Investigation of a new positron emission tomography detector using bismuth germanate, avalanche photodiodes and low noise, wide band CMOS preamplifiers

A new Positron Emission Tomography (PET) detector is investigated which replaces conventional photomultiplier tubes (PMTs) with avalanche photodiodes (APDs). As a part of the investigation a low-noise, wide-band CMOS preamplifier array is designed and tested with APD based detectors.

The fundamental theory of APD operation is first reviewed and the governing equations are derived. The frequency dependent behavior of the APD is calculated and a new treatment of the frequency response of the undepleted p-region based on the classic model for graded base transistors is presented.

A modeling methodology is developed to simulate APD performance based on the device fabrication procedure. Using this methodology, the performance characteristics of the beveled-edge and reach-through APD structures are compared to determine which device is most appropriate for use in a PET detector. The model results indicate the beveled-edge APD has superior characteristics for PET due to its lower excess-noise factor and lower dark current. Commercially available beveled-edge and reach-through APDs are experimentally evaluated to confirm the model predictions.

A new wide-band, low-noise CMOS preamplifier topology is developed for use with APD based PET detectors. A 6-element prototype preamplifier array is designed, fabricated and evaluated in a commercial 2 µm CMOS fabrication line. The preamplifiers in the array have rise-times of 7 ns and an equivalent input noise voltages of 1.1 nV/rt-Hz with an input load capacitance of 6 pF due to the package. When coupled to a 5 mm diameter beveled-edge APD and 3 x 3 x 30 mm³; BGO crystal, the amplifier and detector have a measured pulse height resolution of 14% (511 keV) and a measured timing resolution of 9.1 ns FWHΜ.