Collimation Methods Research Articles

Investigation of a novel 2.5 MV sintered diamond target beam for intracranial linac-based stereotactic treatments.

This work investigates the small-field dosimetric characteristics of a 2.5 MV sintered diamond target beam and its feasibility for use in linac-based intracranial stereotactic treatments. Due to the increased proportion of low energy photons in the low-Z beam, it was hypothesized that this novel beam would provide sharper dose fall-off compared to the 6 MV beam owing to the reduced energy, and therefore range, of secondary electrons. Stereotactic treatments of ocular melanoma and trigeminal neuralgia were simulated for 2.5 MV low-Z and 6 MV beams using Monte Carlo to calculate dose in a voxelized anatomical phantom. Two collimation methods were investigated, including a 5x3 mm2 HDMLC field and a 4 mm cone to demonstrate isolated and combined effects of geometric and radiological contributions to the penumbral width. The measured 2.5 MV low-Z dosimetric profiles demonstrated reduced penumbra by 0.5 mm in both the inline and crossline directions across all depths for both collimation methods, compared to 6 MV. In both treatment cases, the 2.5 MV low-Z beam collimated with the 4 mm cone produced the sharpest dose fall off in profiles captured through isocenter. This improved fall-off resulted in a 59% decrease to the maximum brainstem dose in the trigeminal neuralgia case for the 2.5 MV low-Z MLC collimated beam compared to 6 MV. Reductions to the maximum and mean doses to ipsilateral and contralateral OARs in the ocular melanoma case were observed for the 2.5 MV low-Z beam compared to 6 MV with both collimation methods. While the low dose rate of this novel beam prohibits immediate clinical translation, the results of this study support the further development of this prototype beam to decrease toxicity in intracranial SRS treatments.&#xD.

Technical note: A comparison of in-house 3D-printed and commercially available patient-specific skin collimators for use with electron beam therapy.

Skin collimation is a useful tool in electron beam therapy (EBT) to decrease the penumbra at the field edge and minimize dose to nearby superficial organs at risk (OARs), but manually fabricating these collimation devices in the clinic to conform to the patient's anatomy can be a difficult and time intensive process. This work compares two types of patient-specific skin collimation (in-house 3D printed and vendor-provided machined brass) using clinically relevant metrics. Attenuation measurements were performed to determine the thickness of each material needed to adequately shield both 6 and 9 MeV electron beams. Relative and absolute dose planes at various depths were measured using radiochromic film to compare the surface dose, flatness, and penumbra of the different skin collimation materials. Clinically acceptable thicknesses of each material were determined for both 6 and 9 MeV electron beams. Field width, flatness, and penumbra results between the two systems were very similar and significantly improved compared to measurements performed with no surface collimation. Both skin collimation methods investigated in this work generate sharp penumbras at the field edge and can minimize dose to superficial OARs compared to treatment fields with no surface collimation. The benefits of skin collimation are greatest for lower energy electron beams, and the benefits decrease as the measurement depth increases. Using bolus with skin collimation is recommended to avoid surface dose enhancement seen with collimators placed on the skin surface. Ultimately, the appropriate choice of material will depend on the desire to create these devices in-house or outsource the fabrication to a vendor.

High performance γ-ray imager using dual anti-mask method for the investigation of high-energy nuclear materials

As the γ-ray energy increases, a reconstructed image becomes noisy and blurred due to the penetration of the γ-ray through the coded mask. Therefore, the thickness of the coded mask was increased for high energy regions, resulting in severely decreased the performance of the detection efficiency due to self-collimation by the mask. In order to overcome the limitation, a modified uniformly redundant array γ-ray imaging system using dual anti-mask method was developed, and its performance was compared and evaluated in high-energy radiation region. In the dual anti-mask method, the two shadow images, including the subtraction of background events, can simultaneously contribute to the reconstructed image. Moreover, the reconstructed images using each shadow image were integrated using a hybrid update maximum likelihood expectation maximization (h-MLEM). Using the quantitative evaluation method, the performance of the dual anti-mask method was compared with the previously developed collimation methods. As the shadow image which was subtracted the background events leads to a higher-quality reconstructed image, the reconstructed image of the dual anti-mask method showed high performance among the three collimation methods. Finally, the quantitative evaluation method proves that the performance of the dual anti-mask method was better than that of the previously reconstruction methods.

Compact Laser Collimation System for Simultaneous Measurement of Five-Degree-of-Freedom Motion Errors

A compact laser collimation system is presented for the simultaneous measurement of five-degree-of-freedom motion errors. The optical configuration of the proposed system is designed, and the principle of the measurement of five-degree-of-freedom errors is described in detail. The resolution of the roll and the horizontal straightness is doubled compared with other laser collimation methods. A common optical path compensation method is provided to detect light drift in real time and compensate for straightness and angle errors. An experimental setup is constructed, and a series of experiments are performed to verify the feasibility and stability of the system. Compared with commercial instruments, the pitch and yaw residuals are ± 2.5 ″ and ± 3.5 ″ without correction, and the residuals are ± 1.9 ″ and ± 2.8 ″ after correction, respectively. The comparison deviations of the horizontal straightness and vertical straightness changed from ± 4.8 μ m to ± 2.8 μm and ± 5.9 μm to ± 3.6 μm, respectively. The comparison deviation of the roll is ± 4.3 ″ . The experimental results show that the data of the five-degree-of-freedom measurement system obtained are largely the same as the measurement data of commercial instruments. The common optical path compensation can effectively improve the measurement accuracy of the system.

Compact hybrid gamma camera with a coded aperture for investigation of nuclear materials

Mechanical collimation commonly uses the photoelectric effect to reconstruct radiation images. Electronic collimation (i.e. Compton camera) using Compton scattering has been developed to reconstruct radiation images without utilizing mechanical collimators. Generally, for radiation imaging, electronic and mechanical collimation methods are used individually. In order to increase the quality of imaging and the efficiency of radiation detection, we combined both collimation methods in a single system. Our compact hybrid gamma camera comprised a modified uniformly redundant array (MURA) and a Compton camera, and the information from each modality was obtained simultaneously. The entire system formed a radial shape with detector modules which comprising CsI(Na) scintillators coupled with position-sensitive photomultiplier tubes (PSPMTs) whose anodes were connected to custom-made circuits. For various energy sources, the reconstructed images produced using this hybrid method were obtained and compared with reconstructed images from the two aforementioned methods. The maximum likelihood expectation maximization (MLEM) algorithm was applied for the reconstruction method. Compared with individual imagers at intermediate energies, the hybrid imager showed equal or better performance.

Collimation Sensing With Differential Grating and Talbot Interferometry

The collimation of monochromatic light in optical metrology is of great importance for its determination of the accuracy of physical measurement. This paper proposes a novel collimation testing method with the differential grating (DG) characterized by its high sensitivity. In the process of measurement, the monochromatic light is incident upon a linear grating and is then projected onto a DG to create two sets of linear moiré fringes. By analyzing the relationship between the phase of moiré fringes and the divergence or convergence angle, the collimation of monochromatic light can be easily measured. Compared with the traditional collimation methods, this method is more accurate with an experimental accuracy of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> rad , which is in good agreement with the theoretical prediction.

Portable Active Collimation Imager Using URA Patterned Scintillator

A portable active collimator using systematically patterned scintillators was constructed and its performance evaluated. In the conventional passive radiation collimation method, single or multiple holes are used to limit radiation reaching a detector while radiation scattered in the collimator is not used. However in active collimation where one replaces the passive collimator with a radiation detector, both the radiation that passes through the holes and radiation scattered in the collimator can be used. The active collimator was composed of uniformly redundant array (URA)-patterned Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> (BGO) scintillators with a CsI(Na) detector planar array positioned behind the collimator. Images using radiation passing through the holes of the URA collimator were reconstructed using the correlation method of conventional coded apertures, while the radiation scattered in the active collimator and detected in the planar detector, was imaged using a Compton imaging technique. Since the active collimation method uses both Compton scattered events and photoelectric absorption events, its detection efficiency and energy range are inherently higher than those of conventional collimation methods. The reconstructed images of the portable active collimation methods for various energy sources were obtained and compared with those of conventional methods.

Recent Advances in Cardiac SPECT Instrumentation and System Design

Recent advances in clinical cardiac SPECT instrumentation are reviewed from a systems perspective. New hardware technologies include pixelated scintillator and semiconductor detector elements; photodetectors such as position-sensitive photomultiplier tubes (PSPMT), avalanche photodiodes (APD) and silicon photomultipliers (SiPM); and novel cardiac collimation methods. There are new approaches for positioning detectors and controlling their motion during cardiac imaging. Software technology advances include iterative image reconstruction with modeling of Poisson statistics and depth-dependent collimator response. These new technologies enable faster acquisitions, the lowering of administered activity and radiation dose, and improved image resolution. Higher sensitivity collimators are a significant factor enabling faster acquisitions. Several clinical systems incorporating new technologies are discussed and different system designs can achieve similar performance. With detector elements such as APDs, SiPMs and semiconductors that are insensitive to magnetic fields, the potential for cardiac SPECT imagers that are MRI compatible opens up new frontiers in clinical cardiac research and patient care.

Collimation of H− beam transverse halo by triplets and foil scrapers

Transverse beam halo from a high current linac is an important source of beam losses in a ring as the next stage accelerator. A method is introduced that uses periodic triplet cells and foil scrapers to collimate the beam halo in the beam transport line from the linac to the ring and transports the collimated particles to a beam dump. It has good properties such as being low beam loss in the beam line and avoiding local collimators. Comparisons are made with doublet cells and FODO cells as well as with other collimation methods. In the paper, applying the method to the Chinese Spallation Neutron Source (CSNS) is also presented, together with the simulation results, using macro-particles and taking into account nuclear elastic scattering effect and foil stripping efficiency.

Non-Coronary Cardiac CT Imaging

Cardiac multidetector CT (MDCT) has moved from purely anatomic imaging, to assessment of cardiac function. Significant advances since the advent of multidetector CT now make it feasible to assess not only the coronary arteries, but also ejection fraction, ventricular volumes, myocardial mass and the presence of wall-motion abnormalities. Advances include improvements in EKG-gating, including improvements in temporal resolution, as well as the addition of delayed contrast-enhanced methods. Anatomic imaging has improved as well, with thinner collimation and better reconstruction methods. Three-dimensional software programs now permit excellent surface rendered displays and multiplanar reconstructions suitable as surgical and procedural "road maps."

Three-dimensional level-curve scanning based on intersection of laser lines

A dimensional measurement system that also tracks object movements is presented here. The method directly yields the level curves of an object. It is an extension of collimation methods, coupled with the use of structured lighting with features formed from several luminous planes intersecting in a single line. This line defines a set of points of the space at a fixed distance Z0 from the measuring head. The locus of the points of the object where the lighting is reduced to a single line is the level curve sought. The introduction of an asymmetry into the lighting structure permits one to determine the direction as well as an approximate value of the value of the distance to the level curve from a point on the illuminated object. The experiments performed permit one to evaluate the performance of such a system, and suggest future applications.

パワー半導体レーザと応用の動向

The present state-of-the-art of high-power semiconductor lasers and their application is reviewed. Three phenomena which prevent semiconductor lasers from emitting high power, i.e. kink, roll-over and COD(catastrophic optical damage) in power-current characteristics are discussed and the means to solve are introduced. The two important problems, the removal of heat generated in lasers and collimation methods of diverging light emitted from lasers are described. The recent trends of application such as laser processing (cutting, welding, forming), pumping of solid state lasers, optical memory and display are reviewed.

Effects of copper seedlayer deposition method for electroplating

We have investigated copper seedlayer deposition using both ionized physical vapor deposition (IPVD) and collimation methods by depositing similar films into single and dual damascene structures. Step coverage measurements using transmission electron microscopy indicated that IPVD seedlayers exhibited better bottom and sidewall coverage as compared to collimated seedlayers. Subsequent electroplating of contact structures did not indicate differences in the quality of the filling when observed using scanning electron microscopy (SEM). However electrical testing of 68 000 link dual damascene via chains did show improved chain yield for the IPVD deposited films. Cross sections of the chains revealed small voids at the bottom of some of the vias deposited using collimated seedlayers, while no voiding was observed for the IPVD copper films. Finally SEM examinations of unfilled dual damascene cross sections indicated the IPVD copper seedlayer morphology to be rougher as compared to copper films sputtered using collimation.

Extracranial Stereotactic Radiosurgery of Localized Targets

A method for extracranial stereotactic radiosurgery (ESRS) is described. For this treatment method a stereotactic body frame was developed and is described here. The ESRS method uses a hypofractionation schedule with 2–3 fractions with 10–20 Gy/fraction to the periphery of the target volume, at the 65% isodose. The heterogeneous target dose is used as a means to increase the probability of killing the hypoxic tumor cells. It is argued from in vitro cell survival data that the dose increase used will give a reasonable probability to eradicate at least moderate concentrations of hypoxic cells. A high reproducibility of the target position with the ESRS method is based on the definition of the patient reference system to the stereotactic system and not to the bony anatomical reference points as used in conventional radiotherapy. Important to the ESRS method is that the target position is verified by computed tomography (CT) examinations. The positional accuracy of the target in the stereotactic system is 3.1 mm (mean deviation) in the transversal plane and 5.5 mm (mean deviation) in the longitudinal direction. The setup accuracy, i.e., the correct alignment of the stereotactic system to the isocenter of the treatment unit, is within 1 mm. Other aspects of the ESRS method that are discussed are the treatment technique including collimation methods and dose distributions used, as well as target margins. ESRS has been in clinical use for several years at Radiumhemmet, Karolinska Hospital.

SPECT reconstruction of combined cone beam and parallel hole collimation with experimental data

Three methods for combining parallel and cone beam (P&CB) SPECT data using modified maximum-likelihood expectation-maximization (ML-EM) algorithms are presented. The first method applies both parallel and cone beam data sets to reconstruct a single intermediate image after each iteration using the ML-EM algorithm. The other two are iterative methods that combine the intermediate parallel beam and cone beam source estimates to enhance the uniformity of images. These two methods are ad hoc methods. These combined collimation methods are qualitatively evaluated using experimental data. Attenuation compensation is performed by including the effects of attenuation in the transition matrix as a multiplicative factor. The results indicate that the combined P&CB approaches suppress artifacts caused by truncated projections and corrects for th distortions of the CB-only images. >

Quantitative X-Ray Fluorescence Spectrometry of Irregularly Shaped Samples Using Fundamental Parameters Software

Industrial analytical laboratories often receive a wide variety of specimens that require elemental determinations. For solid samples, x-ray fluorescence spectrometry (XRF) is frequently the method of choice. One usually prepares solid samples, such as alloys, for XRF analysis by cutting pieces from bulk solids to fit into the sample holder. A usual requirement for XRF is that the specimen have a flat, smooth surface. Irregularly shaped specimens are not always amenable to direct analysis. If part of the sample already has a flat area, it can be analyzed with the use of a mask or a selected area aperture. Small fragments can be analyzed by the use of beam collimation or a pinhole aperture. Although masking or beam collimation methods can be used, often they are not practical because of reduced intensity, which leads to excessive analysis times. The effective specimen area must be the same for both standards and unknowns.