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For multipinhole single-photon emission computed tomography (SPECT), iterative reconstruction algorithms are preferred over analytical methods, because of the often complex multipinhole geometries and the ability of iterative algorithms to compensate for effects like spatially variant sensitivity and resolution. Ideally, such compensation methods are based on accurate knowledge of the position-dependent point spread functions (PSFs) specifying the response of the detectors to a point source at every position in the instrument. This paper describes a method for model-based generation of complete PSF lookup tables from a limited number of point-source measurements for stationary SPECT systems and its application to a submillimeter resolution stationary small-animal SPECT system containing 75 pinholes (U-SPECT-I). The method is based on the generalization over the entire object to be reconstructed, of a small number of properties of point-source responses which are obtained at a limited number of measurement positions. The full shape of measured point-source responses can almost be preserved in newly created PSF tables. We show that these PSFs can be used to obtain high-resolution SPECT reconstructions: the reconstructed resolutions judged by rod visibility in a micro-Derenzo phantom are 0.45 mm with 0.6-mm pinholes and below 0.35 mm with 0.3-mm pinholes. In addition, we show that different approximations, such as truncating the PSF kernel, with significant reduction of reconstruction time, can still lead to acceptable reconstructions. Index Terms--Calibration, pinhole, reconstruction, single-photon emission computed tomography (SPECT).

Channel-edge pinhole designs have been proposed in order to reduce the penetration of gamma rays through the edge of the pinhole aperture. A characterization of penetration and scatter in the pinhole aperture metal can be used in the design of small animal SPECT collimators or for model based corrections during micro-SPECT image reconstruction. In this study the penetration and scatter contributions of micropinholes were compared for Tc-99m, 1-123, and 1-125 for knife-edges and channel-edges. To this end, Geant 4 Monte Carlo simulations of 0.3 mm and 0.5 mm diameter apertures with acceptance angles ranging from 20 to 60 degrees were performed. At perpendicular incidence of the photons, channel pinholes had lower penetration and scatter fractions than did knife-edge pinholes. This advantage disappeared at higher angles of incidence. In addition, the total sensitivity decreased substantially with increasing channel height. Planar projection images of a grid of spheres showed that channel-edge pinholes resulted in a slightly higher spatial resolution than knife-edge pinholes with an equal diameter, when combined with a high-resolution detector. However, the channel-edge pinhole's sensitivity and Contrast Inverse Coefficient of Variation were lower than the knife edge pinhole's at the edges of the detector. We conclude that channel pinholes can result in lower imaging performance when used with non-perpendicular incidence photons because of loss of sensitivity. Index Terms--Monte Carlo simulation, penetration, pinhole geometry, pinhole SPECT, scattering.

Cupping and streak artifacts caused by the detection of scattered photons may severely degrade the quantitative accuracy of cone-beam X-ray computed tomography (CT) images. In order to overcome this problem, we propose and validate the following iterative scatter artifact reduction scheme. First, an initial image is reconstructed from the scatter-contaminated projections. Next, the scatter component of the projections is estimated from the initial reconstruction by a Monte Carlo (MC) simulation. The estimate obtained is then utilized during the reconstruction of a scatter-corrected image. The last two steps are repeated until an adequate correction is obtained. The estimation of the noise-free scatter projections in this scheme is accelerated in the following way: first, a rapid (i.e., based on a low number of simulated photon tracks) MC simulation is executed. The noisy result of this simulation is de-noised by a three-dimensional fitting of Gaussian basis functions. We demonstrate that, compared to plain MC, this method shortens the required simulation time by three to four orders of magnitude. Using simulated projections of a small animal phantom, we show that one cycle of the scatter correction scheme is sufficient to produce reconstructed images that barely differ from the reconstructions of scatter-free projections. The reconstructions of data acquired with a charge-coupled device based micro-CT scanner demonstrate a nearly complete removal of the scatter-induced cupping artifact. Quantitative errors in a water phantom are reduced from around 12% for reconstructions without the scatter correction to 1% after the proposed scatter correction has been applied. In conclusion, a general, accurate, and efficient scatter correction algorithm is developed that requires no mechanical modifications of the scanning equipment and results in only a moderate increase in the total reconstruction time. Index Terms--Artifact removal, computed tomography, cone beam, image reconstruction, medical imaging, Monte Carlo simulation, scatter.

Quantitative accuracy of single-photon-emission computed tomography (SPECT) images is highly dependent on the photon scatter model used for image reconstruction. Monte Carlo simulation (MCS) is the most general tool for accurate modeling and correction of scatter, but calculations are notoriously slow. Recently, we proposed an efficient strategy for fully three-dimensional (3-D) statistical reconstruction using highly accelerated MCS as a forward projector. We use convolution forced detection (CFD) which accelerates convergence of reprojection by about two orders of magnitude. Here, we investigate how many photon histories during CFD-based MCS need to be calculated to: i) ensure that the noise in the reconstructed image does not increase markedly because of the stochastic nature of MCS reprojections; and ii) to determine the reconstruction time needed. To this end, phantom studies representing Tc-99m cardiac perfusion SPECT, were carried out. We generated reconstructions with different numbers of photon histories during MCS forward projection. Images and profiles were compared for reconstructions with a different number of photon tracks, for reconstructions from different noise realizations using a repeat measurement and for reconstructions with different random seeds for MCS reprojection. We found that [10.sup.5] photon histories per subset is sufficient to produce accurate images; more photons do not show visible improvement. This amount of photons corresponds to a typical reconstruction time below 5 min for a [64.sup.3] volume image on a dual-cpu PC (2.66 GHz), which is sufficiently short to apply such highly accurate reconstruction in clinical routine. Index Terms--Cardiac imaging, image reconstruction, Monte Carlo simulation, scatter, single photon emission computed tomography.

We have developed a compact ultra-high-resolution gamma camera (UGC), based on an electron-multiplying charge-coupled device (EMCCD) coupled to a columnar CsI(Tl) scintillator. The EMCCD is suitable for photon-counting gamma-camera imaging, since it has an extremely low readout noise, even at high frame rates ( Index Terms--Charge-coupled device (CCD), digital signal processor (DSP), electron-multiplying charge-coupled device (EMCCD), gamma camera, molecular imaging, photon-counting, readout electronics, single photon emission computed tomography (SPECT).

Pinhole collimation of gamma rays to image distributions of radiolabeled tracers is considered promising for use in small animal imaging. The recent availability of transgenic mice, coupled with the development of [sup.125]I and [sup.99m]Tc labeled tracers, has allowed the study of a range of human disease models while creating demand for ultrahigh resolution imaging devices. We have developed a compact gamma camera that, in combination with pinhole collimation, allows for accessible, ultrahigh resolution in vivo single photon emission computed tomography (SPECT) imaging of small animals. The system is based on a pixilated array of NaI(TI) crystals coupled to an array of position sensitive photomultiplier tubes (PSPMTs). Interchangeable tungsten pinholes with diameters ranging from 0.5 to 3 mm are available, allowing the camera to be optimized for a variety of imaging situations. We use a three dimensional maximum likelihood expectation maximization (MLEM) algorithm to reconstruct the images. Our evaluation indicates that high quality, submillimeter spatial resolution images can be achieved in living mice. Reconstructed axial spatial resolution was measured to be 0.53, 0.74, and 0.96 mm full width at half maximum (FWHM) for rotation radii of 1, 2, and 3 cm, respectively, using the 0.5-mm pinhole. In this configuration, sensitivity is comparable to that of a high-resolution parallel hole collimator. SPECT images of hot- and cold-rod phantoms and a highly structured monkey brain phantom illustrate that high quality images can be obtained with the system. Images of living mice demonstrate the ability of the system to obtain high-resolution images in vivo. The effect of object size on the quantitative assessment of isotope distributions in an image was also studied. Index Terms--Animal imaging, pin hole collimation, single photon emission computed tomography (SPECT), three-dimensional (3-D) image reconstruction.

Quantitative accuracy of single photon emission computed tomography (SPECT) images is highly dependent on the photon scatter model used for image reconstruction. Monte Carlo simulation (MCS) is the most general method for detailed modeling of scatter, but to date, fully three-dimensional (3-D) MCS-based statistical SPECT reconstruction approaches have not been realized, due to prohibitively long computation times and excessive computer memory requirements. MCS-based reconstruction has previously been restricted to two--dimensional approaches that are vastly inferior to fully 3-D reconstruction. Instead of MCS, scatter calculations based on simplified but less accurate models are sometimes incorporated in fully 3-D SPECT reconstruction algorithms. We developed a computationally efficient fully 3-D MCS-based reconstruction architecture by combining the following methods: 1) a dual matrix ordered subset (DM-OS) reconstruction algorithm to accelerate the reconstruction and avoid massive transition matrix precalculation and storage; 2) a stochastic photon transport calculation in MCS is combined with an analytic detector modeling step to reduce noise in the Monte Carlo (MC)-based reprojection after only a small number of photon histories have been tracked; and 3) the number of photon histories simulated is reduced by an order of magnitude in early iterations, or photon histories calculated in an early iteration are reused. For a 64 x 64 x 64 image array, the reconstruction time required for ten DM-OS iterations is approximately 30 min on a dual processor (AMD 1.4 GHz) PC, in which case the stochastic nature of MCS modeling is found to have a negligible effect on noise in reconstructions. Since MCS can calculate photon transport for any clinically used photon energy and patient attenuation distribution, the proposed methodology is expected to be useful for obtaining highly accurate quantitative SPECT images within clinically acceptable computation times. Index Terms--Dual matrix reconstruction, image reconstruction, single photon emission computed tomography, monte carlo methods, scattering.