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The photoacoustic effect was employed to generate short-duration quasi-unipolar acoustic pressure pulses in both planar and spherically focused geometries. In the focal region, the temporal profile of a pressure pulse can be approximated by the first derivative of the temporal profile near the front transducer surface, with a time-averaged value equal to zero. This approximation agreed with experimental results acquired from photoacoustic transducers with both rigid and free boundaries. For a free boundary, the acoustic pressure in the focal region is equal to the sum of a positive pressure that follows the spatial profile of the optical energy deposition in the medium and a negative pressure that follows the temporal profile of the laser pulse.

Optoacoustics is a promising modality for biomedical imaging, sensing, and monitoring with high resolution and contrast. In this paper, we present an overview of our studies for the last two decades on optoacoustic effects in tissues and imaging capabilities of the optoacoustic technique. In our earlier optoacoustic works we studied laser ablation of tissues and tissue-like media and proposed to use optoacoustics for imaging in tissues. In mid-90s we demonstrated detection of optoacoustic signals from tissues at depths of up to several centimeters, well deeper than the optical diffusion limit. We then obtained optoacoustic images of tissues both in vitro and in vivo. In late 90s we studied optoacoustic monitoring of thermotherapy: hyperthermia, coagulation, and freezing. Then we proposed and studied optoacoustic monitoring of blood oxygenation, hemoglobin concentration, and other physiologic parameters.

Melanoma is the deadliest skin cancer and is responsible for over 7000 deaths in the US annually. The spread of cancer, or metastasis, is responsible for these deaths, as secondary tumors interrupt normal organ function. Circulating tumor cells, or those cells that spread throughout the body from the primary tumor, are thought to be responsible for metastasis. We developed an optical method, photoacoustic flow cytometry, in order to detect and enumerate circulating melanoma cells (CMCs) from blood samples of patients. We tested the blood of Stage IV melanoma patients to show the ability of the photoacoustic flow cytometer to detect these rare cells in blood. We then tested the system on archived blood samples from Stage III melanoma patients with known outcomes to determine if detection of CMCs can predict future metastasis. We detected between 0 and 66 CMCs in Stage IV patients. For the Stage III study, we found that of those samples with CMCs, two remained disease free and five developed metastasis. Of those without CMCs, six remained disease free and one developed metastasis. We believe that photoacoustic detection of CMCs provides valuable information for the prediction of metastasis and we postulate a system for more accurate prognosis.