Image reconstruction for a multi-layer Compton telescope: an analytical model for three interaction events

Abstract

Compton Cameras are electronically collimated photon imagers suitable for sub-MeV to few MeV gamma-ray detection. Such features are desirable to enable in vivo range verification in hadron therapy, through the detection of secondary Prompt Gammas. A major concern with this technique is the poor image quality obtained when the incoming gamma-ray energy is unknown. Compton Cameras with more than two detector planes (multi-layer Compton Cameras) have been proposed as a solution, given that these devices incorporate more signal sequences of interactions to the conventional two interaction events. In particular, three interaction events convey more spectral information as they allow inferring directly the incident gamma-ray energy. A three-layer Compton Telescope based on continuous Lanthanum (III) Bromide crystals coupled to Silicon Photomultipliers is being developed at the IRIS group of IFIC-Valencia. In a previous work we proposed a spectral reconstruction algorithm for two interaction events based on an analytical model for the formation of the signal. To fully exploit the capabilities of our prototype, we present here an extension of the model for three interaction events. Analytical expressions of the sensitivity and the System Matrix are derived and validated against Monte Carlo simulations. Implemented in a List Mode Maximum Likelihood Expectation Maximization algorithm, the proposed model allows us to obtain four-dimensional (energy and position) images by using exclusively three interaction events. We are able to recover the correct spectrum and spatial distribution of gamma-ray sources when ideal data are employed. However, the uncertainties associated to experimental measurements result in a degradation when real data from complex structures are employed. Incorrect estimation of the incident gamma-ray interaction positions, and missing deposited energy associated with escaping secondaries, have been identified as the causes of such degradation by means of a detailed Monte Carlo study. As expected, our current experimental resolution and efficiency to three interaction events prevents us from correctly recovering complex structures of radioactive sources. However, given the better spectral information conveyed by three interaction events, we expect an improvement of the image quality of conventional Compton imaging when including such events. In this regard, future development includes the incorporation of the model assessed in this work to the two interaction events model in order to allow using simultaneously two and three interaction events in the image reconstruction.

Hybrid PET/Compton-camera imaging: an imager for the next generation

“…Alternatively, three or more interactions could also allow for calculating E 0 [94]. To this aim, specific Compton camera concepts with more than one scatterer are needed, as well as specific modeling [95]. Still, this kind of events is much less frequent than coincidences made of two interaction.…”

System Characterization and Performance Studies with Macaco Iii Compton Camera

“…A FOV of 101 × 101 × 51 mm 3 with voxel size of 1 mm 3 was used in the different studies presented in this work. In addition, a median filter with cubical footprint and 3 × 3 × 3 voxels window size (Roser et al, 2020) was employed after the final iteration in all the studies.…”

Multiple scatter correction for single plane Compton camera imaging in nuclear decommissioning

“…Due to their design without collimation medium, the sensitivity of Compton cameras is greatly increased compared to Anger cameras and allows measurements with a large field of view. Example usages of Compton cameras include nuclear decommissioning [1][2][3], homeland security [4,5], astronomical imaging [6] or medical imaging applications like hadron therapy or single photon emission computed tomography [7][8][9][10][11][12]. Due to these inherently different applications, various sizes and geometries of Compton cameras have been considered, the typical parameters that are considered for optimization in the construction of setups are usually sensitivity (overall or in specific energy ranges), spatial resolution and energy resolution [8,13].…”

“…The induced model mismatch leads to spatial artifacts in the reconstruction of the source activity. Some works have already considered models in which coincidences of three or more interactions are explicitly modeled, but usually for photon interactions between more than two detectors, not multiple non-distinguishable interactions in the same detector element [8,14].…”