A 32 mm  ×  32 mm  ×  22 mm monolithic LYSO:Ce detector with dual-sided digital photon counter readout for ultrahigh-performance TOF-PET and TOF-PET/MRI

Abstract

New applications for positron emission tomography (PET) and combined PET/magnetic resonance imaging (MRI) are currently emerging, for example in the fields of neurological, breast, and pediatric imaging. Such applications require improved image quality, reduced dose, shorter scanning times, and more precise quantification. This can be achieved by means of dedicated scanners based on ultrahigh-performance detectors, which should provide excellent spatial resolution, precise depth-of-interaction (DOI) estimation, outstanding time-of-flight (TOF) capability, and high detection efficiency. Here, we introduce such an ultrahigh-performance TOF/DOI PET detector, based on a 32 mm × 32 mm × 22 mm monolithic LYSO:Ce crystal. The 32 mm × 32 mm front and back faces of the crystal are coupled to a digital photon counter (DPC) array, in so-called dual-sided readout (DSR) configuration. The fully digital detector offers a spatial resolution of ~1.1 mm full width at half maximum (FWHM)/~1.2 mm mean absolute error, together with a DOI resolution of ~2.4 mm FWHM, an energy resolution of 10.2% FWHM, and a coincidence resolving time of 147 ps FWHM. The time resolution closely approaches the best results (135 ps FWHM) obtained to date with small crystals made from the same material coupled to the same DPC arrays, illustrating the excellent correction for optical and electronic transit time spreads that can be achieved in monolithic scintillators using maximum-likelihood techniques for estimating the time of interaction. The performance barely degrades for events with missing data (up to 6 out of 32 DPC dies missing), permitting the use of almost all events registered under realistic acquisition conditions. Moreover, the calibration procedures and computational methods used for position and time estimation follow recently made improvements that make them fast and practical, opening up realistic perspectives for using DSR monolithic scintillator detectors in TOF-PET and TOF-PET/MRI systems.

Depth of interaction determination in monolithic scintillator with double side SiPM readout

“…The easiness of the proposed methods allows the implementation of the DOI reconstruction directly in the readout electronics (i.e., on FPGA), thus reducing the complexity of the acquisition system and of the data elaboration process and making this solution suitable for application in a whole PET scanner. Even if the DOI resolution obtained in this work is close to state of the art results obtained with monolithic crystals [ 29 , 41 ], a comparison with the results obtained by other groups using thicker scintillators, e.g., [ 47 ], is not straightforward due to the different signal-to-noise ratio achievable on the SiPMs with a different dimension of the scintillator.…”

A Novel DOI Positioning Algorithm for Monolithic Scintillator Crystals in PET Based on Gradient Tree Boosting

“…A similar course of the positioning performance employing the same DOI observable is reported for a 22 mm thick monolithic scintillator [15]. The deterioration in the central DOI region is significantly reduced if a dual-sided readout is utilized [24]. In contrast to this work, the calculation of the DOI observable in [15] is based on fully read out events.…”

“…The SO model can be successfully employed as a DOI estimator. In contrast to lookup table-approaches as employed in [15], [24], [25], the IR model allows a continuous DOI estimation. In general, no huge influence on the positioning performance was observed for varying the segmentation of the scintillator because the main differences of the photon detection efficiency between single DPCs are canceled out by the normalization of the DOI observable.…”

“…After fitting two free parameters, the ratio of event energy to the maximum local intensity was utilized as DOI observable. A method based on a single DOI observable only including the photon counts -the sum of the squared pixel intensities -was applied to both single and dual-sided read-out [15], [24]. The expected distribution of events according to the attenuation was simulated and then matched to the distribution of the DOI observable.…”

A High Count-Rate and Depth-of-Interaction Resolving Single-Layered One-Side Readout Pixelated Scintillator Crystal Array for PET Applications

“…To suppress these effects, and reach the target spatial resolutions of 1 mm, compact MR compatible photosensors coupled to crystal arrays with adequate x-y and DoI resolution are required without compromising energy resolution, time resolution and maximum count rate [7], [8]. At present three primary designs of single sided readout scintillator detectors have been developed to solve this issue: mutli-layered pixelated arrays [9]- [14], monolithic [15]- [17], and one/two-axis light sharing patterned reflector foil crystal arrays [18]- [22].…”