Physical Collimation Research Articles

Development and optimisation of grid inserts for a preclinical radiotherapy system and corresponding Monte Carlo beam simulations.

Objective. To develop a physical grid collimator compatible with the X-RAD preclinical radiotherapy system and create a corresponding Monte Carlo (MC) model.Approach. This work presents a methodology for the fabrication of a grid collimator designed for utilisation on the X-RAD preclinical radiotherapy system. Additionally, a MC simulation of the grid is developed, which is compatible with the X-RAD treatment planning system. The grid was manufactured by casting a low melting point alloy, cerrobend, into a silicone mould. The silicone was moulded around a 3D-printed replica of the grid, enabling the production of diverging holes with precise radii and spacing. A MC simulation was conducted on an equivalent 3D grid model and validated using 11 layers of GAFChromic EBT-3 film interspersed in a 3D-printed water-equivalent phantom. A 3D dose distribution was constructed from the film layers, enabling a direct comparison with the MC Simulation.Main results. The film and the MC dose distribution demonstrated a gamma passing rate of 99% for a 1%, 0.5 mm criteria with a 10% threshold applied. The peak-to-valley dose ratio and output factor at the surface were determined to be 20.4 and 0.79, respectively.Significance. The pairing of the grid collimator with a MC simulation can significantly enhance the practicality of grid therapy on the X-RAD. This combination enables further exploration of the biological implications of grid therapy, supported by a knowledge of the complex dose distributions. Moreover, this methodology can be adapted for use in other systems and scenarios.

Virtual-Collimator Based Spatial Dose Modulation for Proton GRID Therapy

Compared to conventional proton therapy, the proton GRID therapy can substantially improve normal tissue protection (with the delivery of spatially-modulated peak-valley dose pattern to normal tissues) while maintaining the tumor control efficacy (with the delivery of uniform dose pattern to tumor targets). The realization of proton GRID often relies on the use of physical collimators to shape the spatial dose distribution. However, the physical collimator may increase neutron dose, decrease delivery efficiency, and limit the freedom for patient positioning. Here we propose a virtual-collimator (VC) method for proton GRID. This new approach can generate peak-to-valley pattern with high peak-to-valley dose ratio (PVDR), without using a physical collimator. The principle behind the VC method to modulate the spatial dose distribution consists of two major steps: (1) the primary beam is essentially halved, i.e., the beamlets are interleaved, so that the organ-at-risk (OAR) plane has the peak-valley dose pattern, while the target plane also has the valley dose; (2) the complementary beam is added with half complementary beamlets to fill in the previously valley-dose positions at the target plane, so that the target dose is uniform, while on the other hand, the complementary beam is angled slightly from the primary beam, so that the OAR still has the peak-valley dose pattern. Moreover, on top of VC, we also utilize sparsity regularization method using total variation and L1 sparsity (TVL1) to further jointly optimize PVDR and dose objectives, namely VC-TVL1. VC and VC-TVL1 were validated in comparison with conventional proton GRID treatment planning method via IMPT ("CONV") and TVL1-based proton GRID treatment planning method without VC ("TVL1"), for a prostate case with single-beam (270° only) or two-beam (90° and 270°) scenarios. As shown in the table, the results show that VC can indeed modulate spatial dose with higher PVDR than CONV or even TVL1. VC had higher spatial modulation frequency with smaller peak-to-peak distance than TVL1. Moreover, VC+TVL1, as the synergy of VC and TVL1, further improved PVDR from VC or TVL1 alone. A new way to deliver proton GRID therapy without a physical collimator is developed using the VC method. The VC method can be synergized with TVL1 optimization algorithm to further jointly optimize PVDR and dose objectives.

Merlin++, a flexible and feature-rich accelerator physics and particle tracking library

Merlin++ is a C++ charged-particle tracking library developed for the simulation and analysis of complex beam dynamics within high energy particle accelerators. Accurate simulation and analysis of particle dynamics is an essential part of the design of new particle accelerators, and for the optimization of existing ones. Merlin++ is a feature-full library with focus on long-term tracking studies. A user may simulate distributions of protons or electrons in either single particle or sliced macro-particle bunches. The tracking code includes both straight and curvilinear coordinate systems allowing for the simulation of either linear or circular accelerator lattice designs, and uses a fast and accurate explicit symplectic integrator. Physics processes for common design studies have been implemented, including RF cavity acceleration, synchrotron radiation damping, on-line physical aperture checks and collimation, proton scattering, wakefield simulation, and spin-tracking. Merlin++ was written using C++ object orientated design practices and has been optimized for speed using multicore processors. This article presents an account of the program, including its functionality and guidance for use. Program summaryProgram Title: Merlin++CPC Library link to program files:https://doi.org/10.17632/4x4nsbhz37.1Developer's repository link: 10.5281/zenodo.3700155Licensing provisions: GPLv2+Programming language: C++Nature of problem: Complexity of particle accelerators beam dynamics over extensive tracking distances.Solution method: Long-term particle accelerator and tracking simulations utilizing explicit symplectic integrators.Additional comments including restrictions and unusual features: For further information see github.com/Merlin-Collaboration

Industrial tomographic gamma scan for demister evaluation

The industrial tomographic 2-D gamma scan is an innovative non-destructive nuclear test designed to troubleshoot problems with process equipment, such as distillation columns and reactors. Due to physical restrictions, normally the gamma ray source and detector can only be positioned laterally to the equipment. The 2-D gamma scan is able to reconstruct the interior of the equipment even under this restriction, generating rectangular 2-D axial density profiles of equipment. To obtain clear reconstructions, it is necessary to collimate the detector, discarding the divergent incoming rays. It is not possible to use physical collimation in an industrial environment, as it is impossible to point the detector precisely in the direction of the gamma-ray emitter. In this article, we describe a technique we call “electronic collimation for gamma scan” that allows obtaining collimated data without physical collimation. We use 2-D gamma scan to evaluate a huge dilution steam drum in a large petrochemical plant to investigate possible problems with three demister pads. The low density of the demisters when compared to the vessel and surroundings makes it difficult to generate a reconstructed image where demisters are clearly visible. We describe how we overcome these difficulties using information known a priori about the equipment. We also ran simulations to show the superiority of 2-D gamma scan over conventional scan. These simulations also showed that, if there were defects in the demisters, we would observe them in the reconstructed images. We conclude that electronic collimation and prior information are essential for evaluating properly equipment with challenging layouts and low density parts.

Quantitative imaging and automated fuel pin identification for passive gamma emission tomography

Compliance of member States to the Treaty on the Non-Proliferation of Nuclear Weapons is monitored through nuclear safeguards. The Passive Gamma Emission Tomography (PGET) system is a novel instrument developed within the framework of the International Atomic Energy Agency (IAEA) project JNT 1510, which included the European Commission, Finland, Hungary and Sweden. The PGET is used for the verification of spent nuclear fuel stored in water pools. Advanced image reconstruction techniques are crucial for obtaining high-quality cross-sectional images of the spent-fuel bundle to allow inspectors of the IAEA to monitor nuclear material and promptly identify its diversion. In this work, we have developed a software suite to accurately reconstruct the spent-fuel cross sectional image, automatically identify present fuel rods, and estimate their activity. Unique image reconstruction challenges are posed by the measurement of spent fuel, due to its high activity and the self-attenuation. While the former is mitigated by detector physical collimation, we implemented a linear forward model to model the detector responses to the fuel rods inside the PGET, to account for the latter. The image reconstruction is performed by solving a regularized linear inverse problem using the fast-iterative shrinkage-thresholding algorithm. We have also implemented the traditional filtered back projection (FBP) method based on the inverse Radon transform for comparison and applied both methods to reconstruct images of simulated mockup fuel assemblies. Higher image resolution and fewer reconstruction artifacts were obtained with the inverse-problem approach, with the mean-square-error reduced by 50%, and the structural-similarity improved by 200%. We then used a convolutional neural network (CNN) to automatically identify the bundle type and extract the pin locations from the images; the estimated activity levels finally being compared with the ground truth. The proposed computational methods accurately estimated the activity levels of the present pins, with an associated uncertainty of approximately 5%.

ASSESSMENT OF AMBIENT NEUTRON DOSE EQUIVALENT IN SPATIALLY FRACTIONATED RADIOTHERAPY WITH PROTONS USING PHYSICAL COLLIMATORS.

New technique is trending in spatially fractionated radiotherapy with protons to utilize the spot scanning together with a physical collimator to obtain minibeams. The primary goal of this study is to quantify ambient neutron dose equivalent (${H}^{\ast }(10)$) due to the secondary neutrons when physical collimator is used to achieve desired minibeams. The ${H}^{\ast }(10)$ per treatment proton dose (D) was assessed using Monte Carlo code TOPAS and measured using WENDI-II detector at different angles (135, 180, 225 and 270 degrees) and distances (11cm, 58 and 105cm) from the phantom for two cases: with and without physical collimation. Without collimation $\frac{H^{\ast }(10)}{D}$ varied from 0.0013 to 0.242mSv/Gy. With collimation $\frac{H^{\ast }(10)}{D}$ varied from 0.017 to 3.23mSv/Gy. Results show that the secondary neutron dose will increase tenfold when the physical collimator is used. Regardless, it will be low and comparable to the neutron dose produced by conventional passive-scattered proton beams.

Dosimetric evaluation of cobalt-60 teletherapy in advanced radiation oncology

AbstractBackgroundRecent investigations demonstrate a strong potential for cobalt-60 (Co-60)-based teletherapy. The influence of the lower energy and penetration of a cobalt-60 beam compared with linear accelerator beams is negligible for intensity-modulated radiotherapy.PurposeThe aim of this research is to investigate source head fluence modulation in cobalt-60 teletherapy by using a three-dimensional (3D) physical compensator and secondary collimator jaw motion.Materials and methodsThe Oncentra treatment planning system was used to develop three hypothetical plans by secondary collimator jaw motion. A clinical MDS Nordion Equinox 80 cobalt-60 teletherapy unit was used to acquire conventional water phantom beam characteristics. Fluence modulation experiments were executed at 5·0 cm depth in a PTW universal intensity-modulated radiation therapy (IMRT) verification phantom using calibrated Gafchromic external beam therapy 2 (EBT2) and RTQA2-1010 film batches. Gafchromic EBT2 film was used to sample intensity maps generated by secondary collimator jaw motion, yet Gafchromic RTQA2-1010 film sampled maps from the 3D physical compensator. The solid-state drives used were 75·0 and 74·3 cm for the Gafchromic EBT2 and Gafchromic RTQA2-1010 film measurements.ResultsA 2D gamma index analysis was coded to compare EBT2 film measurements with Digital Imaging and Communications in Medicine data. This analysis was also used to verify film measurements versus Monte-Carlo simulations.ConclusionLateral beam profiles generated from water phantom measurements were used to establish source head fluence modulation on the film measurements. The source head fluence of a cobalt-60 teletherapy beam could be modulated by secondary collimator jaw motion and using a 3D physical compensator.

Development, commissioning, and evaluation of a new intensity modulated minibeam proton therapy system.

To invent, design, construct, and commission an intensity modulated minibeam proton therapy system (IMMPT) without the need for physical collimation and to compare its resulting conformity to a conventional IMPT system. A proton therapy system (Hitachi, Ltd, Hitachi City, Japan; Model: Probeat-V) was specially modified to produce scanned minibeams without collimation. We performed integral depth dose acquisitions and calibrations using a large diameter parallel-plate ionization chamber in a scanning water phantom (PTW, Freiburg, Germany; Models: Bragg Peak ionization chamber, MP3-P). Spot size and shape was measured using radiochromic film (Ashland Advanced Materials, Bridgewater NJ; Type: EBT3), and a synthetic diamond diode type scanned point by point in air (PTW Models: MicroDiamond, MP3-P). The measured data were used as inputs to generate a Monte Carlo-based model for a commercial radiotherapy planning system (TPS) (Varian Medical Systems, Inc., Palo Alto, CA; Model: Eclipse v13.7.15). The regular ProBeat-V system (sigma ~2.5mm) TPS model was available for comparison. A simulated base of skull case with small and medium targets proximal to brainstem was planned for both systems and compared. The spot sigma is determined to be 1.4mm at 221MeV at the isocenter and below 1mm at proximal distances. Integral depth doses were indistinguishable from the standard spot commissioning data. The TPS fit the spot profiles closely, giving a residual error maximum of 2.5% in the spot penumbra tails (below 5% of maximum) from the commissioned energies 69.4 to 221.3MeV. The resulting IMMPT plans were more conformal than the IMPT plans due to a sharper dose gradient (90-10%) 1.5mm smaller for the small target, and 1.3mm for the large target, at a representative central axial water equivalent depth of 7cm. We developed, implemented, and tested a new IMMPT system. The initial results look promising in cases where treatments can benefit from additional dose sparing to neighboring sensitive structures.

A method to reduce patient's eye lens dose in neuro-interventional radiology procedures

Abstract Complex and prolonged neuro-interventional radiology procedures using the biplane angiography system increase the patient's risk of radiation-induced cataract. Physical collimation is the most effective way of reducing the radiation dose to the patient's eye lens, but in instances where collimation is not possible, an attenuator may be useful in protecting the eyes. In this study, an eye lens protector was designed and fabricated to reduce the radiation dose to the patients’ eye lens during neuro-interventional procedures. The eye protector was characterised before being tested on its effectiveness in a simulated aneurysm procedure on an anthropomorphic phantom. Effects on the automatic dose rate control (ADRC) and image quality are also evaluated. The eye protector reduced the radiation dose by up to 62.1% at the eye lens. The eye protector is faintly visible in the fluoroscopy images and increased the tube current by a maximum of 3.7%. It is completely invisible in the acquisition mode and does not interfere with the clinical procedure. The eye protector placed within the radiation field of view was able to reduce the radiation dose to the eye lens by direct radiation beam of the lateral x-ray tube with minimal effect on the ADRC system.

A prototype fan‐beam optical CT scanner for 3D dosimetry

The objective of this work is to introduce a prototype fan-beam optical computed tomography scanner for three-dimensional (3D) radiation dosimetry. Two techniques of fan-beam creation were evaluated: a helium-neon laser (HeNe, λ = 543 nm) with line-generating lens, and a laser diode module (LDM, λ = 635 nm) with line-creating head module. Two physical collimator designs were assessed: a single-slot collimator and a multihole collimator. Optimal collimator depth was determined by observing the signal of a single photodiode with varying collimator depths. A method of extending the dynamic range of the system is presented. Two sample types were used for evaluations: nondosimetric absorbent solutions and irradiated polymer gel dosimeters, each housed in 1 liter cylindrical plastic flasks. Imaging protocol investigations were performed to address ring artefacts and image noise. Two image artefact removal techniques were performed in sinogram space. Collimator efficacy was evaluated by imaging highly opaque samples of scatter-based and absorption-based solutions. A noise-based flask registration technique was developed. Two protocols for gel manufacture were examined. The LDM proved advantageous over the HeNe laser due to its reduced noise. Also, the LDM uses a wavelength more suitable for the PRESAGE(TM) dosimeter. Collimator depth of 1.5 cm was found to be an optimal balance between scatter rejection, signal strength, and manufacture ease. The multihole collimator is capable of maintaining accurate scatter-rejection to high levels of opacity with scatter-based solutions (T < 0.015%). Imaging protocol investigations support the need for preirradiation and postirradiation scanning to reduce reflection-based ring artefacts and to accommodate flask imperfections and gel inhomogeneities. Artefact removal techniques in sinogram space eliminate streaking artefacts and reduce ring artefacts of up to ∼40% in magnitude. The flask registration technique was shown to achieve submillimetre and subdegree placement accuracy. Dosimetry protocol investigations emphasize the need to allow gel dosimeters to cool gradually and to be scanned while at room temperature. Preliminary tests show that considerable noise reduction can be achieved with sinogram filtering and by binning image pixels into more clinically relevant grid sizes. This paper describes a new optical CT scanner for 3D radiation dosimetry. Tests demonstrate that it is capable of imaging both absorption-based and scatter-based samples of high opacities. Imaging protocol and gel dosimeter manufacture techniques have been adapted to produce optimal reconstruction results. These optimal results will require suitable filtering and binning techniques for noise reduction purposes.

Portable fast-neutron radiography with the nuclear materials identification system for fissile material transfers

The nuclear materials identification system (NMIS) has historically been used with a 252Cf source for bulk interrogation of highly enriched uranium and plutonium. Recently, a portable associated-particle sealed-tube neutron generator (APSTNG) has been added as an interrogation source. This paper reports results from using the NMIS with the neutron generator to perform transmission radiography. In particular, the use of associated-particle techniques allows low-dose imaging to be performed simultaneously with traditional measurements, without physical collimation, and in high background environments.

SU-CC-ValA-03: A Directional Algorithm for An Electronically-Collimated Gamma-Ray Detector for Intraoperative Localization of Radiation Sources

Purpose: An electronically‐collimated gamma‐radiation detector for intraoperative localization of sentinel lymph nodes and metastases is under development. Analogous to Compton telescopes and Compton cameras, localization is achieved using the coincidence detection of Compton‐scattered gamma rays. Electronic collimation allows the device to operate without physical collimation, providing high sensitivity while also allowing directional information to be determined. We report on implementation of algorithms to calculate the direction to the source.Methods and Materials: Two approaches to direction reconstruction were evaluated. The first technique backprojects each event onto the surface of a sphere centered on the device's primary detector. To use Fourier filtering methods for deblurring, the sphere's surface is mapped by stereographic projection onto a plane, filtered in Fourier space, and then projected back onto the sphere. The second technique also backprojects events onto the sphere, then determines the rectangle that circumscribes the backprojected cone; localization is obtained by intersection of all circumscribed rectangles. Results: Performance of the algorithm has been evaluated using randomly generated ideal Compton‐scatter events from point sources for our detector geometry. Direction angles are calculated within 5% accuracy for source positions up to 45° off‐axis for the filtering approach and ∼30° for the circumscription approach. Error in calculated direction angles depends on the arbitrary diameter of the sphere; optimally, the sphere should intersect the source. The circumscription technique converges to an estimate of direction angles in ∼50 events; the filtering approach requires ∼1000 events. Conclusion: The two methods complement each other in speed and field‐of‐view. Monte Carlo simulations and experimental testing of a prototype system are ongoing as a separate part of the overall project; data from these will further supplement evaluation of the algorithms. Acknowledgment: Supported by Homeland Security Advanced Research Projects Agency, and Space and Naval Warfare Systems Center San Diego; Contract No. N66001‐05‐C‐6024.

Potential of PET in oncology and radiotherapy

Positron emission tomography (PET) is a non-invasive imaging technique based on coincidence detection of two simultaneously emitted photons that occurs when a positron annihilates after combination with an electron. Positrons are emitted from proton-rich unstable isotopes. Such short-lived radioactive isotopes are made in a cyclotron and then chemically linked to a probe molecule (e.g. a drug, water or a metabolite) to form a labelled PET tracer that is injected intravenously into a patient. Single emitted positrons combine with an electron to form a positronium, the lifetime of which is very short, resulting in annihilation of the positron and electron (all the mass is converted into electromagnetic radiation). To conserve energy and linear momentum, the electromagnetic radiation appears in the form of two photons of equal energy (511 keV; equal to the rest mass energy of the electron and positron), which are emitted at 180 to each other. The emitted photons are detected by a technique known as coincidence detection. A PET scanner comprises a large number of scintillation detectors; when two scintillation detectors that are separated by 180 are both stimulated simultaneously, they will transmit a coincident signal. This enables localisation of a source without the need for physical (lead) collimation. This fundamental physics forms the basis of dynamic detection and threedimensional localisation of the positron emitter and confers advantages of greater efficiency and resolution for PET over other nuclear imaging methods. Since a number of commonly occurring elements such as carbon, oxygen, nitrogen and fluorine have positron-emitting radionuclides, the stable nuclides of these elements can potentially be replaced by their positron-emitting counterparts in a number of compounds and evaluated using PET (Table 1). Emission of positrons from the compound follows the presence of such compounds when used as tracers of tissue function within the body; hence, PET can provide unique functional imaging data not available with other modalities. Moreover, the ability to correct for attenuation of radiation by body tissue as well as the highly sensitive nature of PET technology makes it possible for accurate quantification of the radiolabelled compound to picogram amounts. These properties of PET can be utilised to provide functional in vivo imaging data and as a result, the utility of PET has increased considerably over the last few years. The wide usage and worldwide acceptability of the radioligand fluorine-18-radiolabelled fluorodeoxyglucose (F-FDG) as a diagnostic PET imaging tool has made FDG-PET synonymous with PET. FDG follows the same route as glucose into cells, where it is phosphorylated by hexokinase to FDG-6-phosphate. Unlike glucose, little further metabolism occurs and FDG-6-phosphate remains essentially trapped within cells, with the rate of accumulation proportional to the rate of glucose utilisation. FDG-6-phosphate has low membrane permeability and although dephosphorylation does occur, it is very slow in brain, heart and tumour, which have very low levels of glucose-6-phosphatase. FDG uptake in tumours probably reflects a combination of factors, including phosphorylating activity of mitochondria, degree of hypoxia and levels of glucose transporters [1–3]. The role of FDG-PET for imaging and staging of malignant tumours is well established in oncology and is used in a routine clinical environment in many countries for diagnostic purposes. In addition to its diagnostic utility, other salient features of PET, notably its capability to image functional changes, make it a potential tool for further research in oncology. Therefore, a number of other PET probes in addition to FDG have been imaged and are being developed. The potential role of PET as a research tool in oncology can be broadly divided into five main areas:

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.

Novel approach to collimated physical vapor deposition

In some applications of physical vapor deposition (PVD), it is desirable to have the atoms arriving at the substrate at angles close to the normal (collimation). Applications of collimated PVD include continuous coverage of high-aspect ratio vias, as used in multilevel integrated circuit metallization, or high resolution shadow masking as used in lift-off technology. Several approaches to collimation have been proposed in recent years: natural (or long throw) collimation, physical collimation, ion beam deposition, ionized PVD cathodic arc deposition and hollow cathode collimation. In this article, we introduce a novel approach to collimation. According to simulations of target-to-substrate atom transport using the SIMBAD program this approach provides narrow angular distribution functions of the sputtered flux at the substrate surface. Furthermore, it is shown experimentally that this approach leads to a strongly improved degree of collimation when compared to the conventional methods based on either physical or natural collimation, through formation of abutted junction permanent magnet stabilization and metallization layers as in applied magnetoresistive thin film recording heads, using a lift-off process.

Void-free chemically vapor-deposited aluminum dual inlaid metallization schemes for ultra-large-scale-integrated via and interconnect applications

This letter reports an investigation of two unique dual inlaid metallization approaches with low pressure chemical vapor deposition (LPCVD) of aluminum (Al) for sub-0.35 μm ultra-large-scale-integration interconnect technology: (1) warm Al/CVD Al/coherent (coh.) PVD Al/coh. PVD Ti and (2) warm PVD Al/CVD Al/coh. PVD Ti or Al/selective CVD Al. The integration of thin coh. PVD Al, deposited with a physical collimator or a variation of ionized metal plasma technique, was found to be the unique and simple solution in providing void-free via and interconnect structures, which have not been reported elsewhere. Excellent electrical and electromigration results have been obtained.

Experience with fully 3D PET and implications for future high-resolution 3D tomographs

The aim of this paper is to report on experience with 3D positron emission tomography (PET) in our institute where we have three 3D scanners, of which two operate exclusively in 3D mode (ECAT ART, EXACT 3D). Fully 3D PET requires attention to a number of factors which are not as problematic in 2D PET. Firstly, 3D tomographs designed for whole-body acquisition suffer from a large single-photon field of view, extending well beyond the coincidence field of view. Single photons from outside the coincidence field of view increase the dead time and random coincidence rates, and contribute scattered events. For brain studies, we have extended the lead side shielding at the ends of the tomograph to mitigate against these effects, and this has dramatically improved the count rate performance. This approach is not as effective for whole-body scanning. In addition, operating in 3D without septa necessitates new approaches to transmission scanning, as measurements using positron emitters such as have the unfavourable characteristics of high dead time and high scatter. Both of our fully 3D scanners use for single-photon transmission measurements, although the data are treated differently. On the ECAT ART, a combination of physical and electronic collimation effectively reduces transmission scatter to acceptable levels. On the EXACT 3D physical collimation is not as readily implemented and therefore segmentation and reassignment of the histogrammed attenuation values is employed to produce unbiased attenuation correction factors in 3D. Many of the lessons learnt with these BGO (bismuth germanate) based tomographs will be applicable to the next generation of systems using faster detectors such as lutetium oxyorthosilicate (LSO).

A high sensitivity, electronically collimated gamma camera

A high sensitivity electronically collimated gamma camera is under development at NOVA. The proposed detector has electronic collimation and will measure the direction of the incident /spl gamma/-ray directly, hence eliminating the need for a physical collimator. This is accomplished by using the Compton scatter imaging technique. A feasibility study was successfully completed and prototype fabrication started. The new detector is expected to have 3 to 10 times improvement over the present gamma cameras. The high sensitivity is expected to allow fast imaging on time scales of 1 to 2 min and may open the way for dynamic metabolism visualization and multi gated acquisition (MUGA) studies. The prototype is designed to have a high spatial resolution which is 2 to 3 times better compared to present gamma cameras. It is expected to have improved energy resolution by a factor of 2 to 4. Good energy resolution will enable discrimination against natural and scattered /spl gamma/-ray background with higher effectiveness than presently available. This will improve the signal-to-noise ratio and improve image quality. The detector can be designed as a single gamma camera or formed into a cylindrical detector and used as a tomographic SPECT imager.

Collimated magnetron sputter deposition with grazing angle ion bombardment

Collimated magnetron sputter deposition with concurrent grazing angle ion bombardment has been developed as a technique for enhancing the ability to deposit films into trenches and vias. The collimated sputtering technique uses a conventional magnetron cathode operating at low pressure (mTorr) and a physical collimator or directional filter mounted between the cathode/discharge plasma and the sample position. The collimator serves to limit the directionality of the transmitted, sputtered atoms to near-normal incidence. A broad-beam Kaufman-type ion source is mounted on the sample side of the collimator such that the ion beam impacts the sample surface at grazing (5°–15°) incidence. This serves to resputter the depositing film selectively from the topmost regions of the sample surface, while lower areas, such as trenches and vias, are geometrically shielded.