FPGA-based digitizer for BGO-based time-of-flight PET

Abstract

We present a novel field-programmable gate array (FPGA)-based bismuth germanate (BGO) time-of-flight (TOF) digitizer, implemented on an FPGA (XC7VX485T-2FFG1761C, Xilinx). This digitizer is designed to address the recently highlighted characteristics of BGO, which generates both scintillation and prompt Cerenkov photons when a 511 keV photon interacts with BGO. The developed digitizer independently processes these two types of photons for precise energy and timing measurements. The digitizer incorporates a noise-resistant binary counter that measures energy signals using the time-over-threshold (TOT) method. For timing measurements, we employ an embedded dual-side monitoring time-to-digital converter, which efficiently captures timing information while maintaining low resource usage. We validated the efficacy of our FPGA-based TOF digitizer through extensive experiments, including both electrical testing and coincidence measurements using BGO pixels. Our evaluations of TOT energy and timing performance utilized two 3 × 3 × 20 mm³BGO pixels coupled to CHK-HD MT silicon photomultipliers. The digitizer achieved a coincidence timing resolution (CTR) of 407 ps full width at half maximum (FWHM) for events within the full width at tenth maximum of the photopeak in the measured TOT energy spectrum. Notably, when measured with an oscilloscope, the same detector pair exhibited a CTR of 403 ps FWHM, confirming that the performance of the developed digitizer is comparable to that of an oscilloscope. With its low resource usage, our design offers significant potential for scalability, making it particularly promising for multi-channel BGO-based PET systems.

Keywords: Cerenkov (Cherenkov); bismuth germanate (BGO); coincidence timing resolution (CTR); tapped delay line (TDL); time-of-flight positron emission tomography (TOF PET); time-over-threshold (TOT); time-to-digital converter (TDC).

Figure 1.

(a) Energy and timing signals of 3 × 3 × 20 mm³ BGO pixels coupled to CHK-HD MT SiPMs. (b) Enlarged views of the rising parts of the energy and timing signals, respectively.

Figure 2.

TOT energy measurement operation using binary counter (BC) and noise-resistant BC (NRBC).

Figure 3.

Comparison of time measurement methods: (a) conventional method focusing solely on start of propagation (SOP), and (b) proposed DSM TDC method incorporating both SOP and end of propagation (EOP).

Figure 4.

Block diagram of the proposed BGO TOF digitizer with peripheral units.

Figure 5.

Operating flowchart for both the channel controller and the main controller.

Figure 6.

Experimental setup for electrical tests and coincidence measurement with 3 × 3 × 20 mm³ BGO pixels.

Figure 7.

Average TOT values measured by the NRBC using three different user-defined period times (UPTs): 1, 2, and 4 clocks. The enlarged view highlights detailed variations in measurements.

Figure 8.

Measured time intervals using the DSM TDC. FWHM values for each measurement are indicated.

Figure 9.

Distribution of measured pulse width jitters at Gated E, caused by local spikes.

Figure 10.

Total energy spectra measured at bias voltages of 49 V (a) and 51 V (b). In (a), spectra are shown for five different VT_Energy thresholds, with Gaussian fits applied to each. The calculated energy resolutions are indicated next to the corresponding fitting results.

Figure 11.

FWHMs (a) and FWTMs (b) in CTRs for the bias and VT_Energy sweeps measured using the developed BGO TOF digitizer.

Figure 12.

Energy spectra (a) and timing spectra (b) for the oscilloscope and the BGO TOF digitizer (with BC and NRBC) measured at a SiPM bias of 49 V and VT_Energy of 250 mV. The same experimental setup was used to minimize variations due to changes in the experimental configuration.