List-mode Maximum Likelihood Expectation Maximization Research Articles

Evaluation of lesion contrast in the walk-through long axial FOV PET scanner simulated with XCAT anthropomorphic phantoms

BackgroundThis study evaluates the lesion contrast in a cost-effective long axial field of view (FOV) PET scanner, called the walk-through PET (WT-PET). The scanner consists of two flat detector panels covering the entire torso and head, scanning patients in an upright position for increased throughput. High-resolution, depth-of-interaction capable, monolithic detector technology is used to provide good spatial resolution and enable detection of smaller lesions.MethodsMonte Carlo GATE simulations are used in conjunction with XCAT anthropomorphic phantoms to evaluate lesion contrast in lung, liver and breast for various lesion diameters (10, 7 and 5 mm), activity concentration ratios (8:1, 4:1 and 2:1) and patient BMIs (18–37). Images were reconstructed iteratively with listmode maximum likelihood expectation maximization, and contrast recovery coefficients (CRCs) were obtained for the reconstructed lesions.ResultsResults shows notable variations in contrast recovery coefficients (CRC) across different lesion sizes and organ locations within the XCAT phantoms. Specifically, our findings reveal that 10 mm lesions consistently exhibit higher CRC compared to 7 mm and 5 mm lesions, with increases of approximately 54% and 330%, respectively, across all investigated organs. Moreover, high contrast recovery is observed in most liver lesions regardless of diameter or activity ratio (average CRC = 42%), as well as in the 10 mm lesions in the lung. Notably, for the 10 mm lesions, the liver demonstrates 42% and 62% higher CRC compared to the lung and breast, respectively. This trend remains consistent across lesion sizes, with the liver consistently exhibiting higher CRC values compared to the lung and breast: 7 mm lesions show an increase of 96% and 41%, while 5 mm lesions exhibit approximately 294% and 302% higher CRC compared to the lung and breast, respectively.ConclusionA comparison with a conventional pixelated LSO long axial FOV PET shows similar performance, achieved at a reduced cost for the WT-PET due to a reduction in required number of detectors.

A Portable Three-Layer Compton Camera for Wide-Energy-Range Gamma-ray Imaging: Design, Simulation and Preliminary Testing

(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.

An Extended List-Mode MLEM Algorithm for 3D Compton Image Reconstruction from Multi-View Data

The lack of parallax in the measurements is a big challenge in 3D image reconstruction for handheld Compton cameras. Our solution to this issue is to extend the conventional list-mode maximum-likelihood expectation-maximization (LM-MLEM) 3D reconstruction algorithm to allow the simultaneous use of multi-view Compton data seeking parallax improvement. It involves building a new list-mode simultaneous data space from multi-view Compton events, formulating the associated probabilistic models for the system response matrix and sensitivity, and developing an extended LM-MLEM algorithm. For the performance assessment, we experiment the extended 3D reconstruction algorithm on real multi-view data conducted with a handheld CeBr3 Compton camera developed by Damavan Imaging and a punctual 0.2 (MBq)22Na source. Various comparative studies with different view numbers, source locations and energy ranges confirm the outperformances of our extended LM-MLEM algorithm.

A hybrid coded-aperture and Compton camera based on cerium-doped Gd3Al2Ga3O12 scintillators coupled with multi-pixel photon counter arrays.

In this study, we propose a hybrid coded-aperture and Compton camera based on cerium-doped Gd3Al2Ga3O12 (GAGG:Ce) scintillator arrays coupled with Multi-Pixel Photon Counter (MPPC) arrays. The sensitive detector of the gamma camera consists of a single GAGG:Ce crystal coupled with a single-chip MPPC unit module. An impedance bridge circuit and a 64-channel data acquisition system were employed to record the code-aperture events and Compton coincidence events. After the calibration of position and energy, the total energy resolution for 662keV gamma-rays from 137Cs was 6.6%. The hybrid camera had the characteristics of mechanical collimation and electronic collimation at the same time. In the code aperture mode, the reconstructed images were obtained by direct deconvolution and maximum likelihood expectation maximization (MLEM) methods. In the Compton imaging mode, the energy-dependent method was applied to order the sequence of Compton scatter events. The simple back-projection algorithm and list-mode MLEM algorithm were adopted for image reconstruction. Practical performances demonstrated that the angular resolutions in two modes were measured as 5.2° and 11.4°, respectively. In addition, the hybrid camera had a desirable imaging capability in a wide energy range (32 keV-2.6MeV) and a wide field of view (∼210° in the horizontal direction). As for the sensitivity, the camera had a commercially available sensitivity level of localizing a 137Cs point source, producing ∼0.026 µSv/h in 5min. Furthermore, the function of distinction for different radiation sources was preliminarily realized.

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

After the nuclear accident at the Fukushima Daiichi Nuclear Power Plant, researchers focused on the accident and studied the effects of leaked radiation based on radiation detection and measurement. Many methods can be used for monitoring and mapping natural and leaked radionuclides; however, portable detectors installed in unmanned ground or aerial vehicles (UAVs) can measure radioactivity near the target point, preventing damage to the human body from radiation in highly contaminated areas. Compton cameras have a wider field-of-view (FOV) and less weight than a detector system that uses a mechanical collimator. In a previous study, a position-sensitive 4π Compton camera based on a lutetium–yttrium oxyorthosilicate (LYSO) scintillator attached to silicon photomultipliers (SiPMs) was developed. Compared with a semiconductor-based Compton camera operating with a power device supplying more than a thousand volts, our system requires a maximum of approximately several tens of volts. In addition, our Compton camera has a single-crystal structure; hence, it has advantages in terms of FOV and efficiency over a Compton camera that utilizes separate detectors. In this study, a modified scintillator-based Compton camera was created using an advanced SiPM model and a gadolinium aluminum gallium garnet (GAGG) scintillator. In the experiment, 133Ba, 22Na, and 137Cs radionuclides were used for data acquisition, and simple back-projection and list-mode maximum likelihood expectation maximization algorithms were used to reconstruct the Compton images. The 4π FOV coverage was evaluated using Compton imaging and angular resolution measure (ARM), and the results indicated that FOV coverage was degraded when the angle between the radiation source and the perpendicular axis of the detector was near 45°. The degradation of the ARM reflected as the bimodal distribution became more severe for the Compton camera based on the LYSO scintillator than that based on the GAGG scintillator, which was attributed to the intrinsic radiation of LYSO from 176Lu considered as background noise. In the evaluation of intrinsic efficiency, the influence of intrinsic radiation was significantly large. Without background noise rejection, the intrinsic efficiencies for 356, 511, and 662 keV radiation were 0.66%–1.28%, and 1.50%–3.05% for the detection system based on the GAGG and LYSO scintillators, respectively. After background noise rejection, the intrinsic efficiencies were re-calculated as 0.64%–1.27% and 1.39%–1.72% for the detection system based on the GAGG and LYSO scintillator, respectively.

An accurate probabilistic model with detector resolution and Doppler broadening correction in list-mode MLEM reconstruction for Compton camera

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.

Region of Interest Image Reconstruction for Compton Imaging Using 3-D Position Sensing CdZnTe

Iterative methods, such as list-mode maximum likelihood expectation maximization (LM-MLEM) provide high signal-to-noise ratio reconstructions for Compton imaging. However, MLEM reconstructions can be computationally expensive when reconstructing a high-resolution image over the full field of view (FOV). When employing MLEM for high-resolution imaging, a vast number of data pixels would be needed. This work proposes a region of interest MLEM algorithm (ROI-MLEM) for Compton imaging, using 3-D position sensing CdZnTe, that allows for image reconstruction to take place within a fixed ROI based on prior knowledge of the approximate location of the source. ROI-MLEM is demonstrated by reconstructing simulated and experimental <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs source measurements. A preliminary study of the ROI-MLEM performance in high-energy imaging is shown by reconstructing the Compton image of the 4.44 MeV photopeak from a PuBe source. ROI-MLEM shows a 44% improvement in the signal-to-noise ratio (SNR) compared to standard MLEM for simulated source reconstructions in a truncated FOV. The experimental results show the capabilities of estimating the source location with submillimeter pixel resolution at a 30-cm source-to-detector distance, resulting in an average error of 1.4 mm in source location estimation.

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

The Compton camera is a gamma ray imaging device expected to provide clinically relevant images in the SPECT applications where collimated cameras are sub-optimal. Its imaging performances depend not only on the design of the detection system but also on choices related to tomographic reconstruction. The aim of this work is to show that the accuracy in modelling the acquisition largely influences the quality of the images. For this purpose, we restrict here to Doppler broadening models in conjunction with the list-mode maximum likelihood expectation maximization (LM-MLEM) algorithm. The study was carried out with Monte-Carlo simulation. We show that the reconstructed point spread function is location-dependent when the model is not accurate, and the usual elongation artefacts well-known in Compton camera imaging will appear. The model we propose allows us to reconstruct isolated point sources and more complex non-uniform sources with improved resolution even in the direction orthogonal to the camera.

Anisotropic neutron response of trans-stilbene and impact on a handheld dual particle imager

This manuscript details measurements of the anisotropic light-output response of trans-stilbene, henceforth referred to as stilbene, along the three primary crystal planes and analyzes how this anisotropic response impacts neutron image reconstruction in a handheld dual particle imager (H2DPI). A prototype H2DPI composed of stilbene pillars (6×6×50 mm3) coupled to silicon photomultipliers was built and is capable of imaging both fast neutrons and gamma rays from kilogram quantities of special nuclear material. Stilbene was chosen as the scintillating medium for the imager because of its pulse shape discrimination capability, time resolution and relatively high light output. A drawback to using stilbene, however, is the anisotropic response. A recoiling proton from a neutron elastic-scattering event in stilbene will yield different amounts of scintillation light depending on the direction of the recoiling proton with respect to the crystal lattice. This manuscript analyzes how this anisotropic response impacts neutron image reconstruction in 4π. The light output in the three primary crystal planes of stilbene were independently measured 6–7 times for proton recoil energies ranging from 0.5–5.0 MeV using a quasi-monoenergetic time-of-flight neutron source. The measured light-output data in the three crystal planes were fit with a semi-empirical function based on Birks’ formula. These fits were used to reconstruct the location of a 252Cf source in front of the imager (0°, −0.8°), to the left of the imager (−90°, −0.8°) and directly above the imager (0°, 90°) to determine if it is necessary to apply the directionally-dependent response of stilbene to accurately reconstruct source locations. A bootstrapping technique was applied to the measured data sets to produce 1000 images composed of 2000 cone projections for each source location and each measured light-output curve. List-mode maximum likelihood expectation maximization was applied to each image; the highest pixel location and the full width at half maximum (FWHM) were then recorded. The average and standard deviations of these parameters were taken for each set of images. The maximum difference in the average azimuthal pixel location when varying between the appropriate and incorrect light-output curves for the source locations at (0°, −0.8°) and (−90°, −0.8°) were respectively found to be 1.85±1.39° and 2.03±1.78°. All other pixel locations and FWHMs were within a single standard deviation of uncertainty. Taken together, the anisotropic response of stilbene has negligible impact on the neutron image reconstruction capability of the H2DPI.

Development of a Position-Sensitive 4π Compton Camera Based on a Single Segmented Scintillator

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 position-sensitive 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 back-projection, 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 $4\pi $ 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 $2.23\times 10^{-2}$ for 511 keV.

3-D Reconstruction Benchmark of a Compton Camera Against a Parallel-Hole Gamma Camera on Ideal Data

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.

A spectral reconstruction algorithm for two-plane Compton cameras

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.

Real-Time Free-Moving Active Coded Mask 3D Gamma-Ray Imaging

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.

Radiation measurement and imaging using 3D position sensitive pixelated CZT detector

Abstract In this study, we evaluated the performance of a commercial pixelated cadmium zinc telluride (CZT) detector for spectroscopy and identified its feasibility as a Compton camera for radiation monitoring in a nuclear power plant. The detection system consisted of a 20 mm × 20 mm × 5 mm CZT crystal with 8 × 8 pixelated anodes and a common cathode, in addition to an application specific integrated circuit. The performance of the various radioisotopes 57Co, 133Ba, 22Na, and 137Cs was evaluated. In general, the amplitude of the induced signal in a CZT crystal depends on the interaction position and material non-uniformity. To minimize this dependency, a drift time correction was applied. The depth of each interaction was calculated by the drift time and the positional dependency of the signal amplitude was corrected based on the depth information. After the correction, the Compton regions of each spectrum were reduced, and energy resolutions of 122 keV, 356 keV, 511 keV, and 662 keV peaks were improved from 13.59%, 9.56%, 6.08%, and 5%–4.61%, 2.94%, 2.08%, and 2.2%, respectively. For the Compton imaging, simulations and experiments using one 137Cs source with various angular positions and two 137Cs sources were performed. Individual and multiple sources of 133Ba, 22Na, and 137Cs were also measured. The images were successfully reconstructed by weighted list-mode maximum likelihood expectation maximization method. The angular resolutions and intrinsic efficiency of the 137Cs experiments were approximately 7°–9° and 5 × 10−4–7 × 10−4, respectively. The distortions of the source distribution were proportional to the offset angle.

Prototype of an array SiPM-based scintillator Compton camera for radioactive materials detection

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.

Accurate modeling of event-by-event backprojection for a germanium semiconductor Compton camera for system response evaluation in the LM-ML-EM image reconstruction method

We develop an image reconstruction method, considering the physical phenomenon in the measurement process of a Compton camera. The image quality is improved by applying an accurate error model of the Compton scattering angle. The angular error has two properties: an error distribution function specific to the detector material and the variation of its function parameters, depending on each measurement event. We incorporate these factors into the backprojection of the list-mode maximum-likelihood expectation-maximization method as the system response function. We apply our image reconstruction method to simulated data assumed to be measured by a Ge-semiconductor Compton camera GREI, and the imaging data of a tumor-bearing live mouse obtained using GREI. This method is evaluated by comparing an image with variable angular error with that having fixed angular error. The consideration of the variable angle estimation error improves the spatial resolution and reduces image roughness.

Optimization of DPC dSiPM-Based DOI Compton Camera by Monte Carlo Simulation

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 137Cs 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~\mu $ Sv/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.

Compton camera study for high efficiency SPECT and benchmark with Anger system

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.

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

Introduction Boron Neutron Capture Therapy (BNCT) [1] is a binary radiotherapy based on 10B(n, α 7Li reaction, whose clinical outcome depends on the deposited dose, associated with both the thermal neutron flux and the boron concentration distribution. However, it’s still an on-going challenge to verify the boron concentration in real time. Potentially, one can measure the 0.478-MeV prompt gamma rays de-excited from 7 Li* to monitor the boron concentration in vivo. Based on our previously optimized Compton camera (CC) [2] , which has higher efficiency than collimator based SPECT techniques [1] , this study focuses on the evaluation of the clinical translation of the CC for Melanoma patients. Materials and Methods A virtual patient with one melanoma lesion was constructed and irradiated with epithermal neutron beam; the boron concentration was set as from 30 to 100 ppm. The List-Mode Maximum Likelihood Expectation Maximization (LM-MLEM) algorithm [3] was applied to reconstruct the 3D image. Results With different boron concentration simulated in melanoma lesion, the detection efficiency of “True” events keeps stable. The reconstructed image matches well with the profile of the capture location scored in the patient geometry. The pass rate of gamma index analysis of 3 mm/3% for the relative distribution is about 90% with the prescribed dose level. Conclusion This study shows that CC can be used to reconstruct the location of the capture reaction, which can be further used for the dose calculation in BNCT. LM-MLEM is an efficient algorithm to perform the reconstruction for this application. The results provide the fundamental basis for further clinical translation of the CC during BNCT.

Double-Layered CZT Compton Imager

In this paper, we propose a double-layered cadmium zinc telluride Compton camera using the weighted list-mode maximum likelihood expectation maximization (MLEM) method. We performed a Monte Carlo simulation of the performance. The detection system comprised two detector arrays arranged in parallel. Each array consisted of $6 \times 6$ crystals with a 7.5 mm pixel pitch. The dimension of a single crystal was $6 \times 6 \times 15$ mm. For Compton imaging, the 137Cs activity was set to 78 kBq and the acquisition time was set to 2 hours. By applying the weighted MLEM method, the events with small angular uncertainty were more emphasized than those with large angular uncertainty. Each angular uncertainty was calculated based on the energy and geometry uncertainties of each event. We compared the traditional list-mode MLEM and the weighted list-mode MLEM for single and double detector arrays. The results showed that the combination of a double-layered imager with the weighted MLEM method represented the best image performance compared to other combinations. The sources at the offset positions were also simulated to estimate the directional dependency of the camera system.