A high resolution and high detection efficiency depth-encoding detector for brain positron emission tomography based on a 0.75 mm pitch scintillator array

Abstract

The quantitative accuracy and precision of brain positron emission tomography (PET) studies can be considerably improved using dedicated brain PET scanners with a uniform high resolution and a high sensitivity across the brain volume. One approach to building such a system is to construct the PET scanner using depth-of-interaction (DOI) encoding detectors with finely segmented and thick crystal arrays. In this paper, the performance of a DOI PET detector based on two 16 × 16 arrays of 2 × 2 mm² SiPMs coupled to both ends of a 44 × 44 array of 0.69 × 0.69 × 30 mm³ polished LYSO crystals was evaluated at different temperatures (-9°C, 0°C, 10°C, and 20°C) for brain PET applications. The pitch size of the LYSO array is 0.75 mm. The flood histograms show that all the crystal elements in the LYSO array can be resolved except some edge crystals, due to the limited light sharing. The average energy resolution, average DOI resolution, and average timing resolution across crystal elements are 21.1 ± 3.0%, 3.47 ± 0.17 mm, and 1.38 ± 0.09 ns, respectively, which were obtained at a bias voltage of 56.5 V and a temperature of 0°C.

Keywords: Detector design and construction technologies and materials; Gamma camera; Gamma detectors (scintillators, CZT, HPGe, HgI etc); PET PET/CT; SPECT; coronary CT angiography (CTA).

Figure 1.

(left) photograph of a 16 × 16 SiPM array, composed of four individual 8 × 8 SiPM arrays, and mounted to PCB and (right) schematic of the SiPM array.

Figure 2.

Photographs of (left) the 44 × 44 LYSO crystal array and (right) the assembled detector module with readout from both sides of the array.

Figure 3.

Distribution of the bias voltages of all the 512 SiPMs needed to obtain a gain of 1.7 × 10⁶ at a temperature of 25° C. The maximum difference of the bias voltages is only 0.13 V and the standard deviation is 0.0297 V. Data was provided by Hamamatsu.

Figure 4.

Schematics of the position encoding circuit. The 16 inputs (In1-16) are the 16 amplified row signals or the 16 amplified column signals. The two outputs (Pos⁺ and Pos⁻) are X⁺ and X⁻, or Y⁺ and Y⁻. All the values of the resistors are in Ohms.

Figure 5.

Photograph of the signal conditioning boards, which were used to amplify the row/column signals and to implement the position encoding circuit shown in figure 4. Two identical boards were stacked together to handle the 64 row/column signals from the DOI detector, and each board can process the 32 row/column signals from one 16 × 16 SiPM array.

Figure 6.

Schematic diagram of the experimental setup for flood histogram measurements. During the DOI resolution measurements, the dual-ended readout detector was irradiated from one side of the DOI detector (section 2.3.3).

Figure 7.

Flood histogram obtained using the data for the DOI resolution measurement.

Figure 8.

Schematic diagram of the experimental setup for timing resolution measurements.

Figure 9.

Flood histogram obtained at temperatures of (from left to right) −9, 0, 10 and 20°C.

Figure 10.

(top) position profiles of the 22^nd crystal row obtained at temperatures of −9 and 20°C. (bottom) flood histogram quality versus bias voltage and temperature. The error bars in the bottom figure are the standard deviation of the flood histogram qualities over all crystals [21].

Figure 11.

(left) energy resolution and (right) 511 keV photopeak position for each LYSO crystal in the LYSO array, which were obtained at a bias voltage of 56.5 V and a temperature of 0°C.

Figure 12.

Average energy resolution (in %) versus temperature. The energy resolutions were obtained at the optimal bias voltage determined by the flood histogram (table 1). The error bars are the standard deviation of the energy resolutions of all crystals.

Figure 13.

Average DOI resolution (in mm) versus temperature. The DOI resolutions were obtained at the optimal bias voltage determined by the flood histogram (table 1). The error bars are the standard deviation of the DOI resolution over the 15 × 15 crystals selected.

Figure 14.

Average CTR (ns) versus temperature. The CTRs were obtained at the optimal bias voltage determined by the flood histogram (table 1). The error bars are the standard deviation of the CTRs of all crystals.