A depth encoding PET detector using four‐crystals‐to‐one‐SiPM coupling and light‐sharing window method

Abstract

Purpose Depth of interaction (DOI) decoding capability is of great importance for positron emission tomography (PET) requiring high resolution. In this study, we presented a novel low‐cost DOI detector design with four crystals coupling to one SiPM, based on the method of rectangular light‐sharing window (RLSW). A prototype detector was constructed, calibrated, and assessed using the methods of homogeneous radiation and flood map analysis. Methods The DOI detector was constructed with a 4 × 4 array of lutetium‐yttrium oxyorthosilicate (LYSO) crystals (2.95 mm × 2.95 mm × 20 mm3), barium sulfate (BaSO4) reflectors, and optical glues. A RLSW 7 mm in height was deployed in the BaSO4 reflectors. A non‐DOI detector with identical dimensions and without RLSW was also constructed for comparison. The light‐output surface of the detector was air‐coupled with a 4 × 4 array of SiPMs (3 mm × 3 mm2). The signals generated from the 16 SiPMs were read out by a custom‐designed electronic system, and the signals from four adjacent 3 mm SiPMs were summed into one signal to emulate a 2 × 2 array of 6 mm SiPMs. The RLSW caused the DOI‐related position shifts of the crystal spots in the flood map. A homogeneous radiation method was used to establish the transfer functions to convert the spot shifts measured from the flood map into DOI measurements. The accuracy of the DOI measurements was assessed with data acquired using the conventional collimated radiation method. Results All 16 crystals are distinctly separated from each other in the flood map. Twelve crystals, including four central crystals and eight edge crystals, have the DOI capability. The full width half maximum (FWHM) of the DOI measurements of the central crystals and the edge crystals are 3.06 ± 0.08 and 3.79 ± 0.15 mm, respectively, for the configuration with four crystals coupling to one SiPM. By contrast, the FWHMs (3.98 ± 0.16 and 5.12 ± 0.38 mm, respectively) are slightly worse for the configuration with one crystal coupling to one SiPM. The average and standard deviation (STD) of the FWHM energy resolutions of the DOI detector and non‐DOI detector were 10.2% ± 0.7% and 10.7% ± 1.7%, respectively. Their FWHM coincidence timing resolutions were 197.0 ± 9.6 and 206.4 ± 13.3 ps, respectively. The RLSW had no significant impact on the energy resolutions and timing resolutions of the DOI detector. Conclusions The novel four‐crystals‐to‐one‐SiPM coupling technology is a cost‐efficient approach to construct high‐performance detector modules with DOI capability. The methods of homogeneous radiation and flood map analysis are easy to perform and of good performance. Those methods can be adapted in the clinic PET scanners to enable the capability of DOI measurements.

Depth of Interaction Measurements Based on Rectangular Light Sharing Window Technology and Nine-Crystals-to-One-SiPM Coupling Method

“…The type name indicates the number of windows that the crystal is adjacent to. The origin of M-type, C-type, E-type follows the naming rule in our previous research [40]. Therefore, it is expected that the 25 C-type crystals have no DOI capability.…”

“…Note that no energy window was implemented because the Compton effect was not induced in the simulation. On the basis of our previous research, it was concluded that when the elongations are longer, the DOI accuracies are better [40]. To assess preliminarily the performance of DOI decoding of the three detector models, we plotted the distributions of event counts along the elongation direction of the decoded map in the blocks marked A and B in the flood maps.…”

“…We presented a novel design for constructing highperformance DOI detectors with a 2:1 ratio of the SiPM pitch to the crystal pitch [40], [41]. In this work, we used three models of LYSO arrays with different window heights to construct the detector and conducted optical simulation.…”

A Novel Portable Gamma Radiation Sensor Based on a Monolithic Lutetium-Yttrium Oxyorthosilicate Ring

“…Lutetium-yttrium oxyorthosilicate (LYSO) is a scintillation crystal with efficient radiation detection characteristics, such as short decay time and high light yield [ 12 ]. LYSO has been used as an alternative to bismuth germanate (BGO) and has become the most popular scintillator in nuclear imaging systems, such as single-photon emission computed tomography (SPECT) and positron emission tomography (PET) [ 13 , 14 ]. For photon detection, photomultiplier tube (PMT) instruments are still the most popular photon sensors used in the last few decades.…”

Development and initial characterization of a high-resolution PET detector module with DOI

“…The measurement of the DOI is of critical importance for a PET scanner with compact geometry to simultaneously achieve high and uniform spatial resolution, and high photon sensitivity ( Bailey et al 2003 , Yang et al 2016 ). Developing a practical DOI-capable PET detector with balanced performance has been studied for over two decades, including but not limited to dual-ended readout method using two detectors coupled to the both ends of crystal ( Moses and Derenzo 1994 , Miyaoka et al 1998 , Yang et al 2006 , Ling et al 2007 , Maas et al 2009 ), single-ended readout method using one detector coupled to one end of crystal ( Berg et al 2016 ), side readout method by reading signal from crystal lateral surface ( Yamaya et al 2006 , Yeom et al 2014 ), light-sharing window-based method ( Zhang et al 2019 ), monolithic scintillator detector based method ( Schaart et al 2009 ), single-ended readout method with light guide on the top surface ( Pizzichemi et al 2016 ). Among all DOI methods, dual-ended readout method has the advantages of a continuous DOI measurement and a balanced detector performance by increasing the total light output ( Shao et al 2002 , Burr et al 2004 , Yang et al 2008 , Kuang et al 2017 ).…”