Evaluation of monolithic crystal detector with dual-ended readout utilizing multiplexing method

Objective.Monolithic crystal detectors are increasingly being applied in positron emission tomography (PET) devices owing to their excellent depth-of-interaction (DOI) resolution capabilities and high detection efficiency. In this study, we constructed and evaluated a dual-ended readout monolithic crystal detector based on a multiplexing method.Approach.We employed two 12 × 12 silicon photomultiplier (SiPM) arrays for readout, and the signals from the 12 × 12 array were merged into 12 X and 12 Y channels using channel multiplexing. In 2D reconstruction, three methods based on the centre of gravity (COG) were compared, and the concept of thresholds was introduced. Furthermore, a light convolutional neural network (CNN) was employed for testing. To enhance depth localization resolution, we proposed a method by utilizing the mutual information from both ends of the SiPMs. The source width and collimation effect were simulated using GEANT4, and the intrinsic spatial resolution was separated from the measured values.Main results.At an operational voltage of 29 V for the SiPM, an energy resolution of approximately 12.5 % was achieved. By subtracting a 0.8 % threshold from the total energy in every channel, a 2D spatial resolution of approximately 0.90 mm full width at half maximum (FWHM) can be obtained. Furthermore, a higher level of resolution, approximately 0.80 mm FWHM, was achieved using a CNN, with some alleviation of edge effects. With the proposed DOI method, a significant 1.36 mm FWHM average DOI resolution can be achieved. Additionally, it was found that polishing and black coating on the crystal surface yielded smaller edge effects compared to a rough surface with a black coating.Significance.The introduction of a threshold in COG method and a dual-ended readout scheme can lead to excellent spatial resolution for monolithic crystal detectors, which can help to develop PET systems with both high sensitivity and high spatial resolution.

Effect of depth of interaction resolution on the spatial resolution of SIAT aPET

Objective.Spatial resolution is a crucial parameter for a positron emission tomography (PET) scanner. The spatial resolution of a high-resolution small animal PET scanner is significantly influenced by the effect of depth of interaction (DOI) uncertainty. The aim of this work is to investigate the impact of DOI resolution on the spatial resolution of a small animal PET scanner called SIAT aPET and determine the required DOI resolution to achieve nearly uniform spatial resolution within the field of view (FOV).Approach. The SIAT aPET detectors utilize 1.0 × 1.0 × 20 mm3crystals, with an average DOI resolution of ∼2 mm. A default number of 16 DOI bins are used during data acquisition. First, a Na-22 point source was scanned in the center of the axial FOV with different radial offsets. Then, a Derenzo phantom was scanned at radial offsets of 0 and 15 mm in the center axial FOV. The measured DOI information was rebinned to 1, 2, 4 and 8 DOI bins to mimic different DOI resolutions of the detectors during image reconstruction.Main results. Significant artifacts were observed in images obtained from both the point source and Derenzo phantom when using only one DOI bin. When accurate measurement of DOI is not achieved, degradation in spatial resolution is more pronounced in the radial direction compared to tangential and axial directions for large radial offsets. The radial spatial resolutions at a 30 mm radial offset are 5.05, 2.62, 1.24, 0.86 and 0.78 mm when using 1, 2, 4, 8, or 16 DOI bins, respectively. The axial spatial resolution improved from ∼1.3 to 0.7 mm as the number of DOI bins increased from 1 to 16 at radial offsets from 0 to 25 mm. Two DOI bins are required to obtain images without significant artifacts. The required DOI resolution is about three times the crystal width of SIAT aPET to achieve a uniform submillimeter spatial resolution within the central 60 mm FOV and resolve the 1 mm rods of the Derenzo phantom at both positions.

Optical Simulation and Experimental Assessment with Time-Walk Correction of TOF-PET Detectors with Multi-Ended Readouts

As a commonly used solution, the multi-ended readout can measure the depth-of-interaction (DOI) for positron emission tomography (PET) detectors. In the present study, the effects of the multi-ended readout design were investigated using the leading-edge discriminator (LED) triggers on the timing performance of time-of-flight (TOF) PET detectors. At the very first, the photon transmission model of the four detectors, namely, single-ended readout, dual-ended readout, side dual-ended readout, and triple-ended readout, was established in Tracepro. The optical simulation revealed that the light output of the multi-ended readout was higher. Meanwhile, the readout circuit could be triggered earlier. Especially, in the triple-ended readout, the light output at 0.5 ns was observed to be nearly twice that of the single-ended readout after the first scintillating photon was generated. Subsequently, a reference detector was applied to test the multi-ended readout detectors that were constructed from a 6 × 6 × 25 mm³ LYSO crystal. Each module is composed of a crystal coupled with multiple SiPMs. Accordingly, its timing performance was improved by approximately 10% after the compensation of fourth-order polynomial fitting. Finally, the compensated full-width-at-half-maximum (FWHM) coincidence timing resolutions (CTR) of the dual-ended readout, side dual-ended readout, and triple-ended readout were 216.9 ps, 231.0 ps, and 203.6 ps, respectively.

Evaluation of dual-ended readout GAGG-based DOI-PET detectors with different surface treatments

PURPOSE

Parallax error is a major issue in small animal positron emission tomography (PET) scanners which are used in preclinical studies or detailed scanning of human organs. Several methods have been proposed and investigated to reduce this radial artifact in PET images by estimating the depth of interaction (DOI) of 511-keV photons in the crystal. Among all, the dual-ended readout method seems to be very simple and effective as it does not have any fabrication and readout complications. In the past, some studies suggested that increasing the roughness of crystal lateral surfaces improves DOI resolution. In this paper, this was experimentally examined for four Ce:GAGG crystals with different surface structures.

METHODS

Four 1.2 × 1.2 × 20 mm³ GAGG crystals with following surface treatment were examined: polished with optical finishing, fine grinding (using a fine surface grinding machine), fine cutting (no treatment), and coarse cutting (no treatment). These crystals were coupled individually to two SiPMs for dual-ended readout and placed in a coincidence detection circuit for electronic collimation of 511 keV incidents. The crystals were compared in terms of energy response and DOI estimation capability.

RESULTS

DOI function for each crystal was extracted and FWHM DOI resolution was calculated. DOI resolution for the polished crystal varied in the range of 0.54-4.14 mm throughout the length of the crystal due to its nonlinear DOI function. The fine grinding crystal showed a linear DOI function within the dynamic range of (-0.75, 0.75), and its DOI resolution varied in the range of 1.24-1.50 mm (1.37 ± 0.13 mm DOI resolution). The fine-cut crystal had almost a linear DOI function and a wider dynamic range of (-0.85, 0.85) and therefore the best performance with 1.2 ± 0.08 mm DOI resolution. However, for the crystal with the roughest surface (coarse-cut crystal), even though the dynamic range expanded to (-0.95, 0.95), its DOI function became nonlinear resulting in 1.24 ± 0.28 mm DOI resolution. This means there is an optimum surface roughness to provide the crystal with the best DOI capability. The pulse-height spectrum measured at each depth was used as a measure to compare the energy performance of the four crystals. The photopeak of 511 keV was observed for all depths, all crystals. The photopeak position for the coarse-cut crystal had extensive depth dependency which results in poorer energy resolution unless the energy window is calibrated for each depth. This variation of photopeak for the fine-cut and fine grinding crystals was comparable with that of polished crystal.

CONCLUSION

This paper reports 1.2 ± 0.08 mm FWHM DOI resolution for a fine-cut unpolished crystal. This resolution is as narrow as the crystal width, resulting in the complete elimination of parallax error in PET images. Results suggest that there is an optimum roughness for the best performance of the dual-ended method and further increase in the roughness, degrades DOI resolution. Thanks to the high light yield of GAGG, the energy performance of the fine-cut crystal is acceptable, and the depth dependency of the spectrum is negligible.