The authors provide evidence of significant extravasation of contrast media responsible for the contrast enhancement of pathological tissue on computed tomography. The tissue-blood ratio of enhancement was calculated by the EMI scanner in 2 patients after injection of contrast material and prior to surgery; tumor-blood ratios for red blood cell and plasma tracers were calculated after surgery. The ratios of enhancement demonstrated the analogy between contrast enhancement and the leaking of radionuclide across the blood-brain barrier. This phenomenon may cause error if this technique is used for the measurement of cerebral blood volume. The area for complementary roles of CT and radionuclide brain imaging seems to be narrower than expected.