A spectral reconstruction algorithm for two-plane Compton cameras

Abstract

One factor limiting the current applicability extent of hadron therapy is the lack of a reliable method for real time treatment monitoring. The use of Compton imaging systems as monitors requires the correct reconstruction of the distribution of prompt gamma productions during patient irradiation. In order to extract the maximum information from all the measurable events, we implemented a spectral reconstruction method that assigns to all events a probability of being either partial or total energy depositions. The method, implemented in a list-mode maximum likelihood expectation maximization algorithm, generates a four dimensional image in the joint spatial-spectral domain, in which the voxels containing the emission positions and energies are obtained. The analytical model used for the system response function is also employed to derive an analytical expression for the sensitivity, which is calculated via Monte Carlo integration. The performance of the method is evaluated through reconstruction of various experimental and simulated sources with different spatial and energy distributions. The results show that the proposed method can recover the spectral and spatial information simultaneously, but only under the assumption of ideal measurements. The analysis of the Monte Carlo simulations has led to the identification of two important degradation sources: the mispositioning of the gamma interaction point and the missing energy recorded in the interaction. Both factors are related to the high energy transferred to the recoil electrons, which can travel far from the interaction point and even escape the detector. These effects prevent the direct application of the current method in more realistic scenarios. Nevertheless, experimental point-like sources have been accurately reconstructed and the spatial distributions and spectral emission of complex simulated phantoms can be identified.

Range Verification by Means of Prompt-Gamma Detection in Particle Therapy 1

“…three-interaction events which allow to determine the energy of the primary gamma [46]. Another approach, reported recently in [55], takes into account the probability of incomplete energy deposition of a primary gamma in the detector. The aspect of computation speed was addressed in [56], where it was shown that a fast line-cone reconstruction technique yields significantly lower precision than an iterative maximum likelihood expectation maximization algorithm.…”

Development of a Two-Layer Staggered GAGG Scatter Detector for Whole Gamma Imaging

“…There are several Compton camera studies that investigated the ARM [61], [62] and spatial resolution [63], [64] of 88 Y, which emits gamma rays of 898 keV, close to 909 keV. Although it is difficult to compare them quantitively because of differences in configuration and application of the measurement system, those papers reported the ARM value was about 4 • and the spatial resolution was about 4-5 mm.…”

Imaging of polychromatic sources through Compton spectral reconstruction

“…The expression in equation ( 3) is modified to include the factors relating the deposited energies with the different possible incident photon energies. After inclusion of these factors, as deduced in (Muñoz et al 2020), the calculation of the system matrix elements is given by the expression:…”

“…For each event, a row of the system matrix is constructed using four-dimensional (three spatial and one spectral) voxels, so that the reconstruction algorithm is able to find both the spatial distribution and the energy of emission of the measured sources. Spectral image reconstruction methods for Compton imaging devices have been previously developed as a means of reconstructing sources with an unknown emission spectrum or a continuous one (Xu & He 2007, Gillam et al 2011, Muñoz et al 2020. The model employed in this work follows the one presented in (Muñoz et al 2020), with several modifications.…”

“…Spectral image reconstruction methods for Compton imaging devices have been previously developed as a means of reconstructing sources with an unknown emission spectrum or a continuous one (Xu & He 2007, Gillam et al 2011, Muñoz et al 2020. The model employed in this work follows the one presented in (Muñoz et al 2020), with several modifications. In the first place, here we focus on the energy range of nuclear medical imaging.…”