Figure 10

Tissue radioactivities as a function of distance (profiles) away from the CSF-brain interface for the caudate-putamen (A) and periaqueductal gray matter (B) at four times. The shapes of the curves and the concentration-distance integrals indicate two different tissue distribution dynamics. To illustrate this, we will consider only the 5 and 9 min profiles and avoid the matter of sizable tissue uptake from blood that affects the later times. For the caudate-putamen (10A), the 5 min points all lie above the 9 min ones out to 1.5 mm, where they both approach zero. The areas under these curves are 2757 (nCi/g) × mm at 5 min and 2002 (nCi/g) × mm at 9 min; clearly there was a loss of radioactivity, probably back into the CSF, over the 5–9 min period. In contrast, periaqueductal gray matter radioactivities (10B) were similar at the edge of the tissue (x = 0) at these two times but the 9 min points were higher than the 5 min ones from 0.25 to 0.75 mm, where both became essentially zero. The area under the 5 min curve was less than that under the 9 min one, 662 vs. 873 (nCi/g) × mm, respectively; periaqueductal gray matter, thus, continued to take up ¹²⁵I-IGF-1 even as concentration in aqueductal CSF was falling (Fig. 7). One explanation for this is greater binding or trapping of IGF-1 in periaqueductal gray matter than in caudate-putamen. Data are shown as mean ± SD.