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) fl ow-through radioactivity detector, designed for use in PET studies of cerebral blood flow. Methods: The Pico-Count detects the two 511-keV p ositron annihilation photons with two bismuth germanate detectors oper ating 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 sam pling 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 t he computer and can vary throughout the study. Approximately 22 cm of arterial tubing (which contains 0.11 ml of blood) is looped between th e detectors. Typically, blood is withdrawn with a syringe pump at a ra te of 2.75 ml/min, which corresponds to a flow rate in the tubing of 9 .2 cm/sec.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 obse rving the response to an input step function and is well-modeled as a monoexponential. Results: The sensitivity is 270 Hz/(mu Ci/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 appro ximately 2100 Hz, with the background being less than 0.2%, The disper sion time constant is 1.3 sec, and the delay between radioactivity pre sent 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 bl ood time radioactivity curves is far less than the corresponding cutof f for the dispersion function, Conclusion: The Pico-Count is an excell ent detector for continuously monitoring positron radioactivity in blo od. Depending on the application, dispersion correction for the detect ion apparatus may not be needed.