CAN THE MICROSPHERE MODEL BE APPLIED TO CEREBRAL BLOOD-FLOW MEASUREMENT USING N-ISOPROPYL-P-[I-123]IODOAMPHETAMINE WITH SPET REGARDLESS OF WASHOUT FROM BRAIN-TISSUE

The microsphere model and the continuous withdrawal of arterial blood have commonly been used in clinical studies when measuring regional ce rebral blood flow (rCBF) by N-isopropyl-p-[I-123]iodoamphetamine (IMP) single photon emission tomography. The method is considered to undere stimate rCBF because of the washout of tracer from brain tissue; howev er, the extent of this underestimation is not known. To assess whether this underestimation can be determined quantitatively, we performed s imulation analysis based on the microsphere model and the two-compartm ent model [influx, K-1(rCBF); and outflux, k(2)(washout)] using the ti me-activity curves of I-123-IMP in arterial blood [C-a(t)] of 10 subje cts. With the microsphere method, rCBF values fell as time post-inject ion increased. The extent of underestimation of rCBF was 4.7 +/- 0.28% (mean +/- S.D.) at 5 min, 10.2 +/- 0.42% at 10 min and 15.2 +/- 0.55% at 15 min. There was little variation in the extent of underestimatio n and it was not dependent on the C-a(t) of the subjects. We therefore considered the results to be generally applicable to various studies of the microsphere model. As 4.7% is considered to be negligible in cl inical studies, we conclude that the microsphere model can be applied to obtain accurate measurement of rCBF up to 5 min regardless of washo ut.