Photoacoustic imaging for prostate brachytherapy

Abstract

Photoacoustic (PA) imaging is an emerging imaging modality that relies on the PA effect. The PA effect is caused by exposing an optically absorbing sample to near-infrared light which causes the sample to experience a temporary temperature increase through optical absorption. The heated region undergoes thermoelastic expansion and produces an abrupt and localized pressure change. This change results in a transient PA wave that propagates out toward the sample surface for collection by an ultrasound (US) transducer. Through image reconstruction, the optical property of the sample can be obtained. PA imaging is promising in detecting brachytherapy seeds during prostate brachytherapy. The high absorption coefficient of the metallic seeds leads to high PA imaging contrast. One major drawback is the limited imaging depth due to high optical attenuation of the excitation light in tissue. One of the goals of this thesis is to conduct initial feasibility tests of enhancing the PA contrast through brachytherapy seeds modifications. Seed coated with a contrast enhancing material shows an increase of 18 dB in signal-to-noise ratio (SNR) and two time increase in the imaging depth (5 cm). Another method of silver coating leads to a 5 dB improvement in the SNR of the modified seeds. An alternative approach in using dyed ethanol solution as a contrast enhancing agent by filling the spaces between two seeds is also reported. The result showed improvement comparable to the black paint method. Another goal is to propose a novel method of tissue typing in PA imaging. A temperature change in tissue can lead to changes of several tissue parameters which can be used for tissue typing. One of the parameters is the speed of sound in tissue, which increases in water-based non-fatty tissue and decreases in fatty tissue as temperature is raised. We show that on average, 6.9±1.5 %/min increase and 4.2±1.5 %/min decrease in PA intensity are observed in porcine liver and bovine fat samples respectively through one minute of laser heating. These results demonstrate that by analyzing the PA intensity change of the illuminated sample, one can extract characteristic information that can lead to tissue type differentiation.