tube-current (320-340-mA) scans were obtained through the kidneys before (axial mode), during (helical mode, 25-seco nd delay, corticomedullary-phase images), and after (axial mode, 120-s econd delay, nephrographic-phase images) administration of a 117-secon d biphasic injection of intravenous contrast material. RESULTS: Eight of 16 neoplasms measured less than 20 HU on CT scans obtained without contrast material enhancement; measurements of two of these correspond ed to ''cyst attenuation'' during the corticomedullary phase. Enhancem ent of 10 HU or greater was demonstrated in 11 neoplasms during the co rticomedullary phase and in all neoplasms in the nephrographic phase. No enhancement was seen in 15 radiologically benign cysts. Both renal neoplasms and normal renal cortex demonstrated significantly greater e nhancement in the nephrographic phase compared with that in the cortic omedullary phase (P = .0002 and P < .0001, respectively). CONCLUSION: Enhancement of renal neoplasms is time dependent and may not be eviden t in hypovascular tumors analyzed during the early corticomedullary ph ase. Reliance on absolute CT attenuation measurements, without use of internal standards as controls, may lead to misdiagnosis of neoplasms as cysts.