The theoretical underpinnings of photoacoustic ultrasound (PAUS) reconstruction tomography are presented. A formal relationship between PAUS signals and the heterogeneous distribution of optical absorption within the object being investigated is developed. Based on this theory, a reconstruction approach, analogous to that used in x-ray computed tomography, is suggested. Initial experimental results suggest that this approach produces "reasonable" reconstructions for absorbers distributed within a narrow plane embedded within a highly scattering medium.
An imaging technology, thermoacoustic tomograpy (TAT), was applied to the visualization of high-intensity focused ultrasound (HIFU)-induced lesions. A single, spherically focused ultrasonic transducer, operating at a central frequency of approximately 4 MHz, was used to generate a HIFU field in fresh porcine muscle. Microwave pulses from a 3-GHz microwave generator were then employed to generate thermoacoustic sources in this tissue sample. The thermoacoustic signals were detected by an unfocused ultrasonic transducer that was scanned around the sample. To emphasize the boundaries between the lesion and its surrounding tissue, a local-tomography-type reconstruction method was applied to reconstruct the TAT images of the lesions. Good contrast was obtained between the lesion and the tissue surrounding it. Gross pathologic photographs of the tissue samples confirmed the TAT images.This work demonstrates that TAT may potentially be used to image HIFU-induced lesions in biological tissues.
In this thesis, photoacoustic ultrasonography (PAUS) and its applicability in breast cancer detection were investigated. PAUS employs a short pulse of electromagnetic energy, at either near‐infrared or microwave frequency, to heat breast tissue. Rapid heating, resulting from inhomogeneous absorption of the energy pulse, generates ultrasonic waves. The energy absorption patterns can be reconstructed from these pressure waves recorded at locations around the periphery of the tissue. This study concentrated on microwave‐induced PAUS (434 MHz). The principle of the photoacoustic signal generation was analyzed, and the image reconstruction method was implemented and validated by imaging experiments. Extensive studies of microwave‐induced PAUS demonstrated that an adequate absorption difference of microwaves at 434 MHz exists between benign and malignant breast tissues. Experiments suggest that adequate ultrasonic signals can be detected using proper instrumentation, which allow the microwave absorption patterns to be reconstructed. I conclude that microwave‐induced PAUS is likely to be a useful imaging modality for breast screening.
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