Collimator Design Research Articles

Temporal analysis of Z-Gradient coil eddy currents in tungsten collimator with different resistivities for SPECT/MRI

Combining Single Photon Emission Computed Tomography (SPECT) with Magnetic Resonance Imaging (MRI) results in an interaction of the time-varying magnetic field gradients with the highly conducting tungsten collimator, which generates a secondary magnetic field causing spatial distortions in reconstructed MR images. Accurate simulations are important for the characterization of these eddy currents and to further optimize the gradient coils and the collimator design. This study investigated the temporal variations of eddy currents in a tungsten collimator for pre-clinical SPECT/MRI due to only the z-gradient coil for different resistivities of tungsten, which can be obtained through additive manufacturing. We modeled a z-gradient coil and a collimator using FEKO, a 3D electromagnetic simulation tool. A time analysis approach was used to generate the pulsed magnetic field gradient. The approach was validated with measurements using a 7T MRI scanner. Simulations show that when tungsten with high resistivity (ρ=370 nΩ.m) is used, eddy currents generate an added magnetic field representing 1.72 % of the nominal gradient field in a Field Of View (FOV) of 3 cm. This percentage increases rapidly for tungsten with lower resistivities, which was expected since a higher resistivity implies lower current densities. A higher density of tungsten is preferred leading to low resistivity but results in stronger induced eddy currents. A compromise needs to be made between the eddy currents strength and the design of the collimator. Using the insights gained from these simulations, the next step is to optimize the design of the gradient coils and the collimator to further reduce the eddy current effects.

Collimator design for a clinical brain SPECT/MRI insert.

This project's goal is to design a SPECT insert for a clinical MRI system for simultaneous brain SPECT/MR imaging. We assume the stationary SPECT insert will consist of two rings of ∼5x5-cm SiPM-based detectors insensitive to magnetic fields, with 0.8-mm intrinsic resolution. The maximum diameter is 44.5 cm, the minimum diameter is 33 cm to accommodate the patient and MRI receive/transmit coil, and the FOV has a 20 cm diameter. We have compared eight collimator designs: single-, 2x2-, 3x3- and 5+2½- pinhole, and single-, 2-, 3- and 1+2½-slit slit-slat, where ½-pinholes/slits are shared between two detectors. Analytical geometric efficiency was calculated for an activity distribution corresponding to a human brain and a target resolution of 10 mm FWHM at the centre of the FOV. Noise-free data were simulated with and without depth-of-interaction (DOI) information, and reconstructed for uniform, Defrise, Derenzo, and Zubal brain phantoms. For DOI it is assumed that the crystal's first and second half can be differentiated. Comparing the multi-pinhole and multi-slit slit-slat collimators, the former gives better reconstructed uniformity and trans-axial resolution, while the latter gives better axial resolution. Although the 2x2-pinhole and 2-slit designs give the highest sensitivities, they result in a sub-optimal utilization of the detector FOV. The best options are therefore the 5+2½-pinhole and the 1+2½-slit systems, with sensitivities of 4.9*10–4 and 4.0*10–4, respectively. The brain phantom reconstructions with multi-pinhole collimator are superior as compared to slit-slat, especially in terms of symmetry and realistic activity distribution. DOI information reduces artefacts and improves uniformity in geometric phantoms, although the difference is small for the brain phantom. These results favour a multi-pinhole configuration. Figure 1 Top left: trans-axial sections of a reconstructed uniform phantom. Top right: sagittal sections of a reconstructed Defrise phantom. Bottom left: transaxial sections of a reconstructed Derenzo phantom. Bottom right: transaxial (left), coronal (middle) ...

Innovative design of light collimator based on a freeform microlens array

In developing a daylighting system, the overall system efficiency is crucial. In the daylighting system, whether the light propagates parallel strongly affects the efficiency. In this paper, we simulate a multicurvature lens to collimate rays propagated from different angles. We describe a method based on a freeform microlens array, which increases transmission efficiency. Results show that with the freeform microlens array collimator, the light propagates provide at least 50.26% parallel and the efficiency increases by 24.76%, enhancing the core values of the daylighting system in building illumination.

Retroreflective imaging system for optical labeling and detection of microorganisms.

A retroreflective imaging system for imaging microscopic targets over macroscopic sampling areas is introduced. Detection of microorganism-bound retroreflector (RR) targets across millimeter-scale samples is implemented according to retroreflection directionality, collimation, and contrast design characteristics. Retroreflection directionality is considered for corner-cube (CC) and spherical geometries. Spherical-RRs improve directionality and reliability. Retroreflection collimation is considered for spherical-RRs. Retroreflective images for micro-CC-RRs and micro-spherical-RRs with varying refractive indices show optimal results for high refractive index BaTiO3 micro-spherical-RRs. A differential imaging technique improves retroreflection contrast by 35 dB. High refractive index micro-spherical-RRs and differential imaging, together, can detect microscopic RR targets across macroscopic areas.

SU-D-19A-03: Monte Carlo Investigation of the Mobetron to Perform Modulated Electron Beam Therapy

Purpose: Modulated electron radiotherapy (MERT) has been proposed as a mean of delivering conformal dose to shallow tumors while sparing distal structures and surrounding tissues. In intraoperative radiotherapy (IORT) utilizing Mobetron, an applicator is placed as closely as possible to the suspected cancerous tissues to be treated. In this study we investigate the characteristics of Mobetron electron beams collimated by an in-house prospective electron multileaf collimator (eMLC) and its feasibility for MERT. Methods: IntraOp Mobetron™ dedicated to perform radiotherapy during surgery was used in the study. It provides several energies (6, 9 and 12 MeV). Dosimetry measurements were performed to obtain percentage depth dose curves (PDD) and profiles for a 10-cm diameter applicator using the PTW MP3/XS 3D-scanning system and the semiflex ion chamber. MCBEAM/MCSIM Monte Carlo codes were used for the treatment head simulation and phantom dose calculation. The design of an electron beam collimation by an eMLC attached to the Mobetron head was also investigated using Monte Carlo simulations. Isodose distributions resulting from eMLC collimated beams were compared to that collimated using cutouts. The design for our Mobetron eMLC is based on our previous experiences with eMLCs designed for clinical linear accelerators. For Mobetron the eMLC is attached to the end of a spacer-mounted rectangular applicator at 50 cm SSD. Steel will be used as the leaf material because other materials would be toxic and will not be suitable for intraoperative applications. Results: Good agreement (within 2%) was achieved between measured and calculated PDD curves and profiles for all available energies. Dose distributiosn provided by the eMLC showed reasonable agreement (∼3%/1mm) with those obtained by conventional cutouts. Conclusion: Monte Carlo simulations are capable of modeling Mobetron electron beams with a reliable accuracy. An eMLC attached to the Mobteron treatment head will allow better treatment options with those machines.

TH‐A‐18C‐06: A Scatter Elimination Scheme for Cone Beam CT Using An Oscillating Narrow Beam

Purpose:While cone beam CT (CBCT) has been widely used in image guided radiation therapy, its low image quality, primarily caused by scattered x‐rays, hinders advanced clinical applications, e.g., CBCT based on‐line adaptive re‐planning. We propose in this abstract a new scheme called oscillating narrow beam CBCT (ONB‐CBCT) to eliminate scatter signals.Methods:ONB‐CBCT consists of two major components. 1) Oscillating narrow beam (ONB) scan and 2) partitioned flat panel containing multiple individual detector strips and their own readouts. Both the beam oscillation and detector partition are along the superior‐inferior (SI) direction. During data acquisition, at a given projection, the narrow beam sweep through the detector region, and different portions of the detector acquires projection data in synchrony with the narrow beam. ONB can be generated by a rotating slit collimator design with conventional tube with single focal spot, or by directly using a new source with multiple focal spots. A proof‐of‐principle study via Monte Carlo simulation is conducted to demonstrate the feasibility of ONB‐CBCT.Results:As the beam becomes narrower, more and more scatter signals are eliminated. For the case with a bowtie filter and using 15 ONBs, the maximum and the average intensity error due to scatter are below 20 and 10 HU, respectively.Conclusion:ONB yields a narrowed exposure field at each snapshot and hence an inherently negligible scatter effect. Meanwhile, the individualized detector units guarantee high frame rate detection and hence a same large volume coverage as that in conventional CBCT. In summary, ONB‐CBCT is a promising design to achieve high‐quality CBCT imaging.This study is supported in part by NIH (1R01CA154747‐01).

Monte Carlo design and simulation of a grid-type multi-layer pixel collimator for radiotherapy: Feasibility study

In order to confirm the possibility of field application of a different type collimator with a multileaf collimator (MLC), we constructed a grid-type multi-layer pixel collimator (GTPC) by using a Monte Carlo n-particle simulation (MCNPX). In this research, a number of factors related to the performance of the GPTC were evaluated using simulated output data of a basic MLC model. A layer was comprised of a 1024-pixel collimator (5.0 × 5.0 mm2) which could operate individually as a grid-type collimator (32 × 32). A 30-layer collimator was constructed for a specific portal form to pass radiation through the opening and closing of each pixel cover. The radiation attenuation level and the leakage were compared between the GTPC modality simulation and MLC modeling (tungsten, 17.50 g/cm3, 5.0 × 70.0 × 160.0 mm3) currently used for a radiation field. Comparisons of the portal imaging, the lateral dose profile from a virtual water phantom, the dependence of the performance on the increase in the number of layers, the radiation intensity modulation verification, and the geometric error between the GTPC and the MLC were done using the MCNPX simulation data. From the simulation data, the intensity modulation of the GTPC showed a faster response than the MLC’s (29.6%). In addition, the agreement between the doses that should be delivered to the target region was measured as 97.0%, and the GTPC system had an error below 0.01%, which is identical to that of MLC. A Monte Carlo simulation of the GTPC could be useful for verification of application possibilities. Because the line artifact is caused by the grid frame and the folded cover, a lineal dose transfer type is chosen for the operation of this system. However, the result of GTPC’s performance showed that the methods of effective intensity modulation and the specific geometric beam shaping differed with the MLC modality.

Performance characteristics of a polyethylene collimator with an EJ-426 detector in neutron source localisation

In this paper, characteristics of neutron source localisation using an EJ-426 detector with a polyethylene collimator are presented. The detector is placed inside of the polyethylene collimator, where the collimator has a constant internal diameter and external diameter gradually increases towards the back end. The combined detector-collimator unit was mounted onto an equatorial mount and, data were collected from an ultra-fast mixed-field analyser. Neutron images have been produced to characterise the area investigated in real-time. Radiation images are fundamentally related to the detector efficiency and to the collimator geometry. In order to characterise the neutron source localisation capability of the polyethylene collimator, with an EJ-426 detector, experiments have been carried out using three different neutron sources. Results indicate that the existing design of the polyethylene collimator is suitable for localisation of low energy neutron sources. The measurements were performed in the low-scatter facility of the Neutron Metrology, National Physical Laboratory, Teddington, UK.

Design and performance investigation of a multi-pinhole collimator for a small field of view gamma imaging system

The aim of this study is to design a collimator for a gamma imaging system that has a small-footprint, a fast-scan-time, and a organ-specific applicability. To achieve such features, the collimator must have a high resolution, a high sensitivity, and a gantry with a simple geometry. For ensuring high resolution and high sensitivity, we designed a multi-pinhole collimator. For realizing a simplified gantry, we carried out studies with limited angle reconstruction. On the designed multipinhole collimator has eight-pinholes, whose diameters are 2 mm. Limited-angle reconstruction has been conducted with angle intervals of 3, 6, 9, and 12° and with an angle coverage of 60, 90, 120, 150, and 180°. The reconstruction of an image was separately developed based on the ray-driven and voxel-driven methods in order to overcome the sampling problem and to reduce the amount of computation required. To evaluate the performance of the designed system, performed studies on three spherical phantoms and a heart phantom by using the Geant4 application for tomographic emission (GATE) simulation tool. The results showed that the full width at half maximum (FWHM) of the center source were 6.25 mm and 7.18 mm for single-pinhole and multi-pinhole collimators, respectively. Moreover, limited angle reconstruction resulted in a higher efficiency of the imaging system because it overcame the limitation of the gantry geometry. Limited-angle reconstruction was optimized at an angle coverage of 120° with an angle interval of 6°, then reconstructed image was shown a 12.18 mm FWHM. This suggests that the designed system needs only one-third the number of projections to acquire a reconstructed image with a slight degradation in image quality. This also suggests that our proposed multi-pinhole collimator is suitable for applications requiring a small-footprint, a fast-scan-time, and organ-specificity.

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.

Neutron collimator design of neutron radiography based on the BNCT facility

For the research of CCD neutron radiography, a neutron collimator was designed based on the exit of thermal neutron of the Boron Neutron Capture Therapy (BNCT) reactor. Based on the Geant4 simulations, the preliminary choice of the size of the collimator was determined. The materials were selected according to the literature data. Then, a collimator was constructed and tested on site. The results of experiment and simulation show that the thermal neutron flux at the end of the neutron collimator is greater than 1.0×106 n/cm2/s, the maximum collimation ratio (L/D) is 58, the Cd-ratio(Mn) is 160 and the diameter of collimator end is 10 cm. This neutron collimator is considered to be applicable for neutron radiography.

基于近零折射率材料准直和分光器件的设计

基于近零折射率材料准直和分光器件的设计

大面积准直型太阳模拟器的设计与研制

大面积准直型太阳模拟器的设计与研制

X-RAY COLLIMATOR DESIGN USING MONTE CARLO SIMULATIONS

The purpose of this study was to apply Monte Carlo (MC) based simulations to design a new collimation system to achieve a larger beam size (2 ± 0.1 cm) and tight penumbra (< 1 mm) with a kV X-ray small animal irradiator. The BEAMnrc and DOSXYZnrc MC-based programs were adapted to simulate the system. First the aperture of the primary collimator was expanded until the desired size of the beam was obtained. Subsequently, the beam was trimmed by reducing the aperture size of the secondary collimator in order to eliminate low energy scattered photons and sharpen the penumbra. Finally, the new collimator was constructed and the resultant dose distributions were evaluated with EBT2 film measurements. From the MC computed dose profiles, a 2.1 cm FWHM (1.9 cm width at 95% of the dose) and a sharp penumbra (< 1.0 mm) at 1.0 cm depth in water were obtained. Dose distribution comparisons between the EBT2 film measurements and the MC calculations showed acceptable agreement. MC-based calculation is an effective tool to expedite the creation of new collimator designs and avoid costly machining.

Simultaneous reduction of radiation dose and scatter for CBCT by using collimators

On-board cone-beam CT (CBCT) imaging has been widely available in radiotherapy clinic for target localization. However, the extra radiation dose from CBCT is always a concern for its frequent use. Additionally, the relatively large scatter in CBCT often degrades the image quality. By using collimators, some of the X-rays can be stopped from reaching the patient and the detectors, hence both the scatter and the patient doses are simultaneously reduced. The authors show in this work that the collimated CBCT data can be reconstructed without any noticeable artifacts for certain collimator blocking ratios and blocking patterns, and the focus of this work is to study the relationship between the image quality and these two collimator factors. A CBCT system with collimators was simulated following the typical geometry used in clinic. Different collimator designs were tested by varying the size and the number of the collimator slits, and at the same time, the ratio of transmitted beams to total beams was varied from 100% to 10%, resulting in hundreds of different simulation scenarios. Lung and pelvis phantoms created from patients CT images were used in the simulations, and an iterative reconstruction algorithm using the compressed sensing technique was adopted. The image quality was examined by root mean square errors (RMSEs) and compared with the conventional CBCT images. The CBCT image quality increases as the amount of beams passing through the collimators increases, and decreases as the size of the collimator slits increases. With ultra-high resolution collimators, the RMSEs were comparable to the conventional CBCT image quality until the beam transmission rate is reduced below 25%. Collimators can reduce the scatters and radiation dose, however, the collimated CBCT image quality is strongly dependent on both the collimator blocking ratio and the blocking pattern. To achieve image quality comparable to the conventional CBCT, the amount of information and data format must be adequate.

Contributions to the inverse problem of radiation transport

We present an integral equation that describes the uncollided particle flux for the case of an inward spherical shell source of radius R. This is a reasonable description, for example, of a point source that moves on a spherical surface located at distance R from the target of a radiation treatment. The additional assumption of conditions for radial symmetry allows the derivation of an integral equation that relates the scalar flux to the description of the beam source as function of the angle between the direction of the source particles and the normal to the sphere.Analytical and numerical solutions for this integral equation are successfully compared with, respectively, known analytical results and with Monte Carlo simulations. The integral equation can then be used for solutions of the inverse problem: given the flux obtain the source, i.e. the shape of the beam. A numerical algorithm was developed for this purpose as well as an analytical solutions based on the solution of the integral equation by the use of the Laplace transform.The optimal shape for the beam is then obtained based on the constraint that the source has to be positive and finite everywhere, allowing the design of appropriate collimators for the beams. Monte Carlo calculations as a function of the number of collisions show that the uncollided flux for the beam so determined behaves as expected and that penumbra effects due to multiple collisions are sufficiently small (∼20%) to consider the beam as a good first guess for an iterative procedure for the design, for example, of 3-D conformal radiotherapy treatment.

Design and performance evaluation of a 20-aperture multipinhole collimator for myocardial perfusion imaging applications

Single photon emission computed tomography (SPECT) myocardial perfusion imaging remains a critical tool in the diagnosis of coronary artery disease. However, after more than three decades of use, photon detection efficiency remains poor and unchanged. This is due to the continued reliance on parallel-hole collimators first introduced in 1964. These collimators possess poor geometric efficiency. Here we present the performance evaluation results of a newly designed multipinhole collimator with 20 pinhole apertures (PH20) for commercial SPECT systems. Computer simulations and numerical observer studies were used to assess the noise, bias and diagnostic imaging performance of a PH20 collimator in comparison with those of a low energy high resolution (LEHR) parallel-hole collimator. Ray-driven projector/backprojector pairs were used to model SPECT imaging acquisitions, including simulation of noiseless projection data and performing MLEM/OSEM image reconstructions. Poisson noise was added to noiseless projections for realistic projection data. Noise and bias performance were investigated for five mathematical cardiac and torso (MCAT) phantom anatomies imaged at two gantry orbit positions (19.5 and 25.0 cm). PH20 and LEHR images were reconstructed with 300 MLEM iterations and 30 OSEM iterations (ten subsets), respectively. Diagnostic imaging performance was assessed by a receiver operating characteristic (ROC) analysis performed on a single MCAT phantom; however, in this case PH20 images were reconstructed with 75 pixel-based OSEM iterations (four subsets). Four PH20 projection views from two positions of a dual-head camera acquisition and 60 LEHR projections were simulated for all studies. At uniformly-imposed resolution of 12.5 mm, significant improvements in SNR and diagnostic sensitivity (represented by the area under the ROC curve, or AUC) were realized when PH20 collimators are substituted for LEHR parallel-hole collimators. SNR improves by factors of 1.94–2.34 for the five patient anatomies and two orbital positions studied. For the ROC analysis the PH20 AUC is larger than the LEHR AUC with a p-value of 0.0067. Bias performance, however, decreases with the use of PH20 collimators. Systematic analyses showed PH20 collimators present improved diagnostic imaging performance over LEHR collimators, requiring only collimator exchange on existing SPECT cameras for their use.

Design, testing and optimization of a neutron radiography system based on a Deuterium–Deuterium (D–D) neutron generator

Simulations show that significant improvement in imaging performance can be achieved through collimator design for thermal and fast neutron radiography with a laboratory neutron generator. The radiography facility used in the measurements and simulations employs a fully high-voltage-shielded, axial D–D neutron generator with a radio frequency driven ion source. The maximum yield of such generators is about 1010 fast neutrons per seconds (E = 2.45 MeV). Both fast and thermal neutron images were acquired with the generator and a Charge Coupled Devices camera. To shorten the imaging time and decrease the noise from gamma radiation, various collimator designs were proposed and simulated using Monte Carlo N-Particle Transport Code (MCNPX 2.7.0). Design considerations included the choice of material, thickness, position and aperture for the collimator. The simulation results and optimal configurations are presented.

Design and simulation of a full-ring multi-lofthole collimator for brain SPECT

Currently, clinical brain single photon emission computed tomography (SPECT) is mostly performed using rotating dual-head gamma cameras equipped with low-energy-high-resolution parallel-beam collimators (LEHR PAR). The resolution of these systems is rather poor (8–10 mm) and the rotation of the heavy gamma cameras can introduce misalignment errors. Therefore, we designed a static full-ring multi-lofthole brain SPECT insert for an existing ring of LaBr3 (5% Ce) detectors. The novelty of the design is found in the shutter mechanism that makes the system very flexible and eliminates the need for rotating parts. A stationary SPECT insert is not only more robust, it is also easier to integrate in a magnetic resonance imaging system (MRI) for simultaneous SPECT-MRI. The target spatial resolution of our design is 6 mm. In this study we used analytical calculations to optimize the collimator for an existing ring of LaBr3 (5% Ce) detectors. We fixed the target spatial resolution at 6 mm in the center of the field-of-view and maximized the volume sensitivity by changing the collimator radius, the aperture and the number of loftholes. Based on these optimal parameters we simulated phantom data and evaluated the image quality of our multi-lofthole system. We simulated a noiseless uniform and Defrise phantom to assess artifacts and sampling completeness and a noiseless hot-rod phantom to assess the reconstructed spatial resolution. We visually evaluated a simulated noisy Hoffman phantom with two lesions. Then, we evaluated the non-prewhitening matched filter signal-to-noise ratio (NPW-SNR) in two lesion detectability phantoms: one with hot lesions and one with cold lesions. Finally, a contrast-to-noise (CNR) study was performed on a phantom with both hot and cold lesions of different sizes (6–16 mm). All results were compared to a LEHR PAR system. The optimization resulted in a final collimator design with a volume sensitivity of 1.55 × 10−4 cps Bq−1, which is 2.5 times lower than the sensitivity of a dual-head system with LEHR PAR collimators. Spatial resolution, on the other hand, has clearly improved compared to LEHR PAR: with the multi-lofthole system we successfully reconstructed 4 mm hot rods. Although this improved resolution did not result in an unambiguous improvement in CNR or NPW-SNR, we believe that the flexibility of the shutter mechanism opens interesting perspectives toward time-multiplexing and integration with MRI.

Innovative Design of Variable Visual Field Collimator Based on Conflict Resolution Theory of TRIZ

Collimators are used in radiosurgery or radiotherapy systems to accurately focus their high-energy rays on the target to avoid causing damage to the collateral healthy tissues. With the development of conformal radiotherapy techniques, variable visual field collimators (VVFC) are widely used and many designs of VVFC have been proposed. However, these designs still exhibit different deficiencies, e.g. unable to completely eliminate the radiation leakage due to the inter-block gaps. Therefore, in this paper, an innovative design of VVFC is proposed through analyzing the original VVFC, determining the conflicts and solving them using the inventive problem solving methods (TRIZ). Compared with the existing designs of VVFC, the proposed design enables the selection of suitable size of field for the treatment of different size of targets, and completely eliminates the radiation leakage due to inter-block gaps.