No abstract available

[en] A set of contiguous transaxial tomographic sections obtained with a rotating-camera tomograph represents the full three-dimensional distribution of activity within a volume of the body. Tomographic sections in planes other than the origianl transverse plane can be produced from these data merely by resorting the data appropriately. The paper presents a simple and efficient algorithm for producing tomograms of the heart oriented either at right angles to the long axis of the left ventricle, or parallel to it. Tomograms in these orientations have specific advantages for imaging the heart and avoid some of the limitations seen in comparable tomograms obtained by the seven-pinhole technique

[en] As investigators consider more comprehensive measurement models for emission tomography, there will be more choices for the complete-data spaces of the associated expectation-maximization (EM) algorithms for maximum-likelihood (ML) estimation. In this paper, the authors show that EM algorithms based on smaller complete-data spaces will typically converge faster. They discuss two practical applications of those concepts: (1) the ML-IA and ML-IB image reconstruction algorithms of Politte and Snyder which are based on measurement models that account for attenuation and accidental coincidences in positron-emissions tomography (PET), and (2) the problem of simultaneous estimation of emission and transmission parameters. Although the PET applications may often violate the necessary regularity conditions, their analysis predicts heuristically that the ML-IB algorithm, which has a smaller complete-data space, should converge faster than ML-IA. This is corroborated by the empirical findings

[en] Monte-Carlo methods have been applied to the design of a detector suitable for use in a SPECT cylindrically shaped scintillation camera. Included in the study are the calculated detection characteristics of two scintillator materials and the optical performance of several geometric configurations. Results include maps of the light distribution for several rectangular crystal-light guide combinations, a comparison of various approaches to specifying the optical properties of detector surfaces, and estimates of relative light output for various geometries

[en] X-ray detectors based on large-area, hydrogenated amorphous silicon photodiode arrays may be useful for performing transmission CT using an x-ray source to acquire the attenuation map for PET and SPECT. Questions regarding the required noise properties of these detectors for current-integration mode transmission CT are answered in this investigation. A simple noise model is developed and from it the first- and second-order statistics are used to examine the propagation of signal and variance through the reconstruction to estimates of the linear attenuation coefficient. Simulation results for a-Si:H detectors are compared with those from a count mode ECT detector used for transmission scanning. For the relatively undemanding application to ECT attenuation correction, these arrays will be usable

[en] A circular ring tomograph, designated SPRINT, is under development for single photon emission tomography of the human head. Most data to date have been gathered using a single rotating slit inside the detector ring to define ray directions. As an alternative method, we have also investigated the use of multiplexed slit patterns consisting of a pseudorandom binary code. From theoretical considerations, one would expect that the substitution of the code for the single slit would have potential advantages in reducing statistical noise in tomographic images of small objects. It has been our experience that this improvement is marginal at best for the types of source distributions encountered in clinical brain imaging. However, the code shows substantial advantage in suppressing uncorrelated backgrounds and the effects of highly penetrating gamma ray components in the source. (orig.)

[en] Maximum likelihood position estimation of a scintillation event in a gamma-ray imaging detector has been described using two independent one-dimensional estimates in x and y. This approximation is only valid when the sampled light-spread function is separable, and spatial distortion is observed when this constraint is violated. The authors have investigated recursive calculation of the 2-D position estimate using a sequence of one-dimensional estimates. Mean phototube response functions have been computed from Monte Carlo simulations of the light-spread function, and position have been estimated for scintillation events using recursive estimation

[en] In single photon emission nuclear medicine, computed tomography systems that employ only a limited angular range of projection data suffer from blurring in the depth (or z) direction. With simulated data, we show that including a second set of projections taken in an orthogonal direction sharpens the image in the z direction. For heart applications, this sharpening improves the sizing of a simulated 2 cm diameter 'hot' infarct by 39%. It also should improve the contrast of 'cold' infarcts by eliminating blurring from other planes, but here we have found a complication, in that, with the present approximate algorithm, effects from the lack of symmetry in two-view tomography may overshadow benefits from contrast improvement. Further simulation study is called for. (orig.)

[en] Historically in nuclear medicine the acceptance window for total-energy pulse-height has been set to maximize contrast and resolution in the image. Recently, there has been increased emphasis on quantification of digital images to calculate, for example, left ventricular volume or tumor radiation absorbed dose. In this paper, we consider specifically what improvements can be made in emission computed tomography simply by employing a window which is offset to the high-energy side of the photopeak in order to reduce the contribution of Compton scattered gamma-rays. The window is offset so as to reduce the count rate for a source in air by 20%. Two lucite phantoms were measured. One was a short cylinder filled with a uniformly-distributed solution of sup(99m)Tc in water. The other was a head phantom with a 0.6 cm³ 'tumor' containing sup(99m)Tc. Water surrounding the tumor could be nonradioactive or contain a dilute background activity. Absolute calibration was accomplished by imaging the simulated tumor in the air-filled phantom. With the offset window, calculated tumor activity is only 3% and 7% high without and with background respectively compared to 20% and 26% high with the symmetric window. However, total activity of the entire slice is still 37% high event without background. For the cylinder containing uniform activity, the error in specific activity drops from 30% to 18% with the offset window. Therefore, an asymmetric window significantly improves quantification, and for certain cases such as an isolated tumor, may be sufficiently accurate without further correction. (orig.)

[en] In this paper, the authors report on the development of timing technique for estimating the arrival time of weak optical signals such as those produced by a scintillation detector. The method makes optimal use of the timing information contained in multiple measurements of the photodetector output by first estimating the joint likelihood function of the measurements given the pulse arrival time and then applying a maximum-likelihood estimation technique. These measurements can be the time-marks produced by present leading-edge or constant fraction timing methods. For a two-measurement system, the technique can be implemented using a simple lookup table approach. Preliminary applications of a system incorporating two leading-edge threshold triggers to estimating the arrival times of LED pulses demonstrated an improvement in timing performance of 20% over a single leading-edge trigger. The technique is applicable to both fast and slow scintillators, implemented using low-cost hardware, and can operate at event rates encountered in PET