Tungsten Collimator Research Articles

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.

Performance evaluation of the 3D-ring cadmium–zinc–telluride (CZT) StarGuide system according to the NEMA NU 1-2018 standard

BackgroundThe application of semi-conductor detectors such as cadmium–zinc–telluride (CZT) in nuclear medicine improves extrinsic energy resolution and count sensitivity due to the direct conversion of gamma photons into electric signals. A 3D-ring pixelated CZT system named StarGuide was recently developed and implemented by GE HealthCare for SPECT acquisition. The system consists of 12 detector columns with seven modules of 16 × 16 CZT pixelated crystals, each with an integrated parallel-hole tungsten collimator. The axial coverage is 27.5 cm. The detector thickness is 7.25 mm, which allows acquisitions in the energy range [40–279] keV. Since there is currently no performance characterization specific to 3D-ring CZT SPECT systems, the National Electrical Manufacturers Association (NEMA) NU 1-2018 clinical standard can be tailored to these cameras. The aim of this study was to evaluate the performance of the SPECT/CT StarGuide system according to the NEMA NU 1-2018 clinical standard specifically adapted to characterize the new 3D-ring CZT.ResultsDue to the integrated collimator, the system geometry and the pixelated nature of the detector, some NEMA tests have been adapted to the features of the system. The extrinsic measured energy resolution was about 5–6% for the tested isotopes (99mTc, 123I and 57Co); the maximum count rate was 760 kcps and the observed count rate at 20% loss was 917 kcps. The system spatial resolution in air extrapolated at 10 cm with 99mTc was 7.2 mm, while the SPECT spatial resolutions with scatter were 4.2, 3.7 and 3.6 mm in a central, radial and tangential direction respectively. Single head sensitivity value for 99mTc was 97 cps/MBq; with 12 detector columns, the system volumetric sensitivity reached 520 kcps MBq−1 cc−1.ConclusionsThe performance tests of the StarGuide can be performed according to the NEMA NU 1-2018 standard with some adaptations. The system has shown promising results, particularly in terms of energy resolution, spatial resolution and volumetric sensitivity, potentially leading to higher quality clinical images.

Minibeam Radiation Therapy Treatment (MBRT): Commissioning and First Clinical Implementation

Minibeam radiation therapy (MBRT) is characterized by the delivery of submillimeter wide regions of high "peak" and low "valley" doses throughout a tumor. Preclinical studies have long shown the promise of this technique, and we report here the first clinical implementation of MBRT. A clinical orthovoltage unit was commissioned for MBRT patient treatments using 3, 4, 5, 8, and 10 cm diameter cones. The 180 kVp output was spatially separated into minibeams using a tungsten collimator with 0.5 mm wide slits spaced 1.1 mm on center. Percentage depth dose (PDD) measurements were obtained using film dosimetry and plastic water for both peak and valley doses. PDDs were measured on central axis for offsets of 0, 0.5, and 1 cm. The peak-to-valley ratio (PVR) was calculated at each depth for all cones and offsets. To mitigate the effects of patient motion on delivered dose, patient-specific 3D printed collimator holders were created. These conformed to the unique anatomy of each patient and affixed the tungsten collimator directly to the body. Two patients were treated with MBRT, both received 2 fractions. Peak PDDs decreased gradually with depth. Valley PDDs initially increased slightly with depth, then decreased gradually beyond 2 cm. PVRs were highest at the surface for smaller cone sizes and offsets. In vivo film dosimetry confirmed a distinct delineation of peak and valley doses on both patients treated with MBRT with no dose blurring. Both patients experienced prompt improvement in symptoms and tumor response. We report commissioning results, treatment processes, and the first two patients treated with MBRT using a clinical orthovoltage unit. While demonstrating feasibility of this approach is a crucial first step toward wider translation, clinical trials are needed to further establish safety and efficacy.

Development and Utility of an Imaging System for Internal Dosimetry of Astatine-211 in Mice.

In targeted radionuclide therapy, determining the absorbed dose of the ligand distributed to the whole body is vital due to its direct influence on therapeutic and adverse effects. However, many targeted alpha therapy drugs present challenges for in vivo quantitative imaging. To address this issue, we developed a planar imaging system equipped with a cadmium telluride semiconductor detector that offers high energy resolution. This system also comprised a 3D-printed tungsten collimator optimized for high sensitivity to astatine-211, an alpha-emitting radionuclide, and adequate spatial resolution for mouse imaging. The imager revealed a spectrum with a distinct peak for X-rays from astatine-211 owing to the high energy resolution, clearly distinguishing these X-rays from the fluorescent X-rays of tungsten. High collimator efficiency (4.5 × 10-4) was achieved, with the maintenance of the spatial resolution required for discerning mouse tissues. Using this system, the activity of astatine-211 in thyroid cancer tumors with and without the expression of the sodium iodide symporter (K1-NIS/K1, respectively) was evaluated through in vivo imaging. The K1-NIS tumors had significantly higher astatine-211 activity (sign test, p = 0.031, n = 6) and significantly decreased post-treatment tumor volume (Student's t-test, p = 0.005, n = 6). The concurrent examination of intratumor drug distribution and treatment outcome could be performed with the same mice.

Near-field coded-mask technique and its potential for proton therapy monitoring

Objective. Prompt-gamma imaging encompasses several approaches to the online monitoring of the beam range or deposited dose distribution in proton therapy. We test one of the imaging techniques - a coded mask approach - both experimentally and via simulations. Approach. Two imaging setups have been investigated experimentally. Each of them comprised a structured tungsten collimator in the form of a modified uniformly redundant array mask and a LYSO:Ce scintillation detector of fine granularity. The setups differed in detector dimensions and operation mode (1D or 2D imaging). A series of measurements with radioactive sources have been conducted, testing the performance of the setups for near-field gamma imaging. Additionally, Monte Carlo simulations of a larger setup of the same type were conducted, investigating its performance with a realistic gamma source distribution occurring during proton therapy. Main results. The images of point-like sources reconstructed from two small-scale prototypes’ data using the maximum-likelihood expectation maximisation algorithm constitute the experimental proof of principle for the near-field coded-mask imaging modality, both in the 1D and the 2D mode. Their precision allowed us to calibrate out certain systematic offsets appearing due to the limited alignment accuracy of setup elements. The simulation of the full-scale setup yielded a mean distal falloff retrieval precision of 0.72 mm in the studies for beam energy range 89.5–107.9 MeV and with 1 × 108 protons (a typical number for distal spots). The implemented algorithm of image reconstruction is relatively fast—a typical procedure needs several seconds. Significance. Coded-mask imaging appears a valid option for proton therapy monitoring. The results of simulations let us conclude that the proposed full-scale setup is competitive with the knife-edge-shaped and the multi-parallel slit cameras investigated by other groups.

Comparing Acute Oral Mucosa Toxicity Associated with Minibeam and Conventional Radiotherapy

Acute oral mucosa toxicity is associated with ulceration, severe pain, and nutritional deficiencies that may lead to treatment interruptions and a decreased quality of life for head and neck cancer patients. Minibeam radiotherapy (MRT) is a type of spatially fractionated radiotherapy that delivers an alternating pattern of sub-millimeter wide peaks of high dose immediately adjacent to much lower valley doses. By delivering a non-uniform dose distribution, MRT allows for elevated peak doses while maintaining extraordinary normal tissue tolerance. In this study, we used an acute oral mucosa toxicity mouse model to test the hypothesis that MRT provides superior tissue sparing compared to conventional radiotherapy (CRT). A total of 33 female C57BLJ6 mice were randomized by body weight (BW) across two CRT groups (n = 4/group) receiving open field radiation of 16 and 20 Gy and five MRT groups (n = 5/group) receiving peak:valley doses of 48:8, 72:12, 96:16, 96:8, and 152:8 Gy. All radiation was given in one fraction. The CRT groups were irradiated with a 10 mm circular field using a 225 kVp PA x-ray beam encompassing the oral cavity and oropharynx. The MRT groups were irradiated with the same arrangement, but the beam was collimated into 0.5 mm wide minibeams spaced 1.1 mm center-to-center using tungsten collimators of 0.5, 1, and 2.5 mm thickness to deliver peak-to-valley ratios (PVRs) of 6:1, 12:1, and 19:1. Acute effects in the oral mucosa were studied using changes in BW as a surrogate for clinically significant oral mucositis. BWs were measured daily starting on the day of treatment, and BW changes were computed relative to the day of treatment. The toxicity endpoint was defined such that mice losing >20% BW were humanely euthanized. Doses of 20 Gy CRT and 96:16 Gy MRT were highly toxic, with all animals in these groups reaching the toxicity endpoint between 9-11 days post-RT, compared to only one animal in the 16 Gy CRT group, and no animals in the other MRT groups. The BWs of all surviving animals returned to baseline within 15 days post-RT. The groups 48:8, 96:8, and 152:8 Gy showed similar BW loss and full recovery patterns. Additionally, the groups 48:8, 72:12, and 96:16 Gy showed that, for the same PVR, increasing the valley dose resulted in increased BW loss. Lastly, animals did not tolerate 96:16 Gy, but those in the 96:8 Gy group fully recovered. Despite the extreme toxicity caused by 20 Gy CRT, animals in the MRT groups tolerated peak doses up to 152 Gy when the valley dose was 8 Gy. These results confirm the superior normal tissue sparing capacity of MRT compared to CRT in an oral mucosa mouse model. We delivered tolerable MRT peak doses an order of magnitude larger than toxic CRT doses. Importantly, our results suggest that valley dose is the most useful parameter to assess acute toxicity when using MRT. Our data may prove useful for eventual clinical applications of MRT in head and neck cancers.

Equivalent Uniform Dose (EUD) and the Evaluation of Cell Survival in Spatially Fractionated Radiotherapy (SFRT)

SFRT has shown promise as a treatment modality to decrease normal tissue sparing without compromising tumor coverage, i.e., an increase in the therapeutic window compared to more conventional uniform radiation therapy (RT). The aim of this work is to examine and test several alternative bio-dosimetric parameters for the prediction of cell survival for normal-tissue and tumor cell lines irradiated in vitro with uniform and microbeam radiotherapy (MRT). A bespoke tungsten collimator with 50 parallel, 50 µm wide slits and 400 µm slit spacing was mounted into an x-ray cabinet. Human fibroblast (MRC5) and two human tumor cell lines (LN18 and A549) were irradiated with a range of doses (< 10 Gy) for uniform and MRT (50um slits, 400um center spacing) using kV X-rays. Average, mean and valley dose as useful predictive metrics of cell survival are compared to the equivalent uniform dose (EUD) with biological parameters estimated from uniform-dose experiments. We find that EUD, with linear-quadratic (LQ) model parameters, is more predictive for survival after SFRT than maximum, minimum or average dose. The maximum and average doses are correlated very poorly with in vitro cell survival. The difference in cell survival between uniform and MRT irradiation as a function of EUD is cell-type and dose dependent. The report results suggest that MRT is more effective at cell killing of tumor-cell lines than uniform irradiation for both tumor cell lines. However, MRT is less effective at killing normal tissue cells than uniform irradiation. EUD is a superior predictor of in vitro cell survival than other metrics sometimes used in the SFRT literature, including mean dose, maximum dose, and valley dose. The reported studies provide some evidence that SFRT may increase the therapeutic ratio by producing spatial dose distributions that effectively reduce normal-tissue damage with little or no change in biological damage to tumor cells. Additional studies are needed to further extend and generalize our results and to test our conclusions against a larger dose range, low and high linear energy transfer (LET) radiations and additional cell lines.

Challenges in Acquiring Clinical Simultaneous SPECT-MRI on a PET-MRI Scanner

The INSERT is the world’s first clinical SPECTMRI brain imaging system based on scintillation detectors with a SiPM readout. Here we demonstrate its use within a clinical MRI environment for the first time. Using a standard transmit-receive head coil, and with an appropriate selection of a custom MRI sequence (GRE), we overcome mutual interference. The INSERT and its bulky 50 kg tungsten collimator introduce magnetic field inhomogeneity. Due to the specific MRI-compatible collimator design, inhomogeneity is compensated by shimming, leading to simultaneous acquisition. We process the SPECT data acquired alongside the MRI sequence to evaluate the SPECT system performance and the impact of the MRI. Finally, we present a set of simultaneous SPECT-MRI acquisitions, demonstrating multimodal imaging capabilities, albeit with a limited MRI sequence.

Simulation studies of a full-ring, CZT SPECT system for whole-body imaging of 99m Tc and 177 Lu.

Single photon emission computed tomography (SPECT) is an imaging modality that has demonstrated its utility in a number of clinical indications. Despite this progress, a high sensitivity, high spatial resolution, multi-tracer SPECT with a large field of view suitable for whole-body imaging of a broad range of radiotracers for theranostics is not available. With the goal of filling this technological gap, we have designed a cadmium zinc telluride (CZT) full-ring SPECT scanner instrumented with a broad-energy tungsten collimator. The final purpose is to provide a multi-tracer solution for brain and whole-body imaging. Our static SPECT does not rely on the dual- and the triple-head rotational SPECT standard paradigm, enabling a larger effective area in each scan to increase the sensitivity. We provide a demonstration of the performance of our design using a realistic model of our detector with simulated body-sized phantoms filled with 99m Tc and 177 Lu. We create a realistic model of our detector by using a combination of a Geant4 Application for Tomographic Emission (GATE) Monte Carlo simulation and a finite element model for the CZT response, accounting for low-energy tail effects in CZT that affects the sensitivity and the scatter correction. We implement a modified dual-energy-window scatter correction adapted for CZT. Other corrections for attenuation, detector and collimator response, and detector gaps and edges are also included. The images are reconstructed using the maximum-likelihood expectation-maximization. Detector and reconstruction performance are characterized with point sources, Derenzo phantoms, and a body-sized National Electrical Manufacturers Association (NEMA) Image Quality (IQ) phantom for both 99m Tc and 177 Lu. Our SPECT design can resolve 7.9mm rods for 99m Tc (140keV) and 9.5mm for 177 Lu (208keV) in a hot-rod Derenzo phantom with a 3-min exposure and reach an image contrast of 78% for 99m Tc and 57% for 177 Lu using the NEMA IQ phantom with a 6-min exposure. Our modified scatter correction shows an improved contrast-recovery ratio compared to a standard correction. In this paper, we demonstrate the good performance of our design for whole-body imaging purposes. This adds to our previous demonstration of improved qualitative and quantitative 99m Tc imaging of brain perfusion and 123 I imaging of dopamine transport with respect to state-of-the-art NaI dual-head cameras. We show that our design provides similar IQ and contrast to the commercial full-ring SPECT VERITON for 99m Tc. Regarding 177 Lu imaging of the 208keV emissions, our design provides similar contrast to that of other state-of-the-art SPECTs with a significant reduction in exposure. The high sensitivity and extended energy range up to 250keV makes our SPECT design a promising alternative for clinical imaging and theranostics of emerging radionuclides.

The Improved X-ray Detector (iXRD) on Sharjah-Sat-1, design principles, tests and ground calibration

The iXRD is the primary science payload on Sharjah-Sat-1, a 3U CubeSat expected to be launched in Q4, 2022. Its main scientific goal is monitoring bright hard X-ray sources and transients in 20 - 200 keV band. The iXRD consists of a CdZnTe crystal (6.45 cm2 area, 5 mm thickness), a Tungsten collimator with square holes with an opening angle of 4.26∘, readout and control electronics and power supply circuitry, a back-shield and mechanical structures. Some of the design elements of iXRD have been inherited from the XRD on BeEagleSat with significant improvements in terms of collecting area, X-ray background and electronic noise. In this article, the design of the iXRD is discussed in detail taking into account mechanical, electronic, control software and data handling aspects. Its expected performance is determined after ground calibration. Depending on the pixel size, the energy resolution is 4 - 7 keV at 60 keV and the minimum detectable energy is 19 - 23 keV.

Experiment of proof-of-principle on prompt gamma-positron emission tomography (PG-PET) system for in-vivo dose distribution verification in proton therapy

In our previous study, we proposed an integrated PG-PET-based imaging method to increase the prediction accuracy for patient dose distributions. The purpose of the present study is to experimentally validate the feasibility of the PG-PET system. Based on the detector geometry optimized in the previous study, we constructed a dual-head PG-PET system consisting of a 16 × 16 GAGG scintillator and KETEK SiPM arrays, BaSO4 reflectors, and an 8 × 8 parallel-hole tungsten collimator. The performance of this system as equipped with a proof of principle, we measured the PG and positron emission (PE) distributions from a 3 × 6 × 10 cm3 PMMA phantom for a 45 MeV proton beam. The measured depth was about 17 mm and the expected depth was 16 mm in the computation simulation under the same conditions as the measurements. In the comparison result, we can find a 1 mm difference between computation simulation and measurement. In this study, our results show the feasibility of the PG-PET system for in-vivo range verification. However, further study should be followed with the consideration of the typical measurement conditions in the clinic application.

Simulation study on carbon-ion beam imaging by measuring secondary electron bremsstrahlung using imaging plate

The purpose of this study is to evaluate the effect of a lead radiation shield on the ability of a beam imaging device consist of an imaging plate (IP) and a collimator by Monte Carlo simulations. Simulations were performed using PHITS. A carbon-ion beam was injected to an acrylic target. A tungsten collimator having a pinhole was placed at the distance of 31.2 cm from the beam. A lead radiation shield was placed on the tungsten collimator. An IP was placed under the collimator. Beam images were acquired by recording the position distribution of energy deposition on the IP. We confirmed that therapeutic carbon-ion beam images could be acquired using the imaging device combining the IP and collimator. It was found that removal of the lead shield had no effect on the imaging results.

Development of an MPPC-based gamma-ray detector onboard a radiation source imager under high-dose environments and initial performance results

Since the accident at the Fukushima Daiichi Nuclear Power Plant ∼10 years ago, various studies have been conducted focusing on the decontamination of radioactive materials and the decommissioning of the remains of the reactor. It is crucial to identify the locations of radioactive materials for the decontamination of a building that is exposed to high radiation doses. One of the most widely used methods is the visualization of gamma rays with energy information using a photon-counting imager. However, the existing imagers have limited functionality, making it difficult to implement them in such an environment. Therefore, we have developed a novel pinhole gamma imager containing multi-pixel photon counters (MPPCs) combined with fast scintillators, which are processed using fast signal-processing analog and digital large integrated circuits under high-dose environments. The two-dimensional sensor array developed in this study can obtain incident gamma-ray photon energies with a counting rate of a few MHz/mm2. Furthermore, we were able to perform two-dimensional gamma-ray imaging of an extremely strong radiation source of 60Co with ∼45 TBq, by combining the sensor array with the dedicated tungsten collimator and housing.

Design of 4π High-Efficiency Directional Radiation Detector Based on Compton Scattering

Obtaining directional information is required in many applications such as nuclear homeland security, contamination mapping after a nuclear incident and radiological events, or during the decontamination work. However, many directional radiation detectors are based on directional shielding, made of lead or tungsten collimators, introducing two main drawbacks. The first is the size and weight, making those detectors too heavy and irrelevant for utilization in handheld devices, drone mapping, or space applications. The second drawback is the limited field of view (FOV), which requires multiple detectors to cover the whole required FOV or machinery to rotate the detector&#x2019;s narrow FOV detector. We propose a novel <inline-formula> <tex-math notation="LaTeX">$4\pi $ </tex-math></inline-formula> directional detector based on a segmented hollow cubic detector, which uses the Compton effect interactions with no heavy collimators. The symmetrical cubical design provides both higher efficiency and <inline-formula> <tex-math notation="LaTeX">$4\pi $ </tex-math></inline-formula> detection ability. Instead of the traditional two types of detectors (scatterer and absorber) structure, we use the same type of detector, based on Gd₃Al₂Ga₃O₁₂ (GAGG) (Ce) scintillator coupled to silicon photomultiplier. An additional advantage of the proposed detector is obtained by locating the photon sensors inside the detector, behind the scintillators, which improves the radiation hardness required for space applications. Furthermore, such an arrangement flattens the temperature variation across the detector, providing better gain stability. The main advantage of the proposed detector is an efficient <inline-formula> <tex-math notation="LaTeX">$4\pi $ </tex-math></inline-formula> radiation detection for high-energy gamma rays without the use of heavy collimators.

Small-field beam data acquisition, detector dependency, and film-based validation for a novel self-shielded stereotactic radiosurgery system.

This study reports a single-institution experience with beam data acquisition and film-based validation for a novel self-shielded sterotactic radiosurgery unit and investigates detector dependency on field output factors (OFs), off-axis ratios (OARs), and percent depth dose (PDD) measurements within the context of small-field dosimetry. The delivery platform for this unit consists of a 2.7-MV S-band linear accelerator mounted on coupled gimbals that rotate around a common isocenter (source-to-axis distance [SAD]=450mm), allowing for more than 260 noncoplanar beam angles. Beam collimation is achieved via a tungsten collimator wheel with eight circular apertures ranging from 4mm to 25mm in diameter. Three diodes (PTW 60012 Diode E, PTW 60018 SRS Diode, and Sun Nuclear EDGE) and a synthetic diamond detector (PTW 60019micro Diamond [µD] detector) were used for OAR, PDD, and OF measurements. OFs were also acquired with a PTW 31022 PinPoint ionization chamber. Beam scanning was performed using a 3D water tank at depths of 7, 50, 100, 200, and 250mm with a source-to-surface distance of 450mm. OFs were measured at the depth of maximum dose (dmax =7mm) with the SAD at 450mm. Gafchromic EBT3 film was used to validate OF and profile measurements and as a reference detector for estimating correction factors for active detector OFs. Deviations in field size, penumbra, and PDDs across the different detectors were quantified. Relative OFs (ROFs) for the diodes were within 1.4% for all collimators except for 5 and 7.5mm, for which SRS Diode measurements were higher by 1.6% and 2.6% versus Diode E. The µD ROFs were within 1.4% of the diode measurements. PinPoint ROFs were lower by >10% for the 4-mm and 5-mm collimators versus the Diode E and µD. Corrections to OFs using EBT3 film as a reference were within 1.2% for all diodes and the µD detector for collimators 10mm and greater and within 2.0%, 2.8%, and 1.1% for the 7.5-, 5-, and 4-mm collimators, respectively. The maximum difference in full width at half maximum (FWHM) between the Diode E and the other active detectors was for the 25-mm collimator and was 0.09mm (µD), 0.16mm (SRS Diode), and 0.65mm (EDGE). Differences seen in PDDs beyond the depth of dmax were <1% across the three diodes and the µD. FWHM and penumbra measurements made using EBT3 film were within 1.34% and 3.26%, respectively, of the processed profile data entered into the treatment planning system. Minimal differences were seen in OAR and PDD measurements acquired with the diodes and the µD. ROFs measured with the three diodes were within 2.6% and within 1.4% versus the µD. Gafchromic Film measurements provided independent verification of the OAR and OF measurements. Estimated corrections to OFs using film as a reference were <1.6% for the Diode E, EDGE, and µD detector.

Design of 4π Directional Radiation Detector based on Compton Scattering Effect

Obtaining directional information is required in many applications such as nuclear homeland security, contamination mapping after a nuclear incident and radiological events, or during the decontamination work. However, many directional radiation detectors are based on directional shielding, made of lead or tungsten collimators, introducing two main drawbacks. The first is the size and weight, making those detectors too heavy and irrelevant for utilization in handheld devices, drone mapping, or space applications. The second drawback is the limited field of view, which requires multiple detectors to cover the whole required field of view or machinery to rotate the narrow field of view detector. We propose a novel 4π directional detector based on a segmented hollow cubic detector, which uses the Compton effect interactions with no heavy collimators. The symmetrical cubical design provides both higher efficiency and 4π detection ability. Instead of traditional two types of detectors (scatterer and absorber) structure, we use the same type of detector, based on GAGG(Ce) scintillator coupled to silicon photomultiplier. Additional advantage of the proposed detector obtained by locating the photon sensors inside the detector, behind the scintillators, which improves the radiation hardness required for space applications. Furthermore, such arrangement flattens the temperature variation across the detector, providing better gain stability. The main advantage of the proposed detector is the ability of 4pi radiation detection for high energy gamma-rays without the use of heavy collimators.

Dual MRI-SPECT imaging system realized with Timepix pixel detector inserted into MR small animal scanner

Although MRI and SPECT are routine tools in medical imaging individually, the combination of these two modalities in multimodal imaging is still unexplored. By hybridizing high-resolution anatomical with functional imaging, combined MRI-SPECT could be a valuable addition to existing medical imaging methods, reducing overall scanning time and image co-registration errors. In contrast to PET, SPECT offers a cost-efficient range of applicable radioisotopes. This proof-of-principle study shows a modified experimental MRI-SPECT insert system consisting of an MR-compatible SPECT unit with hybrid semiconductor detectors Timepix. The insert system is equipped with CdTe pixelated sensors, tungsten collimators and a radiofrequency coil (RF). To increase the number of projections acquired, the setup is also equipped with an electromagnetic step motor Microcon SX 16-0503 for a sample rotation. Measurements were performed our own invented multimodal (1H/99mTc) inhomogeneous phantom filled with a liquid 99mTc radiotracer, emitting 140.5 keV γ-rays inside the RF coil by the Bruker BioSpec 47/20 (4.7 T) MR animal scanner. For the evaluation of measured data, SPECT-back projection software was developed in Matlab. Our results pave the way for a preclinical MRI-SPECT insert system. This research was performed in the framework of the Medipix Collaboration.

Performance characteristics of a tungsten collimator and UVTRON flame sensor in the detection of alpha-induced radioluminescence

It has been established that the UVTRON flame sensor (Hamamatsu) can detect alpha-induced radioluminescence, but that the presence of gamma and beta radiation both interfere with this detection. A UVTRON was placed inside a tungsten collimator and exposed to a range of radioisotopes, 210Po, 241Am, 137Cs, 90Sr and 60Co, to investigate the effect of shielding the UVTRON. The collimator is a cylinder with a hole in the curved wall to allow light and particles to access the interior, without providing a direct shine path to the UVTRON sensor. Ultraviolet C (wavelength 180–280 nm) radioluminescence is reflected onto the UVTRON sensor using a UVC reflecting mirror. It was found that the collimator does not affect the low background count of the UVTRON, but that it does greatly reduce the UVC signal reaching the UVTRON from an alpha source. For example, the collimator reduced the signal by 94% at 60 mm, and 78% at 120 mm, where the signals were still far greater than the background (88 and 84 times background respectively). Beta particles entering the collimator, although not directly impacting on the UVTRON, do increase the count, likely due to bremsstrahlung radiation. The collimator attenuates gamma radiation dependent on the gamma energy, but as expected, does not block it. When using more than one source, the count is cumulative and therefore it may be possible to determine the presence of UVC radioluminescence through subtraction of the gamma and beta element of the signal. The results and findings of the experiments carried out are presented herewithin.

Technical and dosimetric realization of in vivo x-ray microbeam irradiations at the Munich Compact Light Source.

X-ray microbeam radiation therapy is a preclinical concept for tumor treatment promising tissue sparing and enhanced tumor control. With its spatially separated, periodic micrometer-sized pattern, this method requires a high dose rate and a collimated beam typically available at large synchrotron radiation facilities. To treat small animals with microbeams in a laboratory-sized environment, we developed a dedicated irradiation system at the Munich Compact Light Source (MuCLS). A specially made beam collimation optic allows to increase x-ray fluence rate at the position of the target. Monte Carlo simulations and measurements were conducted for accurate microbeam dosimetry. The dose during irradiation is determined by a calibrated flux monitoring system. Moreover, a positioning system including mouse monitoring was built. We successfully commissioned the in vivo microbeam irradiation system for an exemplary xenograft tumor model in the mouse ear. By beam collimation, a dose rate of up to 5.3Gy/min at 25keV was achieved. Microbeam irradiations using a tungsten collimator with 50μm slit size and 350μm center-to-center spacing were performed at a mean dose rate of 0.6Gy/min showing a high peak-to-valley dose ratio of about 200 in the mouse ear. The maximum circular field size of 3.5mm in diameter can be enlarged using field patching. This study shows that we can perform in vivo microbeam experiments at the MuCLS with a dedicated dosimetry and positioning system to advance this promising radiation therapy method at commercially available compact microbeam sources. Peak doses of up to 100Gy per treatment seem feasible considering a recent upgrade for higher photon flux. The system can be adapted for tumor treatment in different animal models, for example, in the hind leg.

Super Spatial Resolution (SSR) method for small animal SPECT imaging: A Monte Carlo study

This work used Monte Carlo (MC) simulations in order to investigate the feasibility of the application of a super-resolution method, namely Super Spatial Resolution (SSR), to a preclinical SPECT system. The SSR method is used to obtain a resolution enhancement by combining several images containing slightly different perspectives of the same scene.MC simulations are a useful tool in the development of new imaging devices, as well as in the assessment of new reconstruction methods. In this respect, GATE (Geant4 Application for Emission Tomography) is an open source platform dedicated to numerical MC simulations that has become a well validated toolkit for the simulation of SPECT and PET systems.We modelled a four-headed preclinical SPECT scanner capable of the proper movements in order to obtain the SSR acquisition sequences. Each detector is based on a CsI(Tl) pixelated scintillator coupled to a low-energy tungsten collimator with parallel square holes.Several phantoms and radioactive sources were simulated, in order to assess the performance of the proposed SPECT scanner. In addition, a complex voxelized phantom of a mouse was used to obtain realistic images with the aim of evaluating the system as a suitable tool for small animal imaging.The whole system sensitivity was 164.1 cps/MBq. In order to assess the impact of the SSR this value must be divided by the number of the images acquired to perform this method. The average value of the trans-axial resolution improves from 2.4 mm to 1.54 mm, 1.21 mm and 1.03 mm by respectively applying the SSR based on two, three or four images. While the average axial resolution changes from 1.69 mm to 1.49 mm, 1.15 mm and 0.98 mm by respectively using the SSR based on two, three or four images. The mouse images obtained by using the voxel phantom have demonstrated the good capability of the system as a suitable tool for small animal imaging.Finally, a comparison with commercial preclinical scanners has proved that the presented SSR scanner provides an alternative to pinhole SPECT systems for many preclinical research studies.