An animal PET scanner using flat-panel position-sensitive PMTs.

OBJECTIVE: To design, build, and evaluate an animal PET scanner, which can be used with non-human primates under conscious condition, incorporating flat-panel position-sensitive photomultiplier tubes (PS-PMTs). METHODS: The system contains 30 detector modules, each having two PS-PMTs and 16×18 lutetium­yttrium oxyortho-silicate scintillation crystal arrays. The system has 17,280 crystals (480 per ring) arranged in 36 rings, with a diameter of 508 mm and axial extent of 108 mm. The gantry tilt mechanism enables PET studies to be performed on a monkey in the sitting position. Data can be acquired in either the 2D or 3D mode, with the slice collimators being retracted in the 3D mode. RESULTS: At the center of the field-of-view, radial resolution is 2.7 mm full width at half maximum (FWHM) and tangential resolution is 2.4 mm FWHM, while axial resolution is 2.5 mm FWHM for direct slices and 2.7 mm FWHM for cross slices. Scatter fraction, count rate capability, and sensitivity were evaluated using a cylindrical phantom 10 cm in diameter. The noise equivalent count rate in the 3D mode is equivalent to that in the 2D mode at a three times higher radioactivity level. Total system sensitivity is 1.3 kcps/(kBq/mL) in 2D mode and 7.4 kcps/(kBq/mL) in the 3D mode. Animal studies with a monkey were performed to evaluate the imaging capabilities of the scanner. CONCLUSION: The new PET scanner will be a useful research tool with non-human primates for pre-clinical drug development.

A new method for preventing pulse pileup in scintillation detectors.

A new method for preventing pulse pileup in scintillation detectors is proposed. In the new method (G-INT), the energy of an event is calculated from the 'gated integral' of the pulse signal and the period of integration. The period of integration is not fixed but is shortened by the arrival of the succeeding pulse so as to avoid post-pulse pileup. The effect of pre-pulse pileup is corrected by subtracting the remnant energy of the preceding pulses, which is calculated from the gated integral of the preceding pulse. To avoid error due to short pulse intervals, pre- and post-pulse deadtimes are imposed. The method is similar to Wong's method (W-SUM) that depicts the energy by the 'weighted sum' of the current signal and the integrated signal. The performance of G-INT has been studied by Monte Carlo simulation in comparison with W-SUM, the variable sampling-time technique and simple delay-line clipping. It is shown that G-INT provides the smallest degradation in pulse height resolution for a given count rate capability. The difference between G-INT and W-SUM is explained by the difference in the amount of statistical noise involved in the gated integral and in the weighted sum.