Collimation Device Research Articles

A Modulating Liquid Collimator for Coded Aperture Adaptive Imaging of Gamma-Rays

A novel, fully reconfigurable liquid collimator device for γ-ray and X-ray imaging was built and tested as a coded aperture. The device contained a 10 × 10 grid of 5 × 5 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> chambers, each 5 cm long. The chambers were either grilled with an attenuating liquid, absorbing photons, or evacuated of the liquid, allowing the photons to pass. As the pattern of “on” and “off” chambers was manipulated, different, semi-independent views of the γ-ray source were found. A random mask sequence outperformed a uniformly redundant array when maximum-likelihood expectation maximization was used. Noise and reconstruction artifacts decreased as the number of masks increased. With ten mask patterns, the signal-to-noise ratio in images of point sources increased by a factor of 2. Images of a segmented, extended source are presented to demonstrate, qualitatively, that image quality increased when more masks were used. Additionally, the system crudely imaged a source using only the photons that Compton scattered within the mask.

A source model for modulated electron radiation therapy using dynamic jaw movements

The development of fast and accurate source models (SMs) might be of crucial importance for the future clinical implementation of modulated electron radiation therapy (MERT). In this study, a SM is presented for reconstructing phase-space information of modulated electron beams using a few-leaf electron collimator (FLEC) and the photon jaws. During a FLEC-based delivery, two collimation devices (jaws and FLEC) modulate the electron beam characteristics dynamically. The SM separates the beam into a primary and a scattered component. The primary component is derived by a fast Monte Carlo (MC) transport calculation in air using the EGSnrc/BEAMnrc code. The scattered beam is modeled analytically. The accelerator was decomposed into its individual leaf components and the scattered beam was characterized at various levels of the accelerator. Scattered particles are assigned an energy and position by sampling pre-calculated probability distributions. The direction is estimated by geometrical arguments. Particles were assumed to emerge from tunable virtual sources on the side of each collimator leaf. A leaf-hit algorithm was developed to dynamically reject particles that are incident on any collimating leaf. Electron transport in air between the two collimation levels was calculated based on a MC-modified version of the Fermi-Eyges scattering theory. Correlations between direction and position were observed and taken into account at the final collimation level. To validate the model, reconstructed phase-space data were compared with the full accelerator MC phase-space data. The model accurately reproduced the beam characteristics and preserved important correlations. Depth and profile dose distributions in water were derived for square, rectangular, and off-axis field sizes and for a range of clinical energies. Discrepancies in the dose distributions and dose output were within 3% in all cases. Fast and accurate SMs open the possibility for fast treatment planning in MERT, based on an inverse optimization MC treatment planning scheme.

A detector response function design in pinhole SPECT including geometrical calibration

Clinical single photon emission computed tomography (SPECT) equipped with pinhole collimators have a magnification factor that results in high spatial resolution images for small animal imaging. Using Monte Carlo simulations to model the acquisition process and the propagation of the photons from their point of emission to their detection point then integrating the model into an iterative reconstruction algorithm improves the signal-to-noise ratio, the contrast and the spatial resolution in the reconstructed images. However, pinhole SPECT systems are known to be very sensitive to geometrical misalignments. Geometrical misalignments are defined as the radial or axial shift of the collimator pinhole and/or twist and tilt of the detector heads and are introduced in the system each time the collimation device is changed (pinhole to parallel holes or vice versa). In this work, we present a flexible detector response function table (DRFT) design that takes into account the geometrical misalignments and avoids performing new Monte Carlo simulations for each exam in order to calculate a geometrical study-dependent system matrix. The utilization of the DRFT for the calculation of the system matrix speeds up its computation time by two orders of magnitude making it acceptable for preclinical and clinical applications.

On the accuracy of localization achievable in fiducial-based stereoscopic image registration system using an electronic portal imaging device

Portal imaging using electronic portal imaging device (EPID) is a well-established image-guided radiation therapy (IGRT) technique for external beam radiation therapy. The aims of this study are threefold; (i) to assess the accuracy of isocentre localization in the fiducial-based stereoscopic image registration, (ii) to investigate the impact of errors in the beam collimation device on stereoscopic registration, and (iii) to evaluate the intra- and inter-observer variability in stereoscopic registration. Portal images of a ball bearing phantom were acquired and stereoscopic image registrations were performed based on a point centred in the ball bearing as the surrogate for registration. Experiments were replicated by applying intentional offsets in the beam collimation device to simulate collimation errors. The accuracy of fiducial markers localization was performed by repeating the experiment using three spherical lead shots implanted in a pelvic phantom. Portal images of pelvis phantom were given to four expert users to assess the inter-observer variability in performing registration. The isocentre localization accuracy tested using ball bearing phantom was within 0.3 mm. Gravity-induced systematic errors of beam collimation device by 2 mm resulted in positioning offsets of the order of 2 mm opposing the simulated errors. Relatively large inter-portal pair projection errors ranges from 1.3 mm to 1.8 mm were observed with simulated errors in the beam collimation device. The intra-user and inter-user variabilities were observed to be 0.8 and 0.4 mm respectively. Fiducial-based stereoscopic image registration using EPID is robust for IGRT procedure.

MO‐G‐213AB‐05: Beam Simulation and Measurement for Energy‐Intensity Modulated Electron Radiotherapy (MERT) with a Computer‐Controlled Electron Multileaf Collimator Device

Purpose: To verify an add‐on computer‐controlled multileaf collimator (eMLC) device on a Varian linac capable of delivering accurate dose for energy‐intensity modulated electron radiotherapy (MERT). Methods: The eMLC has 27 pairs of tungsten leaves (tongue and groove design to reduce intraleaf leakage)with 0.56cm width and 2cm thickness, providing a field size as large as 15 cm × 15 cm defined at 94cm SSD. Measurements were done to determine the appropriate jaw setting for an eMLC shaped field, mainly to reduce the leaf leakage outside the eMLC shaped field. The phase space data were acquired by Monte Carlo (MC) simulations for electron beams of energies 6, 9, 12 and 15 MeV, respectively and used as an input source in MC dose calculations in a phantom. MC calculated PDDs and dose profiles were compared with measurements for large fields (e.g. 10 cm × 10 cm) and small fields (e.g. 3.4 cm × 3.4 cm). The eMLC leakage for various energies was measured both in‐air and in phantom (at dmax) as a ratio of doses with the eMLC closed and completely open. Results: With the jaw position at 0.5 cm beyond the edge of the eMLC shaped field, it was showed to best eliminate the interleaf leakage, especially for high energies, e.g. 15 MeV. The average leaf leakage ranged from 0.3% (6 MeV) to 2.3% (15 MeV), which were consistent with lower in‐phantom values than in‐air values. MC calculated PDDs and dose profiles generally agreed with measurements to within 2mm/2%. Conclusions: This eMLC device is capable of delivering energy and intensity modulated electron beams accurately with acceptable leaf leakage for advanced MERT treatment.

Combined modulated electron and photon beams planned by a Monte-Carlo-based optimization procedure for accelerated partial breast irradiation

The purpose of this study was to present a Monte-Carlo (MC)-based optimization procedure to improve conventional treatment plans for accelerated partial breast irradiation (APBI) using modulated electron beams alone or combined with modulated photon beams, to be delivered by a single collimation device, i.e. a photon multi-leaf collimator (xMLC) already installed in a standard hospital. Five left-sided breast cases were retrospectively planned using modulated photon and/or electron beams with an in-house treatment planning system (TPS), called CARMEN, and based on MC simulations. For comparison, the same cases were also planned by a PINNACLE TPS using conventional inverse intensity modulated radiation therapy (IMRT). Normal tissue complication probability for pericarditis, pneumonitis and breast fibrosis was calculated. CARMEN plans showed similar acceptable planning target volume (PTV) coverage as conventional IMRT plans with 90% of PTV volume covered by the prescribed dose (Dp). Heart and ipsilateral lung receiving 5% Dp and 15% Dp, respectively, was 3.2–3.6 times lower for CARMEN plans. Ipsilateral breast receiving 50% Dp and 100% Dp was an average of 1.4–1.7 times lower for CARMEN plans. Skin and whole body low-dose volume was also reduced. Modulated photon and/or electron beams planned by the CARMEN TPS improve APBI treatments by increasing normal tissue sparing maintaining the same PTV coverage achieved by other techniques. The use of the xMLC, already installed in the linac, to collimate photon and electron beams favors the clinical implementation of APBI with the highest efficiency.

Effects of the sample environment and collimation in the background measurement

Most of diffractometers have collimators in front of detectors either to reduce the background contribution or to increase the angular resolution. These collimation devices are designed to collect only the neutrons coming from the sample position. When a sample environment is required, its walls are close to the sample position and produce some unwanted effects on the diffractograms. A simple geometrical model allows a description of the observed patterns in the two-axis diffractometer D4 (ILL, Grenoble, France). The calculated effects are compared with experimental data for a vanadium bar as measured in the standard cryostat of that instrument. This kind of measurements allows the experimental determination of the sample transmission.

Combinatorial Benders cuts for decomposing IMRT fluence maps using rectangular apertures

We consider the problem of decomposing Intensity Modulated Radiation Therapy (IMRT) fluence maps using rectangular apertures. A fluence map can be represented as an integer matrix, which denotes the intensity profile to be delivered to a patient through a given beam angle. We consider IMRT treatment machinery that can form rectangular apertures using conventional jaws, and hence, do not need sophisticated multi-leaf collimator (MLC) devices. The number of apertures used to deliver the fluence map needs to be minimized in order to treat the patient efficiently. From a mathematical point of view, the problem is equivalent to a minimum cardinality matrix decomposition problem. We propose a combinatorial Benders decomposition approach to solve this problem to optimality. We demonstrate the efficacy of our approach on a set of test instances derived from actual clinical data. We also compare our results with the literature and solutions obtained by solving a mixed-integer programming formulation of the problem.

High efficiency multichannel collimator for structural studies of liquids and low-Z materials at high pressures and temperatures

A high efficiency multichannel collimator (MCC) device has been developed at the high pressure beamline ID27 of the European Synchrotron Radiation Facility to drastically reduce the x-ray background from the sample environment in the Paris-Edinburgh press. The main technical difficulty, which resides in the minimum slits size achievable using the classical mono-bloc design, has been resolved using an original concept based on a set of independent slits. Then, a very small slit size of 50 μm was manufactured resulting in a great improvement of the signal to background ratio. In addition, the transfer function of the MCC has been measured using the x-ray diffusion signal of a metal doped glass and efficiently applied to correct the raw data. The potential of this new device is illustrated in two challenging examples: iron-sulfur liquid structures and C(60) polymerization process at high pressure and high temperature.

Adapting a generic BEAMnrc model of the BrainLAB m3 micro-multileaf collimator to simulate a local collimation device

This work is focussed on developing a commissioning procedure so that a Monte Carlo model, which uses BEAMnrc's standard VARMLC component module, can be adapted to match a specific BrainLAB m3 micro-multileaf collimator (µMLC). A set of measurements are recommended, for use as a reference against which the model can be tested and optimized. These include radiochromic film measurements of dose from small and offset fields, as well as measurements of µMLC transmission and interleaf leakage. Simulations and measurements to obtain µMLC scatter factors are shown to be insensitive to relevant model parameters and are therefore not recommended, unless the output of the linear accelerator model is in doubt. Ultimately, this note provides detailed instructions for those intending to optimize a VARMLC model to match the dose delivered by their local BrainLAB m3 µMLC device.

SU‐GG‐T‐389: Mixed Electron and Photon Beams Modulated by the XMLC for Partial Breast Irradiation

Purpose: To present a modulated radiation therapy technique as a more accurate alternative to current approaches for the planning of partial breast irradiation (PBI). This technique is based on the use of modulated electron beams alone or in combination with intensity modulated photon radiation therapy (MERT+IMRT), delivered with the same collimation device (xMLC). Method and Materials: A Monte Carlo treatment planning system based on BEAMnrc/DOSXYZnrc code, named CARMEN, was developed by our group. A Linear Programming (LP) optimization procedure calculates the weight for each beamlet generated from phase space data below the jaws. An algorithm to reduce beamlets and voxels was developed to make possible the LP formulation and to solve the problem in operative times. A specific sequencer capable to consider electron and photon beams was also developed by taking into account previous full MC simulations of the xMLC. Four PBI cases were planned with CARMEN following the recommendations from the RTOG‐0413/ NSABP‐B39 trial protocol for 3D‐CRT PBI. Two cases were planned by MERT alone and the other two by MERT+IMRT as a function of PTV position in depth. Results: More than 90% of PTV volume received 90% of prescribed dose with a maximum dose ≤ 110%. A reduced dose to ipsilateral breast, lung and heart, was also achieved, especially in the low‐dose range. The addition of IMRT to MERT improved PTV coverage in depth diminishing overdosed superficial areas. MERT and MERT+IMRT solutions improved PTV homogeneity compared with the other techniques evaluated and in most of the cases reduced dose to normal tissues. Conclusions: An improved PBI plan was obtained with MERT+IMRT solution compared with 3D‐CRT and brachytherapy techniques. The accuracy of this technique by means of the xMLC opens the chance to implement the accelerated partial breast irradiation in the clinical practice.

Wireless Synchronization of a Multi-Pinhole Small Animal SPECT Collimation Device With a Clinical Scanner

A multi-pinhole collimation device for small animal single photon emission computed tomography (SPECT) uses the gamma camera detectors of a standard clinical SPECT scanner. The collimator and animal bed move independently of the detectors, and therefore their motions must be synchronized. One approach is manual triggering of the SPECT acquisition simultaneously with a programmed motion sequence for the device. However, some data blurring and loss of image quality result, and true electronic synchronization is preferred. An off-the-shelf digital gyroscope with integrated Bluetooth interface provides a wireless solution to device synchronization. The sensor attaches to the SPECT gantry and reports its rotational speed to a notebook computer controlling the device. Software processes the rotation data in real-time, averaging the signal and issuing triggers while compensating for baseline drift. Motion commands are sent to the collimation device with minimal delay, within approximately 0.5 second of the start of SPECT gantry rotation. Test scans of a point source demonstrate an increase in true counts and a reduction in background counts compared to manual synchronization. The wireless rotation sensor provides robust synchronization of the collimation device with the clinical SPECT scanner and enhances image quality.

Collimation method of measurement of linear magnitudes

Principles for the construction and transformations of signals in optoelectronic collimation devices used in the measurement of linear displacements are considered.

Design and performance of a multi-pinhole collimation device for small animal imaging with clinical SPECT and SPECT–CT scanners

A multi-pinhole collimation device is developed that uses the gamma camera detectors of a clinical SPECT or SPECT–CT scanner to produce high-resolution SPECT images. The device consists of a rotating cylindrical collimator having 22 tungsten pinholes with 0.9 mm diameter apertures and an animal bed inside the collimator that moves linearly to provide helical or ordered-subsets axial sampling. CT images also may be acquired on a SPECT–CT scanner for purposes of image co-registration and SPECT attenuation correction. The device is placed on the patient table of the scanner without attaching to the detectors or scanner gantry. The system geometry is calibrated in-place from point source data and is then used during image reconstruction. The SPECT imaging performance of the device is evaluated with test phantom scans. Spatial resolution from reconstructed point source images is measured to be 0.6 mm full width at half maximum or better. Micro-Derenzo phantom images demonstrate the ability to resolve 0.7 mm diameter rod patterns. The axial slabs of a Micro-Defrise phantom are visualized well. Collimator efficiency exceeds 0.05% at the center of the field of view, and images of a uniform phantom show acceptable uniformity and minimal artifact. The overall simplicity and relatively good imaging performance of the device make it an interesting low-cost alternative to dedicated small animal scanners.

Geometric characterization of multi-axis multi-pinhole SPECT

A geometric model and calibration process are developed for single photon emission computed tomography (SPECT) imaging with multiple pinholes and multiple mechanical axes. Unlike the typical situation where pinhole collimators are mounted directly to rotating gamma ray detectors, this geometric model allows for independent rotation of the detectors and pinholes, for the case where the pinhole collimator is physically detached from the detectors. This geometric model is applied to a prototype small animal SPECT device with a total of 22 pinholes and which uses dual clinical SPECT detectors. All free parameters in the model are estimated from a calibration scan of point sources and without the need for a precision point source phantom. For a full calibration of this device, a scan of four point sources with 360 degrees rotation is suitable for estimating all 95 free parameters of the geometric model. After a full calibration, a rapid calibration scan of two point sources with 180 degrees rotation is suitable for estimating the subset of 22 parameters associated with repositioning the collimation device relative to the detectors. The high accuracy of the calibration process is validated experimentally. Residual differences between predicted and measured coordinates are normally distributed with 0.8 mm full width at half maximum and are estimated to contribute 0.12 mm root mean square to the reconstructed spatial resolution. Since this error is small compared to other contributions arising from the pinhole diameter and the detector, the accuracy of the calibration is sufficient for high resolution small animal SPECT imaging.

Shielding design for a laser-accelerated proton therapy system

In this paper, we present the shielding analysis to determine the necessary neutron and photon shielding for a laser-accelerated proton therapy system. Laser-accelerated protons coming out of a solid high-density target have broad energy and angular spectra leading to dose distributions that cannot be directly used for therapeutic applications. A special particle selection and collimation device is needed to generate desired proton beams for energy- and intensity-modulated proton therapy. A great number of unwanted protons and even more electrons as a side-product of laser acceleration have to be stopped by collimation devices and shielding walls, posing a challenge in radiation shielding. Parameters of primary particles resulting from the laser–target interaction have been investigated by particle-in-cell simulations, which predicted energy spectra with 300 MeV maximum energy for protons and 270 MeV for electrons at a laser intensity of 2 × 1021 W cm−2. Monte Carlo simulations using FLUKA have been performed to design the collimators and shielding walls inside the treatment gantry, which consist of stainless steel, tungsten, polyethylene and lead. A composite primary collimator was designed to effectively reduce high-energy neutron production since their highly penetrating nature makes shielding very difficult. The necessary shielding for the treatment gantry was carefully studied to meet the criteria of head leakage <0.1% of therapeutic absorbed dose. A layer of polyethylene enclosing the whole particle selection and collimation device was used to shield neutrons and an outer layer of lead was used to reduce photon dose from neutron capture and electron bremsstrahlung. It is shown that the two-layer shielding design with 10–12 cm thick polyethylene and 4 cm thick lead can effectively absorb the unwanted particles to meet the shielding requirements.

Monte Carlo optimization of an industrial tomography system

Computed tomography (CT) is a very useful non-destructive testing technique in the industrial field, since it permits the detection of small inner defects in a reliable and accurate way. In order to get very good performance, in terms of image contrast and spatial resolution, the configuration of the tomography system has to be optimized carefully. Monte Carlo simulations can be very helpful for choosing different conditions and selecting the best configurations of a CT system. In this paper, we present a preliminary optimization of an industrial CT apparatus, obtained by means of Monte Carlo simulations. The system is composed of an X-ray tube, filtering and collimation devices, and a detector made of a scintillator coupled to a CCD camera. We focus our attention on large aluminum objects, such as motor heads, and investigate the contribution of the scattered radiation. Some options have been simulated, for reducing the scattering photons, thus improving the overall image quality.

SU‐FF‐T‐309: Laser‐Accelerated Proton Therapy: Target Chamber Design and Shielding Requirements

Purpose: Recent advances in laser technology have facilitated proton (light ion) acceleration using laser‐induced plasmas. In this work, we investigate target chamber designs and their shielding requirements for a laser‐proton therapy system. Method and Materials: This new therapy system employs laser‐accelerated protons. If successfully developed, it may provide a compact and cost‐effective solution to energy‐ and intensity‐modulated proton therapy (EIMPT). Since laser‐accelerated protons have broad energy and angular distributions, which are not suitable for radiotherapy applications directly, we have designed a compact particle selection and beam collimating system for EIMPT beam delivery. The target chamber contains the laser focusing and target assembles and is connected to the proton beam collimation and particle selection device. Monte Carlo simulations using MCNPX and FLUKA have been performed to verify the shielding walls for the experimental setup, which consist of stainless steel, polystyrene and lead. Results: The primary particles resulting from the laser‐target interaction are protons and electrons. Our particle in cell simulation predicted energy spectra with 300 MeV maximum energy for protons and 20 MeV for electrons for a laser intensity of 1021 W/cm2. The maximum number was 1011 and 1012 per pulse for protons and electron, respectively. Our Monte Carlo simulations showed that a combination of 1/4″ 304 stainless steel, 6″ polystyrene and 3″ lead will reduce the combined leakage dose equivalent to 0.32 μSv (0.032 mrem) per laser pulse at 1 m from the chamber, which included the effect of all primary and secondary particles. Conclusion: We have designed an experimental setup to accommodate the laser focusing mirror, the target assemblies, the beam collimator and the particle selection system. Different shielding walls are designed to ensure the leakage dose equivalent to within 20 μSv (2 mrem)/week.

SU‐FF‐T‐295: Intermediate Energy Photon Dramatically Improves Penumbra, Dose Distribution Homogeneity and Conformality for Small Field Radiosurgery

Purpose: Stereotactic radiosurgery ensures highly conformal treatment of small volumes. When treating lesions with no microscopic extension and with efficient immobilisation, the PTV (Planned Target Volume) closely overlaps the GTV (Gross Tumour Volume). When using dedicated beam collimation devices, the radiological penumbra accounts for an additional 2 to 3mm dose gradient extending within the normal tissue. We hypothesized that, for small radiosurgery field sizes, intermediate energy photons (IEP, above orthovoltage and below megavoltage) will dramatically reduce the radiological penumbra. Material and Methods: Monte Carlo simulation was used to investigate the dose distribution characteristics and the radiological penumbra of monoenergetic photons (100 keV to 1 MeV) in a water phantom and for various field sizes (0.5×0.5 to 4×4cm2). A virtual unit based on a simulated optimised IEP spectrum was described in the Pinnacle3 TPS using an extended kernel library in the kilovoltage range. Results: Radiological penumbra below 300μm are generated for field sizes below 2×2 cm2 at all depth and for monoenergetic photons between 200 to 400keV. The depth dose curve is steeper with a 50% relative dose at 6.5 cm depth, the dose homogeneity in the target is dramatically increased, and the dose to the bone is not increased. An 800 kV beam generated in a 0.5mm tungsten target maximizes the photon intensity in this range. Using six coincidental IEP generators allow a dose rate of 1.5Gy per minutes at 5 cm depth. Pinnacle3 confirms the dramatic reduction in penumbra size, and the superiority of IEP over megavoltage beams for radiosurgery applications regarding dose distribution homogeneity and conformality. Conclusions: The reduction of radiological penumbra is linked to the reduced photon scattering using small field sizes and the reduced secondary electron range using IEP. This is the first report of the use of IEP for radiosurgery.

SU‐FF‐T‐190: Dosimetric Impacts of Smaller Pencil Beam Utilizing in Smaller Intracranial Lesions On Intensity Modulation Serial Tomotherapy

Purpose: To test the dosimetric feasibility of using a smaller size circular pencil beam on small intracranial lesions in serial tomotherapy based radiosurgery. Method and Materials: An in‐house post collimation device (Gizz) was developed to refine the NOMOS Peacock pencil beams to an array of 5 mm diameter circular pencil beams. Beam characteristics were investigated and implemented into the treatment planning system. Eighteen patients with small irregular intracranial lesions (0.19 to 3.21c.c, mean 1.21c.c) were selected in groups of arteriovenous malformation (n = 6), acoustic neuroma (n = 6), and metastatic lesion (n = 6). Plans were calculated with the refined smaller size circular pencil beam and a 4 mm x 10 mm rectangular pencil beam in Corvus 6.0. A novel strategy of normalizing plans to 97% target volume covered by 100% prescription line was adopted. Plan quality was characterized, for the purposes of the study, by conformity and homogeneity indices, mean, maximum and minimum does to target and critical structures, and volume of healthy tissue receiving various dose levels. Results: This new pencil beam provided a better two‐dimensional resolution than three available commercial rectangular pencil beams. Due to adjacent gap spaces from this new physical design, a new couch indexing approach was proposed. Clinical selected cases experienced an average 15% conformity improvement utilizing this smaller pencil beam associated with better normal tissue avoidance. Results reflected that the performance of this device is dependent on target coverage, target volume, and beam displacement location. Conclusion: Smaller pencil beam is an option to improve the target comformality for small irregular lesions in serial tomotherapy based radiosurgery. Future works on optimized pencil beam size, shape, isocenter shift should be accomplished.