Design of a microvolumetric blood counter/sampler for metabolic PET imaging in rats and mice

Quantitative metabolic imaging in small animals with positron emission tomography (PET) requires the determination of the tracer concentration in whole blood, arterial plasma and metabolites as a function of time. A blood counting and sampling system was designed to simultaneously measure the time-activity curve as microvolumes of blood are collected. The system consists of a flow-through counter made of a plastic scintillator to detect positrons and of a computer-controlled blood sampler based on the concept of bubble segmentation. The number and size of samples, the withdrawal speed and the sampling time are all programmable and can be modified on-line. Samples as small as 10 μl can be repetitively obtained from an implanted arterial catheter in the femoral vein or artery of small rats (150 g) or the jugular vein of mice (20 g). For medium sampling speed (100 μl/min) at a constant rate, the standard deviation of the sample activity is typically less than 4%. By cutting the tubing at the bubbles at the end of the experiment, samples are made available for further processing and biochemical analysis. This apparatus has become an essential tool for quantitative animal PET studies, allowing easy, reliable sampling at a low cost