Hexagonal Parallel-hole Collimator Research Articles

A SIMIND Monte Carlo Simulation Study on CdTe and NaI (Tl) Thickness as Detectors of a Small Animal SPECT System

Purpose: Micro Single Photon Emission Computed Tomography (Micro-SPECT) system has recently been introduced on nuclear medicine in the preclinical and research in which NaI (Tl) and Cadmium Telluride (CdTe) are used as the gamma-ray detectors with more generally use of NaI (Tl). The present study aimed to evaluate different thicknesses of the NaI (Tl) and (CdTe) detectors on functional parameters of a micro-SPECT system.
 Materials and Methods: A Micro-SPECT system with CdTe semiconductor detector and a hexagonal parallel hole collimator with a hole diameter of 0.11 mm, high of 24.05 mm, and septal thickness of 0.016 mm was simulated by Simulating Medical Imaging Nuclear Detectors (SIMIND) Monte Carlo program. The system performance was assessed by comparing the functional parameters, including system efficiency, sensitivity, energy and spatial resolution with the NaI (Tl) detector. The simulated scans of a 99mTc point source, a digital micro-Jacszack phantom, and a voxelized MOBY mouse phantom with the system were prepared to evaluate image quality.
 Results: The functional parameters; sensitivity, efficiency, planar spatial resolution, and image contrast of CdTe detector were determined 1.4, 1.2, 1.7, and 1.8 times higher than those of NaI (Tl), respectively. Moreover, the calculated energy resolution of CdTe and NaI (Tl) detectors was obtained 6.2% and 10.2% at 141 KeV, respectively. In the filtered back projection (FBP) reconstructed images of the micro-Jacszack phantom, minimum detectable size of the cold rods with CdTe and NaI (Tl) detectors were obtained 0.79 mm and 0.95 mm, respectively.
 Conclusion: The imaging system with a 5.5 mm thickness CdTe detector provided better image quality and showed considerable efficiency.

Validation and optimization studies of small animal SPECT using GATE Monte Carlo simulation

Small animal imaging has been widely considered for the development and evaluation of new radiopharmaceuticals. For this purpose, simulations are primarily used for the design and optimization of new and existing imaging devices, the assessment of acquisition and processing protocols. In this paper, the methodology follows two basic steps: (I) A validation of a small animal SPECT scanner which is based on CsI(Na) pixelated scintillator detector and a lead hexagonal parallel-hole collimator. In this phase, experimentally obtained results were directly compared to Geant4 Toolkit for Emission Tomography (GATE) simulation data. (II) The optimization studies with different collimator and semiconductor materials by using GATE simulation. The simulated and experimental planar sensitivities of the system were obtained 51.06 and 48.65 cps/MBq on the collimator surface, respectively. The difference between simulated spatial resolution and experimental value at 2.5 cm distance from the collimator surface was 3.27 %. Also, sensitivity of lead collimator was 18, 63.37, and 65.21% higher than tantalum, tungsten and gold collimators, respectively but gold collimator had the best spatial resolution. CdTe semiconductor detector usually has some superiority to the other semiconductors in energy resolution, spatial resolution, sensitivity, and detection efficiency. Very good agreement between GATE simulation and experimental results indicates that the GATE Monte Carlo simulation is appropriate and reliable tool for simulating imaging systems. Also, HiReSPECT has high resolution and sensitivity for imaging of small animals such as mice and rats. In the following, our study highlights that the collimator and crystal materials can significantly affect the camera performance. As well as developments in semiconductor detector technology can provide an appropriate substitution for scintillation detectors according to high sensitivity, better energy resolution, and also high spatial resolution.

SU-E-CAMPUS-I-04: Creation and Quantification Analysis of a Simulated SPECT Database Using the 4D-MOBY Phantom for Imaging Applications

Purpose: To develop a database of realistic animal SPECT simulations.Simulations were performed using a wholebody mouse model testing several realistic preclinical situations and compared against real small animal imaging acquisitions. Methods: The GATE Monte Carlo toolkit was used for modeling a small animal SPECT system. The camera has a hexagonal parallel hole collimator and a ∼1.6 mm spatial resolution.The 4D digital MOBY phantom was used in order to model mouse anatomy and activity distribution.Planar and SPECT scans for the biodistribution of the commonly used 99mTc based radiopharmaceuticals (MIBI, MDP and HMPAO) were modeled. Resulting simulated data were reconstructed using the QSPECT opensource software for SPECT reconstruction with a 10% energy window. Results: Simulations were executed in the GateLab grid resulting to: a) MDP bone scan biodistribution (anterior posterior planar images), b) Whole body mouse Sestamibi biodistribution, c) 99mTc HMPAO (whole body scan), d) 2 scans for HMPAO biokinetics within the brain (mouse and rat), and e) 99mTc radiolabled magnetic nanoparticles imaging with the MOBY phantom (the activity distribution was extracted from real data). Quantitative analysis was performed in several regions of interest. Simulated results were compared against real data and theoretical values available in the literature.The HMPAO brain biokinetics of both the mouse and rat brains were compared in order to investigate the impact of the brain size on detecting various brain structures.In the mouse model the brain was observed as a single entity, while in the case of the rat brain model structures were distinguished. Conclusion: GATE provides precise physics modeling based on the Geant4 code, while the 4D MOBY phantom can produce realistic imaging data, against which other imaging systems, reconstruction and correction techniques can be compared and standardized. To our knowledge this is the first small animal SPECT database with corresponding imaging data of realistic preclinical situations.

Optimization of an ultra-high-resolution parallel-hole collimator for CdTe semiconductor SPECT system

Recently, there has been an increase in the demand for semiconductor detectors in the field of nuclear medicine imaging. The development of semiconductor detectors using materials such as CdTe that allowed for improved spatial resolution greatly advanced the field. However, the pinhole collimator that allows for high spatial resolution compromises the sensitivity due to the small size of the hole. An improvement in both sensitivity and spatial resolution may be achieved by using a pixelated parallel-hole collimator where the hole and pixel sizes are the same. The purpose of this study was to optimize the design of a detector and collimator system to achieve excellent resolution and high sensitivity for a SPECT detector based on a CdTe detector. We performed a simulation study of the PID 350 (Ajat Oy Ltd., Finland) CdTe detector by using a Geant4 Application for Tomographic Emission (GATE). In addition to the above-mentioned pixelated parallel-hole collimator, we also designed a hexagonal parallel-hole collimator with similar hole size, and we evaluated the sensitivity and spatial resolution of each to determine which set-up was optimal for the PID 350 CdTe detector. Our results indicated that the average sensitivity and spatial resolution were 33.48% and 10.97% higher for the pixelated parallel-hole collimator than for the hexagonal parallel-hole collimator, respectively. We resolved a diameter of 0.5 mm in hot-rod phantom images with the pixelated parallel-hole collimator at a distance of 2 cm. Based on our results, we recommend the pixelated parallel-hole collimator for improving the sensitivity and spatial resolution of SPECT systems with CdTe semiconductor detectors.

Optimization of a high-resolution collimator for a CdTe detector: Monte Carlo simulation studies

Photon counting detectors using cadmium zinc telluride (CZT) or cadmium telluride (CdTe) have benefits compared to conventional scintillation detectors, and CZT and CdTe have advantageous physical characteristics for nuclear medicine imaging. Recently, many studies have been conducted using these materials to improve the sensitivity and the spatial resolution of the photon counting detector. By using a pixelated parallel-hole collimator, we may be able to improve the sensitivity and the spatial resolution. The purpose of this study was to optimize the design of a collimator to achieve excellent resolution and high sensitivity for a gamma camera system based on the CdTe detector. In this study we simulated a gamma camera system with a photon counting detector based on CdTe and evaluated the system’s performance. We performed a simulation study of the PID 350 (Ajat Oy Ltd., Finland) CdTe detector by using a Geant4 Application for Tomographic Emission (GATE) simulation. This detector consists of small pixels (0.35 × 0.35 mm2). We designed two parallel-hole collimators with different shapes and verified their usefulness. One was the proposed pixelated parallel-hole collimator in which the hole size and the pixel size are the same, and the other was the hexagonal parallel-hole collimator, which had a hole size similar to that of the pixelated parallel-hole collimator. We evaluated the sensitivity, spatial resolution, and contrast resolution to determine which parallel-hole collimator was best for the PID 350 CdTe detector. The average sensitivity was 22.65% higher for the pixelated parallel-hole collimator than for the hexagonal parallel-hole collimator. Also, the pixelated parallel-hole collimator provided 10.7% better spatial resolution than the hexagonal parallel-hole collimator, and the contrast resolution was improved by 8.93%. These results reflect an improvement in sensitivity and spatial resolution, and indicate that the imaging performance of the pixelated parallel-hole collimator is better than that of the hexagonal parallel-hole collimator. In conclusion, we successfully established a high resolution gamma camera system with a pixelated parallel-hole collimator, and based on our results, we recommend using the pixelated parallel-hole collimator to improve the sensitivity and the spatial resolution of gamma camera systems with semiconductor detectors such as CdTe.

Performance Characterization of a Miniature, High Sensitivity Gamma Ray Camera

A compact, hand-held gamma camera with excellent intrinsic and extrinsic performance has been developed for the rapid identification and localization of sentinel lymph nodes during the surgical staging of cancer. The camera comprises a 5×5 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> field of view NaI (Tl) pixellated crystal array, a high sensitivity lead (2.0 cm thick) hexagonal parallel-hole collimator, a position sensitive photomultiplier tube (PSPMT), and a novel highly multiplexed electrical readout. The intrinsic energy resolution (12.1±2.0%) at 140 keV including edge crystals, extrinsic sensitivity (5 cps/μCi from 1-5 cm with a 24% energy window) and extrinsic spatial resolution (1.81±0.1 mm at 0.6 cm) facilitate rapid identification of a radioactive node. Using a node phantom we performed an ideal observer study to estimate the detectability of small spheres in the range of 3 mm to 8 mm with different exposure times. With a 5 seconds exposure the camera can detect a 3 mm diameter sphere at a depth of 3.6 cm containing 1 μCi of Tc-99m within a cold background, and a 4 mm diameter sphere containing 2 μCi at 2.6 cm depth within a warm background (>10:1 contrast) with a 5 second exposure duration.

Online gamma-camera imaging of 103Pd seeds (OGIPS) for permanent breast seed implantation

Permanent brachytherapy seed implantation is being investigated as a mode of accelerated partial breast irradiation for early stage breast cancer patients. Currently, the seeds are poorly visualized during the procedure making it difficult to perform a real-time correction of the implantation if required. The objective was to determine if a customized gamma-camera can accurately localize the seeds during implantation. Monte Carlo simulations of a CZT based gamma-camera were used to assess whether images of suitable quality could be derived by detecting the 21 keV photons emitted from 74 MBq 103Pd brachytherapy seeds. A hexagonal parallel hole collimator with a hole length of 38 mm, hole diameter of 1.2 mm and 0.2 mm septa, was modeled. The design of the gamma-camera was evaluated on a realistic model of the breast and three layers of the seed distribution (55 seeds) based on a pre-implantation CT treatment plan. The Monte Carlo simulations showed that the gamma-camera was able to localize the seeds with a maximum error of 2.0 mm, using only two views and 20 s of imaging. A gamma-camera can potentially be used as an intra-procedural image guidance system for quality assurance for permanent breast seed implantation.

SU‐FF‐I‐81: Gamma Camera Guided Permanent Breast 103Pd Seed Implantation

Objective: To assess whether a proposed SPECT device could address the requirements of resolving distributions of permanent breast brachytherapy seeds following implantation; while maintaining an acceptable imaging time to allow for correction of misplaced seeds. Method: Monte Carlo simulations of a cadmium zinc telluride crystal‐based gamma camera were used to assess whether the detection of 22 keV photons emitted from the seeds was feasible. A hexagonal parallel hole collimator, hole length 38 mm, diameter 1.2 mm with 0.2 mm septa was modeled. The design of the gamma camera device was evaluated on two phantom models. The first model consisted of a simple representation of the clinical problem by simulating the breast as 8 cm diameter sphere of breast tissue containing a central, 1cm cubic distribution of 8 seeds. The second simulation presented a more accurate depiction of the clinical problem, where the breast model was based on the pre‐implant CT scan of a typical breast brachytherapy patient and the activity was simulated from the patient's corresponding treatment plan. Results: The spherical phantom yielded promising results after 24 s of imaging time, where the maximum error between the center of mass of the seeds in the reconstructed image and the simulated seed location was 1.02 mm. The results from the clinically accurate simulation revealed that individual seeds could not be identified from the reconstructed images after 2 minutes of imaging. However, the strands of seeds, arranged in each needle were localized to a maximum error of 1.9 mm. Conclusion: The online gamma‐camera approach to imaging the seeds is feasible for simple seed distributions. Additional improvements to the collimator design and the gamma camera orbit are required before the gamma camera device will be able to distinguish each seed in an implanted seed distribution.

Sci‐AM1 Sat ‐ 04: Gamma‐camera verification of breast brachytherapy seed distributions

Objective: To assess whether a proposed SPECT device could resolve implanted breast brachytherapy seeds within an acceptable imaging time for correction of misplaced seeds. Method: Monte Carlo simulations of a cadmium zinc telluride crystal‐based gamma camera were used to assess whether the detection of 22 keV photons emitted from seeds was feasible. A 5×5 cm detector, fitted with a hexagonal parallel hole collimator (hole length 38 mm, diameter 1.2 mm, septa 0.2 mm) was modeled. Projections were taken every 7.5 degrees, with a radius of rotation of 10 cm, and images reconstructed using an OSEM algorithm. The phantom modeled consisted of an 8 cm diameter sphere of breast tissue containing a central, 1 cm cubic distribution of 8 seeds, which were each 5 mm long and 0.8 mm wide titanium tubes with an inner radius of 0.32 mm. Results: An acquisition duration of 24 seconds yielded images with a FWHM of 6.0 mm and a scatter fraction of 8.2%. The error between the center of mass of the reconstructed image and the physical seed location was 0.81 ± 0.16 mm, when the phantom was imaged for 24 seconds. Conclusion: The online gamma‐camera approach to imaging the seeds is feasible for a simple seed distribution. The high contrast between the seeds and the non‐radioactive background allow a practical acquisition time of a minute or less. Additional simulations will be required to assess the system design for more realistic seed distributions in a geometry more closely modeling a patient.

Caractérisation de la réponse des collimateurs à trous parallèles hexagonaux des caméras à scintillation Characterization of the response of hexagonal parallel-hole collimators of scintillation cameras

Cet article présente une formulation de la réponse en fréquence des collimateurs à trous parallèles hexagonaux des caméras à scintillation. Pour décrire cette réponse, nous proposons une équation déterminée de façon semi-empirique à partir d'une grand nombre de simulations. L'intérêt de cette équation est de pouvoir calculer simplement la réponse à partir des caractéristiques du collimateur en tenant compte de la structure en `nid d'abeille' du collimateur. Du fait que cette équation ne fait pas l'hypothèse de l'invariance par translation, les résultats obtenus peuvent être directement corrélés aux mesures expérimentales réalisées avec une source ponctuelle. Ceci est particulièrement intéressant pour les collimateurs à gros trous, comme ceux à moyenne résolution utilisés avec des sources radioactives de haute énergie. Le contrôle de qualité et la mesure des performances physiques sont donc facilités pour ce type de collimateur. Par ailleurs, nous présentons un nouveau paramètre qui apporte une information quantitative sur l'importance de la pénétration septale. Ce paramètre est mesuré directement sur la réponse en fréquence du collimateur. Il a été étudié par simulation, en tenant compte de l'atténuation des photons dans les parois du collimateur. Les mesures expérimentales qui ont été faites sont en bon accord avec les formulations présentées.