Compton Imaging Research Articles

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.

Measurement of Galactic 26Al with the Compton Spectrometer and Imager

Abstract The Compton Spectrometer and Imager (COSI) is a balloon-borne compact Compton telescope designed to survey the 0.2–5 MeV sky. COSI’s energy resolution of ∼0.2% at 1.8 MeV, single-photon reconstruction, and wide field of view make it capable of studying astrophysical nuclear lines, particularly the 1809 keV γ-ray line from decaying Galactic 26Al. Most 26Al originates in massive stars and core-collapse supernova nucleosynthesis, but the path from stellar evolution models to Galaxy-wide emission remains unconstrained. In 2016, COSI had a successful 46 day flight on a NASA superpressure balloon. Here, we detail the first search for the 1809 keV 26Al line in the COSI 2016 balloon flight using a maximum-likelihood analysis. We find a Galactic 26Al flux of (8.6 ± 2.5) × 10−4 ph cm−2 s−1 within the Inner Galaxy (∣ℓ∣ ≤ 30°, ∣b∣ ≤ 10°) with 3.7σ significance above background. Within uncertainties, this flux is consistent with expectations from previous measurements by SPectrometer on INTEGRAL (SPI) and the Compton Telescope on the Compton Gamma-Ray Observatory (COMPTEL). This analysis demonstrates COSI’s powerful capabilities for studies of γ-ray lines and underscores the scientific potential of future compact Compton telescopes. In particular, the next iteration of COSI as a NASA Small Explorer satellite has recently been approved for launch in 2025.

Calibrations of the Compton Spectrometer and Imager

The Compton Spectrometer and Imager (COSI) is a balloon-borne soft γ-ray telescope (0.2–5MeV) designed to study astrophysical sources. COSI employs a compact Compton telescope design and is comprised of twelve high-purity germanium semiconductor detectors. Tracking the locations and energies of γ-ray scatters within the detectors permits high-resolution spectroscopy, direct imaging over a wide field-of-view, polarization studies, and effective suppression of background events. Critical to the precise determination of each interaction’s energy, position, and the subsequent event reconstruction are several calibrations conducted in the field before launch. Additionally, benchmarking the instrument’s higher-level performance through studies of its angular resolution, effective area, and polarization sensitivity quantifies COSI’s scientific capabilities. In May 2016, COSI became the first science payload to be launched on NASA’s superpressure balloon and was slated for launch again in April 2020. Though the 2020 launch was canceled due to the COVID-19 pandemic, the COSI team took calibration measurements prior to cancellation. In this paper we provide a detailed overview of COSI instrumentation, describe the calibration methods, and compare the calibration and benchmarking results of the 2016 and 2020 balloon campaigns. These procedures will be integral to the calibration and benchmarking of the NASA Small Explorer satellite version of COSI scheduled to launch in 2025.

DOI correction for gamma ray energy reconstruction based on energy segment in 3D position-sensitive CdZnTe detectors

The amplitude of the induced signal in 3D CdZnTe detector depends on the depth of interaction (DOI). Therefore, calibrating the detector by using DOI correction technology plays a crucial role in improving the energy resolution of the detectors to gamma rays. The current DOI correction method focuses on the single energy gamma rays, and its application to multiple energy gamma-rays are not found. In this study, we propose an improved energy correction algorithm with excellent correction results in the multiple energy gamma-ray detection. In the experiment, the DOI correction factors of a CdZnTe detector under different energies are discussed. The energy resolution and peak height of multiple energy peaks in the energy spectrum are significantly improved by using the segment energy correction method. We also extend the DOI correction method to the gamma detectors used in the Compton imaging, and the influence of this method on the Compton imaging quality is also discussed. For a single 60Co point source, the intrinsic efficiency increases from 6.5‰ to 8.3‰.

Unveiling the Origin of the Fermi Bubbles with MeV Photon Telescopes

Abstract The Fermi Bubbles (FBs) are a pair of large-scale ellipsoidal structures extending above and below the Galactic plane almost symmetrically aligned with the Galactic center. After more than 10 yr since their discovery, their nature and origin remain unclear. Unveiling the primary emission mechanisms, whether hadronic or leptonic, is considered to be the main tool to shed light on the topic. We explore the potential key role of MeV observations of the FB, and we provide a recipe to determine the sensitivity of Compton and Compton-pair telescopes to the extended emission of the FB. We illustrate the capabilities of the Imaging Compton Telescope COMPTEL, the newly selected NASA MeV mission Compton Spectrometer and Imager, as well as the expectations for a potential future Compton-pair telescope such as the All-sky Medium Energy Gamma-ray Observatory eXplorer.

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 gamma -ray energies spanning up to 5–6 MeV, rather low gamma -ray emission yields and very intense neutron induced gamma -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 gamma -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_{3} crystals, when compared to other similar systems based on LYSO, CdZnTe or LaBr_{3}. Its high time-resolution (CRT sim 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_{3} monolithic crystals, thereby achieving a high efficiency in coincidence of 0.2% for a point-like 1 MeV gamma -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^{8} 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 gamma -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.

Research on proton beam spot imaging based on pixelated gamma detector

The secondary particles from the spallation target of the China Spallation Neutron Source are mainly gammas and neutrons, which are related to the incident proton. The reconstruction of the proton beam spot could be implemented based on the distribution of the positions of secondary gammas or neutrons. The methods of pinhole imaging and Compton imaging are developed by measuring the position distribution of gammas based on the pixelated detector. The secondary gammas could be detected by the pixelated gamma detector directly. The neutron can be identified by detecting the characteristic (478 keV) γ-rays from the 10B(n, α) reactions. In order to detect secondary neutrons, a layer of 10B converter is added before the pixelated gamma detector. The pixelated gamma detector is sensitive to the characteristic (478 keV) γ-rays and then the neutron imaging could be achieved based on measuring the position distribution of the characteristic (478 keV) γ-rays.

Characterization of time-of-flight double-photon Compton imaging system by simulation

Double-photon emission computed tomography (DPECT) has been proposed to overcome the disadvantage of a low signal-to-background ratio for conventional Compton imaging. This method has shown significant image reconstruction capability in the 2D plane. However, its performance is unsatisfactory when the field of view is 3-dimensional (3D). To solve this problem, we propose application of the time-of-flight (TOF) technique to DPECT as an enhancement. In this research, we used a Geant4 simulation to demonstrate the effectiveness of TOF in large 3D volume image reconstruction.

Compton Imaging and Machine-Learning techniques for an enhanced sensitivity in key stellar (n,γ) measurements

Neutron capture cross-section measurements are fundamental in the study of astrophysical phenomena, such as the slow neutron capture (s-) process of nucleosynthesis operating in red-giant stars. To enhance the sensitivity of such measurements we have developed the i-TED detector. i-TED is an innovative detection system which exploits the Compton imaging technique with the aim of obtaining information about the incoming direction of the detected γ-rays. The imaging capability allows one to reject a large fraction of the dominant γ-ray background, hence enhancing the (n,γ) detection sensitivity. This work summarizes the main results of the first experimental proof-of-concept of the background rejection with i-TED carried out at CERN n_TOF using an early i-TED demonstrator. Two state-of-the-art C6D6 detectors were also used to benchmark the performance of i-TED. The i-TED prototype built for this study shows a factor of ~3 higher detection sensitivity than C6D6 detectors in the ~10 keV neutron-energy range of astrophysical interest. This works also introduces the perspectives of further enhancement in performance attainable with the final i-TED array and new analysis methodologies based on Machine-Learning techniques. The latter provide higher (n,γ) detection efficiency and similar enhancement in the sensitivity than the analytical method based on the Compton scattering law. Finally, we present our proposal to use this detection system for the first time on key astrophysical (n,γ) measurements, in particular on the s-process branching-point 79Se, which is especially well suited to constrain the thermal conditions of Red Giant and Massive Stars.

Coded Aperture and Compton Imaging Capability of Spherical Detector System Design Based on Gagg Scintillators

Coded Aperture and Compton Imaging Capability of Spherical Detector System Design Based on Gagg Scintillators

Coded Aperture and Compton Imaging Capability of Spherical Detector System Design Based on GAGG Scintillators

Coded Aperture and Compton Imaging Capability of Spherical Detector System Design Based on GAGG Scintillators

First in-beam tests on simultaneous PET and Compton imaging aimed at quasi-real-time range verification in hadron therapy

Hadron therapy with protons has advantages with respect to conventional radiotherapy because of the maximization of the dose at the Bragg peak. As a drawback, and because of different systematic uncertainty sources, a quasi-real time monitoring for the proton range verification is required to reduce safety margins. In this respect, two gamma-ray imaging techniques are pursued: prompt gamma-ray monitoring and positron-annihilation tomography (PET). The promising prompt gamma-ray monitoring requires detection systems with large detection efficiency, high time resolution, compactness, fast response, low sensitivity to neutron-induced backgrounds and powerful image reconstruction capabilities. On the other hand, in-beam PET surveys require additionally good γ-ray position reconstruction resolution. In this contribution we show that, to a large extent, both approaches can be simultaneously accomplished by using an array of Compton cameras conveniently arranged around the target volume. Here we demonstrate experimentally the suitability of such an array, named i-TED, for PG monitoring in ion-range monitoring during Hadron Therapy, in-beam PET survey and β+ production yield measurements capability. Furthermore, with the use of GPUs, a quasi-real time PG monitoring and in-beam PET can be achieved.

Perspectives of brain imaging with PET systems

Abstract In this partial review and partial attempt at vision of what may be the future of dedicated brain PET scanners, the key implementations of the PET technique, we postulate that we are still on a development path and there is still a lot to be done in order to develop optimal brain imagers. Optimized for particular imaging tasks and protocols, and also mobile, that can be used outside the PET center, in addition to the expected improvements in sensitivity and resolution. For this multi-application concept to be more practical, flexible, adaptable designs are preferred. This task is greatly facilitated by the improved TOF performance that allows for more open, adjustable, limited angular coverage geometries without creating image artifacts. As achieving uniform very high resolution in the whole body is not practical due to technological limits and high costs, hybrid systems using a moderate-resolution total body scanner (such as J-PET) combined with a very high performing brain imager could be a very attractive approach. As well, as using magnification inserts in the total body or long-axial length imagers to visualize selected targets with higher resolution. In addition, multigamma imagers combining PET with Compton imaging should be developed to enable multitracer imaging.

Performance improvement of Compton imaging of astatine-211 by optimising coincidence time windows

Astatine-211 is one of the promising radioisotopes for targeted alpha therapy. Optimising treatment strategies as well as determining the suitability of a given agent for a particular patient requires to image the time-dependent distribution of the targeted radiotherapeutic agent both in tumours and in normal tissues. Since the biodistribution of astatine is different from that of iodine, imaging astatine-211 directly is essential. In the previous study of astatine-211 Compton imaging, random coincidence events due to polonium K-shell X-rays were dominant and seemed to cause saturation of counts. Thus optimisation of the coincidence time windows is important to reduce random coincidence events. In this study, we have optimised the coincidence time windows of a Compton camera and improved the sensitivity, noise and spatial resolution of astatine-211 imaging.

Scientific Potential of MeV Polarimetry for Relativistic Jets

AbstractRelativistic jets from supermassive black holes or stellar mass black holes are among the most powerful astrophysical phenomena. Magnetic field plays an important role in the jet launching and propagation, as well as particle acceleration and radiation. Polarimetry is the only way to observe the magnetic field evolution. The recent launch of the Imaging X-ray Polarimetry Explorer (IXPE) has opened up the X-ray polarization window, which has revealed very interesting phenomena for relativistic jets. However, the field of MeV gamma-ray polarimetry remains largely unexplored. This paper aims to summarize key scientific potentials for MeV polarimetry for blazars and gamma-ray bursts (GRBs) from recent theoretical modeling. These predictions, which are closely related to the cosmic ray acceleration, neutrino production, radiation mechanism, and the jet evolution, can be examined by future MeV polarimeters, such as the Compton Spectrometer and Imager (COSI), the LargE Area burst Polarimeter (LEAP), and the All-sky Medium-Energy Gamma-ray Observatory eXplorer (AMEGO-X).

Carbon range verification with 718\xa0keV Compton imaging

Carbon ion radiotherapy is a sophisticated radiation treatment modality because of its superiority in achieving precise dosage distribution and high biological effectiveness. However, there exist beam range uncertainties that affect treatment efficiency. This problem can be resolved if the clinical beam could be monitored precisely in real-time, such as by imaging the prompt gamma emission from the target. In this study, we performed real-time detection and imaging of 718 keV prompt gamma emissions using a Si/CdTe Compton camera. We conducted experiments on graphite phantoms using clinical carbon ion beams of 290 MeV/u energy. Compton images were reconstructed using simple back-projection methods from the energy events of 718 keV prompt gamma emissions. The peak intensity position in reconstructed 718 keV prompt gamma images was few millimeters below the Bragg peak position. Moreover, the dual- and triple-energy window images for all positions of phantoms were not affected by scattered gammas, and their peak intensity positions were approximately similar to those observed in the reconstructed 718 keV prompt gamma images. In conclusion, the findings of the current study demonstrate the feasibility of using our Compton camera for real-time beam monitoring of carbon ion beams under clinical beam intensity.

Application of a deep learning algorithm to Compton imaging of radioactive point sources with a single planar CdTe pixelated detector

Compton imaging is the main method for locating radioactive hot spots emitting high-energy gamma-ray photons. In particular, this imaging method is crucial when the photon energy is too high for coded-mask aperture imaging methods to be effective or when a large field of view is required. Reconstruction of the photon source requires advanced Compton event processing algorithms to determine the exact position of the source. In this study, we introduce a novel method based on a Deep Learning algorithm with a Convolutional Neural Network (CNN) to perform Compton imaging. This algorithm is trained on simulated data and tested on real data acquired with Caliste, a single planar CdTe pixelated detector. We show that performance in terms of source location accuracy is equivalent to state-of-the-art algorithms, while computation time is significantly reduced and sensitivity is improved by a factor of ∼5 in the Caliste configuration.

Tomographic reconstruction of a spatially-extended source from the perimeter of a restricted-access zone using a SCoTSS compton gamma imager

It is a standard procedure in many countries that response to a nuclear or radiological accident or incident would involve mobile aerial- or ground-based survey with highly sensitive gamma-ray detectors to map the distribution of radioactivity. There may however arise situations in which ground- or air-based detectors are not able to access an area to survey for radioactive materials, therefore technologies and techniques that can estimate the position and activity of radioactive materials from a distance are under development. Tomographic reconstruction methods, well-known in medical physics, permit the reconstruction of an N-dimensional map or image, from a number of N-1-dimensional cross-sectional images, or back-projections. We are investigating a tomographic reconstruction method to reconstruct the radioactivity distribution within a restricted-access zone using measurements from a Compton gamma imager placed at several locations around the perimeter of the zone. In this work an extended source of La-140 with an activity of 35 GBq was deposited within a 500 m by 500 m zone that was surveyed from the perimeter at six locations using a Silicon photomultiplier-based Compton Telescope for Safety and Security (SCoTSS) gamma imager. The reconstructed Compton images from multiple viewpoints were then projected back into the zone to reconstruct the distribution of La-140 within it. This tomographic method reconstructed high intensity along the known location of the La-140 source, suggesting that the method is able to localize the radioactive material. A simple fit to measured counts using a point-source approximation of the source distribution yielded a strength estimate of (7 ± 2) GBq at time of deposition, a reasonable result given the presence of soil and snow attenuation. Our method provides an expedient estimate of the distribution of radioactivity using tomographic techniques. It may be used to inform decisions made on the scene in urgent situations where the distribution of radioactivity must be reconstructed from a distance.

Polaris-LAMP: Multi-Modal 3-D Image Reconstruction With a Commercial Gamma-Ray Imager

The Polaris-LAMP multi-modal 3-D gamma-ray imager is a radiation mapping and imaging platform which uses a commercial off-the-shelf (COTS) detector integrated with a contextual sensor localization and mapping platform. The integration of these systems enables a free-moving radiation imaging capability with proximity mapping, coded-aperture, and Compton imaging modalities, which can create 3-D reconstruction of photon sources from tens of keV to several MeV. Gamma-ray events are recorded using a segmented cadmium zinc telluride (CZT) detector (Polaris-H Quad by H3D Inc., Ann Arbor, MI, USA), while scene data are derived from a contextual sensor and computation package developed by Lawrence Berkeley National Laboratory which includes GPS, laser ranging, and inertial measurement sensors. An onboard computer uses these inputs to create rapidly updating pose (10 Hz) and 3-D scene estimates using a simultaneous localization and mapping (SLAM) algorithm. The precise gamma-ray event location and timing resolution of the Polaris CZT sensor enables Compton imaging above several hundred keV, while photon sources at lower images are localized using coded-aperture imaging techniques. The multi-modal imaging concept enables imaging of diverse radiation sources spanning from the 59-keV emission of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">241</sup> Am to the 1.1 and 1.3 MeV lines of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sup> Co. This work focuses on the description of the operational principles of the detector system and demonstrating the 3-D imaging performance in a variety of source detection and mapping scenarios. As a proof of concept, we demonstrate mapping complex environments, including both point source and distributed-source environments using proximity, coded-aperture, and Compton imaging modalities. Furthermore, we show the successful use of the system to perform measurements in high-background environments through analysis of arrays of uranium hexafluoride cylinders at the Paducah UF6 project site.