An evaluation of maximum likelihood reconstruction for SPECT

A reconstruction method for SPECT (single photon emission computerized tomography) that uses the maximum likelihood (ML) criterion and an iterative expectation-maximization (EM) algorithm solution is examined. The method is based on a model that incorporates the physical effects of photon statistics, nonuniform photon attenuation, and a camera-dependent point-spread response function. Reconstructions from simulation experiments are presented which illustrate the ability of the ML algorithm to correct for attenuation and point-spread. Standard filtered backprojection method reconstructions, using experimental and simulated data, are included for reference. Three studies were designed to focus on the effects of noise and point-spread, on the effect of nonuniform attenuation, and on the combined effects of all three. The last study uses a chest phantom and simulates Tl-201 imaging of the myocardium. A quantitative analysis of the reconstructed images is used to support the conclusion that the ML algorithm produces reconstructions that exhibit improved signal-to-noise ratios, improved image resolution, and image quantifiability.     

Optimal mean velocity estimation for Doppler weather radars

Optimal Doppler velocity estimation is explored for a standard Gaussian signal measurement model and thematic maximum-likelihood (ML) and Bayes estimation. Because the model considered depends on a vector parameter [velocity, spectrum width (SW), and signal-to-noise ratio (SNR)], the exact formulation of an ML or Bayes solution involves a system of coupled equations that cannot be made explicit for any of the parameters. Simple computational forms are shown to exist when SW and SNR are assumed known. An information-theoretic concept is used to extend these equations to the general case of SW and SNR unknown. A Monte Carlo simulation experiment is used to verify that the method can work, with no a priori information for either SW or SNR and a very small (20 pulse) sample size. The improved performance of this new Doppler velocity estimator is documented by comparison with derived optimal bounds and with the performance of the pulse pair (PP) method. Bayes estimator results are used to provide true performance bounds for comparison. Cramer-Rao bounds are also derived and shown to be inferior to the Bayes bounds in the small sample case considered     

Initialization for improved IIR filter performance

A new method for initializing the memory registers of infinite impulse response (IIR) filters is introduced. In addition to providing improved performance as compared to other methods of initialization, this method is unique in that it makes no prior assumptions regarding the input-signal content. Therefore, the method applies equally well to a variety of IIR filter designs and applications. The method is best suited for signal processing applications in which `batch' processing of the data is used. However, sequential processing, can be accommodated when delays at the beginning of a processing segment can be tolerated