Effects of multiple‐interaction photon events in a high‐resolution PET system that uses 3‐D positioning detectors

Abstract

Purpose:The authors' laboratory is developing a dual-panel, breast-dedicated PET system. The detector panels are built from dual-LSO-position-sensitive avalanche photodiode ͑PSAPD͒ modules-units holding two 8 ϫ 8 arrays of 1 mm 3 LSO crystals, where each array is coupled to a PSAPD. When stacked to form an imaging volume, these modules are capable of recording the 3-D coordinates of individual interactions of a multiple-interaction photon event ͑MIPE͒. The small size of the scintillation crystal elements used increases the likelihood of photon scattering between crystal arrays. In this article, the authors investigate how MIPEs impact the system photon sensitivity, the data acquisition scheme, and the quality and quantitative accuracy of reconstructed PET images. Methods: A Monte Carlo simulated PET scan using the dual-panel system was performed on a uniformly radioactive phantom for the photon sensitivity study. To establish the impact of MIPEs on a proposed PSAPD multiplexing scheme, experimental data were collected from a dual-LSO-PSAPD module edge-irradiated with a 22 Na point source, the data were compared against simulation data based on an identical setup. To assess the impact of MIPEs on the dual-panel PET images, a simulated PET of a phantom comprising a matrix of hot spherical radiation sources of varying diameters immersed in a warm background was performed. The list-mode output data were used for image reconstruction, where various methods were used for estimating the location of the first photon interaction in MIPEs for more accurate line of response positioning. The contrast recovery coefficient ͑CRC͒, contrast to noise ratio ͑CNR͒, and the full width at half maximum spatial resolution of the spheres in the reconstructed images were used as figures of merit to facilitate comparison. Results: Compared to image reconstruction employing only events with interactions confined to one LSO array, a potential single photon sensitivity gain of Ͼ46.9% ͑Ͼ115.7% for coincidence͒ was noted for a uniform phantom when MIPEs with summed-energy falling within a Ϯ12% window around the photopeak were also included. Both experimental and simulation data demonstrate that Ͻ0.4% of the events whose summed-energy deposition falling within that energy window interacted with both crystal arrays within the same dual-LSO-PSAPD module. This result establishes the feasibility of a proposed multiplexed readout of analog output signals of the two PSAPDs within each module. Using MIPEs with summed-energy deposition within the 511 keVϮ 12% photopeak window and a new method for estimating the location of the first photon interaction in MIPEs, the corresponding reconstructed image exhibited a peak CNR of 7.23 for the 8 mm diameter phantom spheres versus a CNR of 6.69 from images based solely on single LSO array interaction events. The improved system photon sensitivity could be exploited to reduce the scan time by up to approximately 10%, while still maintaining image quality comparable to that achieved if MIPEs were excluded. Conclu...

Recovery and normalization of triple coincidences in PET

“…While previous attempts to increase the sensitivity of PET scanners using IDS events have been focused on systems using specialized detectors, 12,13,[21][22][23] this work demonstrates that those events can not only be detected but also effectively used in conventional block-detector-based scanners. Moreover, since our method only uses the distribution of standard PET coincidences to estimate the most probable distribution for IDS events, it does not suffer from limitations imposed by the energy resolution of the detectors as occurs with Comptonscatter kinematics-based methods.…”

“…In most previous studies, 12,13,[21][22][23] no special considerations were made for the normalization of IDS or Rn T coincidences, even though there is no reason to assume that the sensitivity of the scanner for such events should be similar to the sensitivity for double coincidences. In fact, in the mathematical demonstration shown in Sec.…”

“…[21][22][23][24] In some of these studies, [21][22][23] the complete interaction sequence for each annihilation photon of IDS or intercrystal scattered events is delineated in order to select the earliest interactions and, thus, the points defining the LOR containing the original decay. In general, a Bayesian estimator based on Compton kinematics is used in an iterative reconstruction algorithm to ensure that all the interactions within each sequence are consistent with the entire dataset.…”

Physical effects of mechanical design parameters on photon sensitivity and spatial resolution performance of a breast‐dedicated PET system

“…[19][20][21] If successful, the accurate identification of such events will greatly enhance the system's sensitivity while maintaining or even improving the achieved spatial resolution. 25 Thus, in systems where the multi-interaction photon events account for the majority of the recorded coincidences, such as the system under study, identification and eventually inclusion of MIPEs may result in an overall increase in system sensitivity. However, if the actual number of detected MIPEs is reduced due to the interspersion of mechanical structures between the detector arrays ͑reaching up to 20% reduction in the system under study͒, the net increase in the overall system sensitivity ͑due to the inclusion of MIPEs in the analysis͒ may be become less beneficial than initially predicted.…”

A DOI Detector With Crystal Scatter Identification Capability for High Sensitivity and High Spatial Resolution PET Imaging

“…This error in determining the initial interaction location reduces image contrast and degrades spatial resolution. This leads to degradation of the lesion detectability and quantitative characteristics of an imaging system [33], [34]. Therefore, appropriate ICS event identification and correction methods should be pursued if possible.…”