Compton Camera Research Articles

Inversion formulas for the attenuated conical Radon transform: Plane and cylinder case

Since the invention of Compton camera imaging, the conical Radon transform, which maps a given function defined on 3-dimensional Euclidean space to its surface integrals over cones, has been studied intensively. The problem of recovering such a function from its unrestricted conical Radon transform is overdetermined, since the set of all cones depends on the three dimensions of the vertex, the two dimensions of the central axis, and the one-dimensional opening angle. Therefore, various types of restricted conical Radon transforms have also been studied. However, most of these studies have neglected the attenuation of the medium. This article presents the study of attenuated conical Radon transforms with vertices on a plane (referred to as the plane case) or the cylinder (referred to as the cylinder case) in a 3-dimensional space. In all cases, the function is integrated over conical surfaces with an additional weight that decreases with the distance to the vertex of the cones. The main results provide explicit inversion formulas for the attenuated conical Radon transform in the plane and in the cylinder case.

Quantum Entanglement Filtering: A PET feasibility study in imaging dual-positron and prompt gamma emission via Monte Carlo simulation.

In this article, we investigate quantum entanglement (QE) filtering to address the challenges in multi-isotope positron emission tomography (PET) or in PET studies utilizing radiotracers with dual- positron and prompt gamma emissions. Via GATE simulation, we demonstrate the efficacy of QE filtering using a one-of-a-kind cadmium zinc telluride (CZT) PET system - establishing its viability as a multimodal scanner and ability to perform QE filtering. We show the high Compton scattering probability in this CZT-based scanner with 44.2% of gammas undergoing a single scatter prior to absorption. Additionally, the overall system sensitivity as a standard PET scanner (11.29%), QE-PET scanner (6.81%), and Compton Camera (11.29%) is quantified. Further, we find a 23% decrease in the double Compton scatter (DCSc) frequency needed for QE filtering for each mm decrease in crystal resolution and an increase in mean absolute error (MAE) of their Δϕs from 6.8° for 1 mm resolution to 9.5° , 12.2° , and 15.3° for 2, 4, and 8 mm resolution, respectively. These results reinforce the potential of CZT detectors to lead next generation PET systems taking full advantage of the QE information of positron annihilation photons.

High-contrast Compton camera: Challenges to high-quality and broadband imaging

In the field of nuclear medicine, various radiopharmaceuticals require wideband x-ray/gamma-ray imaging devices for clinical and treatment monitoring. Compton cameras, which perform imaging using high-energy gamma rays, have the potential to significantly increase the variety of radioactive nuclides that can be imaged. However, artifacts caused by the so-called “Compton cone” have hindered their clinical use. Therefore, we propose the use of a collimator to improve the contrast of images obtained using Compton cameras. In this study, we developed a high-contrast Compton camera by attaching a tungsten collimator to its front surface. The contrast is improved by applying weighting to the signals based on the distance that the high-energy gamma rays penetrated the collimator walls. As a demonstration, we visualized 198Au plates that emit 412-keV gamma rays with and without the collimator. In addition, low-energy (<200 keV) x-ray/gamma-ray imaging, which is difficult for conventional Compton cameras, was achieved by performing single-photon emission computed tomography (SPECT) using the collimator and scatterer of the Compton camera. We demonstrated broadband gamma-ray imaging by visualizing a 133Ba standard source using 81-keV and 356-keV gamma rays based on the principles of SPECT and Compton cameras, respectively.

Wide-band X-ray and gamma-ray imaging for clinical application; visualization of pharmacokinetics in targeted alpha therapy

Targeted alpha therapy (TAT) is a nuclear medicine therapy which administers alpha-emitting drugs and causes localized and effective damage to tumors. To investigate whether the drugs accumulate in the tumor correctly, in-situ visualization of the distribution of the drugs is essential. Visualization of alpha-emitting drugs using X-rays and gamma rays emitted simultaneously with the alpha-rays is suggested in theranostics, a combination of therapy and diagnostics. Since various X-rays and gamma rays ranging from a few tens of keV to several MeV are emitted from the alpha-emitting radionuclides, wide-band X-ray and gamma-ray imagers are required for a more versatile visualization of alpha-emitting drugs. Therefore, we developed a hybrid Compton camera (HCC) to visualize broadband X-rays and gamma rays. In this study, we visualized representative alpha-emitting drugs, Ra-223 dichloride and At-211 NaAt, using HCC. Ra-223 dichloride in the human body was imaged using gamma rays around 300 keV, and in-vivo imaging of At-211 NaAt in a mouse was performed using 79-keV X-rays. This study demonstrates the potential of HCC as a versatile imaging device for alpha-emitting drugs.

Improving Compton camera imaging of multi-energy radioactive sources by using machine learning algorithms for event selection

Event selection and background reduction for Compton camera imaging of multi-energy radioactive sources has been performed by employing neural networks. A Compton camera prototype with detectors made of LaBr3 crystals coupled to silicon photomultiplier arrays was used to acquire experimental data from a circular array of 22Na sources. The prototype and two arrays of 22Na sources were simulated with GATE v8.2 Monte Carlo code, to obtain data for neural network training. Neural network models were trained on simulated data for event classification. The optimum models were found by using Weights & Biases platform tools. The trained models were used to classify simulated and real data for selecting signal events and rejecting background prior to image reconstruction. The models performed well on simulated data. The image obtained with experimental data showed an improvement with respect to event selection with energy cuts. The method is promising for Compton camera imaging of multi-energy radioactive sources.

Demonstration of in-vivo simultaneous 3D imaging with 18F-FDG and Na131I using Compton–PET system

Simultaneous imaging of the SPECT tracer 131I and PET tracer 18F is important in the diagnosis of high- and low-grade thyroid cancers because high-grade thyroid cancers have high 18F-FDG and low 131I uptake, while low-grade thyroid cancers have high 131I and low 18F-FDG uptake. In this study, Na131I and 18F-FDG were simultaneously imaged using the Compton-PET system, in vivo. The angular resolution and sensitivity of the Compton camera with 356 keV gamma ray measured using a 133Ba point source were 12.3° and 2 × 10−5, respectively. The spatial resolution and sensitivity of PET were measured with a 22Na point source. The transaxial and axial spatial resolutions of the PET at the center of the FOV were 1.15 mm and 2.04 mm, respectively. Its sensitivity was 1.2 × 10−4. In-vivo images of the 18F and 131I isotopes were simultaneously acquired from mice. These showed that 18F-FDG was active in the heart, brown fat, and brain, while Na131I was active in the thyroid, stomach, and bladder. Artifacts were found in the Compton camera images when the activity of 131I was much lower than that of 18F. This study demonstrates the potential of simultaneous clinical imaging of 18F and 131I.

Calibration of a gamma ray Compton camera for radioactivity measurements

AbstractA dual-plane Compton imaging detector previously developed for prompt gamma imaging has been further tested and calibrated for quantitative radioactivity determination, specifically to assess radioactive debris from a reactor fuel element. The debris piece contains uranium isotopes and several long-lived fission products, including cesium-137 (Cs-137). The detector consists of two pixelated planes of cadmium-zinc-telluride (CZT) crystals which record gamma ray events interacting within these volumes. In addition to the energy spectrum obtained by pulse height analysis, the time-stamped events from each detector pixel are sorted by location into 3 categories for post-processing: intra-plane, inter-plane, and coincidence. An image reconstruction software enables spatial localization of the select gamma ray peak(s) from the spectrum, thereby separating the gamma ray by specific isotopes. We used the camera to image a small piece of debris for demonstration purposes. A measurement was performed alongside a high purity germanium (HPGe) detector, which could determine the isotopes' absolute activities and be used as a reference for the camera. In addition, a known isotopic point source was used to calibrate the camera’s energy detection efficiency in the same measurement geometry, correlating the image pixel intensity to the isotopic activity.

A single-layer compton camera for nuclear decommissioning: An improved back-projection algorithm

The decommissioning of nuclear facilities, a critical and hazardous process due to radiation exposure, necessitates the advancement of radiological measurement techniques concerning safety and efficiency. This study presents an optimized back-projection method, integrating a novel single-layer Compton camera and 3D camera setup with the Timepix3 readout chip to improve the precision and efficiency of gamma-ray source localization in decommissioning scenarios. Implementing ‘twin addition’ and ‘twin multiplication’ techniques addressed data ambiguities that arise from event selection in the Compton camera, enhancing the reliability of localization. Introducing a zoom function significantly improved computational efficiency, achieving 163 times faster computation times without sacrificing accuracy. Our method demonstrated enhanced precision with a median angular error of 1.41°, outperforming traditional methods and showing competitive advantages over state-of-the-art technologies, including the Caliste-HD detector. The feasibility of integrating this methodology onto mobile robotic platforms suggests a promising avenue to minimize human radiation exposure and optimize decommissioning tasks, ensuring safer and more effective nuclear facility decommissioning.

Performance evaluation of a GAGG-SiPM based compton camera for gamma-ray astronomy

We fabricated and calibrated a GAGG-SiPM based Compton camera which can provide directional information of radiation sources for gamma-ray astronomy. The proposed Compton camera consists of two layers of 8 × 8 GAGG scintillator detectors, directly read out by an 8 × 8 SiPM (MicroFC-60035) array. Each GAGG crystal in the scatterer layer measures 6 × 6 × 8 mm3 in 7.2 mm pitches, while that in the absorber layer measures 6 × 6 × 12 mm3 in 7.2 mm pitches. A reconstruction software running on multiple GPUs based on the list mode MLEM algorithm for the Compton camera has also been developed. Point source imaging studies demonstrated an angular resolution (Full width at half maximum, FWHM) of 13.5°, and a measured intrinsic efficiency of 0.85% when a137Cs point source is placed 52 mm away from the scatterer with a scatterer-to-absorber distance of 57 mm. The camera's capability to simultaneously image point sources of different energies was assessed by imaging two distinct point sources, 22Na and 137Cs. The phantom imaging study was carried out with a U-shaped tube injected with 18F solution to verify the imaging capability of the system.

Radioactivity estimation of radioactive hotspots using a Compton camera and derivation of dose rates in the surrounding environment

At the Fukushima Daiichi Nuclear Power Station, radiation sources released in the accident were deposited on various equipment and building structures. During decommissioning, it is crucial to understand the distribution of radiation sources and ambient dose equivalent rates to reduce worker exposure and implement detailed work planning. In this study, the author introduces a method for visualizing radiation sources, estimates their radioactivity using a Compton camera, and derives the dose rate around the radiation sources. In the demonstration test, the Compton camera was used to visualize radioactive hotspots caused by 137Cs radiation sources deposited in the outdoor environment and estimated the radioactivity. Furthermore, the dose rate around the hotspots was calculated from the estimated radioactivity, which confirmed that the calculated dose rate correlated with the dose rate measured using a survey meter. This approach is novel, where a series of analyses were conducted using the Compton camera to visualize radioactive hotspots, estimate the radioactivity, and derive the dose rate in the surrounding environment.

ComptonNet: A direct reconstruction model for Compton camera

ComptonNet: A direct reconstruction model for Compton camera

Prompt gamma imaging system in particle therapy: a mini-review

Accurate in-vivo verification of beam range and dose distribution is crucial for the safety and effectiveness of particle therapy. Prompt gamma (PG) imaging, as a method for real-time verification, has gained prominence in this area. Currently, several PG imaging systems are under development, including gamma electron vertex imaging (GEVI), the Compton camera, the slit camera, and the multi-array type collimator camera. However, challenges persist in dose prediction accuracy, largely due to patient positioning uncertainty and anatomical changes. Although each system demonstrates potential in verifying PG range, further improvements in detection efficiency, spatial resolution, background reduction, and integration into clinical workflows are essential.

Single-layer Compton camera based on a 1 mm thick pixelated CDTE detectors

Compton camera is a novel γ-camera paradigm that relies upon the kinematics of Compton scattering for image reconstruction. A Conventional Compton camera uses two layers of sensors - the Absorber and the Scatterer. This work reports the proof of concept of a single-layer Compton camera (SLCC) where simultaneous Compton pairs events are registered in a single High-Z semiconductor sensor. The Hybrid pixel detector (HPD) is 1 mm thick, 256 x 256 square pixels with 55 μm pixel pitch CdTe sensor bonded to a Timepix3 readout ASIC. The superior spectroscopic imaging and fast timing capabilities of the Timepix3 readout ASIC coupled with a microscopic and highly pixelated CdTe enable simultaneous event detection of multi-energies occurring at multiple positions. The concept was exemplified by measuring the 122 KeV γ-ray emitted from a 57Co radioisotope source at two positions. Data were captured in the Timepix3 data-driven mode with the KATHERINE readout system via Gigabyte Ethernet data transfer. A bespoke Compton kinematics criterion algorithm implemented in Python 3 IDE was used for data analysis and Compton’s image reconstruction. Numerous events (5.2 million) were captured for 30-minute acquisitions. However, due to the thin nature of CdTe, fewer events (≈ 0.01 %) met the Compton event selection criteria. Nevertheless, the algorithm accurately pinpointed the radioisotope’s location, demonstrating proof of concept of the SLCC system.

‘Accurate proton range shift verification by using a two-layer dense-pixel LYSO compton camera prototype

ObjectiveCompton camera (CC)-based prompt gamma imaging (PGI) is promising to realize real-time in-vivo range verification during proton therapy. Lutetium–yttrium orthosilicate (LYSO)-based CC has advantages in high detection efficiency and position resolution. However, few LYSO CC-based PGI experimental evaluations have been investigated. ApproachWe built a 46 × 46 two-layer dense-pixel LYSO CC prototype for PGI. We attempt the first-ever effort to conduct the proton range shift verification experiments with the LYSO CC prototype. A 13 MeV proton beam with a cross-section of 1 mm2 produced by the Compact Pulsed Hadron Source (CPHS) and a lead collimator irradiates a block of graphite, a block of polymethacrylates (PMMA) and a block of fresh pork, respectively. A range shift varied from 1 mm to 5 mm of the proton beam is used to evaluate the performance of the prototype in PGI. The peak current intensity was about 640 nA. The pulse width and frequency were 100 μs and 1 Hz, respectively. The rate of incident protons in each measurement is about 3.6 × 108/s, and the corresponding delivered dose is about 5.8 Gy. Main resultsThe built LYSO CC prototype can distinguish the displacement of the Bragg peak in 2 mm (reaching 1 mm) at a distance of about 13 cm, and can accurately reconstruct the positions of the Bragg peak with a mean deviation of 0.2 mm in the eighteen groups of experiments. SignificationThe experiments demonstrate the feasibility of the developed two-layer dense-pixel LYSO CC in realizing accurate range shift monitoring and show its potential in range verification during clinical proton therapy and high particle rate proton therapy (e.g., FLASH proton therapy).

Development of a radioactive substance detection system integrating a Compton camera and a LiDAR camera with a hexapod robot

Herein, a system for locating radioactive substances using a hexapod robot equipped with a Compton camera and light detection and ranging (LiDAR) camera was developed and its performance was evaluated at FUGEN, a nuclear facility owned by the Japan Atomic Energy Agency. During the evaluation test, the system visualized and identified the radioactive hotspot, wherein radioactive substances accumulated due to 60Co. This was achieved by projecting images of the radioactive substances acquired using the Compton camera onto a three-dimensional model of the work environment obtained using the LiDAR camera. Furthermore, the results of dose-rate measurements related to the work area obtained using a survey meter confirmed that the identified radioactive hotspot exhibited a dose rate that was one order of magnitude higher than that exhibited by the surrounding area.

Localization of hotspots via a lightweight system combining Compton imaging with a 3D lidar camera

Efficient and secure decommissioning of nuclear facilities demands advanced technologies. In this context, gamma-ray detection and imaging are crucial in identifying radioactive hotspots and monitoring radiation levels. Our study is dedicated to developing a gamma-ray detection system tailored for integration into robotic platforms for nuclear decommissioning, offering a safe and automated solution for this intricate task and ensuring the safety of human operators by mitigating radiation exposure and streamlining hotspot localization.Our approach integrates a Compton camera based 3D reconstruction algorithm with a single Timepix3 detector. This eliminates the need for a second detector and significantly reduces system weight and cost. Additionally, combining a 3D camera with the setup enhances hotspot visualization and interpretation, rendering it an ideal solution for practical nuclear decommissioning applications.In a proof-of-concept measurement utilizing a 137Cs source, our system accurately localized and visualized the source in 3D with an angular error of 1° and estimated the activity with a 3% relative error. This promising result underscores the system's potential for deployment in real-world decommissioning settings. Future endeavors will expand the technology's applications in authentic decommissioning scenarios and optimize its integration with robotic platforms.The outcomes of our study contribute to heightened safety and accuracy for nuclear decommissioning works through the advancement of cost-effective and efficient gamma-ray detection systems.

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.

High-energy extension of the gamma-ray band observable with an electron-tracking Compton camera

Although the MeV gamma-ray band is a promising energy-band window in astrophysics, the current situation of MeV gamma-ray astronomy significantly lags behind those of the other energy bands in angular resolution and sensitivity. An electron-tracking Compton camera (ETCC), a next-generation MeV detector, is expected to revolutionize the situation. An ETCC tracks each Compton-recoil electron with a gaseous electron tracker and determines the incoming direction of each gamma-ray photon; thus, it has a strong background rejection power and yields a better angular resolution than classical Compton cameras. Here, we study ETCC events in which the Compton-recoil electrons do not deposit all energies to the electron tracker but escape and hit the surrounding pixel scintillator array (PSA). The PSA provides additional information on the electron-recoil direction, which enables us to improve significantly the angular resolution. We developed an analysis method for this untapped class of events and applied it to laboratory and simulation data. We found that the energy spectrum obtained from the simulation agreed with that of the actual data within a factor of 1.2. We then evaluated the detector performance using the simulation data. The angular resolution for the new-class events was found to be twice as good as in the previous study at the energy range 1.0–2.0 MeV, where both analyses overlap. We also found that the total effective area is dominated by the contribution of the double-hit events above an energy of 1.5 MeV. Notably, applying this new method extends the sensitive energy range with the ETCC from 0.2–2.1 MeV in the previous studies to up to 3.5 MeV. Adjusting the PSA dynamic range should improve the sensitivity in even higher energy gamma-rays. The development of this new analysis method would pave the way for future observations by ETCC to fill the MeV-band sensitivity gap in astronomy.

Detailed visualization of radioactive hotspots inside the unit 1 reactor building of the Fukushima Daiichi Nuclear Power Station using an integrated Radiation Imaging System mounted on a Mecanum wheel robot

ABSTRACT In the decommissioning of the Fukushima Daiichi Nuclear Power Station, understanding the distribution of radioactive substances and dose-equivalent rates is crucial to develop detailed decontamination plans and minimize worker exposure. In this study, we remotely visualized radioactive hotspots and dose-equivalent rate distribution in Unit 1 reactor building of the station using a Mecanum wheel robot equipped with a Compton camera, simultaneous localization and mapping device, and survey meter. We successfully visualized high-concentration radioactive hotspots on the U-shaped piping of the drywell humidity control system and the atmospheric control piping in the ceiling in front of the transverse in-core probe room. Furthermore, the hotspot location was identified in three dimensions using the Compton camera used to analyze the atmospheric control piping. By simultaneously analyzing the dose-equivalent rate data acquired by the survey meter and the hotspot locations visualized by the Compton camera, it was confirmed that the hotspots caused elevated dose-equivalent rates in the surrounding area. In the future, if this robotic system is used in unexplored areas, such as the upper floors of reactor buildings, it can provide information about the locations of radioactive hotspots and the distribution of dose-equivalent rates.

Analysis of the weighted conical Radon transform

In this article, we consider the weighted conical Radon transform—the transform is motivated by Compton camera imaging as well as optical tomography. Our contribution involves introducing new inversion formulas for the weighted conical Radon transform, including explicit formulas and properties associated with convolution frames. Furthermore, we propose reconstruction formulas that solve for variety weighted parameters in the two-dimensional space.