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. 2010 Dec 7;55(23):7149-74.
doi: 10.1088/0031-9155/55/23/001. Epub 2010 Nov 16.

Analog signal multiplexing for PSAPD-based PET detectors: simulation and experimental validation

Frances W Y Lau  1 , Arne Vandenbroucke, Paul D Reynolds, Peter D Olcott, Mark A Horowitz, Craig S Levin
Affiliations

Analog signal multiplexing for PSAPD-based PET detectors: simulation and experimental validation

Frances W Y Lau et al. Phys Med Biol. .

Abstract

A 1 mm(3) resolution clinical positron emission tomography (PET) system employing 4608 position-sensitive avalanche photodiodes (PSAPDs) is under development. This paper describes a detector multiplexing technique that simplifies the readout electronics and reduces the density of the circuit board design. The multiplexing scheme was validated using a simulation framework that models the PSAPDs and front-end multiplexing circuits to predict the signal-to-noise ratio and flood histogram performance. Two independent experimental setups measured the energy resolution, time resolution, crystal identification ability and count rate both with and without multiplexing. With multiplexing, there was no significant degradation in energy resolution, time resolution and count rate. There was a relative 6.9 ± 1.0% and 9.4 ± 1.0% degradation in the figure of merit that characterizes the crystal identification ability observed in the measured and simulated ceramic-mounted PSAPD module flood histograms, respectively.

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Figures

Figure 1
Figure 1
Left: breast-dedicated PET system with 9 × 16 cm2 detector heads and data acquisition electronics. Middle: a potential breast imaging orientation for a patient standing/sitting upright. Right: another potential breast imaging orientation for a patient standing/sitting upright (top view).
Figure 2
Figure 2
Depiction of how the dual-PSAPD modules are stacked. (a) Depiction of one dual-PSAPD module, which consists of two scintillation crystal arrays and two PSAPDs configured two units deep with respect to incoming 511 keV photons, mounted directly on a flex circuit. (b) A stack of dual-PSAPD modules with 511 keV photons entering edge-on. The design enables 1 mm3 intrinsic resolution, directly measured 1 mm DOI resolution, the capability to position the 3D interaction coordinates of multiple interaction photon events, and >90% scintillation light collection efficiency, independent of interaction location.
Figure 3
Figure 3
Circuit configurations without and with multiplexing. For simplicity, the dynamic range scaling capacitors (described in section 2.2) and bias resistors are omitted and only the DC blocking capacitor and preamplifier for the spatial channels are shown. The configuration for the common will be discussed in section 2.2. (a) No multiplexing. (b) With multiplexing.
Figure 4
Figure 4
Pictures of un-multiplexed and multiplexed modules. (a) Un-multiplexed dual-PSAPD module. There are a total of ten terminals (two high voltage and eight low voltage terminals). The scintillation crystal arrays will be placed on top of the PSAPDs. The alumina frame provides mechanical support and facilitates thermal regulation. (b) Multiplexed dual-PSAPD module. For this new design, there are a total of six terminals (two high voltage and four low voltage terminals), compared to ten terminals in the un-multiplexed design. The scintillation crystal arrays were already placed on top of the PSAPDs in this picture, and their back sides are showing.
Figure 5
Figure 5
Dynamic range scaling scheme for the common signal. Cc_large and Cc_small form a capacitive divider. The common terminal of the PSAPD has a DC value of − 1750 V, which is blocked by the 1000 pF capacitor. The 1 MΩ resistor is necessary to ensure the DC value of node X remains at 0 V so that Cc_small and Cc_large do not need to be rated for high voltage.
Figure 6
Figure 6
Dynamic range scaling scheme for the spatial channels. Catten and the DC blocking capacitor form a capacitive divider.
Figure 7
Figure 7
Schematic of a PSAPD.
Figure 8
Figure 8
One subunit of the FEM circuit model for PSAPD. Cdetector is the PSAPD capacitance, isig is the photodetection signal and inoise is the combination of all the noise sources. Rresistive_sheet models the high resistivity layer on the n-doped side of the PSAPD that is connected to the spatial channels. Rlow_resistivity models the low resistivity layer that is connected to the common. This FEM section is replicated 100 times.
Figure 9
Figure 9
Face-on versus edge-on configurations. In the face-on configuration, the Na-22 point source is placed above the detector. In the edge-on configuration, the Na-22 source irradiates the edge of the detector. (a) Face-on. (b) Edge-on.
Figure 10
Figure 10
Simulation flow chart for creating a simulated ‘flood histogram’, used to evaluate the crystal identification ability. An example simulated crystal flood histogram is shown at the bottom along with a profile through one crystal row.
Figure 11
Figure 11
Ceramic-mounted PSAPDs.
Figure 12
Figure 12
Schematic of experimental setup for face-on measurements. Coincidence is between a dual-PSAPD module and a LYSO array coupled to a PMT. The space between the edges of the LYSO arrays was 3 cm.
Figure 13
Figure 13
Schematic of experimental setup for ‘edge-on’ measurements. Coincidence is between two dual-PSAPD modules. The distance between the edge of the LYSO arrays was 6 cm.
Figure 14
Figure 14
Simulation results, assuming face-on irradiation of 8 × 8 arrays of 1 mm3 LSO crystals coupled to the PSAPDs. (a) Flood histograms, (b) profile through the fourth row of the flood, (c) figure of merit (FOM) for each row of the flood.
Figure 15
Figure 15
Simulated data: average distance between peaks and average width (FWHM) of the middle peaks for each row in the flood histogram in figure 14, comparing the un-multiplexed and multiplexed results. The values for the distance and width are relative to the grid used for the flood histogram which has a maximum range spanning from −1 to 1.
Figure 16
Figure 16
Overall energy spectrum obtained from the common channel of the ceramic-mounted LSO-PSAPD detectors, produced after calibrating out per-crystal gain differences. The RENA-3 ASIC was used for readout. (a) Overall energy spectrum for the un-multiplexed case. Overall energy resolution is 14.6 % ± 0.8% FWHM for the 511 keV photo-peak. (b) Overall energy spectrum for the multiplexed case. Overall energy resolution is 14.4% ± 0.8% FWHM for the photo-peak.
Figure 17
Figure 17
Experimental results for 8 × 8 arrays of 1 mm3 LSO crystals coupled to the ceramic-mounted PSAPDs read out by the RENA-3 ASIC for events in the photo-peak (i.e. between 440 and 590 keV), with face-on irradiation. (a) Flood histograms. (b) Profile through the fourth row of the flood. The total acquired number of counts that hit the PSAPD of interest was less for the multiplexed case, so those peaks are lower. (c) Figure of merit (FOM) for each row of the flood. For all figures, the overall energy spectra from figure 16 was used to gate for events in the photo-peak.
Figure 18
Figure 18
Ceramic-mounted LSO-PSAPD detector experimental data: average distance between peaks and average width (FWHM) of the middle peaks for each row in the flood histogram. The values for the distance and width are relative to the grid used for the flood histogram which has a maximum range spanning from − 1 to 1.
Figure 19
Figure 19
Flood histograms for ceramic-mounted LSO-PSAPD detectors read out with RENA-3 and FoM with and without multiplexing for events in two energy windows outside the photo-peak.
Figure 20
Figure 20
Coincidence time resolution for events in the photo-peak (i.e. events with energy ~440–590 keV) using the RENA-3 ASIC (leading edge discriminator). (a) Un-multiplexed. (b) Multiplexed.
Figure 21
Figure 21
Flood histograms and FoM for dual-PSAPD modules with flex-mounted detectors read out with NIM electronics with face-on irradiation. Gated for events within three standard deviations of the 511 keV photo-peak.
Figure 22
Figure 22
Dual-PSAPD module experimental data: average distance between peaks and average width (FWHM) of the middle peaks for each row in the flood histogram. As explained in section 5, in addition to the un-multiplexed and multiplexed cases, there is the additional case of multiplexed with a shorter 100 ns shaping time (versus 500 ns in the regular multiplexed case). The values for the distance and width are relative to the grid used for the flood histogram which has a maximum range spanning from −1 to 1.
Figure 23
Figure 23
FoM for multiplexed flex-mounted PSAPDs with a shorter 100 ns shaping time (versus 500 ns in figure 21) under face-on irradiation. Gated for events within three standard deviations of the 511 keV photo-peak.
Figure 24
Figure 24
Backprojected lines of response (LORs) for two flex-mounted modules in coincidence in an edge-on configuration with a point source in the center.
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