Recovery and normalization of triple coincidences in PET

Abstract

Purpose: Triple coincidences in positron emission tomography (PET) are events in which three γ-rays are detected simultaneously. These events, though potentially useful for enhancing the sensitivity of PET scanners, are discarded or processed without special consideration in current systems, because there is not a clear criterion for assigning them to a unique line-of-response (LOR). Methods proposed for recovering such events usually rely on the use of highly specialized detection systems, hampering general adoption, and/or are based on Compton-scatter kinematics and, consequently, are limited in accuracy by the energy resolution of standard PET detectors. In this work, the authors propose a simple and general solution for recovering triple coincidences, which does not require specialized detectors or additional energy resolution requirements. Methods: To recover triple coincidences, the authors' method distributes such events among their possible LORs using the relative proportions of double coincidences in these LORs. The authors show analytically that this assignment scheme represents the maximum-likelihood solution for the triple-coincidence distribution problem. The PET component of a preclinical PET/CT scanner was adapted to enable the acquisition and processing of triple coincidences. Since the efficiencies for detecting double and triple events were found to be different throughout the scanner field-of-view, a normalization procedure specific for triple coincidences was also developed. The effect of including triple coincidences using their method was compared against the cases of equally weighting the triples among their possible LORs and discarding all the triple events. The authors used as figures of merit for this comparison sensitivity, noise-equivalent count (NEC) rates and image quality calculated as described in the NEMA NU-4 protocol for the assessment of preclinical PET scanners. Results: The addition of triple-coincidence events with the authors' method increased peak NEC rates of the scanner by 26.6% and 32% for mouse-and rat-sized objects, respectively. This increase in NEC-rate performance was also reflected in the image-quality metrics. Images reconstructed using double and triple coincidences recovered using their method had better signal-to-noise ratio than those obtained using only double coincidences, while preserving spatial resolution and contrast. Distribution of triple coincidences using an equal-weighting scheme increased apparent system sensitivity but degraded image quality. The performance boost provided by the inclusion of triple coincidences using their method allowed to reduce the acquisition time of standard imaging procedures by up to ∼25%. parameters like spatial resolution or contrast.

Positron emission tomography with additional γ -ray detectors for multiple-tracer imaging

“…These events are the same as the random coincidence events in a conventional PET and can be corrected using the delayed coincidence method. Thus, in our system, only energy discrimination is used to distinguish prompt and annihilation γ ‐rays, although another method could distinguish between them based on the collinearity of annihilation photons …”

Systematic study on factors influencing the performance of interdetector scatter recovery in small‐animal PET

“…The NN scheme further requires calculations in a network trained by simulation data. However, it is reported that the computational burden of the NN scheme is manageable, even in existing PET systems …”

“…First, we tested different energy criteria for triple coincidence for IDS validation because they are likely to determine the number of IDS events to be recovered. Second, we compared different recovery schemes that are expected to be efficient for small ‐ animal PET geometry and relatively simple for real implementation . An accurate recovery scheme has to be applied to correctly find the true LORs of the IDS events .…”

Positron Emission Tomography: Current Challenges and Opportunities for Technological Advances in Clinical and Preclinical Imaging Systems

“…A recent proposal is to use what has been termed the triple coincidence, which works with any third photon because it can be distinguished from random or scatter; therefore, the triple coincidences can be included in iterative reconstruction models (96). Events coming from triple-photon isotopes can be detected separately from pure positron emitters using precalculated double-coincidence probabilities that are used to assign the triple event to one of the possible LORs.…”