Thermoacoustic emissions induced by 2450 MHz microwave pulses in water, tissue-simulating phantoms and dog kidneys have been detected. The analytic signal magnitude has been employed in generating 'A-mode' images with excellent depth resolution. Thermoacoustic emissions have also been detected from the dose-gradient at the beam edges of a 4 MeV x-ray beam in water. These results establish the feasibility of employing thermoacoustic signals in generating diagnostic images, and in locating x-ray beam edges during radiation therapy. A theoretical model for thermoacoustic imaging using a directional transducer has been developed, which may be used in the design of future thermoacoustic imaging system, and in facilitating comparisons with other types of imaging systems. A method of characterizing biological tissues has been proposed, which relates the power spectrum of the detected thermoacoustic signals to the autocorrelation function of the thermoacoustic source distribution in the tissues. The temperature dependence of acoustic signals induced by microwave pulses in water has been investigated. The signal amplitudes vary with temperature as the thermal expansion of water, except near 4°C. The signal waveforms show a gradual phase change as the temperature changes from below 4° to above 4°C. This anomaly is due to the presence of a nonthermal component detected near 4°C, whose waveform is similar to the derivative of the room temperature signal. The results are compared to a model based on a nonequilibrium relaxation mechanism proposed by Pierce and Hsieh. The relaxation time was found to be (0.20±0.02) ns and (0.13±0.02) ns for 200 ns and 400 ns microwave pulse widths, respectively. A microwave-induced thermoacoustic source capable of launching large aperture, unipolar ultrasonic plane wave pulses in water has been constructed. This source consists of a thin water layer trapped between two dielectric media. Due to the large mismatch in the dielectric constants, the incident microwaves undergo multiple reflections between the dielectric boundaries trapping the water, resulting in an enhanced specific microwave absorption in the thin water layer. This source may be useful in ultrasonic scattering and attenuation experiments.

This dissertation presents realistic mathematical models of normal and diseased livers and a nuclear medicine camera. The mathematical model of a normal liver is developed by creating a data set of points on the surface of the liver and fitting it to a truncated set of spherical harmonics. We model the depth-dependent MTF of a scintillation camera taking into account the effects of Compton scatter, linear attenuation, intrinsic detector resolution, collimator resolution, and Poisson noise. The differential diagnosis on a liver scan includes normal, focal disease, and diffuse disease. Object classes of normal livers are created by randomly perturbing the spherical harmonic coefficients. Object classes of livers with focal disease are created by introducing cold ellipsoids within the liver volume. Cirrhotic livers are created by modelling the gross morphological changes, heterogenous uptake, and decreased overall uptake. Simulated nuclear medicine images are made by projecting livers through nuclear imaging systems. The combination of object classes of simulated livers and models of different imaging systems is applied to imaging-system design optimization in a psycho-physical study. Human observer performance on simulated liver images made on nine different systems is compared to the Hotelling trace criterion (HTC). The system with the best observer performance is judged to be the best system. The correlation between the human performance metric dₐ and the HTC for this study was 0.829, suggesting that the HTC may have value as a predictor of observer performance. Texture in a liver scan is related to the three-dimensional distribution of functional acini, which changes with disease. One measure of texture is the fractal dimension, related to the Fourier power spectrum. We measured the average radial power spectra of 70 liver scans. All of these scans yield straight lines when plotted on a log-log scale, a characteristic of fractal objects. The slope of the line is related to the fractal dimension of the acini. The slopes are significantly higher for normal than abnormal livers (t = 4.04, df = 29, p = 0.005). On 32 liver scans with confirmed diagnoses, receiver operating characteristics (ROC) analysis was performed using power spectral slope as a feature. Analysis of the ROC curve yielded an area under the curve of 85, suggesting that power spectral slope may be a useful classifier of disease.

Imaging in nuclear medicine involves the injection of a radioactive tracer into the body and subsequent detection of the radiation emanating from an organ of interest. Single-photon emission computed tomography (SPECT) is the branch of nuclear medicine that yields three-dimensional maps of the distribution of a tracer, most commonly as a series of two-dimensional slices. One major drawback to transaxial tomographic imaging in SPECT today is the rotation required of a gamma camera to collect the tomographic data set. Transaxial SPECT usually involves a large, single-crystal scintillation camera and an aperture (collimator) that together only satisfy a small portion of the spatial sampling requirements simultaneously. It would be very desirable to have a stationary data-collection apparatus that allows all spatial sampling in the data set to occur simultaneously. Aperture or detector motion (or both) is merely an inconvenience in most imaging situations where the patient is stationary. However, aperture or detector motion (or both) enormously complicate the prospect of tomograhically recording dynamic events, such as the beating heart, with radioactive pharmaceuticals. By substituting a set of small modular detectors for the large single-crystal detector, we can arrange the usable detector area in such a way as to collect all spatial samples simultaneously. The modular detectors allow for the possibility of using other types of stationary apertures. We demonstrate the capabilities of one such aperture, the pinhole array. The pinhole array is one of many kinds of collimators known as coded apertures. Coded apertures differ from conventional apertures in nuclear medicine in that they allow for overlapping projections of the object on the detector. Although overlapping projections is not a requirement when using pinhole arrays, there are potential benefits in terms of collection efficiency. There are also potential drawbacks in terms of the position uncertainty of emissions in the reconstruction object. The long-term goal of the research presented is dynamic SPECT imaging of the heart. The basic concepts and tasks involved in transaxial SPECT imaging with pinhole arrays are presented along with arguments for the combination of modular gamma cameras and pinhole arrays. We demonstrate by emulation two methods of tomographically imaging a stationary single object slice and present results for these two systems on object space grids of 10cm x 10cm and 20cm x 20cm.

High quality three-dimensional bioluminescence tomographic (BLT) images, if available, would constitute a major advance and provide much more useful information than the two-dimensional bioluminescence images that are frequently used today. To-date, high quality BLT images have not been available, largely because of the poor quality of the data being input into the reconstruction process. Many significant confounds are not routinely corrected for and the noise in this data is unnecessarily large and poorly distributed. Moreover, many of the design choices affecting image quality are not well considered, including choices regarding the number and type of filters used when making multispectral measurements and choices regarding the frequency and uniformity of the sampling of both the range and domain of the BLT inverse problem. Finally, progress in BLT image quality is difficult to gauge owing to a lack of realistic gold-standard references that engage the full complexity and uncertainty within a small animal BLT imaging experiment. Within this dissertation, I address all of these issues. I develop a Cerenkov-based gold-standard wherein a Positron Emission Tomography (PET) image can be used to gauge improvements in the accuracy of BLT reconstruction algorithms. In the process of creating this reference, I discover and describe corrections for several confounds that if left uncorrected would introduce artifacts into the BLT images. This includes corrections for the angle of the animal’s skin surface relative to the camera, for the height of each point on the skin surface relative to the focal plane, and for the variation in bioluminescence intensity as a function of luciferin concentration over time. Once applied, I go on to derive equations and algorithms that when employed are able to minimize the noise in the final images under the constraints of a multispectral BLT data acquisition. These equations and algorithms allow for an optimal choice of filters to be made and for the acquisition time to be optimally distributed among those filtered measurements. These optimizations make use of Barrett’s and Moore-Penrose pseudoinverse matrices which also come into play in a paradigm I describe that can be used to guide choices regarding sampling of the domain and range.

No description available.

No description available.

From one treatment to the next, considerable effort is made to accurately position radiotherapy patients according to their treatment plans. However, some variation is unavoidable. The target volume and the organs at risk may also move within the patient and/or change shape during the treatment. Thus, it is important to be able to verify the success of the treatment by determining the position of patient and the dose deposited in the patient at each fraction. One possibility to achieve this, particularly when equipment, time and budgets are limited, would be to collect limited information while the patient is on the treatment couch. This research was aimed to develop a method for optimum beam position determination, for each patient-specific case. The optimized beam positions would balance the both treatment planning and image reconstruction, so that the patient???s image can be obtained during the treatment delivery using the information collected from the same angles as used for treatment. This will allow verification of the dose deposited in the patient for every fraction. Using a limited number of angles for image reconstruction, the dose to the patient can be minimized. This work has two major parts, beam position optimization for image reconstruction and beam position optimization for treatment planning. These two optimizations are then combined to obtain the optimum beam position for both image reconstruction and treatment planning. An objective function, projection correlation, was developed to investigate the image reconstruction method using limited information. Another objective function, the average optimization quality factor, was also introduced to optimize beam positions for treatment planning. Two optimization methods, the gradient descent method and the simulated annealing based on these objective functions were used to determine the beam angles. The results show that the projection correlation presents several advantages. It can be applied without any iterations, and it produces a fast algorithm. The present research will allow selection of the optimum beam positions without excessive computational cost for treatment planning and imaging. By combining the projection correlation and the average optimization quality factor together with more advanced image reconstruction software this could potentially be used in a clinical environment.

Thesis (Ph.D)--Stanford University, 1999. / Title from pdf t.p. (viewed April 3, 2002). "January 1999." "Adminitrivia V1/Prg/20000907"--Metadata.

Thesis (M. Phil.)--University of Hong Kong, 2001. / Includes bibliographical references (leaves 97-100).

Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: imaging; phosphorescence; eye; retina. Includes bibliographical references (p. 50-55).