Focusing Collimator Research Articles

Fast quantitative reconstruction with focusing collimators for liver SPECT

BackgroundGeneration of a SPECT scan during procedure may aid in the optimization of treatments as liver radioembolization by offering image-guided dosimetry. This, however, requires both shortened acquisition times and fast quantitative reconstruction. Focusing collimators increase sensitivity and thus may speed up imaging. Monte Carlo-based iterative reconstruction has shown to provide quantitative results for parallel hole collimators but may be slow. The purpose of this work is to develop fast Monte Carlo-based reconstruction for focusing collimators and to evaluate the impact of reconstruction and collimator choice on quantitative accuracy of liver dosimetry by means of simulations.ResultsThe developed fast Monte Carlo simulator was found to accurately generate projections compared to a full Monte Carlo simulation, providing projections in several seconds instead of several days. Monte Carlo-based scatter correction was superior to other scatter correction methods in describing recovered activity and reached similar noise levels as dual-energy window scatter correction. Although truncation artifacts were present in the cone beam collimator (50 cm), the region inside the field of view (FOV) could be reconstructed without loss of accuracy. Provided the object to image is inside the FOV, the focusing collimator with 50 cm focal distance could retrieve the same noise levels as a parallel hole collimator in 68% of the total scanning time, the multifocal collimator in 73% of the time, and the 100-cm focal distance collimator in 84% of the time.ConclusionFocusing collimators combined with Monte Carlo-based reconstruction have the ability to enable quantitative imaging of the FOV in a significantly shorter timeframe. The proposed approach to the forward projector will additionally make it possible to reconstruct within minutes. These are crucial steps in moving toward real-time dosimetry during interventions.

Murine cardiac images obtained with focusing pinhole SPECT are barely influenced by extra-cardiac activity

Ultra-high-resolution SPECT images can be obtained with focused multipinhole collimators. Here we investigate the influence of unwanted high tracer uptake outside the scan volume on reconstructed tracer distributions inside the scan volume, for 99mTc-tetrofosmin myocardial perfusion scanning in mice. Simulated projections of a digital mouse phantom (MOBY) in a focusing multipinhole SPECT system (U-SPECT-II, MILabs, The Netherlands) were generated. With this system differently sized user-defined scan volumes can be selected, by translating the animal in 3D through the focusing collimators. Scan volume selections were set to (i) a minimal volume containing just the heart, acquired without translating the animal during scanning, (ii) a slightly larger scan volume as is typically applied for the heart, requiring only small XYZ translations during scanning, (iii) same as (ii), but extended further transaxially, and (iv) same as (ii), but extended transaxially to cover the full thorax width (gold standard). Despite an overall negative bias that is significant for the minimal scan volume, all selected volumes resulted in visually similar images. Quantitative differences in the reconstructed myocardium between gold standard and the results from the smaller scan volume selections were small; the 17 standardized myocardial segments of a bull's eye plot, normalized to the myocardial mean of the gold standard, deviated on average 6.0%, 2.5% and 1.9% for respectively the minimal, the typical and the extended scan volume, while maximum absolute deviations were respectively 18.6%, 9.0% and 5.2%. Averaged over ten low-count noisy simulations, the mean absolute deviations were respectively 7.9%, 3.2% and 1.9%. In low-count noisy simulations, the mean and maximum absolute deviations for the minimal scan volume could be reduced to respectively 4.2% and 12.5% by performing a short survey scan of the exterior activity and focusing the remaining scan time at the organ of interest. We conclude that reconstructed tracer distribution in the myocardium can be influenced by activity in surrounding organs when a too narrow scan volume is used. With slightly larger scan volumes this problem is adequately suppressed. This approach produced a smaller mean deviation and may be more effective than employing a narrow scan volume with an additional survey scan.

A pinhole gamma camera with optical depth-of-interaction elimination

The performance of pinhole single photon emission computed tomography (SPECT) depends on the spatial resolution of the gamma-ray detectors used. Pinhole cameras suffer from strong resolution loss due to the varying depth-of-interaction (DOI) of gamma quanta that enter the detector material at an angle. We eliminate DOI effects in a scintillation gamma camera via a dedicated optic fiber bundle that acts as a focusing collimator for light generated in a scintillation crystal. A curved crystal is connected to a concavely shaped fiber-optic bundle such that the fibers connect perpendicularly to the crystal's convex surface and point straight at the pinhole opening. Limiting the fiber numerical apertures can be used to suppress resolution losses due to light spread. Here we demonstrate experimentally that this prototype position-sensitive gamma sensor successfully eliminates DOI effects, and has an intrinsic resolution of better than 280 µm full width at half maximum with an interaction probability of 67% for 140 keV photons. Therefore, the detector has great potential for increasing the resolution of pinhole SPECT.

Microwave focusing and beam collimation using negative index of refraction lenses

Negative index of refraction materials (NIMs) were first postulated by Veselago in 1968 and have recently been realised using structures formed with rings and wires deposited on printed circuit boards. The proof of the existence of negative index of refraction was established using a Snell's law experiment with a wedge. The predicted and measured refraction angles were found to be consistent for a negative index material and in excellent agreement with the theoretical expectations. For microwave lenses NIMs have the advantage of being lighter, having better focusing properties and potentially lower aberrations. Simulation and experimental results on NIM configurations including gradient index of refraction and spherical 3D lenses are presented. Both focusing and beam collimating applications will be considered. These results will be compared to normal positive index of refraction material lenses.

Current status of the 32 m small-angle neutron scattering instrument, SANS-U

The 32m small-angle neutron scattering instrument, SANS-U, owned by the Institute for Solid State Physics, The University of Tokyo, was upgraded in 2004 with a new multi-wired two-dimensional detector, a focusing collimation system, and a LabVIEW based controlling system. The performance of the new SANS-U, such as (1) linearity and uniformity of the detector, (2) the incident beam flux, (3) the flux and collimation relationship, (4) the absolute intensity correction and so on, was examined. Sample thickness dependence of incoherent scattering intensity from standard polyethylene samples was discussed by considering multiple scattering. The q-resolution of SANS-U was estimated with a series of Bragg reflections from a microphase-separated structure in a block copolymer film, where q is the momentum transfer.

Analytical derivation of the point spread function for pinhole collimators

The point spread function (PSF) of a pinhole collimator plays an important role in determining the resolution and characterizing the sensitivity of the accepted photons from a given point in the image space. The focus of this paper is to derive an analytical expression for the PSF of two different types of focusing pinhole collimators that are based on (1) right-circular double cones and (2) oblique-circular double cones. Conventionally, focusing pinhole collimators used in multi-pinhole SPECT were designed using right-circular double cones, as they were easier to fabricate. In this work, a novel focusing collimator consisting of oblique-circular double cones was designed and its properties were studied in detail with respect to right-circular double-cone based collimators. The main advantage of determining the PSF is the fact that they can be used to accurately model the PSF during the reconstruction, thereby improving the resolution of the reconstructed image. The PSF of the focusing collimators based on oblique-circular cones were found to be almost shift invariant for low and medium energy photons (below 200 keV). This property is very advantageous as algorithms such as slice-by-slice reconstruction can be used for resolution recovery thereby drastically reducing the reconstruction time. However, the PSF of focusing oblique-circular double cones (FOCDC) for higher energy photons were found to be asymmetric and hence need to be modelled more accurately during the reconstruction. On the other hand, the PSF for the right-circular cone based collimators were found to be asymmetric for all energy levels. However, due to the smaller acceptance angle used, the number of penetration photons was found to be far less than that observed for oblique-circular cones. This results in a smaller PSF making right-circular cone based collimators preferable for high-resolution small animal imaging, especially where very small pinhole diameters are used. The analytically derived PSF for both collimators were validated using a ray-tracing based Monte Carlo approach and found to agree well with a mean square error of less than 1%.

SALSA: Advances in Residual Stress Measurement at ILL

SALSA (Strain Analyser for Large Scale engineering Applications) is a novel instrument for strain imaging at the high flux neutron reactor of the Institut Max von Laue – Paul Langevin (ILL) in Grenoble, France. It is the first of its kind that uses a precise robotic sample stage for sample manipulation. In addition to standard xyz-translation it provides tilts up to ±30° about any horizontal axis. Its load capacity is more than 500 kg and samples up to 1.5 m in length can be scanned with high accuracy. Thanks to a double focusing monochromator and supermirror guide count times are very short. A broad wavelength range, variable beam optics, including radial focusing collimators for high lateral resolution, make it a very flexible instrument for a large variety of strain imaging applications in small and large specimens. The instrument has been commissioned in 2005 and is open for access by a peer review proposal system and as well by industry. The paper describes the important aspects of the instrument and results from the first experiments.

Performance of a novel collimator for high‐sensitivity brain SPECT

We assessed improvements in performance in detection and estimation tasks due to a novel brain single photon computed tomography collimator. Data were acquired on the CeraSPECT scanner using both new and standard collimators. The new variable focusing collimator SensOgrade samples the projections unequally, with central regions more heavily represented, to compensate for attenuation of counts from central brain structures. Furthermore, it utilizes more of the cylindrical crystal surface. Two phantom studies were performed. The first phantom was a 21-cm-diameter cylindrical background containing nine spheres ranging from 0.5 to 5 cm3 in volume. 99mTc sphere to background activity ratio was 10:1. Twenty-nine 10-min datasets were acquired with each collimator. The second phantom was the Radiology Support Devices (Long Beach, CA) striatal phantom with striatal-background ratios of 10:1 on the left and 5:1 on the right. Twenty-nine 4-min datasets were acquired with each collimator. Perfusion imaging using 99mTc-HMPAO was also performed in three healthy volunteers using both collimators under identical simulations. Projections were reconstructed by filtered backprojection with an unwindowed ramp filter. The nonprewhitening matched filter signal-to-noise ratio (NPW-SNR) was computed as a surrogate for human performance in detecting spherical lesions. Sphere activity concentration, radius, and location coordinates were simultaneously estimated by fitting images to an assumed model using an iterative nonlinear algorithm. Resolution recovery was implicit in the estimation procedure, as the point spread function was incorporated into the model. NPW-SNR for sphere detection was 1.5 to 2 times greater with the new collimator; for the striatal phantom the improvement in SNR was 54%. The SNR for estimating sphere activity concentration improved by 46 to 89% for spheres located more than 5 cm from the phantom center. Images acquired with the standard collimator were too noisy in the central regions to allow estimation of sphere activity. In 99mTc-HMPAO human studies, SNR was improved by 21 to 41% in the cortex, 66% in the basal ganglia, and 74% in the thalamus. The new collimator leads to substantially improved detection and estimation performance throughout the brain. The higher sensitivity will be particularly important for dynamic imaging.

An original emission tomograph for in vivo brain imaging of small animals

The principle of a new tomograph (TOHR) dedicated for small volume analysis with very high resolution is presented. The authors use uncorrelated multi-photon (X- or gamma rays) radioisotopes and a large solid angle focusing collimator to produce tomographic images without a reconstruction algorithm. With this device, detection efficiency and resolution are independent and submillimeter resolution can be achieved. A feasibility study shows that collimators made with a stack of chemically etched plates can achieve the predicted performances of TOHR. The authors discuss its potential in rat brain tomography by simulating a realistic neuropharmacological experiment using a 1.4 mm resolution prototype of the TOHR under development.

Shock focusing and jet collimation in young stars

We study the collimation of a initially uncollimated outflow from a young star by its environment, a cloud core with a toroidal density distribution. It is shown that when cooling is unimportant the shape of the inner shock and the internal structure of the bubble will collimate the outflow and it is shown that in some cases cooling is unimportant. The effects of the focusing by an elliptical shock are studied. Numerical simulations show that when cooling is important, the collimation is actually much better and interesting combinations of energy- and momentum-conserving flow occur.

Development of high resolution focusing collimators intended for nondestructive testing by the Compton scattering tomography technique : 43209 Duvauchelle, P.; Girier, P.; Peix, G. Applied Radiation and Isotopes, Vol. 41, No. 2, pp. 199–205 (1990)

Development of high resolution focusing collimators intended for nondestructive testing by the Compton scattering tomography technique : 43209 Duvauchelle, P.; Girier, P.; Peix, G. Applied Radiation and Isotopes, Vol. 41, No. 2, pp. 199–205 (1990)

Maximum likelihood positioning in the scintillation camera using depth of interaction

Specific effects of the depth of interaction (DOI) on the photomultiplier (PM) response in an Auger gamma camera were quantified. The method was implemented and tested on a Monte Carlo simulator with special care to the noise modeling. Two models were developed, one considering only the geometric aspects of the camera and used for comparison, and one describing a more realistic camera environment. In a typical camera configuration and 140-keV photons, the DOI alone can account for a 6.4-mm discrepancy in position and 12% in energy between two scintillations. Variation of the DOI can still bring additional distortions when photons do not enter the crystal perpendicularly such as in slant hole, cone beam and other focusing collimators. With a 0.95-cm crystal and a 30 degrees slant angle, the obliquity factor can be responsible for a 5.5-mm variation in the event position. Results indicate that both geometrical and stochastic effects of the DOI are definitely reducing the camera performances and should be included in the image formation process.

The development of a rectilinear scanner utilizing high resolution gamma-ray detection

A rectilinear scanner has been constructed which incorporates high resolution gamma-ray spectroscopy. This scanner was built to simultaneously image the distributions of multiple gamma-emitting radionuclides in small animals, such as rabbits and rats. The instrument design is described, followed by studies of the Ge(Li) detector response and calibration tests of a single straight-hole collimator. Based on these results, two new collimators were designed and constructed; (1) a six tapered-hole focusing collimator and (2) a nine straight-hole focusing collimator. These new collimators are advantageous because they offer an increased efficiency of gamma-ray detection during scanning. They also give better image resolution in the differentiation of two points (or organs) on a plane. The results of the calibration tests and comparisons of the resolution and efficiency among collimators are illustrated and discussed.

An improvement of Compton scatter imaging with wide aperture detectors a Monte Carlo study

A novel NDT (nondestructive testing) imaging technique which is based on the measurement of Compton-scattered photons and uses wide-aperture detectors has been developed. With this technique, the inspection speed can be improved because the scanning activities can be reduced by one degree of freedom. Using wide-aperture detectors serves to simplify the design of measurement systems, and the data collection time at each step in the scanning process can be reduced due to the removal of the usual efficiency-reducing focusing collimator. To study the imaging process of the novel NDT system, a number of Monte Carlo calculations have been performed. The Monte Carlo code has been validated by comparison to the widely accepted general-purpose MCNP (Monte Carlo neutron-photon) code and has been found to run over ten times faster than the MCNP code for this particular research. As a result, the NDT system has been designed to deal, in particular, with the pipe wall thinning problems widely observed in nuclear power plants and some fossil power plants. In this application, the contour of a pipe inner surface can be reconstructed in a very straightforward manner, and the scanning activities can be reduced from three dimensions to two dimensions. >

3-D computer simulations of resolution effects of multidetector point focusing SPECT imaging

The authors present an efficient algorithm and the results of its application in simulating the three-dimensional (3-D) projection data resulting from a 3-D distribution of radioactivity. The algorithm was applied to a series of geometrical mathematical phantoms and to a realistic mathematical brain phantom. The authors simulated the projection data from a multidetector single-photon emission computed tomography (SPECT) system with point focusing collimators. The simulated projection data were then reconstructed using the manufacturer's software. The objects simulated included simple geometrical solids such as spheres and sheets, as well as the distribution of muscarinic cholinergic receptors in a realistic brain slice. Spheres were chosen as a model for brain structures such as caudate nucleus, thalamus, and cerebellum; sheets were selected as representing lateral cortical gray matter regions. The results of these simulations indicate the existence of significant qualitative and quantitative artifacts in reconstructed human brain images.

Development of high resolution focusing collimators intended for nondestructive testing by the Compton scattering tomography technique 43209 Duvauchelle P.; Girier, P.; Peix, G. applied radiation and isotopes, vol. 41, No. 2, pp. 199–205 (1990)

Development of high resolution focusing collimators intended for nondestructive testing by the Compton scattering tomography technique 43209 Duvauchelle P.; Girier, P.; Peix, G. applied radiation and isotopes, vol. 41, No. 2, pp. 199–205 (1990)

Development of high resolution focusing collimators intended for nondestructive testing by the Compton scattering tomography technique: 43209 Duvauchelle P.; Girier, P.; Peix, G. applied radiation and isotopes, vol. 41, No. 2, pp. 199–205 (1990)

Development of high resolution focusing collimators intended for nondestructive testing by the Compton scattering tomography technique: 43209 Duvauchelle P.; Girier, P.; Peix, G. applied radiation and isotopes, vol. 41, No. 2, pp. 199–205 (1990)

Development of high resolution focusing collimators intended for nondestructive testing by the Compton scattering tomography technique

An original method for building high resolution focusing collimators with a great number of channels is described. The method consists of filling a mould with tungsten powder and impregnating it with a fluid epoxy resin, by pumping through the powder. The fine strips, used as cores for the channels, are then pulled out. The collimator that was built had 49 fine slots, all converging in one little bar-shaped volume, to convey x-ray photons from this volume to the entrance window of a large scintillation crystal detector. The collimator exhibits a very good precision of the relative positions of slots, allowing the volume V to be very sharply shaped. Use of this collimator for nondestructive testing by the Compton scattering tomography (CST) technique allowed us to detect plane defects down to 0.1 mm, inside composite plates.

The influence of penetrating radiation on collimator performance

Geometric and penetration responses of a multihole focusing collimator to a point source have been determined using a digital computer and a ray tracing technique. Results are presented as point and line spread functions for collimated rays and penetrating rays, and as plane sensitivities and modulation transfer functions derived from the line spread functions. Responses to a 141Ce line source have been measured for a lead collimator and for a cadmium collimator of practically identical dimensions. A comparison of measured and computed responses shows that while computed penetration approximates measured penetration reasonable well, there is a measurable contribution from scattered rays. Plane sensitivities and the modulation transfer function derived from the computed responses are shown. The effects of penetration are compared with predictions made by other investigators. This analysis of collimator performance provides information not previously available and should be useful for evaluating collimators of existing or proposed design.

A New Development in Single Gamma Transaxial Tomography Union Carbide Focused Collimator Scanner

The evolution of gamma-ray imaging is traced and a new gamma-ray transverse section tomographic instrument is described which matches the favorable properties of wide-aperture focusing collimators to a form of the Radon equation which requires only angular averages of line integrals for reconstructions. Single slice scans of less than 5 minutes are accomplished with amounts of radioactivity that are standard for procedures using gamma cameras.