Parallel Collimator Research Articles

Tomographie par ouverture codée sous deux incidences orthogonales

Standard collimators have the disadvantage of providing limited resolution and sensitivity for gamma detection. Coded-aperture tomography is a method that can help to improve these characteristics: the standard LEUHR parallel collimator is replaced by a multi-pinhole mask that increases the spatial resolution and sensitivity and provides the possibility of obtaining dynamic and multi-spectral tomographic images. Reconstruction is obtained via a planar projection of a single object through the mask, thereby permitting contiguous sections parallel to the detector to be obtained. This reconstruction was based on an itérative Maximum Likelihood-Expectation Maximization (ML-EM) algorithm that takes into account this particular acquisition geometry. The acquisition sensitivity of this method was estimated to be ten times higher than that of a Low Energy-High Resolution (LEUHR) collimator, and allowed reconstruction of the following volume: 133.30 × 176.70 × 130.33 mm 3, (l × w × h) divided into nineteen sections of 19 × 25 pixels, corresponding to 7.0 voxels. Reconstructions of numerical and physical simulations of a single object were made. The resolution in the section and in depth was measured at 7 mm, i.e., the same size as one voxel. The uniformity was still not satisfactory. Scattering and attenuation correction still had to be carried out to obtain quantitative values.

A combined half-cone beam and parallel hole collimation system for SPECT brain imaging

The authors' research group has recently examined new types of collimator designs with the goal of improving sensitivity and lesion detection for head imaging. One of these collimator designs is a half-cone beam collimator. However, the sensitivity is reduced as objects are removed from the focal line and it does not satisfy Tuy's sufficiency condition. Parallel hole collimation does not have this problem with axial blurring, however, values for sensitivity are not as large as for half-cone collimators. A potential configuration is to use one parallel hole collimator in conjunction with two half-cone beam collimators on a triple camera single photon emission computed tomography (SPECT) system. This might produce results that represent the best of both collimation systems. The authors acquired projection data with a Defrise disk phantom filled with Tc-99m. A half-cone beam collimator with a focal length of 50 cm was placed on one head of a triple camera SPECT system. A low energy super high resolution (LESR) parallel hole collimator was placed on a second head. Different projection data sets were combined to model acquisition in the three headed gamma camera with three half-cone beam collimators, two half-cone beam and one parallel beam collimators, one half-cone beam and two parallel beam collimators and three parallel beam collimators. Image reconstruction used a modified maximum likelihood maximization-expectation (ML-EM) algorithm. For the reconstruction with three half-cone beam collimators, the authors observe axial blurring. This is largely reduced with two half-cone beam and one parallel beam collimators. Graphs of the image profiles demonstrate that the blurring along the axial direction is decreased with the addition of parallel hole collimators.

137 パラレルコリメータと長焦点型ファンビ-ムコリメータの組み合わせによる体躯部SPECTの検討

137 パラレルコリメータと長焦点型ファンビ-ムコリメータの組み合わせによる体躯部SPECTの検討

Bone scintigraphy equipped with a pinhole collimator for diagnosis of avascular necrosis of the femoral head.

The objective was to compare the sensitivities for diagnosis of avascular necrosis of the femoral head of bone scintigraphy equipped with a pinhole collimator and with an high resolution parallel collimator. Bone scintigraphy equipped with a pinhole collimator and with an high resolution parallel collimator were performed in 16 patients with bilateral (n=7) or unilateral (n=9) avascular necrosis of the femoral head. Bone scintigraphy equipped with a pinhole collimator documented a photopenic defect in 78.3% of the necrotic hips, while bone scintigraphy equipped with an high resolution parallel collimator documented a defect in 47.8%. There was no false-positive diagnosis of avascular necrosis of the femoral head on either bone scintigraphy equipped with a pinhole or with an high resolution parallel collimator. In conclusion, bone scintigraphy equipped with a pinhole collimator has a greater sensitivity for diagnosis of avascular necrosis of the femoral head than bone scintigraphy equipped with an high resolution parallel collimator.

An electronically collimated gamma camera with a parallel plate collimator for Tc-99m imaging

We present a new idea to apply a parallel plate collimator to an electronically collimated gamma camera, to reduce the complexity of image reconstruction. Back projected images obtained experimentally and using Monte Carlo simulations are shown. Using the collimator, backprojected images of single low-energy gamma-ray emitters can be obtained with larger signal-to-noise ratio than those obtained without the collimator. The results show that this method is useful in obtaining Tc-99m images.

Advantage of fan beam collimators for contrast recovery of hyperfixation in clinical SPECT

The influence of the collimator on the contrast recovery of hyperfixation was studied using a dual-headed single photon emission computed tomography (SPECT) system with standard clinical acquisition parameters. Three parallel collimator sets and two fan beam collimator sets were tested with a Jaszczak phantom. The six spheres of the phantom were filled with /sup 99m/Tc, and four background levels were progressively obtained by adding radioactivity to the cylinder of the phantom, providing four hyperfixation levels. The effects of angular sampling and reconstruction filters have been tested. The statistical analysis was performed with analysis of variance (ANOVA). This study demonstrates the advantage of ultra-high resolution fan beam collimators for contrast recovery of hyperfixation with SPECT when using 64 projections over 360/spl deg/, in particular when the contrast is low. The authors also demonstrate that fan beam collimators permit smaller size hyperfixation detection.

Design, construction and performance of one-dimensional lead foil parallel plate and fan-beam collimators for gamma ray imaging

Precise manufacturing of lead foil collimators for gamma-ray imaging is critical to obtain acceptable resolution, sensitivity and uniformity. Actual fabrication poses numerous technical challenges because of the mechanical characteristics of lead foil and the requirement to maintain 50 μm tolerances or better over large areas. This paper describes the materials and techniques used to construct precision one-dimensional parallel and converging slice collimators for a full-ring SPECT imaging system. The methods devised are generally applicable in the construction of similar gamma-ray or X-ray lead foil collimators. Materials used include lead foil with 2% antimony and 0.5% tin, a machinable rigid polymethacrylimide foam and thin film adhesive. A carefully controlled fabrication process was developed to maximize the uniformity of the collimator slices. Sheets of precise thickness foam, adhesive and lead foil were precut and bonded using a heated press. A specially designed vacuum chuck was used for cutting and shaping the slices. Tolerances on foam thickness could be held to 50 μm. Finished slices were stacked together on a surface plate with precut adhesive film placed between each slice. A completed stack of finished slices was heated to bond the entire collimator. Three collimators have been constructed to date. Two are parallel plate collimators, one designed for 140 keV 99mTc gamma rays and one designed for 511 keV annihilation radiation. The third is a converging collimator with 2 : 1 magnification designed for imaging 131I. The measured uniformity and resolution are in good agreement with the design objectives and reflect the variance in the material dimensions. The uniformity varied from 5% standard deviation for the 99mTc collimator to 14% standard deviation for the 131I collimator. The intrinsic resolution varied from 8.5 mm FWHM for the 99mTc parallel collimator to 1.9 mm FWHM for the 131I collimator. The sensitivity of the 99mTc collimator is comparable to current dual head SPECT cameras with LEAP collimators, 670 cpm/μCi. The sensitivity of the 131I converging and 511 keV parallel collimators are an order of magnitude less than that of the 99mTc collimator.

First results from a YAP:Ce gamma camera for small animal studies

The YAP (yttrium aluminum perovskite) camera is a novel gamma camera with intrinsic submillimeter spatial resolution and detection efficiency comparable to a standard Anger camera. At the first stage, it is a miniature gamma camera with a field of view of 4/spl times/4 cm/sup 2/ and is currently utilized for radio tracer studies on small animals. The YAP camera consists of a multicrystal array coupled to a position sensitive photomultiplier tube (Hamamatsu R2486) with a parallel hole collimator. The preliminary results are presented and discussed, in particular the intrinsic characteristics of the scintillating array, which are measured, as well as the position linearity and the spatial resolution, with a parallel collimator. Images were obtained from /sup 99m/Tc line source and nude Balb C mice, which have been injected with /sup 99m/Tc MDP, a bone-seeking agent. The observed images show the importance of the small-field YAP camera in radiopharmaceutical research.

Reduction of truncation artifacts in fan beam transmission by using parallel beam emission data

We describe a method which uses the measurement of both photopeak and Compton scatter energy window images from a parallel beam collimation to augment the truncated attenuation map reconstruction in a fan beam transmission system. The method first estimates the body and lung outlines from the reconstructed emission data and truncated attenuation map. Based on the outline information, an assigned attenuation map is created and reprojected to estimate the missed projection data, which are then combined with the truncated projection data for the set of complete data without truncation. Finally, a reconstruction using the combined complete data is performed to obtain the attenuation map with no truncation. We demonstrate that this method can significantly reduce the truncation artifacts in two phantom studies and one patient study. When some portion of the heart walls stays outside the densely sampled region (defined as the region of the object that has no truncation in any projection angle), the attenuation map estimated from this method can more effectively correct for the attenuation in the emission data than the truncated attenuation map.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The geometric modulation transfer function of a transmission imaging system that uses a SPECT scintillation camera and parallel hole collimation

An analytic expression has been derived to calculate the geometric modulation transfer function of a transmission imaging system that uses parallel hole collimation for both the source and the SPECT camera. This expression describes the resolution of the transmission imaging system and replaces the need to use computer intensive Monte Carlo simulations for the system design. The geometric modulation transfer function, denoted as MTFg(rho) = [A2sc(Ssc rho)**A2cc(Scc rho)]D(rho), where ** denotes two-dimensional convolution; Asc(rho) and Acc(rho) are the Fourier transforms (FT) of the aperture functions for the parallel hole source collimator (SC) and the camera collimator (CC) holes, respectively; D(rho) is the FT of the camera response; and ssc and scc are scaling constants that depend on the respective collimator dimensions, the system dimensions, the object distance above camera collimator and whether MTFg(rho) is calculated for the object or image plane. The theoretical MTFg(rho) was verified with Monte Carlo simulations and experimental results. The formalism shows that the system resolution is characterized by the camera resolution and a combination of the resolutions of the source and camera collimators. This expression can be used to optimize the design of transmission imaging systems to be used in nuclear medicine.

A high energy gamma camera using a multiple hole collimator and PSPMT

A high energy gamma camera using a parallel hole tungsten collimator and a 7.62 cm square Position Sensitive Photomultiplier Tube (PSPMT) has been assembled and tested. The measured energy resolution is 12–20% FWHM for 137Cs (662 keV) gamma rays, and 17–23% FWHM for 57Co (122 keV). The measured spatial resolution measured is 2.8–3.2 mm for 122 keV and 3.4–5.3 mm for 662 keV. The variation in the energy and spatial resolution is due to the continuously varying gain across the PSPMT. Measured energy spectra and images obtained are presented.

Monte Carlo simulation of transmission studies using a planar source with a parallel collimator and a line source with a fan-beam collimator

A knowledge of the distribution of the attenuation coefficient is generally required for a successful correction of attenuation in nonhomogeneous regions. This can be accomplished by performing transmission measurement with either parallel or fan-bean collimation. The Monte Carlo method makes it possible to fully evaluate the results of such a correction. This study shows results from such simulations for both parallel and fan-beam transmission imaging. Also, examples of the effect of down-scatter from an emission /sup 99m/Tc radionuclide into the energy window for a transmission /sup 153/Gd radionuclide are shown.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Using simultaneous transmission and scatter SPECT imaging from external sources for the determination of the thoracic /spl mu/-map

The authors describe a new technique that uses simultaneous transmission and scatter SPECT imaging for determination of segmented attenuation /spl mu/-maps. These /spl mu/-maps could provide the basis for attenuation correction and quantification of radiopharmaceutical uptake in cardiac SPECT. For transmission imaging, a line source and a fan-beam collimator on one head of a triple-head SPECT system are used. For scatter imaging, two external ring sources and parallel collimators on one or two of the other heads are used. The fan-beam transmission data are truncated, while the parallel-beam data are not. The proposed technique uses both sets of tomographic data to complement each other and to obtain a composite image. A /spl mu/-map with three segmented regions of distinct /spl mu/-values could be derived from the composite image. This final /spl mu/-map can then be used as the basis for attenuation correction.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Monte Carlo modelling of a multiple-hole collimator for high energy gamma-ray imaging

To develop a second generation gamma-ray camera for industrial applications, various multiple-hole collimator designs that meet high-energy, high-intensity and far-field imaging requirements have been studied. The collection of parallel holes each parallel to one another (parallel-hole, parallel collimator) is ineffective for imaging high energy far-field sources, whereas a collimator with parallel holes arranged in a diverging manner (parallel-hole, diverging collimator) is a good candidate for our imaging system. The response of a proposed collimator to different source conditions is modelled and presented in this work. >

Information analysis for 3D SPECT imaging of radionuclide distributions in attenuating objects

The existing communication model of SPECT studies is extended by including the attenuating medium in the communication channel. Based on the extended model two fundamental theorems are presented to demonstrate the significance of the mutual information in system design and imaging reconstruction. In addition, the authors present a mutual information formula for SPECT systems to image radionuclide distributions in attenuating objects using single photon emission computed tomography (SPECT) collimators, The formula is, in turn, expressed in terms of the source distribution, attenuation coefficients, and collimator parameters such as collimator thickness, hole size, septal thickness, hole shape factor, and focal length. The polyhedral collimator is employed for the authors' mutual information analysis in this report. Based on imaging surface decomposition, the analysis of the polyhedral collimator is reduced to the analysis of a single-headed collimator for planar imaging. Conventional collimators including parallel and cone beam collimators are used for the analysis of the single-headed collimator. The information loss due to attenuation is evaluated for myocardial imaging using technetium-99m and thalium-201.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Quantification of technetium-99m hexamethylpropylene amine oxime brain uptake in routine clinical practice using calibrated point sources as an external standard: phantom and human studies

Quantitative methods for calculation of regional cerebral blood flow with technetium-99m hexamethylpropylene amine oxime (99mTc-HMPAO) have been proposed. These methods are very labour intensive and therefore are not useful in routine clinical practice. We describe a simple alternative method, using calibrated point sources as a scaling factor, whereby the tomographic slices are displayed as regional 99mTc-HMPAO brain uptake per cm3 brain tissue in 10(-6) of the injected lipophilic dose. The method was validated on Jaszczak and Hoffman phantoms using a three-detector system with HR parallel and HR fan-beam collimators. Under the optimal conditions described in this paper, the measured to real activity ratio was 1.00 (SD = 0.06). The reproducibility of the cerebellar uptake in a group of ten normal volunteers and five patients was studied. Intra-individually a mean deviation of 12.6% was observed for the total group. For those persons with a heart rate difference of less than 5 units between the two studies, a mean deviation of 7.2% was obtained. Quantitative 99mTc-HMPAO brain uptake images can be useful for longitudinal studies, especially for follow-up, activation and pharmacological studies.

Simultaneous SPECT and CT with shutter controlled radionuclide sources and parallel collimator geometry

A method for combined single photon emission computed tomography (SPECT) and transmission tomography has been developed for application to parallel collimator geometry. The technique is based on a specially designed, shutter operated device with radionuclide line sources for the transmission studies. The present device operates with four 1-GBq, 40-cm-long line sources of /sup 99/Tc/sup m/. Using the same radionuclide for SPECT, emission data are acquired with the shutter closed. At the same angle, transmission data are subsequently acquired with the shutter open. The total acquisition time is prolonged and the transmission data must be corrected before reconstruction. Using different emission and transmission photon energies, the shutter may be open during most of the SPECT acquisition time and closed only for collecting crosstalk correction data. Experiments are performed with a transmission source with lower photon energy, 0.8 GBq /sup 153/Gd line sources (97 and 103 keV). Simulations of transmission data indicate that the activity of /sup 153/Gd might be about 2.5 times that of /sup 99/Tc/sup m/ and about six times higher for /sup 241/Am for the same image noise. Simulations show that the total activity in uniform line sources may be reduced without increasing the noise by choosing line sources with an activity distribution that is averaged inversed to the photon transmission through the object.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Frequency domain implementation of the three-dimensional geometric point response correction in SPECT imaging

Analytical expressions exist for the geometric point response functions for parallel, fan, and cone beam collimators. The geometric point response functions are shown to be shift-invariant for these three collimators if the distance between the point source and the detection plane is constant. For single photon emission computed tomography (SPECT), this property is used in the projector/backprojector to correct for geometric distortion using the iterative EM reconstruction algorithm. In the implementation, the system geometric response, the crystal intrinsic response and the attenuation were compensated. The attenuation compensation was accomplished in the spatial domain. The point response corrections were accomplished using frequency domain filtering, taking advantage of the shift-invariance of the system point response function at a fixed depth. The frequency domain implementation significantly speeded up the algorithm. Phantom studies showed the increase of resolution and the reduction of density nonuniformity artifacts which were due to the spatially varying system response function and the attenuation effect.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An in situ x-ray diffraction method for the structure of amorphous thin films using shallow angles of incidence

A technique for the structural characterization of thin amorphous films employing synchrotron radiation parallel beam x-ray optics at grazing angles of incidence is detailed. At incident angles near to the critical angle for total external reflection, sampling of specimens may be achieved via the evanescent mode. The parallel beam geometry allows the use of a technique in which a 2θ detector, incorporating a parallel plate collimator, scans diffraction data for a given incident angle. For a specified wavelength, the incident angle chosen will determine the penetration of the radiation into the sample (∼10–1000 Å). The data must be corrected for significant peak shifting resulting from x-ray refraction, as well as for the effects associated with conventional θ:2θ scans. Preliminary data resulting from the first application of this technique to amorphous hydrogenated silicon:carbon thin films, deposited onto crystalline silicon substrates, will be presented and discussed. Conventional θ:2θ powder diffraction data will also be presented as a comparative standard.

Images obtained with a compact gamma camera

A design for a compact gamma camera based on the use of a position-sensitive photomultiplier is presented. Tests have been carried out on a prototype detector system, having a sensitive area of 25 cm 2, using both a simple pinhole aperture and a parallel collimator. Images of a thyroid phantom are presented, and after processing to reduce the artefacts introduced by the use of a pinhole aperture, the quality is compared with that obtained using a standard Anger camera.