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In a previous simulation study, we demonstrated the feasibility of using coded apertures together with pixelated detectors for small animal SPECT. In this paper, we further explore the potential of this approach with a prototype detector and simulated multipinhole apertures. We also investigated the effect of multiplexing due to overlapped projections on convergence properties, image signal-to-noise ratio (SNR) and spatial resolution. The detector comprises a 48 x 44 array of NaI(TI) crystals, each 1 mm x 1 mm x 5 mm on a 1.25-mm pitch. The crystal array is directly coupled to a Hamamatsu R3941 8 cm position sensitive photomultiplier tube. Multipinhole apertures were simulated by performing repeated SPECT acquisitions of the same object with a single tungsten pinhole translated to different positions in the aperture plane. Image reconstruction is based on a three-dimensional ray driven projector which is an extension of a method described for single pinhole SPECT with a displaced center of rotation. Image estimates are updated using the maximum likelihood expectation maximization (ML-EM) algorithm. The effect of multiplexing was to slow convergence and reduce the achievable SNR by approximately 15% compared with nonmultiplexed data (but the result may be achieved in a fraction of the time). The reconstructed resolution obtained with a resolution phantom was 1.5-mm full width at half maximum and there was no appreciable difference between the resolution of multiplexed and nonmultiplexed data. These results encourage us to develop a prototype coded aperture system for high sensitivity, high resolution small animal SPECT. Index Terms--Coded aperature, iterature reconstruction, single photon emission computed tomography (SPECT), small animal imaging.

Accurate determination of the input function is essential for absolute quantification of physiological parameters in positron emission tomography and single-photon emission computed tomography imaging, but it requires an invasive and tedious procedure of blood sampling that is impractical in clinical studies. We previously proposed a technique that estimates simultaneously kinetic parameters and the input function from the tissue impulse response functions and requires two blood samples. A nonlinear least squares method estimated all the parameters in the impulse response functions and the input function but failed occasionally due to high noise levels in the data, causing an ill-conditioned cost function. This paper investigates the feasibility of applying a Monte Carlo method called simulated annealing to estimate kinetic parameters in the impulse response functions and the input function. Time-activity curves of teboroxime, which is very sensitive to changes in the input function, were simulated based on published data obtained from a canine model. The equations describing the tracer kinetics in different regions were minimized simultaneously by simulated annealing and nonlinear least squares. We found that the physiological parameters obtained with simulated annealing are accurate, and the estimated input function more closely resembled the simulated curve. We conclude that simulated annealing reduces bias in the estimation of physiological parameters and determination of the input function. Index Terms--Impulse response function, input function, kinetic parameter estimation, nonlinear least squares, simulated annealing, single-photon emission computed tomography (SPECT), teboroxime.

Quantitative positron emission tomography (PET) studies provide in vivo measurements of dynamic physiological and biochemical processes in humans. A limitation of PET is an inability to provide precise anatomic localization due to relatively poor spatial resolution when compared to magnetic resonance (MR) imaging. Manual placement of region-of-interest (ROI) is commonly used in clinical and research settings in analysis of PET datasets. However, this approach is operator dependent and time-consuming. A semi- or fully-automated ROI delineation (or segmentation) method offers advantages by reducing operator error/subjectivity and thereby improving reproducibility. In this work, we describe an approach to automatically segment dynamic PET images using cluster analysis and we validate our approach with a simulated phantom study and assess its performance with real dynamic PET data. Our preliminary results suggest that cluster analysis can automatically segment tissues in dynamic PET studies and has the potential to replace manual ROI delineation for some applications. Index Terms--Cluster analysis, functional imaging, positron emission tomography (PET), segmentation.

Methods capable of correcting for head motion in all six degrees of freedom have been proposed for positron emission tomography (PET) brain imaging but not yet demonstrated in human studies. These methods rely on the accurate measurement of head motion in relation to the reconstruction coordinate frame. We present methodology for the direct calibration of an optical motion-tracking system to the reconstruction coordinate frame using paired coordinate measurements obtained simultaneously from a PET scanner and tracking system. We also describe the implementation of motion correction, based on the multiple acquisition frame method originally described by Picard and Thompson, using data provided by the motion tracking system. Effective compensation for multiple six-degree-of-freedom movements is demonstrated in dynamic PET scans of the Hoffman brain phantom and a normal volunteer. We conclude that reduced distortion and improved quantitative accuracy can be achieved with this method in PET brain studies degraded by head movements. Index Terms--Motion correction, motion tracking, positron emission tomography (PET).

We investigated potential sources of bias due to simultaneous emission and transmission (SET) scanning and their effect on parameter estimation in dynamic positron emission tomography (PET) oncology studies. The sources of bias considered include: i) variation in transmission spillover (into the emission window) throughout the field of view, ii) increased scatter arising from rod sources, and iii) inaccurate deadtime correction. Net bias was calculated as a function of the emission count rate and used to predict distortion in [18F]2-fluoro-2-deoxy-D-glucose (FDG) and [11C]thymidine tissue curves simulating the normal liver and metastatic involvement of the liver. The effect on parameter estimates was assessed by spectral analysis and compartmental modeling. The various sources of bias approximately cancel during the early part of the study when count rate is maximal. Scatter dominates in the latter part of the study, causing apparently decreased tracer clearance which is more marked for thymidine than for FDG. The irreversible disposal rate constant, [K.sub.i], was overestimated by 30% for thymidine. We conclude that SET has a potential role in dynamic FDG PET but is not suitable for 11C-labeled compounds. Index Terms - Oncology, positron emission tomography, transmission scanning.

Coded apertures provide a substantial gain in detection efficiency compared with conventional collimation and are well suited to imaging small volumes. In this work, we investigated several aspects of coded aperture design for a small animal SPECT system, including aperture/detector configuration, sampling requirements, and susceptibility to scatter. We simulated various source distributions and detection systems which included one, two, and four stationary detectors placed around the object, each with a pinhole array or a Fresnel zone plate in front of the detector. Image volumes were reconstructed using an iterative successive over-relaxation algorithm with a penalized weighted least squares cost function. Multiple pinhole arrays performed better than Fresnel zone plates in terms of reconstructed mean squared error and signal-to-noise. Our design goals of [is less than] 2 mm spatial resolution (full width at half maximum) and [is greater than] 1% detection efficiency can be achieved with a four-detector system with arrays of 100 pinholes per detector and the scatter fraction for a 2-cm diameter object is [is less than] 5%. We conclude that a coded aperture design shows great promise for small animal SPECT. Index Terms--Animals, coded apertures, image reconstruction, iterative methods, single-photon emission computed tomography (SPECT).

Presents a study that described an approach to automatically segment dynamic positron emission tomography (PET) images using cluster analysis. Information on the segmentation scheme; Details of the cluster validation; Results of the validation of the segmentation scheme.

Presents corrections to two articles which appeared in the journal 'Nuclear Science.' 'High-Level Triggers in ATLAS'; 'Correction for Head Movements in Positron Emission Tomography Using an Optical Motion-Tracking System.'