AUR Memorial Award 1990. The physiologic basis of the radiodense renal medulla after the administration of blood pool contrast agent PFOB.

The hypothesis that the greater CT enhancement of the renal medulla relative to cortex after the administration of perfluorooctylbromide (PFOB) was due to the renal medullary osmotic gradient was tested in eight dogs infused with 10 g/kg PFOB. Urine osmolarity and CT attenuation of the cortex and medulla of each kidney under the full effect of antidiuretic hormone were measured before and after 40 mg of furosemide given intravenously, which is known to destroy the renal osmotic gradient. In an attempt to measure the fractional blood volume of the cortex and medulla, which could account for the observed difference on CT, 10 mCi of 99mTc-labeled erythrocytes were given and cortical and papillary tip tissue samples harvested within 1 minute of the animals' death. Cortical blood volume was eight times that of the medulla (21.6 +/- 5.1% vs 2.7 +/- 0.5%, P less than 0.01) and was not significantly affected by furosemide. Furosemide markedly decreased urine osmolarity (1473 +/- 176 mOsm to 454 +/- 45 mOsm, P less than 0.01), had a minor effect on cortical attenuation (decreased from 90.4 +/- 8.2 to 85.1 +/- 8.1 HU, P less than 0.01), and a marked effect on medullary attenuation (decreased from 140.9 +/- 12.4 to 85.9 +/- 8.9 HU, P less than 0.01) which resulted in total loss of corticomedullary contrast (decreased from 20.0 +/- 3.1% to 0.1 +/- 2.2%, P less than 0.01). Thus, the observed greater CT attenuation of the medulla than cortex following the administration of PFOB, a blood pool agent, is due to the osmotic gradient across the medulla which increases the concentration of the agent in the vasa rectae.