Precision imaging of 4.4 MeV gamma rays using a 3-D position sensitive Compton camera

Machine learning approach for proton range verification using real-time prompt gamma imaging with Compton cameras: addressing the total deposited energy information gap

Objective.Compton camera imaging shows promise as a range verification technique in proton therapy. This work aims to assess the performance of a machine learning model in Compton camera imaging for proton beam range verification improvement.Approach.The presented approach was used to recognize Compton events and estimate more accurately the prompt gamma (PG) energy in the Compton camera to reconstruct the PGs emission profile during proton therapy. This work reports the results obtained from the Geant4 simulation for a proton beam impinging on a polymethyl methacrylate (PMMA) target. To validate the versatility of such an approach, the produced PG emissions interact with a scintillating fiber-based Compton camera.Main results.A trained multilayer perceptron (MLP) neural network shows that it was possible to achieve a notable three-fold increase in the signal-to-total ratio. Furthermore, after event selection by the trained MLP, the loss of full-energy PGs was compensated by means of fitting an MLP energy regression model to the available data from true Compton (signal) events, predicting more precisely the total deposited energy for Compton events with incomplete energy deposition.Significance.A considerable improvement in the Compton camera's performance was demonstrated in determining the distal falloff and identifying a few millimeters of target displacements. This approach has shown great potential for enhancing online proton range monitoring with Compton cameras in future clinical applications.

Microwave photonics doppler speed measurement based on sagnac loops and four-wave mixing effect in a highly nonlinear fiber

Photonic radars are increasingly being developed and offer a promising replacement for traditional RF radars. They feature higher precision, and smaller size compared to the current microwave radars. One important part of a moving target indicating (MTI) radar is the Doppler shift measurement used to measure the radial velocity of a moving target. Therefore, for any photonic radar operating at MTI mode, it is necessary to have a Doppler measurement subsystem. In this paper, a microwave photonic Doppler frequency measurement system is conceived and implemented for this purpose specifically. The operation is based on making a Doppler shift-dependent yet low-frequency voltage component. It is all-optical and hence has the potential to be integrated into many electronic warfare systems. This feature not only makes the system independent of any sophisticated electrical device but also makes the measurement time lower than that of the electrical counterparts. The specific design presented here provides a much better stability compared to the recent works. An error as low as 0.012 Hz at a 10 GHz radar frequency was obtained, and the system performance was demonstrated up to 40 GHz, at which a 4.75 Hz error was recorded.

A simulation study of in-beam visualization system for proton therapy by monitoring scattered protons

Recently, in-beam positron emission tomography (PET) has been actively researched for reducing biological washout effects and dose monitoring during irradiation. However, the positron distribution does not precisely reflect the dose distribution since positron production and ionization are completely different physical processes. Thus, a novel in-beam system was proposed to determine proton dose range by measuring scattered protons with dozens of scintillation detectors surrounding the body surface. While previous studies conducted a preliminary experiment with a simple phantom, we simulated more complex situations in this paper. Especially, we conducted three stepwise simulation studies to demonstrate the feasibility of the proposed method. First, a simple rectangular phantom was reproduced on simulation and irradiated with protons for obtaining current values and Monte Carlo (MC) dose. Next, we trained a deep learning model to estimate 2-dimensional-dose range (2D-DL dose) from measured current values for simulation (A). We simulated plastic scintillators as detectors to measure the scattered protons. Second, a rectangular phantom with an air layer was used, and 3D-DL dose was estimated in simulation (B). Finally, a cylindrical phantom that mimics the human body was used for confirming the estimation quality of the simulation (C). Consequently, the position of the Bragg peak was estimated with an error of 1.0 mm in simulation (A). In addition, the position of the air layer, as well as the verifying peak position with an error of 2.1 mm, was successfully estimated in simulation (B). Although the estimation error of the peak position was 12.6 mm in simulation (C), the quality was successfully further improved to 9.3 mm by incorporating the mass density distribution obtained from the computed tomography (CT). These simulation results demonstrated the potential of the as-proposed verification system. Additionally, the effectiveness of CT utilization for estimating the DL dose was also indicated.

Influence of the background in Compton camera images for proton therapy treatment monitoring

Objective. Background events are one of the most relevant contributions to image degradation in Compton camera imaging for hadron therapy treatment monitoring. A study of the background and its contribution to image degradation is important to define future strategies to reduce the background in the system.Approach. In this simulation study, the percentage of different kinds of events and their contribution to the reconstructed image in a two-layer Compton camera have been evaluated. To this end, GATE v8.2 simulations of a proton beam impinging on a PMMA phantom have been carried out, for different proton beam energies and at different beam intensities.Main results. For a simulated Compton camera made of Lanthanum (III) Bromide monolithic crystals, coincidences caused by neutrons arriving from the phantom are the most common type of background produced by secondary radiations in the Compton camera, causing between 13% and 33% of the detected coincidences, depending on the beam energy. Results also show that random coincidences are a significant cause of image degradation at high beam intensities, and their influence in the reconstructed images is studied for values of the time coincidence windows from 500 ps to 100 ns.Significance. Results indicate the timing capabilities required to retrieve the fall-off position with good precision. Still, the noise observed in the image when no randoms are considered make us consider further background rejection methods.

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

Compton cameras can offer advantages over gamma cameras for some applications, since they are well suited for multitracer imaging and for imaging high-energy radiotracers, such as those employed in radionuclide therapy. While in conventional clinical settings state-of-the-art Compton cameras cannot compete with well-established methods such as PET and SPECT, there are specific scenarios in which they can constitute an advantageous alternative. The combination of PET and Compton imaging can benefit from the improved resolution and sensitivity of current PET technology and, at the same time, overcome PET limitations in the use of multiple radiotracers. Such a system can provide simultaneous assessment of different radiotracers under identical conditions and reduce errors associated with physical factors that can change between acquisitions. Advances are being made both in instrumentation developments combining PET and Compton cameras for multimodal or three-gamma imaging systems, and in image reconstruction, addressing the challenges imposed by the combination of the two modalities or the new techniques. This review article summarizes the advances made in Compton cameras for medical imaging and their combination with PET.

Development of a two-layer dense-pixel LYSO Compton camera prototype for prompt gamma imaging

Objective.Lutetium-yttrium orthosilicate (LYSO)-based Compton camera (CC) has been proposed for prompt gamma imaging due to its high detection efficiency and position resolution. However, very few LYSO CC prototypes have been built and used for practical evaluation. In this study, we built a lightweight dense-pixel silicon photomultiplier-based two-layer LYSO CC prototype for future prompt gamma imaging.Approach.We attempt the first-ever effort to use the double-encoding with the thick light guide and coding circuit structure for 46 × 46 dense-pixel LYSO detectors construction and use pixel segmentation based on centroid mapping to obtain 4232 spectral calibrations. We also present a framework for list-mode projection data acquisition based on the decoding of the time series data obtained by data acquisition card in this study. Finally, the standard source calibration, ring-like²²Na source with non-uniform intensity, and mixed point-like source with a wide energy spectrum experiments were implemented to evaluate the resolution metrics and imaging performance of the prototype.Main results.The lateral position resolution of the prototype was 1 mm, and the maximum measurement deviation is 2.5 mm and 5 mm in the depth direction for the scatterer and absorber, respectively. In the experiments, the measured energy resolution was 9.63% @ 1.33 MeV for the scatterer and 10.8% @ 1.33 MeV for the absorber. And the detection efficiency of the prototype for a spherical⁶⁰Co source with a diameter of 2.8 mm at 10 cm far was 5.7 × 10^-3@ 1.33 MeV and the full width at half maximum of the reconstruction was 5.5 mm. Besides, the spatial position offset within 2 mm of the radioactive source at 10 cm can be distinguished.Signification.The developed two-layer dense-pixel LYSO CC contributes to incorporating Compton imaging techniques for prompt gamma detection and multiple energy sources into nuclear medical imaging.

Joint image reconstruction algorithm in Compton cameras

Objective. To demonstrate the benefits of using an joint image reconstruction algorithm based on the List Mode Maximum Likelihood Expectation Maximization that combines events measured in different channels of information of a Compton camera. Approach. Both simulations and experimental data are employed to show the algorithm performance. Main results. The obtained joint images present improved image quality and yield better estimates of displacements of high-energy gamma-ray emitting sources. The algorithm also provides images that are more stable than any individual channel against the noisy convergence that characterizes Maximum Likelihood based algorithms. Significance. The joint reconstruction algorithm can improve the quality and robustness of Compton camera images. It also has high versatility, as it can be easily adapted to any Compton camera geometry. It is thus expected to represent an important step in the optimization of Compton camera imaging.

Machine learning-based event recognition in SiFi Compton camera imaging for proton therapy monitoring

Objective. Online monitoring of dose distribution in proton therapy is currently being investigated with the detection of prompt gamma (PG) radiation emitted from a patient during irradiation. The SiPM and scintillation Fiber based Compton Camera (SiFi-CC) setup is being developed for this aim. Approach. A machine learning approach to recognize Compton events is proposed, reconstructing the PG emission profile during proton therapy. The proposed method was verified on pseudo-data generated by a Geant4 simulation for a single proton beam impinging on a polymethyl methacrylate (PMMA) phantom. Three different models including the boosted decision tree (BDT), multilayer perception (MLP) neural network, and k-nearest neighbour (k-NN) were trained using 10-fold cross-validation and then their performances were assessed using the receiver operating characteristic (ROI) curves. Subsequently, after event selection by the most robust model, a software based on the List-Mode Maximum Likelihood Estimation Maximization (LM-MLEM) algorithm was applied for the reconstruction of the PG emission distribution profile. Main results. It was demonstrated that the BDT model excels in signal/background separation compared to the other two. Furthermore, the reconstructed PG vertex distribution after event selection showed a significant improvement in distal falloff position determination. Significance. A highly satisfactory agreement between the reconstructed distal edge position and that of the simulated Compton events was achieved. It was also shown that a position resolution of 3.5 mm full width at half maximum (FWHM) in distal edge position determination is feasible with the proposed setup.

Compton imaging for medical applications

Compton imaging exploits inelastic scattering, known as Compton scattering, using a Compton camera consisting of a scatterer detector in the front layer and an absorber detector in the back layer. This method was developed for astronomy, and in recent years, research and development for environmental and medical applications has been actively conducted. Compton imaging can discriminate gamma rays over a wide energy range from several hundred keV to several MeV. Therefore, it is expected to be applied to the simultaneous imaging of multiple nuclides in nuclear medicine and prompt gamma ray imaging for range verification in particle therapy. In addition, multiple gamma coincidence imaging is expected to be realized, which allows the source position to be determined from a single coincidence event using nuclides that emit multiple gamma rays simultaneously, such as nuclides that emit a single gamma ray simultaneously with positron decay. This review introduces various efforts toward the practical application of Compton imaging in the medical field, including in vivo studies, and discusses its prospects.

Towards machine learning aided real-time range imaging in proton therapy

Compton imaging represents a promising technique for range verification in proton therapy treatments. In this work, we report on the advantageous aspects of the i-TED detector for proton-range monitoring, based on the results of the first Monte Carlo study of its applicability to this field. i-TED is an array of Compton cameras, that have been specifically designed for neutron-capture nuclear physics experiments, which are characterized by [Formula: see text]-ray energies spanning up to 5-6 MeV, rather low [Formula: see text]-ray emission yields and very intense neutron induced [Formula: see text]-ray backgrounds. Our developments to cope with these three aspects are concomitant with those required in the field of hadron therapy, especially in terms of high efficiency for real-time monitoring, low sensitivity to neutron backgrounds and reliable performance at the high [Formula: see text]-ray energies. We find that signal-to-background ratios can be appreciably improved with i-TED thanks to its light-weight design and the low neutron-capture cross sections of its LaCl[Formula: see text] crystals, when compared to other similar systems based on LYSO, CdZnTe or LaBr[Formula: see text]. Its high time-resolution (CRT [Formula: see text] 500 ps) represents an additional advantage for background suppression when operated in pulsed HT mode. Each i-TED Compton module features two detection planes of very large LaCl[Formula: see text] monolithic crystals, thereby achieving a high efficiency in coincidence of 0.2% for a point-like 1 MeV [Formula: see text]-ray source at 5 cm distance. This leads to sufficient statistics for reliable image reconstruction with an array of four i-TED detectors assuming clinical intensities of 10[Formula: see text] protons per treatment point. The use of a two-plane design instead of three-planes has been preferred owing to the higher attainable efficiency for double time-coincidences than for threefold events. The loss of full-energy events for high energy [Formula: see text]-rays is compensated by means of machine-learning based algorithms, which allow one to enhance the signal-to-total ratio up to a factor of 2.

Multi-modal 3D imaging of radionuclides using multiple hybrid Compton cameras

For radiological diagnosis and radionuclide therapy, X-ray and gamma-ray imaging technologies are essential. Single-photon emission tomography (SPECT) and positron emission tomography (PET) play essential roles in radiological diagnosis, such as the early detection of tumors. Radionuclide therapy is also rapidly developing with the use of these modalities. Nevertheless, a limited number of radioactive tracers are imaged owing to the limitations of the imaging devices. In a previous study, we developed a hybrid Compton camera that conducts simultaneous Compton and pinhole imaging within a single system. In this study, we developed a system that simultaneously realizes three modalities: Compton, pinhole, and PET imaging in 3D space using multiple hybrid Compton cameras. We achieved the simultaneous imaging of Cs-137 (Compton mode targeting 662 keV), Na-22 (PET mode targeting 511 keV), and Am-241 (pinhole mode targeting 60 keV) within the same field of view. In addition, the imaging of Ga-67 and In-111, which are used in various diagnostic scenarios, was conducted. We also verified that the 3D distribution of the At-211 tracer inside a mouse could be imaged using the pinhole mode.

Influence of sub-nanosecond time of flight resolution for online range verification in proton therapy using the line-cone reconstruction in Compton imaging

Online ion range monitoring in hadron therapy can be performed via detection of secondary radiation, such as promptγ-rays, emitted during treatment. The promptγemission profile is correlated with the ion depth-dose profile and can be reconstructed via Compton imaging. The line-cone reconstruction, using the intersection between the primary beam trajectory and the cone reconstructed via a Compton camera, requires negligible computation time compared to iterative algorithms. A recent report hypothesised that time of flight (TOF) based discrimination could improve the precision of theγfall-off position (FOP) measured via line-cone reconstruction, where TOF comprises both the proton transit time from the phantom entrance untilγemission, and the flight time of theγ-ray to the detector. The aim of this study was to implement such a method and investigate the influence of temporal resolution on the precision of the FOP. Monte Carlo simulations of a 160 MeV proton beam incident on a homogeneous PMMA phantom were performed using GATE. The Compton camera consisted of a silicon-based scatterer and CeBr₃scintillator absorber. The temporal resolution of the detection system (absorber + beam trigger) was varied between 0.1 and 1.3 ns rms and a TOF-based discrimination method applied to eliminate unlikely solution(s) from the line-cone reconstruction. The FOP was obtained for varying temporal resolutions and its precision obtained from its shift across 100 independentγemission profiles compared to a high statistics reference profile. The optimal temporal resolution for the given camera geometry and 10⁸primary protons was 0.2 ns where a precision of 2.30 ± 0.15 mm (1σ) on the FOP was found. This precision is comparable to current state-of-the-art Compton imaging using iterative reconstruction methods or 1D imaging with mechanically collimated devices, and satisfies the requirement of being smaller than the clinical safety margins.

Performance demonstration of a hybrid Compton camera with an active pinhole for wide-band X-ray and gamma-ray imaging

X-ray and gamma-ray imaging are technologies with several applications in nuclear medicine, homeland security, and high-energy astrophysics. However, it is generally difficult to realize simultaneous wide-band imaging ranging from a few tens of keV to MeV because different interactions between photons and the detector material occur, depending on the photon energies. For instance, photoabsorption occurs below 100 keV, whereas Compton scattering dominates above a few hundreds of keV. Moreover, radioactive sources generally emit both X-ray and gamma-ray photons. In this study, we develop a “hybrid” Compton camera that can simultaneously achieve X-ray and gamma-ray imaging by combining features of “Compton” and “pinhole” cameras in a single detector system. Similar to conventional Compton cameras, the detector consists of two layers of scintillator arrays with the forward layer acting as a scatterer for high-energy photons (> 200 keV) and an active pinhole for low-energy photons (< 200 keV). The experimental results on the performance of the hybrid camera were consistent with those from the Geant4 simulation. We simultaneously imaged \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{241}$$\end{document}241Am (60 keV) and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{137}$$\end{document}137Cs (662 keV) in the same field of view, achieving an angular resolution of 10\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^\circ $$\end{document}∘ (FWHM) for both sources. In addition, imaging of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{211}$$\end{document}211At was conducted for the application in future nuclear medicine, particularly radionuclide therapy. The initial demonstrative images of the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{211}$$\end{document}211At phantom were reconstructed using the pinhole mode (using 79 keV) and Compton mode (using 570 keV), exhibiting significant similarities in source-position localization. We also verified that a mouse injected with 1 MBq of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{211}$$\end{document}211At can be imaged via pinhole-mode measurement in an hour.

Compton imaging reconstruction methods: a comparative performance study of direct back-projection, SOE, a new Bayesian algorithm and a new Compton inversion method applied to real data with Caliste

Compton imaging is one of the main methods to localize radioactive hotspots, which emit high-energy gamma-ray photons, above 200 keV. Most of the Compton imaging systems are composed by at least two detection layers or one 3D position sensitive detector. In this study, we demonstrate the application of a new miniature pixelated single plane detector to Compton imaging. In this configuration, we do not have the information on interaction depth but we successfully test its ability to perform Compton localization by means of comparing different Compton reconstruction algorithms applied to real data measured with our single plane detection system.

Development and performance verification of a 3-D position-sensitive Compton camera for imaging MeV gamma rays

In gamma-ray astronomy, the 1-10 MeV range is one of the most challenging energy bands to observe owing to low photon signals and a considerable amount of background contamination. This energy band, however, comprises a substantial number of nuclear gamma-ray lines that may hold the key to understanding the nucleosynthesis at the core of stars, spatial distribution of cosmic rays, and interstellar medium. Although several studies have attempted to improve observation of this energy window, development of a detector for astronomy has not progressed since NASA launched the Compton Gamma Ray Observatory (CGRO) in 1991. In this work, we first developed a prototype 3-D position-sensitive Compton camera (3D-PSCC), and then conducted a performance verification at NewSUBARU, Hyogo in Japan. To mimic the situation of astronomical observation, we used a MeV gamma-ray beam produced by laser inverse Compton scattering. As a result, we obtained sharp peak images of incident gamma rays irradiating from incident angles of 0° and 20°. The angular resolution of the prototype 3D-PSCC was measured by the Angular Resolution Measure and estimated to be 3.4° ± 0.1° (full width at half maximum (FWHM)) at 1.7 MeV and 4.0° ± 0.5° (FWHM) at 3.9 MeV. Subsequently, we conceived a new geometry of the 3D-PSCC optimized for future astronomical observations, assuming a 50-kg class small satellite mission. The SΩ of the 3D-PSCC is 11 cm²sr, anticipated at 1 MeV, which is small but provides an interesting possibility to observe bright gamma-ray sources owing to the high intrinsic efficiency and large field of view (FoV).