Sensitivity recovery for the AX-PET prototype using inter-crystal scattering events

Abstract

The development of novel detection devices and systems such as the AX-positron emission tomography (PET) demonstrator often introduce or increase the measurement of atypical coincidence events such as inter-crystal scattering (ICS). In more standard systems, ICS events often go undetected and the small measured fraction may be ignored. As the measured quantity of such events in the data increases, so too does the importance of considering them during image reconstruction. Generally, treatment of ICS events will attempt to determine which of the possible candidate lines of response (LoRs) correctly determine the annihilation photon trajectory. However, methods of assessment often have low success rates or are computationally demanding. In this investigation alternative approaches are considered. Experimental data was taken using the AX-PET prototype and a NEMA phantom. Three methods of ICS treatment were assessed--each of which considered all possible candidate LoRs during image reconstruction. Maximum likelihood expectation maximization was used in conjunction with both standard (line-like) and novel (V-like in this investigation) detection responses modeled within the system matrix. The investigation assumed that no information other than interaction locations was available to distinguish between candidates, yet the methods assessed all provided means by which such information could be included. In all cases it was shown that the signal to noise ratio is increased using ICS events. However, only one method, which used full modeling of the ICS response in the system matrix--the V-like model--provided enhancement in all figures of merit assessed in this investigation. Finally, the optimal method of ICS incorporation was demonstrated using data from two small animals measured using the AX-PET demonstrator.

Recovery and normalization of triple coincidences in PET

“…To our knowledge, only a contemporary study 24 took into account the possibility of having different sensitivity profiles for double and IDS coincidences, while in previous works, standard normalizations were applied to datasets containing double and IDS events. 12,13 Importantly, adding incorrectly normalized data to the standard PET coincidences can adversely affect image-quality metrics such as SNR or contrast-to-noise ratio, which may further explain results found in the earlier studies 13 that utilized real data from a scanner.…”

“…[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.…”

“…Unlike the earlier studies, triple-coincidences and higher-order events were taken into consideration because the nonstandard design of these scanners means that IDS events are a significant fraction of the total events. 21,23 In another recent work, 24 the possibility of encoding into the system matrix on-the-fly calculated probabilities for the LORs defined by triple interactions was also investigated. Although the inclusion of IDS coincidences using these methodologies produced PET images with better image quality and spatial resolution comparable to those obtained using only double coincidences, the requirement of using a highly specialized detection system hampers general adoption of those approaches.…”

“…Although the inclusion of IDS coincidences using these methodologies produced PET images with better image quality and spatial resolution comparable to those obtained using only double coincidences, the requirement of using a highly specialized detection system hampers general adoption of those approaches. Furthermore, the use of iterative reconstruction algorithms with such 3D detection systems and/or onthe-fly calculations of the elements of a system matrix requires a significant computational load 24 even when implemented on fast graphic processing units. 21 In this work, we present a general methodology for assigning triple coincidences to LORs that can be used in both blockand specialized-detector-based scanners and has a negligible computational cost.…”

AX-PET: from Demonstrator towards a Full-Ring Brain Scanner

“…A detailed description of the technique can be found in Ref. [4]. Images were reconstructed with and without ICS for a rat injected by 18 F, as shown in Fig.…”