In vivo simultaneous imaging with 99mTc and 18F using a Compton camera

Influence of spill-over for 99mTc images and the effect of scatter correction for dual-isotope simultaneous acquisition with 99mTc and 18F using small-animal SPECT-PET/CT system

A dual-isotope simultaneous acquisition (DISA) of 99mTc and 18F affects the image quality of 99mTc by crosstalk and spill-over from 18F. We demonstrated the influence of spill-over and crosstalk on image quality and its correction effect for DISA SPECT with 99mTc and 18F. A fillable cylindrical chamber of 30 mm with NEMA-NU4 image quality phantom was filled with 99mTc only or a mixed 99mTc and 18F solution (C100). Two small-region chambers were filled with 99mTc only or a mixed 99mTc and 18F solution made at half the radioactivity concentration of C100 (C50) and non-radioactive water (C0). The 18F/99mTc ratio for DISA was set at approximately 0.4-12. Two types of 99mTc transverse images with and without scatter correction (SC and nonSC) were created. The 99mTc images of single-isotope acquisition (SIA) were created as a reference. The DISA/SIA ratio and contrast of 99mTc were compared between SIA and DISA. Although the DISA/SIA ratios with nonSC of C100, C50 and C0 gradually increased with increasing 18F/99mTc ratio, it was nearly constant by SC. The contrasts of C100 and C50 were similar to a reference value for both nonSC and SC. In conclusion, DISA images showed lower image quality as the 18F/99mTc ratio increased. The image quality in hot-spot regions such as C100 and C50 was improved by SC, whereas cold-spot regions such as C0 could not completely remove the influence of spill-over even with SC.

Experimental study on Compton camera for boron neutron capture therapy applications

Boron neutron capture therapy (BNCT) is a high-dose-intensive radiation therapy that has gained popularity due to advancements in accelerator neutron sources. To determine the dose for BNCT, it is necessary to know the difficult-to-determine boron concentration and neutron fluence. To estimate this dose, we propose a method of measuring the prompt γ-rays (PGs) from the boron neutron capture reaction (BNCR) using a Compton camera. We performed a fundamental experiment to verify basic imaging performance and the ability to discern the PGs from 511 keV annihilation γ-rays. A Si/CdTe Compton camera was used to image the BNCR and showed an energy peak of 478 keV PGs, separate from the annihilation γ-ray peak. The Compton camera could visualize the boron target with low neutron intensity and high boron concentration. This study experimentally confirms the ability of Si/CdTe Compton cameras to image BNCRs.

First Demonstration of Compton Camera Used for X-Ray Fluorescence Imaging

X-ray fluorescence computed tomography (XFCT) is a promising approach used for obtaining the distribution of high-Z elements in the target object. The characteristic energy of X-ray fluorescence (XRF) photons makes XFCT have higher sensitivity and contrast ratio. Conventional XFCT systems usually require mechanical collimators, which leads to a huge loss of incident photons and reduce photon collection efficiency. The Compton camera is an imaging modality that uses electronic collimation to obtain the incident direction of the photons, which brings advantages in the detection area and X-ray photon collection efficiency. Compton cameras can also achieve three-dimensional (3D) imaging with one or several views of scanning without rotation. Therefore, it is a good idea to realize XRF imaging by using Compton cameras, which may provide more potential applications and imaging possibilities, such as handheld XRF imaging or image-guided interventional operation. In this work, we demonstrate the first Compton camera platform which is used for XRF imaging. The proposed X-ray fluorescence Compton camera (XFCC) mainly consists of a conventional X-ray tube (150kVp) and a Timepix3 photon-counting detector (PCD). A PMMA phantom with insertions containing different concentrations of 4%, 6%, 8%, and 10% (weight/volume) Gd solution is scanned by a fan-beam X-ray. Besides, a Compton camera-based X-ray fluorescence imaging reconstruction method (CCFIRM) is developed to solve specific problems in XFCC reconstruction. The reconstruction images and the quantitative analyses of the contrast-to-noise ratio (CNR) are presented. The results indicate that the detectability limit of the proposed XFCC platform is 3.5139 wt.% (CNR =4 ).

Multi-molecule imaging and inter-molecular imaging in nuclear medicine

Multi-molecule imaging and inter-molecular imaging are not fully implemented yet, however, can become an alternative in nuclear medicine. In this review article, we present arguments demonstrating that the advent of the Compton positron emission tomography (Compton-PET) system and the invention of the quantum chemical sensing method with double photon emission imaging (DPEI) provide realistic perspectives for visualizing inter-molecular and multi-molecule in nuclear medicine with MeV photon. In particular, the pH change of InCl3 solutions can be detected and visualized in a three-dimensional image by combining the hyperfine electric quadrupole interaction sensing and DPEI. Moreover, chemical states, such as chelating, can be detected through angular correlation sensing. We argue that multi-molecule and chemical sensing could be a realistic stream of research in future nuclear medicine.

Technical note: Rapid and high-resolution deep learning-based radiopharmaceutical imaging with 3D-CZT Compton camera and sparse projection data

BACKGROUND

The Compton camera (CC) has great potential in nuclear medicine imaging due to the high detection efficiency and the ability to simultaneously detect multi-energy radioactive sources. However, the finite resolution of the detectors will degrade the images that the real-world CC can obtain. Besides, the CC sometimes can be limited by the detection efficiency, leading to difficulty in using sparse projection data to realize high-resolution reconstruction with short-time measurement, which limits its clinical application for real-time or rapid radiopharmaceutical imaging.

PURPOSE

To overcome the difficulty and promote the usage of the CC in radiopharmaceutical imaging, we present a deep learning (DL)-based CC reconstruction method to realize rapid and high-resolution imaging with short-time measurement.

METHODS

We developed a DL-based algorithm MCBP-CCnet via Monte Carlo sampling-based back projection and a dedicated convolutional neural network, called CC-Net, to realize the rapid and high-resolution reconstruction with sparse projection data. A CC prototype based on a single three-dimensional position-sensitive CdZnTe (3D-CZT) detector was used to demonstrate the feasibility of our proposed method. The simulations and experiments of radiopharmaceutical imaging used the 3D-CZT CC and [¹⁸ F]NaF. A 3D-printing mouse phantom was also further used to evaluate the performance of the proposed method in animal molecular imaging.

RESULTS

The simulation and experimental results showed that the proposed method could realize the images reconstruction within 5 s for list-mode projection data and realized a rapid reconstruction within 35 s for experimental radiopharmaceutical imaging based on the 3D-printing mouse phantom, as well as realized the high-resolution imaging with an accuracy of within 0.78 mm in terms of the sparse projection data that only contained hundreds of events. Besides, the deviations between the reconstructed radiative activities and the exact values were less than 1.51%.

CONCLUSION

The results demonstrated that the proposed method could realize the rapid and high-resolution CC reconstruction with sparse projection data obtained by the 3D-CZT CC and realize the high-resolution radiopharmaceutical imaging. The study in this paper also demonstrated the potential and feasibility of future applications of a 3D-CZT CC for real-time high-resolution radiopharmaceutical imaging with short-time measurement.

Development and Applications of Compton Camera-A Review

The history of Compton cameras began with the detection of radiation sources originally for applications in astronomy. A Compton camera is a promising γ-ray detector that operates in the wide energy range of a few tens of keV to MeV. The γ-ray detection method of a Compton camera is based on Compton scattering kinematics, which is used to determine the direction and energy of the γ-rays without using a mechanical collimator. Although the Compton camera was originally designed for astrophysical applications, it was later applied in medical imaging as well. Moreover, its application in environmental radiation measurements is also under study. Although a few review papers regarding Compton cameras have been published, they either focus very specifically on the detectors used in such cameras or the particular applications of Compton cameras. Thus, the aim of this paper is to review the features and types of Compton cameras and introduce their applications, associated imaging algorithms, improvement scopes, and their future aspects.

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.

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.

Radiopharmaceutical imaging based on 3D-CZT Compton camera with 3D-printed mouse phantom

PURPOSE

The study proposes the use of three-dimensional CdZnTe Compton camera (3D-CZT CC) for radiopharmaceutical imaging and investigates the influence factors using a 3D-printed mouse phantom.

METHODS

The event selection method and image reconstruction algorithm are optimized by Monte Carlo simulations and mouse phantom experiments.

RESULTS

Simulation results show that the intrinsic energy resolution and spatial resolution of 3D-CZT cause a certain deviation in the calculated Compton scattering angle and Compton axis. Such deviation causes the imaging quality to deteriorate. By selecting events whose distance between Compton and photoelectronic interactions are larger than 10 mm, the mean deviation of the Compton axis could be reduced to less than 10%. Using the ordered origin ensemble algorithm with resolution recovery, the artifacts around organs where the radiopharmaceutical was placed are reduced, and the quality of the reconstruction results are improved compared to the results with simple back projection and origin ensembles algorithms. The phantom study shows that the 3D-CZT CC imaging device could visualize the radiopharmaceuticals distribution by 15 min detection.

CONCLUSIONS

Through the analysis of this study, the feasibility of 3D-CZT CC for in-vivo distribution measurement of radiopharmaceuticals is demonstrated, and the quality of reconstruction result has been improved.

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.

Carbon range verification with 718 keV 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.

Simultaneous in vivo imaging with PET and SPECT tracers using a Compton-PET hybrid camera

Positron-emission tomography (PET) and single-photon-emission computed tomography (SPECT) are well-established nuclear-medicine imaging methods used in modern medical diagnoses. Combining PET with ¹⁸F-fluorodeoxyglucose (FDG) and SPECT with an ¹¹¹In-labelled ligand provides clinicians with information about the aggressiveness and specific types of tumors. However, it is difficult to integrate a SPECT system with a PET system because SPECT requires a collimator. Herein, we describe a novel method that provides simultaneous imaging with PET and SPECT nuclides by combining PET imaging and Compton imaging. The latter is an imaging method that utilizes Compton scattering to visualize gamma rays over a wide range of energies without requiring a collimator. Using Compton imaging with SPECT nuclides, instead of the conventional SPECT imaging method, enables PET imaging and Compton imaging to be performed with one system. In this research, we have demonstrated simultaneous in vivo imaging of a tumor-bearing mouse injected with ¹⁸F-FDG and an ¹¹¹In-antibody by using a prototype Compton-PET hybrid camera. We have succeeded in visualizing accumulations of ¹⁸F-FDG and ¹¹¹In-antibody by performing PET imaging and Compton imaging simultaneously. As simultaneous imaging utilizes the same coordinate axes, it is expected to improve the accuracy of diagnoses.

Self-Collimating SPECT With Multi-Layer Interspaced Mosaic Detectors

Conventional single photon emission computed tomography (SPECT) relies on mechanical collimation whose resolution and sensitivity are interdependent, the best performance a SPECT system can attain is only a compromise of these two equally desired properties. To simultaneously achieve high resolution and sensitivity, we propose to use sensitive detectors constructed in a multi-layer in ter spaced mosaicdetectors (MATRICES) architecture to accomplish part of the collimation needed. We name this new approach self-collimation. We evaluate three self-collimating SPECT systems and report their imaging performance: 1) A simulated human brain SPECT achieves 3.88% sensitivity, it clearly resolves 0.5-mm and 1.0-mm hot-rod patterns at noise-free and realistic count-levels, respectively; 2) a simulated mouse SPECT achieves 1.25% sensitivity, it clearly resolves 50- [Formula: see text] and 100- [Formula: see text] hot-rod patterns at noise-free and realistic count-levels, respectively; 3) a SPECT prototype achieves 0.14% sensitivity and clearly separates 0.3-mm-diameter point sources of which the center-to-center neighbor distance is also 0.3 mm. Simulated contrast phantom studies show excellent resolution and signal-to-noise performance. The unprecedented system performance demonstrated by these 3 SPECT scanners is a clear manifestation of the superiority of the self-collimating approach over conventional mechanical collimation. It represents a potential paradigm shift in SPECT technology development.

Compton-camera-based SPECT for thyroid cancer imaging

Thyroid cancer is the most common type of endocrine-related cancer and the most common cancer in young women. Currently, single photon emission computed tomography (SPECT) and computed tomography (CT) are used with radioiodine scintigraphy to evaluate patients with thyroid cancer. The gamma camera for SPECT contains a mechanical collimator that greatly compromises dose efficiency and limits diagnostic sensitivity. Fortunately, the Compton camera is emerging as an ideal approach for mapping the distribution of radiopharmaceuticals inside the thyroid. In this preliminary study, based on the state-of-the-art readout chip Timepix3, we investigate the feasibility of using Compton camera for radiotracer SPECT imaging in thyroid cancer. A thyroid phantom is designed to mimic human neck, the mechanism of Compton camera-based event detection is simulated to generate realistic list-mode data, and a weighted back-projection method is developed to reconstruct the original distribution of the emission source. Study results show that the Compton camera can improve the detection efficiency for two or higher orders of magnitude comparing with the conventional gamma cameras. The thyroid gland regions can be reconstructed from the Compton camera measurements in terms of radiotracer distribution. This makes the Compton-camera-based SPECT imaging a promising modality for future clinical applications with significant benefits for dose reduction, scattering artifact reduction, temporal resolution enhancement, scan throughput increment, and others.

3D Compton image reconstruction method for whole gamma imaging

Compton imaging or Compton camera imaging has been studied well, but its advantages in nuclear medicine and molecular imaging have not been demonstrated yet. Therefore, the aim of this work was to compare Compton imaging with positron emission tomography (PET) by using the same imaging platform of whole gamma imaging (WGI). WGI is a concept that combines PET with Compton imaging by inserting a scatterer ring into a PET ring. This concept utilizes diverse types of gamma rays for 3D tomographic imaging. In this paper, we remodeled our previous WGI prototype for small animal imaging, and we developed an image reconstruction method based on a list-mode ordered subset expectation maximization algorithm incorporating detector response function modeling, random correction and normalization (sensitivity correction) for either PET and Compton imaging. To the best of our knowledge, this is the world's first realization of a full-ring Compton imaging system. We selected ⁸⁹Zr as an imaging target because a ⁸⁹Zr nuclide emits a 909 keV single-gamma ray as well as a positron, and we can directly compare Compton imaging of 909 keV photons with PET, a well-established modality. We measured a cylindrical phantom and a small rod phantom filled with ⁸⁹Zr solutions of 10.3 MBq and 10.2 MBq activity, respectively, for 1 h each. The uniform radioactivity distribution of the cylindrical phantom was reconstructed with normalization in both PET and Compton imaging. Coefficients of variation for region-of-interest values were 4.2% for Compton imaging and 3.3% for PET; the difference might be explained by the difference in the detected count number. The small rod phantom experiment showed that the WGI Compton imaging had spatial resolution better than 3.0 mm at the peripheral region although the center region had lower resolution. PET resolved 2.2 mm rods clearly at any location. We measured a mouse for 1 h, 1 d after injection of 9.8 MBq ⁸⁹Zr oxalate. The ⁸⁹Zr assimilated in the mouse bony structures was clearly depicted, and Compton imaging results agreed well with PET images, especially for the region inside the scatterer ring. In conclusion, we demonstrated the performance of WGI using the developed Compton image reconstruction method. We realized Compton imaging with a quality approaching that of PET, which is supporting a future expectation that Compton imaging outperforms PET.

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.

Use of a Si/CdTe Compton Camera for In vivo Real-Time Monitoring of Annihilation Gamma Rays Generated by Carbon Ion Beam Irradiation

The application of annihilation gamma-ray monitoring to the adaptive therapy of carbon ion radiotherapy (C-ion RT) requires identification of the peak intensity position and confirmation of activated elements with annihilation gamma-rays generated at the C-ion-irradiated site from those transported to unirradiated sites. Real-time monitoring of C-ion-induced annihilation gamma-rays was implemented using a Compton camera in a mouse model. An adult C57BL/6 mouse was anesthetized, and C-ion beams were directed into the abdomen at 1 × 10⁹ particles/s for 20 s. The 511 keV annihilation gamma-rays, generated by the interaction between the irradiated C-ion beam and the target mouse, were detected using a silicon/cadmium telluride (Si/CdTe) Compton camera for 20 min immediately after irradiation. The irradiated site and the peak intensity position of 511 keV gamma emissions due to C-ion beam irradiation on a mouse were observed at the abdomen of the mouse by developing Compton images. Moreover, the positron emitter transport was observed by evaluating the range of gamma-ray emission after the C-ion beam irradiation on the mouse. Our data suggest that by confirming the peak intensity and beam range of C-ion RT with Si/CdTe-based Compton camera, it would be possible to reduce the intra-fractional and inter-fractional dose distribution degradation. Therefore, the results of this study would contribute to the future development of adaptive therapy with C-ion RT for humans.

Crosstalk Reduction Using a Dual Energy Window Scatter Correction in Compton Imaging

Compton cameras can simultaneously detect multi-isotopes; however, when simultaneous imaging is performed, crosstalk artifacts appear on the images obtained using a low-energy window. In conventional single-photon emission computed tomography, a dual energy window (DEW) subtraction method is used to reduce crosstalk. This study aimed to evaluate the effectiveness of employing the DEW technique to reduce crosstalk artifacts in Compton images obtained using low-energy windows. To this end, in this study, we compared reconstructed images obtained using either a photo-peak window or a scatter window by performing image subtraction based on the differences between the two images. Simulation calculations were performed to obtain the list data for the Compton camera using a 171 and a 511 keV point source. In the images reconstructed using these data, crosstalk artifacts were clearly observed in the images obtained using a 171 keV photo-peak energy window. In the images obtained using a scatter window (176-186 keV), only crosstalk artifacts were visible. The DEW method could eliminate the influence of high-energy sources on the images obtained with a photo-peak window, thereby improving quantitative capability. This was also observed when the DEW method was used on experimentally obtained images.

Improved iterative reconstruction method for Compton imaging using median filter

A Compton camera is a device for imaging a radio-source distribution without using a mechanical collimator. Ordered-subset expectation-maximization (OS-EM) is widely used to reconstruct Compton images. However, the OS-EM algorithm tends to over-concentrate and amplify noise in the reconstructed image. It is, thus, necessary to optimize the number of iterations to develop high-quality images, but this has not yet been achieved. In this paper, we apply a median filter to an OS-EM algorithm and introduce a median root prior expectation-maximization (MRP-EM) algorithm to overcome this problem. In MRP-EM, the median filter is used to update the image in each iteration. We evaluated the quality of images reconstructed by our proposed method and compared them with those reconstructed by conventional algorithms using mathematical phantoms. The spatial resolution was estimated using the images of two point sources. Reproducibility was evaluated on an ellipsoidal phantom by calculating the residual sum of squares, zero-mean normalized cross-correlation, and mutual information. In addition, we evaluated the semi-quantitative performance and uniformity on the ellipsoidal phantom. MRP-EM reduces the generated noise and is robust with respect to the number of iterations. An evaluation of the reconstructed image quality using some statistical indices shows that our proposed method delivers better results than conventional techniques.

Compton imaging with 99mTc for human imaging

We have been developing a medical imaging system using a Compton camera and demonstrated the imaging ability of Compton camera for ^99mTc-DMSA accumulated in rat kidneys. In this study, we performed imaging experiments using a human body phantom to confirm its applicability to human imaging. Preliminary simulations were conducted using a digital phantom with varying activity ratios between the kidney and body trunk regions. Gamma rays (141 keV) were generated and detected by a Compton camera based on a silicon and cadmium telluride (Si/CdTe) detector. Compton images were reconstructed with the list mode median root prior expectation maximization method. The appropriate number of iterations of the condition was confirmed through simulations. The reconstructed Compton images revealed two bright points in the kidney regions. Furthermore, the numerical value calculated by integrating pixel values inside the region of interest correlated well with the activity of the kidney regions. Finally, experimental studies were conducted to ascertain whether the results of the simulation studies could be reproduced. The kidneys could be successfully visualised. In conclusion, considering that the conditions in this study agree with those of typical human bodies and imaginable experimental setup, the Si/CdTe Compton camera has a high probability of success in human imaging. In addition, our results indicate the capability of (semi-) quantitative analysis using Compton images.

Annihilation gamma imaging for carbon ion beam range monitoring using Si/CdTe Compton camera

In this study, we performed on-beam monitoring of 511 keV annihilation gamma emissions using a Compton camera. Beam monitoring experiments were conducted using carbon ion beams of 290 MeV/u irradiated on a polymethyl methacrylate (PMMA) phantom. The intensity of the beams was 3  ×  10⁹ particles per pulse, with 20 pulses per minute. A Compton camera based on a silicon/cadmium telluride (Si/CdTe) detector was used to monitor the annihilation gamma rays emitted from the phantom. We successfully reconstructed the energy events of 511 keV annihilation gamma rays and developed Compton images using a simple back-projection method. The distribution of the annihilation gamma ray generation traced the beam trajectory and the peak intensity position was a few millimeters shorter than the Bragg peak position. Moreover, the effect of the beam range shifter with 30, 60, and 90 mm water equivalent thickness (WET) was clearly visualized in the reconstructed Compton images. The experimentally measured values of the corresponding range shifts in the PMMA phantom (28.70 mm, 52.49 mm, and 76.77 mm, respectively) were consistent with the shifts of the Bragg peak position (25.50 mm, 51.30 mm and 76.70 mm, respectively) evaluated by Monte Carlo simulation. The results show that the Si/CdTe Compton camera has strong potential for on-beam monitoring of annihilation gamma rays in particle therapy in clinical situations.