Stochastic Simulations for the Detection of Objects in Three Dimensional Volumes: Applications in Medical Imaging and Ocean Acoustics

by Shorey, Jamie Margaret

Abstract (Summary)
Given a known signal and perfect knowledge of the environment there exist few

detection and estimation problems that cannot be solved. Detection performance is

limited by uncertainty in the signal, an imperfect model, uncertainty in environmental

parameters, or noise. Complex environments such as the ocean acoustic waveguide and

the human anatomy are difficult to model exactly as they can differ, change with time,

or are difficult to measure. We address the uncertainty in the model or parameters by

incorporating their possibilities in our detection algorithm. Noise in the signal is not so

easily dismissed and we set out to provide cases in which what is frequently termed a

nuisance parameter might increase detection performance. If the signal and the noise

component originate from the same system then it might be reasonable to assume that

the noise contains information about the system as well.

Because of the negative effects of ionizing radiation it is of interest to maximize

the amount of diagnostic information obtained from a single exposure. Scattered

radiation is typically considered image degrading noise. However it is also dependent

on the structure of the medium and can be estimated using stochastic simulation. We

describe a novel Bayesian approach to signal detection that increases performance by

including some of the characteristics of the scattered signal. This dissertation examines

medical imaging problems specific to mammography. In order to model environmental

uncertainty we have written software to produce realistic voxel phantoms of the breast.

The software includes a novel algorithm for producing three dimensional distributions

of fat and glandular tissue as well as a stochastic ductal branching model.

The image produced by a radiographic system cannot be determined analytically

since the interactions of particles are a random process. We have developed a particle

transport software package to model a complete radiographic system including a

realistic x-ray spectrum model, an arbitrary voxel-based medium, and an accurate

material library. Novel features include an efficient voxel ray tracing algorithm that

reflects the true statistics of the system as well as the ability to produce separable images

of scattered and direct radiation.

Similarly, the ocean environment includes a high degree of uncertainty. A

pressure wave propagating through a channel produces a measurable collection of

multipath arrivals. By modeling changes in the pressure wave front we can estimate the

expected pattern that appears at a given location. For this purpose we have created an

ocean acoustic ray tracing code that produces time-domain multipath arrival patterns

for arbitrary 3-dimensional environments. This iterative algorithm is based on a

generalized recursive ray acoustics algorithm. To produce a significant gain in

computation speed we model the ocean channel as a linear, time invariant system. It

differs from other ocean propagation codes in that it uses time as the dependent variable

and can compute sound pressure levels along a ray path effectively measuring the

spatial impulse response of the ocean medium.

This dissertation also investigates Bayesian approaches to source localization in a

3-D uncertain ocean environment. A time-domain-based optimal a posteriori probability

bistatic source localization method is presented. This algorithm uses a collection of

acoustic time arrival patterns that have been propagated through a 3-D acoustic model

as the observable data. These replica patterns are collected for a possible range of

unknown environmental parameters. Receiver operating characteristics for a bistatic

detection problem are presented using both simulated and measured data.

Bibliographical Information:

Advisor:Nolte, Loren W.; Krolik, Jeffery L.; Liu, Qing H.; Lo, Joseph Y.; Samei, Ehsan

School:Duke University

School Location:USA - North Carolina

Source Type:Master's Thesis

Keywords:monte carlo detection theory ocean acoustics radiography particle transport image processing


Date of Publication:05/10/2007

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