In this paper, we demonstrate a temporal-domain intensity-based photoacoustic imaging method that can differentiate between fatty and non-fatty tissues. PA pressure intensity is partly dependent on the tissue’s speed of sound, which increases as temperature increases in non-fatty tissue and decreases in fatty tissue. Therefore, by introducing a temperature change in the tissue and subsequently monitoring the change of the PA intensity, it is possible to distinguish between the two types of tissue. A commercial ultrasound system with a 128-element 5-14 MHz linear array transducer and a tunable ND:YAG laser were used to produce PA images. Ex-vivo bovine fat and porcine liver tissues were precooled to below 10°C and then warmed to room-temperature over ~1 hour period. A thermocouple monitored the temperature rise while PA images were acquired at 0.5°C intervals. The averaged intensity of the illuminated tissue region at each temperature interval was plotted and linearly fitted. Liver samples showed a mean increase of 2.82 %/°C, whereas bovine fat had a mean decrease of 6.24 %/°C. These results demonstrate that this method has the potential to perform tissue differentiation in the temporal-domain.
Prostate brachytherapy is a form of radiotherapy for treating prostate cancer where the radiation sources are seeds
inserted into the prostate. Accurate localization of seeds during prostate brachytherapy is essential to the success of
intraoperative treatment planning. The current standard modality used in intraoperative seeds localization is transrectal
ultrasound. Transrectal ultrasound, however, suffers in image quality due to several factors such speckle, shadowing, and
off-axis seed orientation. Photoacoustic imaging, based on the photoacoustic phenomenon, is an emerging imaging
modality. The contrast generating mechanism in photoacoustic imaging is optical absorption that is fundamentally
different from conventional B-mode ultrasound which depicts changes in acoustic impedance. A photoacoustic imaging
system is developed using a commercial ultrasound system. To improve imaging contrast and depth penetration,
absorption enhancing coating is applied to the seeds. In comparison to bare seeds, approximately 18.5 dB increase in
signal-to-noise ratio as well as a doubling of imaging depth are achieved. Our results demonstrate that the coating of the
seeds can further improve the discernibility of the seeds.
This paper introduces a novel alignment and calibration method for high frequency ultrasound (HFUS) and optical
coherence tomography (OCT) 1D transducers. 2D images are constructed by means of translation of the transducers
using a linear motor stage. Physical alignment of the transducers is needed in order to capture images of the same crosssectional
plane, and calibration is needed to determine the relative coordinates of the images, including the image skew.
A dual wedge-tri step phantom is created for both alignment and calibration. This phantom includes two symmetrical
wedges and three steps that provide the user with visual feedback on how well the scan plane is aligned with the midplane
of the phantom. The phantom image consists of five line segments, each of which corresponds to one of the
wedges or steps. The slopes and positions of the lines are extracted from the image and compared with the phantom
model. The scan plane parameters are found so that the difference between the model and extracted features is
minimized. The main advantage of this phantom is that only one frame is required to determine translations, orientations,
and skew parameters of the scan plane with respect to the phantom. Experimental results with ocular imaging show the
ability to achieve alignment based on this method and its potential for medical applications.
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