RETENTION OF TC-99M-BICISATE IN THE HUMAN BRAIN AFTER INTRACAROTID INJECTION

Tc-99m-bicisate (ECD) was injected as a bolus into the internal caroti d artery, and cerebral uptake and retention were recorded with fast-ro tating single photon emission computed tomography (SPECT) equipment in four patients suffering from temporal lobe epilepsy. Quantitative reg ional cerebral blood flow (rCBF) was measured tomographically with the Xe-133 inhalation technique. We applied a three-compartment kinetic m odel and algorithms modified from a previous analysis of Tc-99m d,l-he xamethylpropyleneamine oxide (HM-PAO) kinetics. The bicisate brain upt ake and retention curve was very similar to that of HM-PAO, and it can be described by a triexponential function including an initial steep component representing the vascular transmitted spike, a second less s teep component representing backdiffusion from brain tissue to blood, and a third, very slow component, representing the very slow loss due to incomplete retention of the deesterified hydrophilic metabolites. C omputerized curve-fitting on data from three patients gave average kin etic values for the first-passage (unilateral) extraction of E = 0.60 (range, 0.59-0.61); the overall retained fraction of the tracer suppli ed was R = 0.44 (0.43-0.45), and the conversion/clearance ratio was al pha = k(3)/k(2) = 2.59 (2.38-2.77). This alpha is higher than that for HM-PAO, and therefore bicisate uptake as a function of blood flow is more linear than in HM-PAO. Less correction for backdiffusion is there fore needed. From 1 to 24 h there was an average loss of hydrophilic t racer of 3.5%/h, but the late distribution images were essentially unc hanged over time, pointing to practically the same rate of loss in all regions.