FPGA-Based Pulse Parameter Discovery for Positron Emission Tomography

Abstract

Modern Field Programmable Gate Arrays (FPGAs) are capable of performing complex digital signal processing algorithms with clock rates well above 100MHz. This, combined with FPGA's low expense and ease of use make them an ideal technology for a data acquisition system for a positron emission tomography (PET) scanner. The University of Washington is producing a series of high-resolution, small-animal PET scanners that utilize FPGAs as the core of the front-end electronics. For these next generation scanners, functions that are typically performed in dedicated circuits, or offline, are being migrated to the FPGA. This will not only simplify the electronics, but the features of modern FPGAs can be utilizes to add significant signal processing power to produce higher resolution images. In this paper we report how we utilize the reconfigurable property of an FPGA to self-calibrate itself to determine pulse parameters necessary for some of the pulse processing steps. Specifically, we show how the FPGA can generate a reference pulse based on actual pulse data instead of a model. We also report how other properties of the photodetector pulse (baseline, pulse length, average pulse energy and event triggers) can be determined automatically by the FPGA.

Fig. 1

Illustration of the timing jitter introduced by time walk when two pulsed have varying amplitudes.

Fig. 2

Typical data acquisition chain for a PET scanner.

Fig. 3

Block diagram of the overall reference pulse discovery algorithm.

Fig. 4

Illustration of a baseline shift greater than the baseline window.

Fig. 5

Illustration of the range of possible sampling of photodetector pulses.

Fig. 6

Histogram of the distribution of pulse samples over one ADC interval (0-255) for the higher resolution pulse.

Fig. 7

Matlab plot of 12ns of the leading edge of the derived reference pulse (a) before smoothing and (b) after smoothing.

Fig. 8

Matlab plot of two reference pulses. The green pulse is derived from our algorithm while the blue pulse is derived from 25GHz oscilloscope data.

Fig. 9

Result of coincidental timing for the hand-tuned reference pulse derived from 25GHz oscilloscope data (grey) and a reference pulse from out automated algorithm (blue).