A Microvolumetric β Blood Counter for Pharmacokinetic PET Studies in Small Animals

Radiotracer kinetic modeling in small animals with Positron Emission Tomography (PET) requires the determination of the blood tracer concentration as a function of time. A continuous blood counting system was designed to measure the input function from rats and mice in real time. The system consists of a flow-through beta counter made of silicon PIN photodiodes and a mul syringe pump. The latter draws blood continuously from an implanted venous or arterial catheter, at a user selected rate. The direct beta detection by photodiodes minimizes the shield footprint next to the animal and reduces the counter sensitivity to ambient gamma radiation. The device is entirely remote controlled for sampling protocol selection, tuning, and real time monitoring of measured parameters. It can be hooked to a computer or fully integrated with the LabPETtrade scanner for blood counting during dynamic PET imaging experiments. The counter sensitivity to the most popular PET radioisotopes (¹⁸F, ⁶⁴Cu, ¹³N, ¹¹ C) ranges from 7.1 to 46.8 Bq/mul. Its linearity is better than 98% up to 46 kBq/mul for ¹⁸F, and a 1.4 s dispersion constant was measured at a rate of 250 mul/min with rat whole blood in a PE10 catheter. Due to its optimized mechanical design and compact shielding, the counter sensitivity to radioactive background is only 5 counts per second (cps) for a 37 MBq ¹⁸F source 10 cm away from the detector. Accurate time-activity curves have been obtained from rats and mice in dynamic PET imaging studies