Conventional Parallel-hole Collimator Research Articles

Integration of advanced 3D SPECT modelling for pinhole collimators into the open-source STIR framework

Single-photon emission computed tomography (SPECT) systems with pinhole collimators are becoming increasingly important in clinical and preclinical nuclear medicine investigations as they can provide a superior resolution-sensitivity trade-off compared to conventional parallel-hole and fanbeam collimators. Previously, open-source software did not exist for reconstructing tomographic images from pinhole-SPECT datasets. A 3D SPECT system matrix modelling library specific for pinhole collimators has recently been integrated into STIR—an open-source software package for tomographic image reconstruction. The pinhole-SPECT library enables corrections for attenuation and the spatially variant collimator–detector response by incorporating their effects into the system matrix. Attenuation correction can be calculated with a simple single line-of-response or a full model. The spatially variant collimator–detector response can be modelled with point spread function and depth of interaction corrections for increased system matrix accuracy. In addition, improvements to computational speed and memory requirements can be made with image masking. This work demonstrates the flexibility and accuracy of STIR’s support for pinhole-SPECT datasets using measured and simulated single-pinhole SPECT data from which reconstructed images were analysed quantitatively and qualitatively. The extension of the open-source STIR project with advanced pinhole-SPECT modelling will enable the research community to study the impact of pinhole collimators in several SPECT imaging scenarios and with different scanners.

Point-spread-function enhancement via designing new configuration of collimator in nuclear medicine

BackgroundThe collimators in which the various geometrical configurations have been suggested to optimize the sensitivity and resolution have a key role in acquiring the qualified images in nuclear medicine towards a better recognition of some diseases. MethodsIn this study, a new configuration as a geometrical combination of the conical, cylindrical and spherical (CCS) volumes for parallel-hole collimators (PCs) which is investigated by using the volumetric-parametric method has been introduced to enhance point spread function (PSF). The obtained PSF is evaluated via collimators response, which is derived from a radioactive point source. The configuration was simulated by MCNPX code at different amounts of energy from the point source together with the traditional collimator which contained only the cylindrical volume. ResultsThe PSF will transmogrify from a delta function to a distribution that can correlate to a Gaussian distribution while increasing the scattered gamma rays. The simulation results shown that the PSF in CCS configuration is narrower than that of the cylindrical one at all the energies, which led to an enhancement in the resolution. Also, the theoretical results were agreement with the simulated ones. The higher the energy amount of the source, the wider the PSF becomes due to the higher penetration and scattering potency. Also, the percentage of scattering and penetration (S+P)% of the CCS collimator was less than the conventional PC one because of more undesirable photons elimination in the annexed conical-spherical denticles. The better the qualified image had a narrower PSF. ConclusionThe suggested geometry and method may also be used to determine the exact attenuation coefficient of absorbers.

The clinical utilities of multi-pinhole single photon emission computed tomography.

Single photon emission computed tomography (SPECT) is an important imaging modality for various applications in nuclear medicine. The use of multi-pinhole (MPH) collimators can provide superior resolution-sensitivity trade-off when imaging small field-of-view compared to conventional parallel-hole and fan-beam collimators. Besides the very successful application in small animal imaging, there has been a resurgence of the use of MPH collimators for clinical cardiac and brain studies, as well as other small field-of-view applications. This article reviews the basic principles of MPH collimators and introduces currently available and proposed clinical MPH SPECT systems.

Compared vulnerabilities to small cardiac motions between different cameras used for myocardial perfusion imaging

This phantom-based study was aimed to determine whether cardiac CZT-cameras, which provide an enhanced spatial resolution and image contrast compared to Anger cameras, are similarly affected by small cardiac motions. Translations of a left ventricular (LV) insert at half-SPECT acquisitions through six possible orthogonal directions and with 5- or 10-mm amplitude were simulated on the Discovery NM-530c and DSPECT CZT-cameras and on an Anger Symbia T2 camera equipped with an astigmatic (IQ.SPECT) or conventional parallel-hole collimator (Conv.SPECT). SPECT images were initially reconstructed as currently recommended for clinical routine. The heterogeneity in recorded activity from the 17 LV segments gradually increased between baseline and motions simulated at 5- and 10-mm amplitudes with all cameras, although being higher for Anger- than CZT-cameras at each step and resulting in a higher mean number of artifactual abnormal segments (at 10-mm amplitude, Conv.SPECT: 3.7; IQ.SPECT: 1.8, Discovery: 0.7, DSPECT: 0). However, this vulnerability to motion was markedly (1) decreased for Conv.SPECT reconstructed without the recommended Resolution Recovery algorithm and (2) increased for DSPECT reconstructed without the recommended cardiac model. CZT-cameras and especially the DSPECT appear less vulnerable to small cardiac motions than Anger-cameras although these differences are strongly dependent on reconstruction parameters.

Evaluation of tomographic image quality of extended and conventional parallel hole collimators using maximum likelihood expectation maximization algorithm by Monte Carlo simulations.

One of the major problems associated with parallel hole collimators (PCs) is the trade-off between their resolution and sensitivity. To solve this problem, a novel PC - namely, extended parallel hole collimator (EPC) - was proposed, in which particular trapezoidal denticles were increased upon septa on the side of the detector. In this study, an EPC was designed and its performance was compared with that of two PCs, PC35 and PC41, with a hole size of 1.5 mm and hole lengths of 35 and 41 mm, respectively. The Monte Carlo method was used to calculate the important parameters such as resolution, sensitivity, scattering, and penetration ratio. A Jaszczak phantom was also simulated to evaluate the resolution and contrast of tomographic images, which were produced by the EPC6, PC35, and PC41 using the Monte Carlo N-particle version 5 code, and tomographic images were reconstructed by using maximum likelihood expectation maximization algorithm. Sensitivity of the EPC6 was increased by 20.3% in comparison with that of the PC41 at the identical spatial resolution and full-width at tenth of maximum here. Moreover, the penetration and scattering ratio of the EPC6 was 1.2% less than that of the PC41. The simulated phantom images show that the EPC6 increases contrast-resolution and contrast-to-noise ratio compared with those of PC41 and PC35. When compared with PC41 and PC35, EPC6 improved trade-off between resolution and sensitivity, reduced penetrating and scattering ratios, and produced images with higher quality. EPC6 can be used to increase detectability of more details in nuclear medicine images.

Design and evaluation of two multi-pinhole collimators for brain SPECT

SPECT is a powerful tool for diagnosing or staging brain diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD) but is limited by its inferior resolution and sensitivity. At the same time, pinhole SPECT provides superior resolution and detection efficiency trade-off as compared to the conventional parallel-hole collimator for imaging small field-of-view (FOV), which fits for the case of brain imaging. In this study, we propose to develop and evaluate two multi-pinhole (MPH) collimator designsto improve the imaging of cerebral blood flow and striatum. We set the target resolutions to be 12 and 8mm, respectively, and the FOV at 200mm which is large enough to cover the whole brain. The constraints for system optimization include maximum and minimum detector-to-center-of-FOV (CFOV) distances of 344 and 294mm, respectively, and minimal radius-of-rotation (ROR) of 135mm to accommodate patients' shoulder. According to the targeted FOV, resolutions, and constraints, we determined the pinhole number, ROR, focal length, apertureacceptance angle, and aperturediameter which maximized the system sensitivity. We then assessed the imaging performance of the proposedMPH and standard low-energy high-resolution (LEHR) collimators using analytical simulations of a digital NCAT brain phantom with 99mTc-HMPAO/99mTc-TRODAT-1 distributions; Monte Carlo simulations of a hot-rod phantom; and a Defrise phantom using GATE v6.1. Projections were generated over 360° and reconstructed using the 3D MPH/LEHR OS-EM methods with up to 720 updates. The normalized mean square error (NMSE) was calculated over the cerebral and striatal regions extracted from the reconstructed images for 99mTc-HMPAO and 99mTc-TRODAT-1 simulations, respectively, and average normalized standard deviation (NSD) based on 20 noise realizations was assessed on selected uniform 3D regions as the noise index. Visual assessment and image profiles were applied to the results of Monte Carlo simulations. The optimized design parameters of the MPH collimators were 9 pinholes with 4.7 and 2.8mm pinhole diameter, 73° acceptance angle, 127mm focal length, 167mm ROR for 12mm and 8mm target resolution, respectively. According to the optimization results, the detection efficiencies of the proposed collimators were 270 and 40% more as compared to LEHR. The Monte Carlo simulations showed that 7.9 and 6.4mm rods can be discriminated for the MPH collimators with target resolutions of 12 and 8mm, respectively. The eight 12mm-thick discs of the Defrise phantom can also be resolved clearly in the axial plane as demonstrated by the image profiles generated with the MPH collimators. The two collimator designs provide superior image quality as compared to the conventional LEHR, and shows potential to improve current brain SPECT imaging based on a conventional SPECT scanner.

Advances in pinhole and multi-pinhole collimators for single photon emission computed tomography imaging.

The collimator in single photon emission computed tomography (SPECT), is an important part of the imaging chain. One of the most important collimators that used in research, preclinical study, small animal, and organ imaging is the pinhole collimator. Pinhole collimator can improve the tradeoff between sensitivity and resolution in comparison with conventional parallel-hole collimator and facilities diagnosis. However, a major problem with pinhole collimator is a small field of view (FOV). Multi-pinhole collimator has been investigated in order to increase the sensitivity and FOV with a preserved spatial resolution. The geometry of pinhole and multi-pinhole collimators is a critical factor in the image quality and plays a key role in SPECT imaging. The issue of the material and geometry for pinhole and multi-pinhole collimators have been a controversial and much disputed subject within the field of SPECT imaging. On the other hand, recent developments in collimator optimization have heightened the need for appropriate reconstruction algorithms for pinhole SPECT imaging. Therefore, iterative reconstruction algorithms were introduced to minimize the undesirable effect on image quality. Current researches have focused on geometry and configuration of pinhole and multi-pinhole collimation rather than reconstruction algorithm. The lofthole and multi-lofthole collimator are samples of novel designs. The purpose of this paper is to provide a review on recent researches in the pinhole and multi-pinhole collimators for SPECT imaging.

Comparison of a newly-designed stack-up collimator with conventional parallel-hole collimators in pre-clinical CZT gamma camera systems: a Monte Carlo simulation study

Recently, many studies have been conducted with pixelated semiconductor detectors that use cadmium zinc telluride (CZT) because these detectors have many advantages in pre-clinical gamma imaging. Collimators play an extremely important role in the imaging performance of pixelated semiconductor gamma cameras. In our previous study, based on the pixelated semiconductor gamma camera system we recommended the use of a pixelated parallel-hole collimator with equal hole and pixel sizes; this approach improved both the sensitivity and the spatial resolution. However, the pixelated parallel-hole collimator had two major limitations: (a) Although its sensitivity was higher than that of pinhole systems, the pixelated parallel-hole collimator may have still resulted in a partial loss of sensitivity due to its small collimator hole size. (b) The pixelated parallel-hole collimator with an adequate septal height was difficult to manufacture due to its small holes. Here, we present a new design for a parallel-hole collimator, which uses the stack-up method and a CZT pixelated semiconductor gamma camera system. The purpose of this study was to compare the performances of various geometric designs of our newly-designed parallel-hole collimator with those of conventional parallel-hole collimators [low-energy high-resolution (LEHR) and low-energy high-sensitivity (LEHS)]. The detector was modeled as an eValuator-2500 (eV Microelectronics Inc., Saxonburg, PA, USA) (3-mm thick, 0.5-mm pixel size) by using a Geant4 Application for Tomographic Emission (GATE) simulation. The proposed parallel-hole collimator consisted of two overlapping parallel-hole collimators. The size of each hole in the proposed parallel-hole collimator was four times that of the hole in the pixelated parallel-hole collimator. The overlap ratios of these collimators were 1 : 1, 1 : 2, 2 : 1, 1 : 5, and 5 : 1. To evaluate and compare the performances of these systems, we evaluated the sensitivity and the spatial resolution of each system. The average sensitivities of the proposed parallel-hole collimators with overlap ratios of 1 : 1, 1 : 2 or 2 : 1, and 1 : 5 or 5 : 1 and of the LEHS conventional parallel-hole collimator were 2.17, 3.45, 13.90, and 3.68 times higher than the average sensitivities of the LEHR conventional parallel-hole collimator, respectively. The average spatial resolution also varied depending on the distance from the collimator. In conclusion, we successfully designed a novel parallel-hole collimator for pre-clinical imaging; this novel collimator employs the stack-up method with a CZT pixelated semiconductor gamma camera.

Will high-resolution/high-sensitivity SPECT ensure that PET is not the only survivor in nuclear medicine during the next decade?

Dear Sir, In a recent issue of the journal, Alavi and Basu [1] on the one hand, and Mariani et al. [2] on the other, debated the future of planar scintigraphy and single photon emission computed tomography (SPECT) in the face of ever better performing positron emission tomography (PET). The pros and cons for the use of single photon emitters, positron emitters and the related tracers were thoroughly discussed. It is, however, our opinion that the discussion relating to the physics and technology of SPECT was biased by the sidestepping of the most recent advances in SPECT, although some points were briefly raised by Mariani et al. [2]. From the beginning, PET was developed as a fully tomographic technique [3]. In the pioneering studies, the detectors, although very limited in number in those early times, were already being placed around the patient’s body. Breakthroughs in crystal technology, electronics, attenuation correction, scatter correction and tomographic reconstruction (Fourier rebinning, 2-D and 3-D iterative reconstruction including resolution and/or time-of-flight consideration) have allowed us to benefit from all the information collected by the detectors. This has led to the modern PET scanners with annular detectors and an axial field-of-view of 15–25 cm with full quantitative 3-D tomographic capabilities [4, 5]. Meanwhile, SPECT continued to be performed with a rotating parallel collimated Anger camera. The only major advances were the dual-head and triple-head cameras, which increased the sensitivity, and the iterative reconstruction algorithms, which improved the overall image quality [6]. Since the beginning of the 1990s, considerable effort has been focused on developing high-resolution SPECT systems by replacing the traditional parallel hole collimators with converging collimators such as fan-beams, cone-beams, cardio-focals, slit-slat collimators and, last but not least, pinholes [7]. Pinhole SPECT was primarily studied in the context of small-animal imaging [8, 9]. In the beginning, a single pinhole mounted on a commercial Anger camera was used [8]. Systems with multiple pinholes with or without the multiplexing of the projections were quickly developed and finally nonrotating systems with a large number of small pinholes came onto the market [9]. Such systems combined high resolution, high sensitivity and collection of the data needed for tomographic reconstruction, without the need to rotate the detectors. This meant that dynamic studies became as easy in SPECT as they were in PET, and with an even better spatial resolution [9]. Meanwhile, a few studies were being devoted to the application of single pinhole SPECT to humans. They demonstrated the feasibility of pinhole SPECT studies of limited volumes of interest (VOI) under not too restrictive conditions: projections could only be collected over 180° around the VOI [10, 11]; the distance from the pinhole aperture to the VOI centre did not necessarily need to be kept constant [12]; the pinhole could be moderately tilted [10–12]; and the detector uniformity requirements were shown to be comparable to those needed for SPECT with conventional parallel hole collimators [13]. Moreover, the introduction of resolution recovery in the iterative reconstruction process was shown to be dramatically effective both in improving the resolution and lowering the noise content of the images [14]. The studies were not limited to the technical feasibility of pinhole SPECT in humans Eur J Nucl Med Mol Imaging (2009) 36:533–535 DOI 10.1007/s00259-008-1026-4

3D RBI-EM reconstruction with spherically-symmetric basis function for SPECT rotating slat collimator

A single photon emission computed tomography (SPECT) rotating slat collimator with strip detector acquires distance-weighted plane integral data, along with the attenuation factor and distance-dependent detector response. In order to image a 3D object, the slat collimator device has first to spin around its axis and then rotate around the object to produce 3D projection measurements. Compared to the slice-by-slice 2D reconstruction for the parallel-hole collimator and line integral data, a more complex 3D reconstruction is needed for the slat collimator and plane integral data. In this paper, we propose a 3D RBI-EM reconstruction algorithm with spherically-symmetric basis function, also called ‘blobs’, for the slat collimator. It has a closed and spherically symmetric analytical expression for the 3D Radon transform, which makes it easier to compute the plane integral than the voxel. It is completely localized in the spatial domain and nearly band-limited in the frequency domain. Its size and shape can be controlled by several parameters to have desired reconstructed image quality. A mathematical lesion phantom study has demonstrated that the blob reconstruction can achieve better contrast-noise trade-offs than the voxel reconstruction without greatly degrading the image resolution. A real lesion phantom study further confirmed this and showed that a slat collimator with CZT detector has better image quality than the conventional parallel-hole collimator with NaI detector. The improvement might be due to both the slat collimation and the better energy resolution of the CZT detector.

A fast algorithm for computer aided collimation gamma camera (CACAO)

The computer aided collimation gamma camera is aimed at breaking down the resolution sensitivity trade-off of the conventional parallel hole collimator. It uses larger and longer holes, having an added linear movement at the acquisition sequence. A dedicated algorithm including shift and sum, deconvolution, parabolic filtering and rotation is described. Examples of reconstruction are given. This work shows that a simple and fast algorithm, based on a diagonal dominant approximation of the problem can be derived. Its gives a practical solution to the CACAO reconstruction problem.

Optimal design of planar-concave collimators for SPECT - an analytical approach

In this paper we present an analytical tool for the design and optimization of planar-concave collimators for SPECT. We conclude that a single general planar-concave collimator that eliminates the non-isotropic blurring for all SPECT applications does not exist. On the other hand, it is possible to achieve pseudo-optimal collimators for different clinical applications by a careful choice of the design parameters. By classifying the clinical applications into two groups, for instance body and brain studies, the non-isotropic blurring for most SPECT situations may be dramatically reduced by means of planar-concave collimators. The results based on Monte Carlo simulations show that the shape of the point source response function of the reconstructed image obtained from the planar-concave collimator is more isotropic than that obtained from a conventional parallel-hole collimator. Specifically, the ratio of the FWHM of the reconstructed point response function in the radial and tangential direction is increased from 0.5 for the parallel-hole to 0.85 for the planar-concave collimator for sources 200 mm away from the centre of rotation.

Evaluation of optimally designed planar-concave collimators in single-photon emission tomography.

The imaging properties of optimally designed planar-concave (PC) collimators were evaluated by means of Monte Carlo simulations. The evaluation was done with respect to total system spatial resolution and the overall image noise distribution in single-photon emission tomography. The results showed that the non-isotropy with PC collimators, assessed by the ratio of the full-width at half-maximum in the radial and tangential directions, was reduced by about 60% as compared with a conventional parallel-hole collimator for sources located 200 mm away from the centre of rotation. Furthermore, the image noise distribution along the object radius became more uniform when the curved collimator was used. The maximum increase in noise due to use of the curved collimator was about 45% close to the edge of the phantom, where the hole length was about 3 times longer. We also showed with Monte Carlo simulations that the spatial resolution of the lateral cortex when using the curved collimator was significantly improved due to improved radial resolution.

Simultaneous SPECT and CT with an opposed dual head gamma camera system and conventional parallel hole collimators

All examinations aiming at quantitative assessment of activity concentrations suffer from non-uniform attenuation due to inhomogeneous tissue density in various body regions. In such applications, a combined transmission tomography with SPECT has demonstrated great clinical advantages since it allows a pixel by pixel correction for attenuation based on measured data. The authors have developed a method for transmission studies that is applicable to dual head detector systems with conventional parallel-hole collimators. A scanning line-source (Am-241) is mounted close to one camera head with its beam directed towards the other head. Two energy discrimination windows are used for each camera head. The camera-head near to the transmission source registrates SPECT-data in one window and the spill-over of scattered emission photons into the Am-window. The opposed camera registrates SPECT-data in one window and the transmission data from the Am-source in another window. Post acquisition corrections are made for differences in camera uniformity and for loss of SPECT-data due to shadowing of the field-of-view during the transverse movement of the transmission device (about 5%). The height of the collimated slit source is only 30 mm to allow a setting of the camera head with the transmission device close to the patient.

Evaluation of a pinhole collimator for I-131 SPECT head imaging

The performance of a pinhole collimator for I-131 SPECT of the head was evaluated. The evaluation included planar and SPECT spatial resolution, sensitivity in air and in water, septal penetration, reconstructed image quality, and activity quantitation within a simple phantom that models tumor uptake in the head. The pinhole collimator was compared to medium and high energy parallel hole collimators. The pinhole collimator demonstrated improved resolution/sensitivity tradeoff compared with the parallel hole collimators over the range of distances relevant to head imaging. In planar point source images the pinhole collimator showed reduced penetration effects although in reconstructed images penetration effects were not apparent for either pinhole or high energy parallel hole collimators. Comparable activity quantitation accuracy was observed with all collimators. The accuracy was dependent on the segmentation threshold and calibration procedure. These results indicate that the pinhole collimator can provide improved performance conventional parallel hole collimators for I-131 imaging in the head.

Surgical probe design for a coincidence imaging system without a collimator

Surgical probes used in nuclear medicine can be maneuvered close to a suspected tumor site, thereby achieving higher resolution and sensitivity than external gamma cameras. Tomography with handheld probes is not possible, however, so it is difficult to determine whether an increase in the photon count rate is caused by features near the probe or by variations in the distant background radiation. Our group has experimented with several probe designs that address this problem. This paper describes a probe that images nearby objects without a collimator, and which is insensitive to inhomogeneities in the distant background level. The probe, which is used with a radionuclide that emits multiple photons per decay, such as 111 In, consists of a collimatorless array of gamma-ray detectors connected by coincidence circuitry. We used a Monte Carlo routine to simulate data collection from such a system and reconstructed the images using the pseudoinverse obtained by singular value decomposition. The images show a significant suppression of distant sources when compared to a probe equipped with a conventional parallel-hole collimator.

Monte Carlo modelling of the performance of a rotating slit-collimator for improved planar gamma-camera imaging

The authors propose a rotating slit-collimator which collects one-dimensional projections from which the planar image may be reconstructed by the theory of computed tomography. The performance of such a collimator is modelled by Monte Carlo methods and images are reconstructed by a convolution and backprojection technique. The performance is compared with that of a conventional parallel-hole collimator and it is shown that higher spatial resolution with increased sensitivity is possible with the slit-collimator. For a point source a spatial resolution of some 6 mm at a distance of 100 mm from the collimator with a *7 sensitivity compared with a parallel-hole collimator was achieved. Applications to bone scintigraphy are modelled and an improved performance in hot-spot imaging is demonstrated. The expected performance in cold-spot imaging is analytically investigated. The slit-collimator is not expected to improve cold-spot imaging. Practical design considerations are discussed.

Rotating conventional gamma camera single-photon tomographic system: physical characterization.

A whole-body single-photon emission computed tomography system has been evaluated in terms of its physical performance. It consists of a standard GE 400 T maxi camera rotating on a ring stand, coupled to an Informatek Simis 3 computer system. In its standard mode of operation, 64 or 128 successive views of 64 X 64 or 128 X 64 matrices are collected at regular angular samples. The reconstruction of up to 64 possible adjacent transverse sections, sorted into coronal, sagittal, and oblique sections, is obtained by filtered backprojection. A conventional parallel-hole collimator is used. The uniformity of the camera field of view is corrected by the GE hardware module. The resolution in the transverse plane is typically of 15.5 mm in a 20 cm Lucite phantom, and independent of radial distance. The physical slice thickness is 19 mm (full width at half maximum of the longitudinal response using a point source) with only slight variations along the diameter of the image. The sensitivity is approximately 7,000 cps mCi-1 as measured using a 99m Tc thin source, enabling images of reasonable signal-to-noise ratio to be obtained in 10 min. The effect of the energy window, the number of angular samples, and the linear sampling modes on the transverse resolution and contrast is measured and discussed.

Whole-body single-photon emission computed tomography using dual, large-field-of-view scintillation cameras

A whole-body single-photon emission computed tomography system (SPECT) consisting of two large-field-of-view scintillation cameras mounted on a rotatable gantry, a minicomputer and a display station has been designed, constructed and evaluated. In its usual mode of operation, eleven contiguous transverse sections, each 12.5 or 25 mm thick, are reconstructed from projection data acquired during a single, continuous 360 degrees rotation lasting from 2 to 22 min. A generalised filtered and weighted backprojection algorithm is used to reconstruct data obtained with conventional parallel-hole collimators in the case of body scanning, or with specially designed fan beam collimators in the case of centrally positioned organs. A simple yet effective, correction is used to compensate for the effects of gamma ray attenuation within the patient.

Single Photon Emission Computed Tomography Using Multi-Slice Fan Beam Collimators

Scintillation camera systems using parallel collimation have been used for single-photon emission computed tomography (SP/ECT). A major problem is the limited resolution of present parallel-hole collimators. We have built and evaluated collimators for a SP/ECT system, consisting of two opposing LFOV scintillation cameras and a gantry, which have parallel collimation along the cranial-caudal axis of the patient and converging collimation in a direction perpendicular to this axis. The collimator converges to a line of focus located 58 cm from the collimator face and parallel to the cranial-caudal axis of the patient. Two converging collimators combined with opposing LFOV cameras are capable of simultaneously imaging 11 contiguous, 2.6 (or 1.3) cm thick, transverse sections from data collected during a single revolution of the cameras about the patient. A fan beam convolution algorithm reconstructs the transverse slices and provides partial compensation for ?-ray attenuation. It is concluded that the fan-beam collimator system, when compared with conventional parallel-hole collimator systems, produces simultaneous improvements in SP/ECT system sensitivity and resolution. This results in improved visualization of small centrally located organs.