W.L. Rogers

Model-based estimation for dynamic cardiac studies using ECT

The authors develop a strategy for joint estimation of physiological parameters and myocardial boundaries using ECT (emission computed tomography). They construct an observation model to relate parameters of interest to the projection data and to account for limited ECT system resolution and measurement noise. The authors then use a maximum likelihood (ML) estimator to jointly estimate all the parameters directly from the projection data without reconstruction of intermediate images. They also simulate myocardial perfusion studies based on a simplified heart model to evaluate the performance of the model-based joint ML estimator and compare this performance to the Cramer-Rao lower bound. Finally, the authors discuss model assumptions and potential uses of the joint estimation strategy.

List-mode maximum likelihood reconstruction of Compton scatter camera images in nuclear medicine

A Maximum Likelihood (ML) image reconstruction technique using list-mode data has been applied to Compton scattering camera imaging. List-mode methods are appealing in Compton camera image reconstruction because the total number of data elements in the list (the number of detected photons) is significantly smaller than the number of possible combinations of position and energy measurements, leading to a much smaller problem than that faced by traditional iterative reconstruction techniques. For a realistic size device, the number of possible detector bins can be as large as 10 billion per pixel of the image space, while the number of counted photons would typically be a very small fraction of that. The primary difficulty in applying the list-mode technique is in determining the parameters which describe the response of the imaging system. In this work, a simple method for determining the required system matrix coefficients is employed, in which a back-projection is performed in list-mode, and response coefficients determined for only tallied pixels. Projection data has been generated for a representative Compton camera system by Monte Carlo simulation for disk sources with hot and cold spots and energies of 141, 364, and 511 keV, and reconstructions performed.

Improved modeling of system response in list mode EM reconstruction of Compton scatter camera images

An improved List Mode EM method for reconstructing Compton scattering camera images has been developed. First, an approximate method for computation of the spatial variation in the detector sensitivity has been derived and validated by Monte Carlo computation. A technique for estimating the relative weight of system matrix coefficients for each gamma in the list has also been employed, as has a method for determining the relative probabilities of emission having some from pixels tallied in each list-mode back-projection. Finally, a technique has been developed for modeling the effects of Doppler broadening and finite detector energy resolution on the relative weights for pixels neighbor to those intersected by the back-projection, based on values for the FWHM of the spread in the cone angle computed by Monte Carlo. Memory issues typically associated with list mode reconstruction are circumvented by storing only a list of the pixels intersected by the back-projections, and computing the weights of the neighboring pixels at each iteration step. Simulated projection data has been generated for a representative Compton camera system (CSPRINT) for several source distributions and reconstructions performed. Reconstructions have also been performed for experimental data for distributed sources.

Acquaintance Rape: The Effect of Race of Defendant and Race of Victim on White Juror Decisions

Racial bias appears to lead jurors in trials of stranger rape to convict Black defendants more readily and to sentence them more harshly than White defendants. It was hypothesized that jurors in an acquaintance rape case would demonstrate a different pattern of bias, based not only on the race of the defendant but also on the racial nature of the defendant-victim relationship. White American undergraduates read a trial transcript that established defendant-victim familiarity and sexual contact but was ambiguous about the victims consent. Race of defendant and of victim (Black or White) were varied on a 2 x 2 design. The participants were asked to rate the guilt of the defendant and to recommend a sentence. Both Black and White defendants were rated as more guilty when the victims race differed from their own.

Model-based estimation with boundary side information or boundary regularization [cardiac emission CT]

The authors have previously developed a model-based strategy for joint estimation of myocardial perfusion and boundaries using ECT (emission computed tomography). They have also reported difficulties with boundary estimation in low contrast and low count rate situations. Here they propose using boundary side information (obtainable from high resolution MRI and CT images) or boundary regularization to improve both perfusion and boundary estimation in these situations. To fuse boundary side information into the emission measurements, the authors formulate a joint log-likelihood function to include auxiliary boundary measurements as well as ECT projection measurements. In addition, they introduce registration parameters to align auxiliary boundary measurements with ECT measurements and jointly estimate these parameters with other parameters of interest from the composite measurements. In simulated PET O-15 water myocardial perfusion studies using a simplified model, the authors show that the joint estimation improves perfusion estimation performance and gives boundary alignment accuracy of <0.5 mm even at 0.2 million counts. They implement boundary regularization through formulating a penalized log-likelihood function. They also demonstrate in simulations that simultaneous regularization of the epicardial boundary and myocardial thickness gives comparable perfusion estimation accuracy with the use of boundary side information.

SPRINT: A Stationary Detector Single Photon Ring Tomograph for Brain Imaging

During the last two decades, various Doppler methods have been successfully used to screen patients with significant cerebral and peripheral vascular disease. In general terms, the principal advantages of Doppler ultrasound techniques in the evaluation of atherosclerotic lesions are that they: 1) are noninvasive, 2) are nontraumatic, 3) are relatively inexpensive, 4) provide anatomical and physiological data, and 5) provide direct and dynamic measurements. Nevertheless, the general limitations of the techniques are of equal importance: 1) the techniques are difficult in some subjects due to obesity and anatomical variations; 2) the technique cannot examine tissues surrounded by air or bone; 3) the techniques require operator skill and a thorough knowledge of human anatomy and cardiovascular dynamics; 4) the techniques have finite spatial resolutions which may compromise the important measurement of vessel diameter, ulceration, and percent stenosis; and 5) the techniques have finite velocity measuring capabilities which may compromise some measurements of highly disturbed blood velocities outside the range of 2-200 cm/sec. As clinical demands for the early diagnosis and quantification of vascular lesions increased, improvements in Doppler ultrasonics and spectra analysis significantly increased the technical and clinical capabilities of existing simple, inexpensive instruments. Presently, both anatomical and physiological images along with quantitative Doppler spectra from superficial and deep-lying vessels can be obtained. Consequently, the ability of Vascular Laboratory, Clinical Research Division; Lovelace Medical Foundation, and the University of New Mexico School of Medicine, Albuquerque, New Mexico 87108 Manuscript received at IEEE April 30, 1982. new expensive imaging equipment to quantitate atherosclerotic lesions using spectral analysis techniques compares favorably with the interpretational precision of standard invasive or intravenous digital angiography. New data suggest that unique hemodynamic information which reflects the effects of cardiac output and vascular input impedance on the hemodynamic consequences of an anatomical lesion can also be obtained. This paper will 1) briefly discuss the general considerations of Doppler ultrasonics; 2) critique the specific characteristics and utility of standard clinical Doppler units; and 3) discuss the ability of new, multipurpose equipment to quantitate (both anatomically and physiologically) atherosclerotic lesions throughout the cardiovascular system.

Experimental results from the C-SPRINT prototype Compton camera

A Compton camera is being tested for nuclear medicine applications. Our design uses a single 3 cm by 3 cm silicon pad detector as the first detector system, and SPRINT, an array of position-sensitive sodium iodide modules, as the second detector. Experimental results with a /sup 99m/Tc point source show coincidence energy spectra agreeing with theoretical predictions. The coincidence energy spectra for both silicon and SPRINT detectors correspond to the geometry-determined scattering angle range. Recorded energy falls outside of strict geometric limits because of Doppler broadening and detector energy resolution effects. The summed energy peak in the initial data run for a /sup 99m/Tc source has a FWHM energy resolution of 33 keV, primarily due to energy uncertainty in the SPRINT modules. A second data run showed an improvement to 25 keV in summed energy resolution due to careful calibration of, and correction for, significant first and second detector gain non-uniformities. Images generated from the second acquired data set result in a backprojection image resolution of 1.5 cm at a source distance of 10 cm. Analytical and Monte Carlo calculations show a very close agreement of 1.6 cm. Using a list-mode maximum likelihood EM reconstruction algorithm, the image resolution is improved to 7 mm, although the resolution recovery is at the expense of increased noise in the image.

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 better spatial resolution and larger signal-to-noise ratio than those obtained without the collimator. The results show that this method is useful in obtaining Tc-99m images.

Incorporating MRI region information into SPECT reconstruction using joint estimation

Single photon emission computed tomographic images (SPECT) have relatively poor resolution. In an attempt to improve SPECT image quality, many methods have been developed for including anatomic information, extracted from higher resolution, structurally correlated magnetic resonance images (MRI), into the SPECT reconstruction process. These methods provide improved SPECT reconstruction accuracy if the anatomic information is perfectly correlated with the SPECT functional information. However there exist mismatches between MRI anatomical structures and SPECT functional structures due to different imaging mechanisms. It has been reported that if the MR structures are applied into SPECT, the mismatched part will cause artifacts. The paper describes a joint estimation approach which unifies MR information extraction and SPECT reconstruction processes to avoid these artifacts. Both qualitative and quantitative evaluations show that the method improves the SPECT reconstruction where the MR information matches and is robust to mismatched MR information.

Joint estimation for incorporating MRI anatomic images into SPECT reconstruction

To improve SPECT reconstruction using spatially-correlated magnetic resonance (MR) images as a source of side information, one must account for mismatch between MRI anatomical information and SPECT functional information. The authors investigate an approach which incorporates the anatomical information into SPECT reconstruction by using region labels representing the anatomical regions extracted from MRI. Each SPECT pixel corresponds to one region label. Both SPECT pixel mean intensities and region labels are jointly estimated by a penalized Maximum-Likelihood criterion using an iterative Space-Alternating Generalized EM algorithm. The likelihood function incorporates both the SPECT noise distribution and the MRI side information measurement statistics. Since the region labels are estimated jointly from both segmented MRI and SPECT projection data, only those anatomical regions that match SPECT functional regions are represented by the estimated labels, and are used to constrain the SPECT reconstruction. The artifacts due to the mismatched MR anatomical region information are reduced using joint estimation. By comparing image quality and the Bias vs. Variance tradeoffs, the authors see that the joint estimation has the potential to improve the SPECT estimation result.<<ETX>>