Studies on the kinetics of Tc-99m-D,L-hexamethylpropylene amine oxime (Tc-99m-HMPAO) in adults have shown that it is not an ideal tracer of CBF because it underestimates high-flow areas. Knowledge of the kinetics of the tracer is important in evaluating the studies. The kinetics of Tc-99m-HMPAO in infants may be different from that in adults, therefore, we examined the cerebral uptake and retention of Tc-99m-HMPAO in neonates and estimated the degree of brain-to-blood back diffusion by comparing corresponding Xe-133 flow images and Tc-99m-HMPAO distribution images. In addition, we measured the urinary excretion of Tc-99m-HMPAO. Regional CBF was measured using a mobile brain-dedicated, fast-rotating, four-head multidetector system specially designed for neonatal studies. Tracers were Tc-99m-HMPAO (4 MBq/kg) and Xe-133 (500 MBq/kg). Cerebral uptake and leak-out of Tc-99m-HMPAO were measured by a single scintillation crystal placed over the frontoparietal part of the infant's head. The cerebral retention of Tc-99m-HMPAO was analyzed in 50 infants. The mean gestational age and birth weight (95% confidence interval) were 34.4 weeks (32.2-35.7) and 2,326 g (1,954-2,995), respectively. The cerebral uptake of Tc-99m-HMPAO was examined in 16 of the 50 infants, and activity during 24 h was monitored in five. In 11 infants, corresponding Xe-133 studies were performed. Urinary excretion was studied in 12 infants. The maximal activity in the brain was reached 90s after i.v. injection and was 104% (98-111) of the stable level. which was reached approximately 3 min after the injection. The decay corrected leakout of the tracer during the following 24 h was 1.0% (0.4-1.5) per hour. The cerebral retention was calculated at 6.8% (6.1-7.6), highest in the group of ictal studies and lowest in premature infants with intracranial hemorrhage. The mean value of the fixation/clearance ratio oc was estimated at 3.4 (2.8-4.4). The mean urinary excretion over 24 h was 19.5% (11.4-27.7) and was significantly related to renal function as indicated by serum urea (p = 0.02 r(2) = 0.55). A four-compartment model describing the kinetics of Tc-99m-HMPAO is shown to be valid in neonates. The cerebral retention of the tracer is higher in neonates because of higher extraction and lower initial back diffusion from brain to blood. In linearizing Tc-99m-HMPAO distribution images, a smaller correction is necessary, and we propose a value of the correction factor of 3.4. In this way, Tc-99m-HMPAO is a more reliable tracer of the distribution of CBF in neonates compared with adults. The urinary excretion is significantly reduced compared with adults, and the radiation dose to the bladder wall is reduced, The effective dose is 0.3 mSv/MBq/kg.
