Technetium-99m-pertechnetate as a whole blood marker for brain perfusion studies

In the brain, diffusible 99mTc-pertechnetate behaves as an intravascular indicator because it is confined within the circulation by the blood-brain barrier, allowing its use for noninvasive dynamic evaluation of cerebral circulation. For this application 99mTc has often been claimed to be a plasma marker. This study examines the validity of such a claim which has not yet been proven in vivo. METHODS: The relative amount of 99mTc in the red cells circulating in large vessels was compared to the corresponding hematocrit (LV Hct) during the rapid (t/2 = 1.98 min) and slow (t/2 = 84 min) phases of 99mTc disappearance from the circulation after bolus intravenous injection. These comparisons were performed on rats at 2 (n = 3), 5 (n = 6), 10 (n = 6) and 20 (n = 9) sec after intravenous injection for the rapid phase and 5 (n = 5), 30 (n = 4), 60 (n = 6) and 120 (n = 6) min after intravenous injection for the slow phase. RESULTS: The results show that the relative amount of intravascular 99mTc fixed to red cells did not differ statistically from LV Hct until at least 1 hr after intravenous administration. This homogeneous distribution of 99mTc in blood was indisputable during the first 20 sec but became progressively less evident and disappeared after 2 hr. Such behavior was attributed to a progressive increase of free 99mTc, which, in whole blood, amounted to 4% at 20 sec and 25% at 2 hr after injection. CONCLUSION: Because it is a 96% whole blood marker early after intravenous administration, 99mTc is a reliable agent for first-pass studies of whole blood circulation in the brain

Non-invasive quantification of liver perfusion with dynamic computed tomography and a dual-input one-compartmental model.

Various liver diseases lead to significant alterations of the hepatic microcirculation. Therefore, quantification of hepatic perfusion has the potential to improve the assessment and management of liver diseases. Most methods used to quantify liver perfusion are invasive or controversial. This paper describes and validates a non-invasive method for the quantification of liver perfusion using computed tomography (CT). Dynamic single-section CT of the liver was performed after intravenous bolus administration of a low-molecular-mass iodinated contrast agent. Hepatic, aortic and portal-venous time-density curves were fitted with a dual-input one-compartmental model to calculate liver perfusion. Validation studies consisted of simultaneous measurements of hepatic perfusion with CT and with radiolabelled microspheres in rabbits at rest and after adenosine infusion. The feasibility and reproducibility of the CT method in humans was assessed by three observers in 10 patients without liver disease. In rabbits, significant correlations were observed between perfusion measurements obtained with CT and with microspheres (r=0.92 for total liver perfusion, r=0.81 for arterial perfusion and r=0.85 for portal perfusion). In patients, total liver plasma perfusion measured with CT was 112+/-28 ml.min(-1).100 ml(-1), arterial plasma perfusion was 18+/-12 ml.min(-1).100 ml(-1) and portal plasma perfusion was 93+/-31 ml.min(-1).100 ml(-1). The measurements obtained by the three observers were not significantly different from each other (P>0.1). Our results indicate that dynamic CT combined with a dual-input one-compartmental model provides a valid and reliable method for the non-invasive quantification of perfusion in the normal liver