A technique for a rapid imaging of regional CBF and partition coefficient using dynamic SPECT and N-isopropyl-p-[123I]iodoamphetamine (123I-IMP)

IMP (iodoamphetamine) is a flow tracer due to a large first pass extraction fraction and high affinity in the brain, but significant clearance from the brain causes change of distribution when the beginning time of scan is delayed. The purpose of the present study was to develop a new method to rapidly calculate a quantitative cerebral blood flow (CBF) image by taking clearance effects into account. A dynamic SPECT scan was performed on 5 subjects (4 patients with cerebral infarction and one normal volunteer) following slow intravenous infusion of 123I-IMP. The arterial input function was obtained by frequent blood sampling and by measuring an octanol extraction ratio for each sample. Firstly, non-linear least square fitting (NLS) was performed to investigate the tracer kinetics of 123I-IMP. The 3 compartment model analysis yielded negligibly small k3 (retaining rate constant) (0.0056±0.0128 (ml/ml/min)), and consistent k1 (transport rate constant) with those determined by 2 compartment model (2CM) analysis (r=0.96, p<0.001). In addition, k1 was consistent with CBF measured by 15O water PET technique. These observations suggested the validity of using 2CM for describing the IMP tracer kinetics. Secondly, a weighted integration (WI) technique has been implemented to calculate rapidly images of CBF and partition coefficient (Vd). The WI technique yielded values of CBF (k1) and Vd (k1/k2). They were confirmed to be consistent with those determined by NLS technique (CBF; r=0.99, p<0.001, Vd; r=0.99, p<0.001), and calculated k1 agreed well with PET CBF (r=0.91, p<0.001). We observed changed Vd in infarcted patients. This supports an importance for calculating Vd image. Vd image will provide additional clinical information because 123I-IMP binding mechanism may be related to cell viability. (author)