Quantitation of small-animal (124)I activity distributions using a clinical PET/CT scanner

Time-dependent PET imaging can be an important tool in the assessment of radiotracer performance in murine models. We have performed a quantitative analysis of PET images of (124)I, acquired on a clinical PET system using a small-animal phantom. We then compared the recovered activity concentrations with the known activity concentration in the phantom spheres. The recovery coefficients found from the phantom data were applied to in vivo (124)I anti-HER2/neu C6.5 diabody PET data and compared with necropsy biodistribution data from the same tumor-bearing immunodeficient mouse.

The small-animal phantom consisted of a 4 x 8 cm water-filled acrylic cylinder with hollow spheres filled with water ranging in volume from 0.0625 to 1.0 mL and activity concentration of 27 +/- 2 kBq/mL. The background activity concentrations varied from 0 to 0.05 to 0.10 of the spheres. Data were acquired at 0, 5, and 10 cm from the scanner longitudinal axis. Recovery coefficients were theoretically calculated for spheres of different volume, background-to-target concentrations, and distance from the scanner's longitudinal axis. The theoretic recovery coefficients were applied to the maximum sphere activity concentration measured from the PET images, thus obtaining a recovered activity concentration to be compared with the known activity concentration of the spheres.

The mean recovered activity concentration for the phantom spheres was 25 +/- 2 kBq/mL. The (124)I diabody PET image of a mouse with a tumor xenograft was then analyzed using the techniques described. The tumor percentage injected dose per gram estimated from the murine PET image (4.8 +/- 0.4) compared well with those obtained from necropsy studies (5.1).

This study indicates the feasibility of performing quantitative imaging on murine (124)I antibody fragment PET images using a large-bore clinical scanner, which enables high-throughput studies to evaluate the performance of PET tracers in a timely and cost-effective manner by imaging multiple animals simultaneously. Tracers deemed promising by this screening method can then be further evaluated using traditional necropsy studies. Our group is currently conducting time-dependent (124)I diabody PET and necropsy comparative studies with larger numbers of mice.