Mechanical Collimation Research Articles

β + surgical radio-guidance using a small Compton-angles collimation probe

The fields of radio-guided surgery and interventional nuclear medicine benefit from a growing array of technologies that aid novel healthcare interventions. Among them, surgery probes able to efficiently detect β + emitters are essential to localize tumors previously detected in PET images. Different methods have been proposed for 18F radio-guidance within the body such as detection of 511 keV annihilation photons using electronically or mechanically collimated probes. The main limitations to the widespread use of these techniques are the lack of precise directional capabilities and inadequate sensitivity. We first used the Monte Carlo simulation platform GATE to determine the optimal signal to noise ratio that can be achieved with an ideal probe. Then, we focused on investigating a small Compton-angles collimation prototype probe followed by the construction of an initial proof of concept demonstrator. The performance of the small Compton-angles collimation probe was compared with a commercial probe based on mechanical collimation in terms of sensitivity and directionality. Monte Carlo simulations showed that in case of an ideal probe, with a tumor of 1 cm diameter positioned 5 mm under the skin and with a SUV of 2, the measured signal to background ratio would be of the order of 25%. The small Compton-angles collimated probe prototype showed significantly improved directionality compared to the commercial probe with mechanical collimation, despite having a sensitivity lower than the commercial probe. Monte Carlo simulations provide insights into the substantial impact of the background on the measured signal. Furthermore, the application of small Compton-angles collimation yields promising outcomes, particularly in terms of improving the directionality, with the objective of enhancing the detection of tumors. In addition, the studied prototype probe sensitivity can possibly be improved by upgrading the detector crystal material and geometry.

Development of focusing multi-knife-edge camera for proton beam range verification: A Monte Carlo study

Proton range uncertainties limit the ability to make full clinical use of the sharp distal falloff of the proton beam. To overcome this limitation, prompt gamma (PG) imaging system with mechanical collimation can be used in non-invasive proton beam range verification. In this study, we developed a focusing multi-knife-edge collimator to improve the counting efficiency and accuracy in detecting proton beam range. Unlike the common design of a single-knife-edge slit (SKE) collimator, the multi-knife-edge slit (MKE) collimator consists of three slits and focuses on the target detection area. To compare the performances of these two collimator designs, Monte Carlo simulation was unitized to model the cameras for measuring the PGs during proton irradiation. The simulation was performed using the GATE v9.1. The setup included a pencil beam of 100 MeV, 160 MeV, and 230 MV protons incident upon a cylindrical PMMA phantom with three different proton numbers of 107, 108, and 109. The detection PG profiles were fitted with a 3-line-segment fitting method to estimate the range of proton beams. Both cameras were aimed at the expected range depth for the corresponding proton beam energies. Experiments for range shifts were conducted by shifting the phantom between −20 mm and 20 mm along the beam axis. Results demonstrate that the MKE camera exhibits approximately twice the efficiency of the SKE camera in detecting PGs. In range shift retrieval, with the delivery of 108 protons, the MKE camera achieves submillimeter accuracy (except for incident protons at 230 MeV), while the SKE camera achieves accuracy within 2 mm. In conclusion, this study assesses the feasibility of the MKE camera and illustrates its potential application for range shift detection, contributing to precise range monitoring in proton therapy.

High-sensitivity cardiac SPECT system design with collimator-less interspaced mosaic-patterned scintillators.

Single-photon emission computed tomography (SPECT) is an important tool for myocardial perfusion imaging (MPI). Mechanical collimators cause the resolution-sensitivity trade-off in the existing cardiac SPECT systems, which hinders fast cardiac scan capability. In this work, we propose a novel collimator-less cardiac SPECT system with interspaced mosaic-patterned scintillators, aiming to significantly improve sensitivity and reduce scan time without trading-off image resolution. We propose to assemble a collimator-less cardiac SPECT with 7 mosaic-patterned detector modules forming a half-ring geometry. The detector module consists of 10 blocks, each of which is assembled with 768 sparsely distributed scintillators with a size of 1.68 mm × 1.68 mm × 20 mm, forming a mosaic pattern in the trans-axial direction. Each scintillator bar contains 5 GAGG(Ce) scintillators and 5 optical-guide elements, forming a mosaic pattern in the axial direction. In the Monte Carlo simulations, the in-plane resolution and axial resolution are evaluated using a hot-rod phantom and 5 disk phantoms, respectively. We simulate a cardiac phantom that is placed in a water-filled cylinder and evaluate the image performance with different data acquisition time. We perform image reconstruction with the expectation-maximization algorithm using system matrices derived from the simulation of a uniform cylindrical source filling the field-of-view (FOV). Besides, a 2-D prototype system is designed to demonstrate the feasibility of the collimator-less imaging concept. In the simulation system, the sensitivity is 16.31% ± 8.85% in a 180 mm (Φ) × 100 mm (L) FOV. The 6-mm rods in the hot rod phantom and the 5-mm disks in the disk phantom are clearly separable. Satisfactory MPI image quality is achieved in the cardiac phantom study with an acquisition time of 30 s. In prototype experiments, the point sources with an 8 mm center-to-center distance are clearly separable at different positions across the FOV. The study reveals a promising approach to high-sensitivity SPECT imaging without a heavy-metal collimator. In cardiac imaging, this approach opens the way to a very fast cardiac scan with good resolution. Further works are ongoing to build a practical 3-D imaging system based on the existing design.

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 ).

A double photon coincidence detection method for medical gamma-ray imaging

Abstract Cascade nuclides emit two or more gamma rays successively through an intermediate state. The coincidence detection of cascade gamma rays provides several advantages in gamma-ray imaging. In this review article, three applications of the double photon coincidence method are reviewed. Double-photon emission imaging with mechanical collimators and Compton double-photon emission imaging can identify radioactive source positions with their angular-resolving detectors, and reduce the crosstalk between nuclides. In addition, a novel method of coincidence Compton imaging is proposed by taking coincidence detection between a Compton event and a photopeak events. Although this type of coincidence Compton imaging cannot specify the location, it can be useful in multi-nuclide Compton imaging.

Self-Alignment FSOC System With Miniaturized Structure for Small Mobile Platform

This paper presents a miniaturized equipment used in tracking and pointing, and provides a method for free space optical communication among small platform. This scheme uses simple scan mirror and receiver with single photo detector to achieve tracking function. Signal beacon is controlled to rotate around target receiver. Transmitter constantly adjusts emitting direction to track receiver according to received signal intensity, which is send back by reverse laser link. The prototype system is successfully established and its transmitting part consisted by directly modulated laser diode, mechanical collimation lens, and scan mirror is also proved. The receiving end is mainly composed of large area photo detector and short-focal lens. Field-programmable gate array chips set in both ends are used to implement tracking algorithm. This system can track target at speed of 40 mm/s within 2 m distance in an angle range of ±30°. For further application, the system has great potential for miniaturization because of elimination of motors, beacons and cameras in its scheme.

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

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

Development 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.

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

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

Technical aspects of proton minibeam radiation therapy: Minibeam generation and delivery

Proton minibeam radiation therapy (pMBRT) is a novel therapeutic strategy that combines the normal tissue sparing of sub-millimetric, spatially fractionated beams with the improved ballistics of protons. This may allow a safe dose escalation in the tumour and has already proven to provide a remarkable increase of the therapeutic index for high-grade gliomas in animal experiments. One of the main challenges in pMBRT concerns the generation of minibeams and the implementation in a clinical environment. This article reviews the different approaches for generating minibeams, using mechanical collimators and focussing magnets, and discusses the technical aspects of the implementation and delivery of pMBRT.

Design and performance evaluation of a 4[formula omitted]-view gamma camera with mosaic-patterned 3D position-sensitive scintillators

Gamma hotspots imaging is of dramatic demand in nuclear-related industrial and homeland security activities such as nuclear facilities monitoring, nuclear emergency response and border inspection. Gamma cameras with mechanical collimation (i.e., coded-aperture) have reduced sensitivity, limited FOV and are typically heavy-weighted. Compton cameras with electrical collimation can achieve higher sensitivity as well as being capable of performing portable 4π-view imaging. However, imageable isotopes of Compton cameras are limited to middle- or high-energy ones. We have previously proposed the concept to image gamma hotspots with a 3-D position-sensitive scintillation detector. This approach achieves high sensitivity, 4π-view FOV and is effective in a wide energy range. In this work, we present a novel gamma camera design aiming for high-resolution gamma imaging. The proposed detector consists of interspaced GAGG(Ce) scintillators that form a mosaic pattern, which is coupled to SiPM arrays on both ends. 3-D photon interaction position information is measured with a dual-end-readout technique. The measured interaction positions for detected photon events are rebinned into the projection data, and the gamma image is reconstructed using a maximum likelihood expectation maximization (MLEM) algorithm. We perform Monte Carlo simulations and experiments to evaluate the performance of the proposed gamma camera design for imaging 99mTc and 137Cs sources. Results show that the proposed gamma camera achieves a positioning accuracy of less than 1° for a 99mTc source and 3° for a 137Cs source. It can clearly resolve two 99mTc sources with 10° separation and two 137Cs point sources with 23° separation, as well as a 2 × 6 99mTc point-source-array with 20° separation clearly. We conclude that the proposed gamma camera design is profitable in portability, FOV, sensitivity, image resolution and energy range. It has promising application potential in fast and accurate radiation monitoring tasks in nuclear security applications.

Development of Image Reconstruction Algorithm in Realtime for Compton Camera Systems

Compton Camera is a imaging technique that uses the principle of Compton scattering interaction to reproduce the distribution image of the radiation source. Compton Camera is a 3-D imaging technique, which has a high sensitivity, wide viewing angle and a range of shooting energy that is much higher than the transmissive imaging technique. In addition, the Compton Camera imaging technique does not use a mechanical collimation system, so it is compact in size, suitable for mobile applications, security control, radiation source detection and medical applications. In this paper, the authors built an image reconstruction algorithm for Compton Camera imaging equipment using data from Monte Carlo simulation. The image quality and the parameters of the algorithm will be evaluated in detail to determine the practical applicability.

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.

Monte Carlo Comparison of Proton and Helium-ion Minibeam Generation Techniques

Proton minibeam radiation therapy (pMBRT) is a novel therapeutic strategy that combines the normal tissue sparing of submillimetric, spatially fractionated beams with the improved dose deposition of protons. In contrast to conventional approaches which work with comparatively large beam diameters (5 mm to several centimetres) producing laterally homogeneous fields, pMBRT uses submillimetric minibeams to create a distinct spatial modulation of the dose featuring alternating regions of high dose (peaks) and low dose (valleys). This spatial fractionation can increase the tolerance of normal tissue and may allow a safe dose escalation in the tumour. Important quantities in this context are the valley dose as well as the peak-to-valley dose ratio (PVDR). Creating submillimetric proton beams for clinical applications is a challenging task that until now has been realized with mechanical collimators (metal blocks with thin slits or holes). However, this method is inherently inefficient, inflexible and creates undesirable secondary neutrons. We therefore recently proposed a method for obtaining clinical minibeams using only magnetic focusing. In this study, we performed Monte Carlo simulations in order to compare minibeams generated using the new method of magnetic focusing with two techniques involving mechanical collimators (collimator and broad beam irradiation, collimator and pencil beam scanning). The dose deposition in water was simulated and dosimetric aspects [beam broadening, depth-dose profiles, PVDR and Bragg-peak-to-entrance dose ratio (BEDR)] as well as irradiation efficiencies were evaluated. Apart from protons, we also considered helium ions which, due to their reduced lateral scattering and sharper Bragg peak, may present a promising alternative for minibeam radiation therapy. Magnetically focused minibeams exhibited a 20–60 times higher PVDR than mechanically collimated minibeams and yielded an increase in irradiation efficiency of up to two orders of magnitude. Compared to proton minibeams, helium ion minibeams were found to broaden at a slower rate and yield an even higher PVDR (at the same minibeam spacing) as well as a more favourable BEDR. Moreover, the simulations showed that methods developed for proton minibeams are suitable for the generation of helium ion minibeams.

Development and performance evaluation of large-area hybrid gamma imager (LAHGI)

We report the development of a gamma-ray imaging device, named Large-Area Hybrid Gamma Imager (LAHGI), featuring high imaging sensitivity and good imaging resolution over a broad energy range. A hybrid collimation method, which combines mechanical and electronic collimation, is employed for a stable imaging performance based on large-area scintillation detectors for high imaging sensitivity. The system comprises two monolithic position-sensitive NaI(Tl) scintillation detectors with a crystal area of 27 × 27 cm2 and a tungsten coded aperture mask with a modified uniformly redundant array (MURA) pattern. The performance of the system was evaluated under several source conditions. The system showed good imaging resolution (i.e., 6.0–8.9° FWHM) for the entire energy range of 59.5–1330 keV considered in the present study. It also showed very high imaging sensitivity, successfully imaging a 253 μCi 137Cs source located 15 m away in 1 min; this performance is notable considering that the dose rate at the front surface of the system, due to the existence of the 137Cs source, was only 0.003 μSv/h, which corresponds to ∼3% of the background level.

Collimator-Less Passive Gamma Scanning for Radioactive Waste Drums

In this article, a new method to quantify the activity of spatially distributed gamma-emitting isotopes (hotspots) in homogenous content waste drums without the use of a collimator is presented. The method utilizes a spatial digital filter derived using maximum likelihood (ML) to determine multiple sources’ positions and then calculate their activities. To solve the multidimensional maximization problem, we use an alternating projection (AP) technique, which transforms the problem into a considerably simpler 1-D maximization problem. A dynamic grid search was developed to further decrease the computational load. The mathematical simulations demonstrate the improved accuracy when compared to that of industrial segmented gamma scanning (SGS) systems and the same accuracy as that of newer methods. Furthermore, the new method offers the benefit of replacing the heavy mechanical collimator with a “virtual collimator” formed by digital filters and an advanced algorithm to create a “digital virtual scan” of drum volume to locate hotspots.

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.

Advancing proton minibeam radiation therapy: magnetically focussed proton minibeams at a clinical centre

Proton minibeam radiation therapy (pMBRT) is a novel therapeutic strategy that has proven to significantly increase dose tolerances and sparing of normal tissue. It uses very narrow proton beams (diameter ≤1 mm), roughly one order of magnitude smaller than state-of-the-art pencil beams. The current implementation of pMBRT with mechanical collimators is suboptimal as it is inflexible, decreases efficiency and produces additional secondary neutrons. As a potential solution, we explore in this article minibeam generation through magnetic focussing and investigate possibilities for the integration of such a technique at existing clinical centres. For this, a model of the pencil beam scanning (PBS) nozzle and beam at the Orsay Proton Therapy Centre was established and Monte Carlo simulations were performed to determine its focussing capabilities. Moreover, various modifications of the nozzle geometry were considered. It was found that the PBS nozzle in its current state is not suitable for magnetic minibeam generation. Instead, a new, optimised nozzle design has been proposed and conditions necessary for minibeam generation were benchmarked. In addition, dose simulations in a water phantom were performed which showed improved dose distributions compared to those obtained with mechanical collimators.

Verification of a Monte Carlo dose calculation engine in proton minibeam radiotherapy in a passive scattering beamline for preclinical trials.

Proton minibeam radiation therapy (pMBRT) is a novel therapeutic strategy that combines the benefits of proton therapy with the remarkable normal tissue preservation observed with the use of submillimetric spatially fractionated beams. This promising technique has been implemented at the Institut Curie-Proton therapy centre (ICPO) using a first prototype of a multislit collimator. The purpose of this work was to develop a Monte Carlo-based dose calculation engine to reliably guide preclinical studies at ICPO. The whole "Y1"-passive beamline at the ICPO, including pMBRT implementation, was modelled using the Monte Carlo GATE v. 7.0 code. A clinically relevant proton energy (100 MeV) was used as starting point. Minibeam generation by means of the brass collimator used in the first experiments was modelled. A virtual source was modelled at the exit of the beamline nozzle and outcomes were compared with dosimetric measurements performed with EBT3 gafchromic films and a diamond detector in water. Dose distributions were recorded in a water phantom and in rat CT images (7-week-old male Fischer rats). The dose calculation engine was benchmarked against experimental data and was then used to assess dose distributions in CT images of a rat, resulting from different irradiation configurations used in several experiments. It reduced computational time by an order of magnitude. This allows us to speed up simulations for in vivo trials, where we obtained peak-to-valley dose ratios of 1.20 ± 0.05 and 6.1 ± 0.2 for proton minibeam irradiations targeting the tumour and crossing the rat head. Tumour eradication was observed in the 67 and 22% of the animals treated respectively. A Monte Carlo dose calculation engine for pMBRT implementation with mechanical collimation has been developed. This tool can be used to guide and interpret the results of in vivo trials. This is the first Monte Carlo dose engine for pMBRT that is being used to guide preclinical trials in a clinical proton therapy centre.

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.