Influence of Doppler broadening model accuracy in Compton camera list-mode MLEM reconstruction

International audience

Objective. The Compton cameras have been researched for medical applications and radioactive material detection. It is challenging for the Compton camera to realize high-resolution reconstruction when the incident photon energy is below 200 keV. However, multiple kinds of nuclear medical radionuclides are in this energy range, such as 201Tl, 67Ga, 99mTc, and 123I. In this work, we propose an improved probabilistic model with correction of detector energy resolution, detector spatial resolution, and Doppler broadening effect. The proposed model is used for numerical calculation of the system matrix in the list-mode maximum likelihood expectation maximization (LM-MLEM) algorithm. Approach. The model can improve the imaging resolution of LM-MLEM reconstruction by taking Doppler broadening effect into account. It performs well, especially in the following situations: low-energy photon incidence below 200 keV or (and) small distance between scattering and absorbing positions. Main results. Firstly, three main factors that affect the angular resolution of the Compton camera are theoretically analyzed and quantitatively calculated. The results of the analysis indicate the necessity of including the Doppler broadening effect in the model. Secondly, the details and derivation of the proposed probabilistic model are described. Thirdly, both Monte Carlo (MC) simulations and experiments are carried out to verify the performance of the proposed algorithm. The simulations focus on the low-energy reconstruction in which 201Tl (70 keV) and 99mTc (141 keV) are simulated. And the experiments are based on a single-layer Compton camera composed of a Timepix3 detector. Significance. The results of the simulations and the Timepix3-based experiments are presented to verify the effectiveness of the proposed algorithm. The model improves the Compton imaging resolution when the photon energy is below 200 keV.

In this paper, a Compton camera capable of estimating depth of interaction (DOI) in the detector was designed and optimized using Geant4. The proposed DOI Compton camera consists of two pixelated CsI(Tl) scintillators coupled with digital silicon photomultipliers [Digital Photon Counter (DPC)-3200-22]. The parameters affecting the performance of a Compton camera were evaluated for optimization in terms of image sensitivity and image resolution with a measure of figure of merit. The considered parameters were: 1) type of the reflector surface; 2) thickness of the scintillator; and 3) the distance between the scatter and the absorber detectors. The simulation results showed that a ground white surface is better than a polished white one. The optimal thickness and the scatter-absorber distance were found to be 15 mm and 4 cm, respectively. With the optimized parameters, performance of the DOI Compton camera was estimated for a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs source at 1 m using Geant4. The image resolutions were 6.06° full-width at half-maximum with the list-mode maximum likelihood expectation maximization algorithm. It took 20 s to take Compton images for the gamma ray sources of 0.1 μSv/h intensity with the DOI Compton camera. It was shown that there was a clear improvement by employing DOI estimation algorithm in combination with the use of thick scintillators for a Compton camera.

Development of modified scintillator-based single-crystal position-sensitive 4[formula omitted] Compton camera for a portable radiation imaging device

The Compton camera, which visualizes the distribution of gamma-ray sources based on the kinematics of Compton scattering, has advantage of wide field of view, broad range of energy and compact structure. In this study, we proposed a prototype of Compton camera, which included array silicon photomultiplier (SiPM)-based position-sensitive detectors, data acquisition (DAQ) system and image reconstruction system. The detectors were composed of Ce-doped Gd3Al2Ga3O12 scintillator arrays and pixel Si-PM arrays. In DAQ system, symmetric charge division circuit, impedance bridge circuit and the delay coincidence algorithm were designed to record coincidence events. Simple back-projection algorithm and list-mode maximum likelihood expectation maximization algorithm were adopted for image reconstruction. The coordinate of longitude and latitude was used for image fusion. The performance of this Compton camera prototype system was evaluated. The results indicated that this system was able to locate a 137Cs point source within 20 s with the corresponding radiation dose of ~ 1.0 μSv/h. The angular resolution of point source was ~ 7° (FWHM), and the total energy resolution of 662 keV was 7.2%. Furthermore, we succeeded in separating two point sources of different energy [22Na (511 keV), 137Cs (662 keV)] in laboratory test. This prototype of scintillator Compton camera offers capabilities for applications like source term investigation and radioactive materials detection.

Single photon emission computed tomography (SPECT) is at present one of the major techniques for non-invasive diagnostics in nuclear medicine. The clinical routine is mostly based on collimated cameras, originally proposed by Hal Anger. Due to the presence of mechanical collimation, detection efficiency and energy acceptance are limited and fixed by the system’s geometrical features. In order to overcome these limitations, the application of Compton cameras for SPECT has been investigated for several years.In this study we compare a commercial SPECT-Anger device, the General Electric HealthCare Infinia system with a High Energy General Purpose (HEGP) collimator, and the Compton camera prototype under development by the French collaboration CLaRyS, through Monte Carlo simulations (GATE—GEANT4 Application for Tomographic Emission—version 7.1 and GEANT4 version 9.6, respectively). Given the possible introduction of new radio-emitters at higher energies intrinsically allowed by the Compton camera detection principle, the two detectors are exposed to point-like sources at increasing primary gamma energies, from actual isotopes already suggested for nuclear medicine applications. The Compton camera prototype is first characterized for SPECT application by studying the main parameters affecting its imaging performance: detector energy resolution and random coincidence rate. The two detector performances are then compared in terms of radial event distribution, detection efficiency and final image, obtained by gamma transmission analysis for the Anger system, and with an iterative List Mode-Maximum Likelihood Expectation Maximization (LM-MLEM) algorithm for the Compton reconstruction.The results show for the Compton camera a detection efficiency increased by a factor larger than an order of magnitude with respect to the Anger camera, associated with an enhanced spatial resolution for energies beyond 500 keV. We discuss the advantages of Compton camera application for SPECT if compared to present commercial Anger systems, with particular focus on dose delivered to the patient, examination time, and spatial uncertainties.

(1) Background: The imaging energy range of a typical Compton camera is limited due to the fact that scattered gamma photons are seldom fully absorbed when the incident energies are above 3 MeV. Further improving the upper energy limit of gamma-ray imaging has important application significance in the active interrogation of special nuclear materials and chemical warfare agents, as well as range verification of proton therapy. (2) Methods: To realize gamma-ray imaging in a wide energy range of 0.3~7 MeV, a principle prototype, named a portable three-layer Compton camera, is developed using the scintillation detector that consists of an silicon photomultiplier array coupled with a Gd3Al2Ga3O12:Ce pixelated scintillator array. Implemented in a list-mode maximum likelihood expectation maximization algorithm, a far-field energy-domain imaging method based on the two interaction events is applied to estimate the initial energy and spatial distribution of gamma-ray sources. The simulation model of the detectors is established based on the Monte Carlo simulation toolkit Geant4. The reconstructed images of a 133Ba, a 137Cs and a 60Co point-like sources have been successfully obtained with our prototype in laboratory tests and compared with simulation studies. (3) Results: The proportion of effective imaging events accounts for about 2%, which allows our prototype to realize the reconstruction of the distribution of a 0.05 μSv/h 137Cs source in 10 s. The angular resolution for resolving two 137Cs point-like sources is 15°. Additional simulated imaging of the 6.13 MeV gamma-rays from 14.1 MeV neutron scattering with water preliminarily demonstrates the imaging capability for high incident energy. (4) Conclusions: We conclude that the prototype has a good imaging performance in a wide energy range (0.3~7 MeV), which shows potential in several MeV gamma-ray imaging applications.

Proposed in the mid-seventies as a tool to retrieve the direction of gamma rays emerging from tumors marked by a radio-tracer, the Compton Camera (CC) was also proposed more recently to localize radioactivity for nuclear dismantling operations. For the TEMPORAL project, CeBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> scintillation crystals coupled with SiPM arrays will be used both for the scatterer and the absorber. The sensitivity of the CC, as well as its angular resolution, depend on the distance between the scatterer and the absorber, on their sizes and on the energies of the detected gamma rays. Its angular resolution also depends on the scatterer and the absorber spatial and energy resolutions. Monte Carlo simulations have shown that CeBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> is suitable for detection of energies from 200 keV to 2 MeV with a scatterer thickness of 1.5 cm and a distance between the scatterer and absorber front faces of 7.5 cm. To reconstruct source positions and activities, the most commonly used algorithm is the list-mode Maximum Likelihood Expectation Maximization (MLEM), which is limited to image reconstruction of mono-energetic gamma rays. We also propose an extension of the list-mode MLEM algorithm in order to allow poly-energetic imaging of radioactive sources (PE-LM-MLEM).

ID: 126 Evaluation of the clinical translation of an optimized Compton camera during Boron Neutron Capture Therapy for melanoma patients

Scintillator-based Compton cameras, which comprise of a separated scatter and absorption detectors, neglect the Compton scattering that is followed by a photoelectric effect in a single set of scintillators, wherein the detection efficiency is severely limited. In this study, we propose a 3-D positionsensitive Compton camera that uses a single set of scintillators, wherein radiation interactions inside the scintillator can be discriminated. The proposed Compton camera comprises a segmented lutetium-yttrium oxyorthosilicate scintillator coupled with two position-sensitive silicon photomultipliers on both sides. Compton image reconstruction algorithms, such as simple backprojection, list-mode maximum likelihood expectation maximization (MLEM), and filtered back-projection, were applied and compared. The single position-sensitive Compton camera identified the positions of multiple radiation sources with various energies in a 4π field of view. The signal-to-noise ratio (SNR) and full-width at half-maximum (FWHM) of a reconstructed 511-keV point source were approximately 12° and 22°, respectively, after MLEM was applied, and the two 511-keV sources with an angle difference of 40° were separately reconstructed. The intrinsic efficiency of the proposed Compton camera was 2.23 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> for 511 keV.

Study of 3D fast Compton camera image reconstruction method by algebraic spatial sampling

Compton camera has been proposed as a potential imaging tool in astronomy, industry, homeland security, and medical diagnostics. Due to the inherent geometrical complexity of Compton camera data, image reconstruction of distributed sources can be ineffective and/or time-consuming when using standard techniques such as filtered backprojection or maximum likelihood-expectation maximization (ML-EM). In this article, the authors demonstrate a fast reconstruction of Compton camera data using a novel stochastic origin ensembles (SOE) approach based on Markov chains. During image reconstruction, the origins of the measured events are randomly assigned to locations on conical surfaces, which are the Compton camera analogs of lines-of-responses in PET. Therefore, the image is defined as an ensemble of origin locations of all possible event origins. During the course of reconstruction, the origins of events are stochastically moved and the acceptance of the new event origin is determined by the predefined acceptance probability, which is proportional to the change in event density. For example, if the event density at the new location is higher than in the previous location, the new position is always accepted. After several iterations, the reconstructed distribution of origins converges to a quasistationary state which can be voxelized and displayed. Comparison with the list-mode ML-EM reveals that the postfiltered SOE algorithm has similar performance in terms of image quality while clearly outperforming ML-EM in relation to reconstruction time. In this study, the authors have implemented and tested a new image reconstruction algorithm for the Compton camera based on the stochastic origin ensembles with Markov chains. The algorithm uses list-mode data, is parallelizable, and can be used for any Compton camera geometry. SOE algorithm clearly outperforms list-mode ML-EM for simple Compton camera geometry in terms of reconstruction time. The difference in computational time will be much larger when full Compton camera system model, including resolution recovery, is implemented and realistic Compton camera geometries are used. It was also shown in this article that while correctly reconstructing the relative distribution of the activity in the object, the SOE algorithm tends to underestimate the intensity values and increase variance in the images; improvements to the SOE reconstruction algorithm will be considered in future work.

The ability to localize and map the distribution of gamma-ray emitting radionuclides in 3D has applications ranging from medical imaging to nuclear security. In the case of radiological source search and nuclear contamination remediation, the deployment of freely moving detection systems such as handheld instruments or ground/aerial-based vehicles is critical in overcoming the inverse square law and complex shielding scenarios. Using contextual sensors, these systems can simultaneously generate 3D maps of the surrounding environment and track the position and orientation of the gamma-ray sensitive detectors in that environment. The fusion of contextual scene data and gamma-ray detector data to facilitate real-time 3D gamma-ray image reconstruction has been demonstrated with mobile high purity germanium (HPGe) and CdZnTe-based Compton cameras for gamma-ray energies ranging from a few hundred keV to several MeV. Here we apply this approach for lower energy (50–400 keV) gamma rays, using a handheld CdZnTe-based omnidirectional imaging system and an active coded mask imaging modality. We present our approach to real-time reconstruction using a scene-data-constrained graphics processing unit (GPU)-accelerated list-mode maximum likelihood expectation maximization (MLEM) algorithm and show results from several measurements in the lab and in the field.

Compton cameras (CCs) and collimated gamma cameras are competing devices suitable for prompt gamma detection in range verification of particle therapy. In this article, we evaluate the first approach from the point of view of the tomographic reconstruction step by comparing it to the second. We clear any technological constraints by considering a simple geometry, ideal detecting stages, a mono-energetic synthetic phantom. To this end, both analytic (filtered back-projection) and iterative (list-mode maximum-likelihood expectation-maximization) algorithms are applied in conjunction with total variation denoising. It was shown in the previous studies that compared to the mechanically collimated camera, the CC has a higher efficiency. Factors between ten and one hundred were reported. Meanwhile, for each detected event, the emission position of the original photon lies on a conical surface for CCs, instead of a line for collimated cameras. This leads to a supplementary degree of freedom that logically calls during image reconstruction for a larger data set and may cancel out the benefits of the superior efficiency. We consider here a static CC and a rotating collimated camera with similar angular coverage. We show empirically that reconstruction from conical projections requires ten times more detected events to obtain at least the same image quality. In some experiments, the CC allows to avoid severe artifacts produced by the limited-angle geometry of the collimated camera. In addition, the CC should be better suited for imaging high-energy and poly-energetic sources.

Super-spatial resolution method combined with the maximum-likelihood expectation maximization (MLEM) algorithm for alpha imaging detector