Pulse width modulation: A novel readout scheme for high energy photon detection

In standard PET scintillation detection, the energy, timing, and location of the incoming photon are recovered using analog signal processing techniques. The energy and location information are processed using an analog-to-digital (ADC) converter that samples an analog value that is proportional to the integral of the charge created by the scintillation event. We propose to change the paradigm and modulate the width (rather than amplitude) of a digital pulse to be proportional to the integral of the charge created. The analog value of the outgoing digital pulses is recovered by using a time-to-digital converter (TDC) in the back-end electronics, without the need for an ADC. Note that in this new scenario the same TDC used to record the time of the event is used to recover the amplitude. The main performance parameter that must be optimized is the dynamic range versus the dead-time of the front-end detector. The goal is an 8-bit dynamic range for this pulse-width modulation (PWM) scheme, which is adequate for high resolution PET systems based on semiconductor detectors such as avalanche photodiodes (APD) or cadmium zinc telluride (CZT). A novel circuit has been designed, fabricated, and tested for the proposed PWM readout scheme. This circuit is different than previously developed time over threshold pulse width modulation circuits used in high energy physics. PWM techniques simplify the routing to the back end electronics without degrading the performance of the system. A readout architecture based on PWM processes digital rather than analog pulses, which can be easily multiplexed, enabling one to achieve very high channel density required for ultra-high resolution, 3-D positioning PET detector systems.