Collimator Surface Research Articles

Analytical and experimental FWHM of a gamma camera: theoretical and practical issues.

Introduction. It is well known that resolution on a gamma camera varies as a function of distance, scatter and the camera’s characteristics (collimator type, crystal thickness, intrinsic resolution etc). Manufacturers frequently provide only a few pre-calculated resolution values (using a line source in air, 10–15 cm from the collimator surface and without scattering). However, these are typically not obtained in situations resembling a clinical setting. From a diagnostic point of view, it is useful to know the expected resolution of a gamma camera at a given distance from the collimator surface for a particular setting in order to decide whether it is worth scanning patients with “small lesion” or not. When dealing with absolute quantification it is also mandatory to know precisely the expected resolution and its uncertainty in order to make appropriate corrections.Aim. Our aims are: to test a novel mathematical approach, the cubic spline interpolation, for the extraction of the full width at half maximum (FWHM) from the acquisition of a line source (experimental resolution) also considering measurement uncertainty; to compare it with the usually adopted methods such as the gaussian approach; to compare it with the theoretical resolution (analytical resolution) of a gamma camera at different distances; to create a web-based educational program with which to test these theories.Methods. Three mathematical methods (direct calculation, global interpolation using gaussian and local interpolation using splines) for calculating FWHM from a line source (planar scintigraphy) were tested and compared. A NEMA Triple Line Source Phantom was used to obtain static images both in air and with different scattering levels. An advanced, open-source software (MATLAB/Octave and PHP based) was created “ad hoc” to obtain and compare FWHM values and relative uncertainty.Results and Conclusion. Local interpolation using splines proved faster and more reliable than the usually-adopted Gaussian interpolation. The proposed freely available software proved effective in assessing both FWHM and its uncertainty.

Development of an Optical Fiber-Based MR Compatible Gamma Camera for SPECT/MRI Systems

Optical fiber is a promising material for integrated positron emission tomography (PET) and magnetic resonance imaging (MRI) PET/MRI systems. Because its material is plastic, it has no interference between MRI. However, it is unclear whether this material can also be used for a single photon emission tomography (SPECT)/MRI system. For this purpose, we developed an optical fiber-based block detector for a SPECT/MRI system and tested its performance by combining $1.2 \times 1.2 \times 6~\hbox{mm} {{\rm Y}_2}{\rm SiO}_5$ (YSO) pixels into a $15 \times 15$ block and was coupled it to an optical fiber image guide that used was 0.5-mm in diameter with 80-cm long double clad fibers. The image guide had $22 \times 22~\hbox{mm}$ rectangular input and an equal size output. The input of the optical fiber-based image guide was bent at 90 degrees, and the output was optically coupled to a 1-in square high quantum efficiency position sensitive photomultiplier tube (HQE-PSPMT). The parallel hole, 7-mm-thick collimator made of tungsten plastic was mounted on a YSO block. The diameter of the collimator holes was 0.8 mm which was positioned one-to-one coupled to the YSO pixels. We evaluated the intrinsic and system performances. We resolved most of the YSO pixels in a two-dimensional histogram for Co-57 gamma photons (122-keV) with an average peak-to-value ratio of 1.5. The energy resolution was 38% full-width at half-maximum (FWHM). The system resolution was 1.7-mm FWHM, 1.5 mm from the collimator surface, and the sensitivity was 0.06%. Images of a Co-57 point source could be successfully obtained inside 0.3 T MRI without serious interference. We conclude that the developed optical fiber-based YSO block detector is promising for SPECT/MRI systems.

Radiation dose enhancement at tissue-tungsten interfaces in HDR brachytherapy

HDR BrachyView is a novel in-body dosimetric imaging system for real-time monitoring and verification of the source position in high dose rate (HDR) prostate brachytherapy treatment. It is based on a high-resolution pixelated detector array with a semi-cylindrical multi-pinhole tungsten collimator and is designed to fit inside a compact rectal probe, and is able to resolve the 3D position of the source with a maximum error of 1.5 mm. This paper presents an evaluation of the additional dose that will be delivered to the patient as a result of backscatter radiation from the collimator. Monte Carlo simulations of planar and cylindrical collimators embedded in a tissue-equivalent phantom were performed using Geant4, with an 192Ir source placed at two different source-collimator distances. The planar configuration was replicated experimentally to validate the simulations, with a MOSkin dosimetry probe used to measure dose at three distances from the collimator. For the cylindrical collimator simulation, backscatter dose enhancement was calculated as a function of axial and azimuthal displacement, and dose distribution maps were generated at three distances from the collimator surface. Although significant backscatter dose enhancement was observed for both geometries immediately adjacent to the collimator, simulations and experiments indicate that backscatter dose is negligible at distances beyond 1 mm from the collimator. Since HDR BrachyView is enclosed within a 1 mm thick tissue-equivalent plastic shell, all backscatter radiation resulting from its use will therefore be absorbed before reaching the rectal wall or other tissues. dosimetry, brachytherapy, HDR

Modulation transfer function assessment in parallel beam and fan beam collimators with square and cylindrical holes

This study investigates modulation transfer function (MTF) in parallel beam (PB) and fan beam (FB) collimators using the Monte Carlo method with full width at half maximum (FWHM), square and circular-shaped holes, and scatter and penetration (S + P) components. A regulation similar to the lead-to-air ratio was used for both collimators to estimate output data. The hole pattern was designed to compare FB by PB parameters. The radioactive source in air and in a water phantom placed in front of the collimators was simulated using MCNP5 code. The test results indicated that the square holes in PB (PBs) had better FWHM than did the cylindrical (PBc) holes. In contrast, the cylindrical holes in the FB (FBc) had better FWHM than the square holes. In general, the resolution of FBc was better than that of the PBc in air and scatter mediums. The S + P decreased for all collimators as the distance from the source to the collimator surface (z) increased. The FBc had a lower S + P than FBs, but PBc had a higher S + P than PBs. Of the FB and PB collimators with the identical hole shapes, PBs had a smaller S + P than FBs, and FBc had a smaller S + P than PBc. The MTF value for the FB was greater than for the PB and had increased spatial frequency; the FBc had higher MTF than the FBs and PB collimators. Estimating the FB using PB parameters and diverse hole shapes may be useful in collimator design to improve the resolution and efficiency of SPECT images.

Development of a high-resolution YSO gamma camera system that employs 0.8-mm pixels

YSO (Ce-doped Y₂SiO₅) is a promising scintillator for a single-photon imaging system since it has relatively high light output and does not contain any natural radioactivity. Since YSO is not hygroscopic, it may be possible to fabricate a block with small pixels for a high-resolution system. For this purpose, we developed a high-resolution gamma camera system that employs smaller than 1-mm YSO pixels. The gamma camera's detector used 0.8 × 0.8 × 7-mm YSO pixels. All the surfaces of these YSO pixels were mechanically polished, combined with a 0.1-mm-thick BaSO4 reflector to form a 48 × 48 matrix, and optically coupled to a high quantum efficiency, 2-inch square position sensitive photomultiplier tube (Hamamatsu Photonics H10966 A-100). The YSO block was 43.2 × 43.2 mm. The YSO gamma camera was encased in a 5-mm-thick tungsten container, and a parallel collimator was mounted on its front. The parallel hole collimator was made of a 3-layer (each layer was 5-mm thick) tungsten plate, and each plate had 48 × 48, 0.6-mm holes that were positioned by one-to-one coupling with the YSO pixels. Even with the 0.8-mm YSO pixels, we clearly resolved most of the pixels in a 2-dimensional histogram with a peak-to-valley ratio of 2.9 for the 122-keV gamma photons. The energy resolution was 20.4% FWHM. The spatial resolutions with a parallel hole collimator 2 mm from the collimator surface were 0.7- and 1.3-mm FWHM for the 122- and ~35-keV gamma photons, respectively. We successfully obtained phantoms and small animal images with our YSO gamma camera system. Our high-resolution system has a potential to be useful for molecular imaging research.

Design and development of a high resolution animal SPECT scanner dedicated for rat and mouse imaging

A dedicated small-animal SPECT system, HiReSPECT, was designed and developed to provide a high resolution molecular imaging modality in response to growing research demands. HiReSPECT is a dual-head system mounted on a rotating gantry. The detection system is based on pixelated CsI(Na) scintillator crystals coupled to two Hamamatsu H8500 Position Sensitive Photomultiplier Tubes in each head. Also, a high resolution parallel-hole collimator is applied to every head. The dimensions of each head are 50mm×100mm, enabling sufficient transaxial and axial fields-of-view (TFOV and AFOV), respectively, for coverage of the entire mouse in single-bed position imaging. However, a 50mm TFOV is not sufficient for transaxial coverage of rats. To address this, each head can be rotated by 90 degrees in order to align the larger dimension of the heads with the short body axis, allowing tomographic data acquisition for rats. An innovative non-linear recursive filter was used for signal processing/detection. Resolution recovery was also embedded in the modified Maximum-Likelihood Expectation Maximization (MLEM) image reconstruction code to compensate for Collimator-Detector Response (CDR). Moreover, an innovative interpolation algorithm was developed to speed up the reconstruction code. The planar spatial resolution at the head surface and the image spatial resolutions were 1.7mm and 1.2–1.6mm, respectively. The measurements followed by post-processing showed that the observed count rate at 20% count loss is about 42kcps. The system sensitivity at the collimator surface for heads 1 and 2 were 1.32cps/µCi and 1.25cps/µCi, respectively. The corresponding values were 1.18cps/µCi and 1.02cps/µCi at 8cm distance from the collimator surfaces. In addition, whole-body scans of mice demonstrated appropriate imaging capability of the HiReSPECT.

Optimization of the angle between detector modules in a dual-head cardiac SPECT

In recent years, dedicated cardiac single photon emission computed tomography (SPECT) systems have been undergoing a profound change in design with multiple detectors and various angles between the modules to improve the sensitivity and the resolution by reducing the distance between the heart and the detector. The performance of a dual-head cardiac SPECT for small-animal imaging was characterized as a function of the angle between two detector heads by using GATE simulations, and simulation data were validated with experimental results. Each detector head consists of 50 × 50 × 6 mm3 NaI(Tl) optically coupled to a Hamamatsu H8500 position sensitive photomultiplier (PSPMT) and a low-energy high-resolution parallel-hole collimator (LEHR, septal thickness: 0.2 mm, diameter: 1.9 mm). The distance between the collimator surface and the center of rotation was set as 20, 20, 20, 25, or 31.5 mm for 70°, 80°, 90°, 100°, or 110°, respectively, based on a 40-mm field of view (FOV). A point source and a rat cardiac phantom of Tc-99m in scattering media were simulated. Projection data were acquired for 180 angular views in steps of 2° and were reconstructed by using a filtered back-projection algorithm. Results demonstrated that the angle between the detector heads did not make a big difference in the image quality when scattering media were not presented, but the dual heads in the 80° geometry provided the best spatial resolution in the cardiac phantom study. The peak-to-valley ratio between the myocardial wall and the cavity was measured as 1.87, 11.01, 3.28, 3.40, or 2.46 for 70°, 80°, 90°, 100°, or 110°, respectively. Experiments were performed with a dual-head SPECT in the 80° geometry, and the results agreed well with these from the simulations. In this study, the impact of the angle between dual detector heads on the imaging performance was evaluated, and the optimal angle was derived for a dedicated cardiac SPECT.

Implementation and evaluation of an expectation maximization reconstruction algorithm for gamma emission breast tomosynthesis

We are developing a dual modality tomosynthesis breast scanner in which x-ray transmission tomosynthesis and gamma emission tomosynthesis are performed sequentially with the breast in a common configuration. In both modalities projection data are obtained over an angular range of less than 180° from one side of the mildly compressed breast resulting in incomplete and asymmetrical sampling. The objective of this work is to implement and evaluate a maximum likelihood expectation maximization (MLEM) reconstruction algorithm for gamma emission breast tomosynthesis (GEBT). A combination of Monte Carlo simulations and phantom experiments was used to test the MLEM algorithm for GEBT. The algorithm utilizes prior information obtained from the x-ray breast tomosynthesis scan to partially compensate for the incomplete angular sampling and to perform attenuation correction (AC) and resolution recovery (RR). System spatial resolution, image artifacts, lesion contrast, and signal to noise ratio (SNR) were measured as image quality figures of merit. To test the robustness of the reconstruction algorithm and to assess the relative impacts of correction techniques with changing angular range, simulations and experiments were both performed using acquisition angular ranges of 45°, 90° and 135°. For comparison, a single projection containing the same total number of counts as the full GEBT scan was also obtained to simulate planar breast scintigraphy. The in-plane spatial resolution of the reconstructed GEBT images is independent of source position within the reconstructed volume and independent of acquisition angular range. For 45° acquisitions, spatial resolution in the depth dimension (the direction of breast compression) is degraded with increasing source depth (increasing distance from the collimator surface). Increasing the acquisition angular range from 45° to 135° both greatly reduces this depth dependence and improves the average depth dimension resolution from 10.8 to 4.8 mm. The 135° acquisition results in a near-isotropic, spatially uniform 3D resolution of approximately 4.3 mm full width at half maximum. Background nonuniformity (cupping) artifacts arise primarily from angular incompleteness for small angular range acquisition but primarily from gamma ray attenuation at larger angular range. However, background artifacts can be largely eliminated if both prior information regularization and AC are applied. An artificial decrease in lesion voxel value with increasing lesion depth can also be substantially reduced through a combination of AC and RR. In experiments using compressible gelatin breast phantoms, lesion contrast and SNR are about 2.6-8.8 times and 2.3-5.6 times higher, respectively, in GEBT than in planar breast scintigraphy depending on the acquisition angle, the gamma camera trajectory, and the lesion location. In addition, the strong reduction in lesion contrast and SNR with increasing lesion depth that is observed in planar breast scintigraphy can be largely overcome in GEBT. The authors have demonstrated a promising EM-based reconstruction scheme for use in GEBT. Compared to planar breast scintigraphy GEBT provides superior and less position-dependent lesion contrast, lesion SNR, and spatial resolution as well as more accurate quantification of lesion-to-background activity concentration ratio.

Performance characteristics of a new pixelated portable gamma camera

To evaluate and characterize the performance of a new commercially available pixelated portable gamma camera Ergo (Digirad, Poway, CA). The authors evaluated a pixelated portable gamma camera system, Ergo, that consists of 11 520 elements of 3 × 3 mm(2) CsI(Tl) crystals that are 6-mm thick and are coupled to silicon photodiodes. The detector element has a size of 3.31 × 3.24 mm(2). The gamma camera performance was evaluated for both low-energy all-purpose (LEAP) and low-energy high-resolution (LEHR) collimators. The flood-field uniformity for (99m)Tc and (201)Tl was assessed using fillable uniform flood phantoms. Energy spectra were acquired for (99m)Tc, (111)In, (201)Tl, and (67)Ga to evaluate energy linearity and energy resolution. Spectral fits were performed to calculate the photopeak energies and resolutions. The pixel size and multiwindow spatial registration (MWSR) was evaluated by measuring mixed (99m)Tc and (201)Tl point sources placed at known distances apart. The system's sensitivity was measured according to the National Electrical Manufacturer's Association (NEMA) NU1-2007 standards for both LEAP and LEHR collimators as a function of distance from the collimator surface (5, 10, 15, 20, 25, 30, and 40 cm). The system resolution without scatter was measured for both LEAP and LEHR using (99m)Tc-filled capillary tubes located at 0, 2, 4, 6, 10, and 12 cm away from the surface of the collimator. As a measure of the spatial resolution, the full width at half maximum (FWHM) at a given distance was calculated from the presampling line spread function (LSF), constructed from the line profiles of the capillary tubes at the same distance. As a comparison, the FWHM at 10 cm away from LEHR and LEAP collimators was also calculated from linear interpolation as described by NEMA NU-1 2007 and from fitting the profiles to a Gaussian-plus-constant model. All isotope-collimator pairs demonstrated good flood-field uniformity with an integral uniformity of ≤5% and a differential uniformity of ≤3%. The system demonstrated excellent energy linearity with maximum discrepancy of measured keV from true keV of <1%. The energy resolution of the (99m)Tc 140-keV photopeak was 7.4%. The image pixel size was measured as 3.23 × 3.18 mm(2), and the MWSR was within 0.3 mm (or ~10% of the nominal pixel size). The system sensitivity at 10 cm was 112.6 cps/MBq (249.9 cpm/μCi) for LEAP and 63.1 cps/MBq (140.1 cpm/μCi) for LEHR. The system spatial resolution varied linearly with distance from the collimator and the FWHM were measured to be 7.2 and 8.9 mm at 10 cm for LEHR and LEAP, respectively. Herein, the authors describe detailed performance evaluation procedures of a new pixelated portable gamma camera system, which can also be applied to evaluate other pixelated gamma camera system. Spatial resolution assessment in near-field imaging condition offers a unique challenge where the measured FWHM is highly dependent on relative position between the capillary tube and the detector element. The evaluations of the Ergo gamma camera suggest suitable clinical imaging performance. This portable gamma camera has a high (LEAP) planar sensitivity, high energy and spatial resolutions that are comparable to other available gamma cameras, and it exhibits superior count rate performance that is linear up to tens of millions count per second. The Ergo imaging performance, however, can still be improved, for example, by optimizing collimator design for near field imaging.

Development of a pixelated GSO gamma camera system with tungsten parallel hole collimator for single photon imaging

In small animal imaging using a single photon emitting radionuclide, a high resolution gamma camera is required. Recently, position sensitive photomultiplier tubes (PSPMTs) with high quantum efficiency have been developed. By combining these with nonhygroscopic scintillators with a relatively low light output, a high resolution gamma camera can become useful for low energy gamma photons. Therefore, the authors developed a gamma camera by combining a pixelated Ce-doped Gd(2)SiO(5) (GSO) block with a high quantum efficiency PSPMT. GSO was selected for the scintillator, because it is not hygroscopic and does not contain any natural radioactivity. An array of 1.9 mm × 1.9 mm × 7 mm individual GSO crystal elements was constructed. These GSOs were combined with a 0.1-mm thick reflector to form a 22 × 22 matrix and optically coupled to a high quantum efficiency PSPMT (H8500C-100 MOD8). The GSO gamma camera was encased in a tungsten gamma-ray shield with tungsten pixelated parallel hole collimator, and the basic performance was measured for Co-57 gamma photons (122 keV). In a two-dimensional position histogram, all pixels were clearly resolved. The energy resolution was ∼15% FWHM. With the 20-mm thick tungsten pixelated collimator, the spatial resolution was 4.4-mm FWHM 40 mm from the collimator surface, and the sensitivity was ∼0.05%. Phantom and small animal images were successfully obtained with our developed gamma camera. These results confirmed that the developed pixelated GSO gamma camera has potential as an effective instrument for low energy gamma photon imaging.

Estimation of Fan Beam and Parallel Beam Parameters in a Wire Mesh Design

Special collimators used in imaging systems play an important part in obtaining qualified images to improve diagnosis in medicine. The primary aim of this study was to compare resolution between fan beam and parallel beam collimators using Monte Carlo simulation in the shape of cubic holes. Also, parameters such as geometric efficiency, geometric resolution, scatter, penetration, and full width at half maximum were studied to compare their special characteristics. The simulation results demonstrated that the geometry efficiency of a fan beam collimator increased as the angle of the slant hole increased, and the geometric resolution decreased as the angle of the slant hole increased, at a distinct distance from a monoenergetic source of γ-rays. In contrast, at a distinct angle, geometric resolution increased as the distance between the source and the collimator surface increased. For both collimators, scatter and penetration decreased as the distance increased. These results were in agreement with ADAC company data. Finally, fan beam collimators were found to have better resolution than parallel beam collimators with a cubic hole shape in a wire mesh design. Estimation of the fan beam by parallel beam parameters as cubic holes can be suitable in collimator design to improve resolution and efficiency.

Development of a high-resolution Si-PM-based gamma camera system

A silicon photomultiplier (Si-PM) is a promising photodetector for PET, especially for PET/MRI combined systems, due to its high gain, small size, and lower sensitivity to static magnetic fields. However, these properties are also promising for gamma camera systems for single-photon imaging. We developed an ultra-high-resolution Si-PM-based compact gamma camera system for small animals. Y2SiO5:Ce (YSO) was selected as scintillators because of its high light output and no natural radioactivity. The gamma camera consists of 0.6 mm × 0.6 mm × 6 mm YSO pixels combined with a 0.1 mm thick reflector to form a 17 × 17 matrix that was optically coupled to a Si-PM array (Hamamatsu multi-pixel photon counter S11064-050P) with a 2 mm thick light guide. The YSO block size was 12 mm × 12 mm. The YSO gamma camera was encased in a 5 mm thick gamma shield, and a parallel hole collimator was mounted in front of the camera (0.5 mm hole, 0.7 mm separation, 5 mm thick). The two-dimensional distribution for the Co-57 gamma photons (122 keV) was almost resolved. The energy resolution was 24.4% full-width at half-maximum (FWHM) for the Co-57 gamma photons. The spatial resolution at 1.5 mm from the collimator surface was 1.25 mm FWHM measured using a 1 mm diameter Co-57 point source. Phantom and small animal images were successfully obtained. We conclude that a Si-PM-based gamma camera is promising for molecular imaging research.

Theory of ultrahigh magnetoresistance achieved byk-space filtering without a tunnel barrier

Calculations of the current-perpendicular-to-plane magnetoresistance (CPP GMR) of an epitaxial Fe/Ag/Fe(001) trilayer attached to an $n$-type InAs lead (collimator) are presented. They show that for realistic InAs carrier densities, the CPP GMR of this $k$-space selective system can reach peak values of 10${}^{5}$%. The very high values of the CPP GMR are achieved because the small Fermi surface of the semiconductor collimator selects only electrons traveling perpendicular to the Fe/Ag interface ($\mathrm{\ensuremath{\Gamma}\ifmmode \bar{}\else \={}\fi{}}$ point) where the magnetic contrast is as high as that of the Fe/MgO interface. The high calculated CPP GMR is very robust to disorder at the Fe/InAs interface and within the InAs layer itself. It is also much higher than the highest observed tunneling magnetoresistance of MgO-based junctions but the total resistance of the Fe/Ag/Fe trilayer with an InAs collimator is much lower than that of the tunneling junction. Nevertheless, the resistance is high enough to obviate the need for nanopillar geometry necessary for conventional CPP GMR systems.

Effects of Crystal Pixel Size and Collimator Geometry on the Performance of a Pixelated Crystal -Camera Using Monte Carlo Simulation

Dedicated γ-cameras based on pixelated scintillators have long been used for breast tumor imaging. Intercrystal scattering (ICS) increases the background counting rate and degrades the image quality when small crystal pixels are used. Because of the small size of applied collimators, scattered radiation and septal penetration are high, and therefore collimator characteristics must be carefully considered. In our study, we investigated the influence of ICS events on position-detection accuracy (PDA) for pixelated crystals and the effects of different geometries of hexagonal-hole collimators on the performance of these cameras, using Monte Carlo simulation to optimize camera design. The arrays of thallium-doped cesium iodide detectors with different pixel dimensions that had been exposed to 140-keV photons of isotropic point source, 50 mm from the collimator surface, were simulated. Hexagonal-hole collimators were 10.5, 15, and 21 mm long. The septal thickness varied from 0.1 to 0.5 mm, with 3 different hole diameters. The results confirmed that by increasing the crystal pixel size, ICS was decreased and change of detection efficiency was negligible, but PDA, contrast-to-noise ratio, and spatial resolution (full width at half maximum) were increased. Our experiences confirmed that 2 × 2 mm was an optimum crystal pixel size, especially for a lower ICS fraction and an appropriate full width at half maximum. Because collimators are the limiting factor for spatial resolution and sensitivity, careful collimator design is of great importance.

SU‐GG‐T‐331: A New Monte Carlo Code for Gamma Knife Simulations

Purpose: Gamma Knife is widely used in stereotactic radiosurgery (SRS) to treat brain tumors. In this study, a new Monte Carlo code was developed to simulate Gamma Knife system, which can be used to calculate Gamma Knife collimator factors, to verify Gamma Knife monthly and annual QA results and to evaluate treatment plan from Leksell GammaPlan system. Method and Materials: An EGSnrc based user code DOSIMETER was used for this study. The Cobalt‐60 source and collimators were simulated based on their material, size and location in the system. Because of the equal geometries and materials of the sources and collimators, one source system is generated for all 201 sources in corresponded angle. For each collimator (4, 8, 14 or 18 mm), a phase space file was produced at the final helmet collimator surface. A CT image of a round QA phantom with 160 mm in diameter was introduced in the simulation system. Source linearity and profiles at isocenter were calculated using Monte Carlo simulations and compared with pin chamber and film measurement results. The relative collimator factors for 4, 8 and 14 mm were calculated and compared with ultra‐thin TLD and OSL measurements. Finally, a plan from Leksell GammaPlan system was evaluated by the Monte Carlo simulations. Results: A Monte Carlo code was implemented for Gamma knife simulation. It showed good dose linearity. The profiles for each collimator were consistent with film measurements. The relative collimator factors from Monte Carlo simulations are 0.970, 0.938 and 0.873, which are within 3% against TLD and OSL results. The DVHs from Monte Carlo are in good agreement with GammaPlan system. The treatment plan dose difference at isocenter between Monte Carlo and the planning system is less than 1%.

Small Angle Gamma Ray Scatter: What Is The Impact On Image Quality

Not unlike the potential small translocation errors associated with the angle of variation from perpendicular defined by the collimator 'hole' length and diameter, limitations of energy discrimination in scintigraphy might also result in a small degradation of resolution and/or contrast. Methods: A series of 99mTc line sources were acquired with multiple energy windows to provide simultaneous acquisition of data. Each energy window combination was acquired with the line source on the collimator surface, against 4.0cm of water attenuation and against 8.0cm of water attenuation. A series of 99mTc thyroid and brain phantoms were also acquired using dual energy windows positioned to capture a 10% window below 140keV and a 10% window above 140keV. The thyroid and the brain phantoms were each positioned on the collimator surface, against 4.0cm of water attenuation and against 8.0cm of water attenuation. Results: The mean improvement in FWHM for 140keV + 10% over 140keV – 10% was 0.26mm (95% CI: 0.16 – 0.36) or 4.0% (95% CI: 2.5 – 5.5) which showed a statistically significant variation from the hypothetical zero difference (P < 0.001). While a 15% window centred on 144keV provided superior image contrast compared to both 15% and 20% centred on 140keV, the improvement is associated with a decrease in accumulated counts. The increase in sampling error associated with these decreased counts, however, is 0.1% to 0.2%. Conclusion: Energy discrimination with a symmetric window about the photopeak is ineffective in eliminating photons scattered by as much as 45 to 60 degrees. While the direct impact on image resolution is perhaps negligible, low angle scatter is a major source of potential artifact and contrast degradation. A 15% energy window centered on 144keV might provide an optimized trade-off between contrast and total counts.

脳血管ＩＶＲにおける水晶体被曝に対する防護シールドの作成と被曝低減効果

A variety of radiation hazards resulting from interventional radiology (IVR) have been reported in recent years. Particularly affected are the skin and the crystalline lens, with their high radiation sensitivity. During neurological interventions, the radiological technologist should consider decreasing radiation exposure. We found exposure projections where the exposure dose became a radiation hazard for the crystalline lens, and examined an efficient method of cover for the exposure projections used for neurological interventions. The exposure projection for maximum crystalline lens radiation exposure was a lateral projection. In the crystalline lens the maximum exposure to radiation was on the X-ray tube side. The method of defense adopted was that of installing a lead plate of the appropriate shape on the surface of the X-ray tube collimator. In other exposure projections, this cover did not become a redundant shadow. With the cover that was created, the X-ray side crystalline lens lateral projection could be defended effectively.

Performance evaluation of a mouse-sized camera for dynamic studies in small animals

A mouse sized camera has been built in terms of collaboration between the presenting institutions. The system is used for the performance of dynamic studies in small animals, in order to evaluate novel radiopharmaceuticals. The active area of the detector is approximately 48×96 mm allowing depiction of the entire mouse in a single view. The system is based on two flat-panel Hamamatsu H8500 position sensitive photomultiplier tubes (PSPMT), a pixellated NaI(Tl) scintillator and a copper–beryllium (CuBe) parallel-hole collimator. In this work, the evaluation results of the system are presented, using phantoms and small animals injected with conventional radiophrmaceuticals. Average resolution was ∼1.6 mm on the collimator surface and increased to ∼4.1 mm in 12 cm distance from the detector. The average energy resolution was measured and found to be ∼15.6% for Tc 99m. Results from imaging thin capillaries demonstrated system's high resolution and sensitivity in activity variations was shown. Initial dynamic studies have been carried out in small animals injected with Tc 99m-DTPA and Tc 99m-MDP. The results show system's ability to perform kinetic imaging in small animals.

Effects of hole tapering on cone-beam collimation for brain SPECT imaging

New collimator manufacturing technologies, such as photoetching, electrical discharge machining, and stereolithography, expand the range of possible cone-beam collimator configurations. For example, it might now be possible for brain SPECT to make a short-focusing cone-beam collimator with tapered holes that increase in size with distance from the collimator surface; conventional lead-casting techniques produce holes of constant size and, consequently, varying septal thicknesses. Moreover, the changes in hole shape and loss of close packing due to focusing leads to thicker septa in the collimator periphery, especially for shorter focal lengths. We investigated the potential advantages of new cone-beam collimator manufacturing processes, and proposed a new design for very short focal-length collimators for brain SPECT imaging. We compared three cone-beam collimators, a conventional collimator manufactured using casting techniques (CC), a novel collimator with uniform hole sizes on the collimator surface and constant hole size through the collimator thickness (FC), and a novel collimator with uniform hole sizes and tapered holes (TC). We determined the resolution of each collimator analytically for focal lengths ranging from 20–50 cm, and adjusted the entrance hole sizes of FC and TC to equalize resolution of all collimators. Sensitivity was calculated at several locations by Monte Carlo simulation. Sensitivity was higher at all points for TC and FC than for CC, and higher for TC than for FC. The differences in sensitivity were larger for shorter focal lengths. For a point on the focal line at 10 cm in front of the collimator entrance surface, the sensitivity gain for TC compared to CC was 7% and 45% for focal lengths of 50 and 20 cm, respectively. The sensitivity gain for a 20-cm focal length, compared to CC, averaged over all locations, was 44% for TC and 23% for FC. We have shown that the new collimator designs made possible by new manufacturing techniques will provide gains in sensitivity that depend strongly on focal length; the sensitivity gains are large for the very short focal lengths that, as we have previously shown, are beneficial for brain imaging.

System characteristics of SPECT with a slat collimated strip detector

In classical SPECT with parallel hole collimation, the sensitivity is constant over the field of view (FOV). This is no longer the case if a rotating slat collimator with planar photon collection is used: there will be a significant variation of the sensitivity within the FOV. Since not compensating for this inhomogeneous sensitivity distribution would result in non-quantitative images, an accurate knowledge of the sensitivity is mandatory to account for it during reconstruction. On the other hand, the spatial resolution versus distance dependency remains unaltered compared to parallel hole collimation. For deriving the sensitivity, different factors have to be taken into account: a first factor concerns the intrinsic detector properties and will be incorporated into the calculations as a detection efficiency term depending on the incident angle. The calculations are based on a second and more pronounced factor: the collimator and detector geometry. Several assumptions will be made for the calculation of the sensitivity formulae and it will be proven that these calculations deliver a valid prediction of the sensitivity at points far enough from the collimator. To derive a close field model which also accounts for points close to the collimator surface, a modified calculation method is used. After calculating the sensitivity in one plane it is easy to obtain the tomographic sensitivity. This is done by rotating the sensitivity maps for spin and camera rotation. The results derived from the calculations are then compared to simulation results and both show good agreement after including the aforementioned detection efficiency term. The validity of the calculations is also proven by measuring the sensitivity in the FOV of a prototype rotating slat gamma camera. An expression for the resolution of these planar collimation systems is obtained. It is shown that for equal collimator dimensions the same resolution–distance relationship is obtained as for parallel hole collimators. Although, a better spatial resolution can be obtained with our prototype camera due to the smaller pitch of the slats. This can be achieved without a major drop in system sensitivity due to the fact that the slats consist of less collimator material compared to a parallel hole collimator. The accuracy of the calculated resolution is proven by comparison with Monte Carlo simulation and measurement resolution values.