Noninvasive Estimation of Normalized Distribution Volume: Application to the Muscarinic-2 Ligand [(18)F]FP-TZTP

Reference tissue methods to estimate neuroreceptor binding are not applicable to [(18)F]FP-TZTP (a muscarinic-2 cholinergic receptor ligand), because there is no suitable receptor-free reference region. We evaluated a new method to estimate, without using arterial data or a receptor-free reference region, a receptor parameter called the normalized distribution volume, V(T)(*), using a region containing receptors as the input tissue. V(T)(*) is defined as V(T)/K'(1) (distribution volume (V(T)) normalized by K'(1) of the input region). We used a two-parameter multilinear reference tissue model (MRTM2) to generate parametric images of V(T)(*) and R(1) (R(1)=K(1)/K'(1)) from [(18)F]FP-TZTP PET data of healthy aged subjects (10 with apolipoprotein E-epsilon4 alleles (APOE-epsilon4(+)) and nine without (APOE-epsilon4(-)). V(T)(*) and V(T) were normalized by plasma-free fraction, f(P). By one-tissue kinetic analysis (1TKA) with metabolite-corrected plasma data, V(T) was previously reported as higher in the APOE-epsilon4(+) group. The noise magnitude of MRTM2 V(T)(*) and R(1) images were nearly identical to those of 1TKA V(T) and K(1) images. K'(1) or f(P) was not different between the two groups. V(T)(*) (mins) (1,659+/-497) and V(T) (mL/cm(3)) (701+/-99) in APOE-epsilon4(+) were higher by 38 and 22% than those (1,209+/-233 and 577+/-112) in APOE-epsilon4(-), respectively. The statistical significance for V(T)(*) (0.041) was lower than that for V(T) (0.025), due to the higher intersubject variability of V(T)(*) (25%) than that of V(T) (17%). We conclude that MRTM2 V(T)(*) allows detection of group differences in receptor binding without arterial blood or a receptor-free reference region.