Performance Evaluation of the Pico-Count Flow-Through Detector for Use in Cerebral Blood Flow PET Studies

This study evaluated the Pico-Count (Bioscan, Inc., Washington, DC) flow-through radioactivity detector, designed for use in PET studies of cerebral blood flow. The Pico-Count detects the two 511-keV positron annihilation photons with two bismuth germanate detectors operating in coincidence. The detectors, photomultipliers and preamps are housed within a 12 cm x 9 cm x 22 cm box, which includes 16 mm of lead shielding, to allow placement of the detector within 15 cm of the sampling site. The counting electronics are housed in a remote box, which is connected to a laptop computer for process control. The dwell time per sample and the number of samples to collect are entered through the computer and can vary throughout the study. Approximately 22 cm of arterial tubing (which contains 0.11 ml of blood) is looped between the detectors. Typically, blood is withdrawn with a syringe pump at a rate of 2.75 ml/min, which corresponds to a flow rate in the tubing of 9.2 cm/sec. Dispersion within the arterial catheter is measured by observing the response to an input step function and is well-modeled as a monoexponential. The sensitivity is 270 Hz/(microCi/ml), which corresponds to detecting 6.9% of the positron decays occurring within the detector. The peak counting rate after a 12-mCi injection is approximately 2100 Hz, with the background being less than 0.2%. The dispersion time constant is 1.3 sec, and the delay between radioactivity present at the catheter tip and that measured by the detector is 4.1 sec. The cutoff in the power spectral density of typical human arterial blood time radioactivity curves is far less than the corresponding cutoff for the dispersion function. The Pico-Count is an excellent detector for continuously monitoring positron radioactivity in blood. Depending on the application, dispersion correction for the detection apparatus may not be needed.