Optical imaging and photothermal therapy have been applied in biomedical field for decades. However, the strong scattering of light in biological tissue hinders the focal light delivery and thus restricts their clinical applications because of the resultant limited penetration. We hypothesize that the photon scattering is reduced in the cylindrical heating zone of high intensity focused ultrasound (HIFU) and thus the efficiency of light delivery can be improved via transmission of light through the heating cylindrical tunnel, enabling photoacoustic signal enhancement at the targeted region. In this study, Monte Carlo simulation and intralipid-phantom experiments were used to verify our hypothesis. The thermal effect could increase the laser fluence at the targeted region by at least 10% no matter in the simulation or the experiment. Similar results were also presented in the measured photoacoustic signal. Note that special care had been taken to keep the Gruneisen coefficient at the targeted region constant so that the photoacoustic signal change solely depended on delivered laser fluence. In addition, the simulation results indicate that with the local cylindrical heating tunnel, the fluence at the targeted region is at least 10% higher than that with global heating, suggesting that HIFU heating tissue tunnel owns the potential in enhancing the light delivery efficiency, the light penetration and thus the photoacoustic signal at the targeted region as well. It is expected that our finding is not only applicable to photoacoustic imaging but also photothermal therapy which also requires more focal light delivery.
In this study, we propose to use gas-filled microbubbles (MBs) simultaneously actuated by the acoustic wave to enhance the imaging contrast of optical coherence tomography (OCT)-based angiography. In the phantom experiments, MBs can result in stronger backscattered intensity, enabling to enhance the contrast of OCT intensity image. Moreover, simultaneous application of low-intensity acoustic wave enables to temporally induce local vibration of particles and MBs in the vessels, resulting in time-variant OCT intensity which can be used for enhancing the contrast of OCT intensitybased angiography. Additionally, different acoustic modes and different acoustic powers to actuate MBs are performed and compared to investigate the feasibility of contrast enhancement. Finally, animal experiments are performed. The findings suggest that acoustic-actuated MBs can effectively enhance the imaging contrast of OCT-based angiography and the imaging depth of OCT angiography is also extended.
Microcirculation volumetric flow rate is a significant index in diseases diagnosis and treatment such as diabetes and
cancer. In this study, we propose an integrated algorithm to assess microcirculation volumetric flow rate including
estimation of blood perfused area and corresponding flow velocity maps based on high frequency destruction/contrast
replenishment imaging technique. The perfused area indicates the blood flow regions including capillaries, arterioles
and venules. Due to the echo variance changes between ultrasonic contrast agents (UCAs) pre- and post-destruction two
images, the perfused area can be estimated by the correlation-based approach. The flow velocity distribution within the
perfused area can be estimated by refilling time-intensity curves (TICs) after UCAs destruction. Most studies introduced
the rising exponential model proposed by Wei (1998) to fit the TICs. Nevertheless, we found the TICs profile has a
great resemblance to sigmoid function in simulations and in vitro experiments results. Good fitting correlation reveals
that sigmoid model was more close to actual fact in describing destruction/contrast replenishment phenomenon. We
derived that the saddle point of sigmoid model is proportional to blood flow velocity. A strong linear relationship (R =
0.97) between the actual flow velocities (0.4-2.1 mm/s) and the estimated saddle constants was found in M-mode and B-mode
flow phantom experiments. Potential applications of this technique include high-resolution volumetric flow rate
assessment in small animal tumor and the evaluation of superficial vasculature in clinical studies.
It is difficult to automatically detect tumors and extract lesion boundaries in ultrasound images due to the variance in
shape, the interference from speckle noise, and the low contrast between objects and background. The enhancement of
ultrasonic image becomes a significant task before performing lesion classification, which was usually done with
manual delineation of the tumor boundaries in the previous works. In this study, a linear support vector machine (SVM)
based algorithm is proposed for ultrasound breast image training and classification. Then a disk expansion algorithm is
applied for automatically detecting lesions boundary. A set of sub-images including smooth and irregular boundaries in
tumor objects and those in speckle-noised background are trained by the SVM algorithm to produce an optimal
classification function. Based on this classification model, each pixel within an ultrasound image is classified into either
object or background oriented pixel. This enhanced binary image can highlight the object and suppress the speckle
noise; and it can be regarded as degraded paint character (DPC) image containing closure noise, which is well known in
perceptual organization of psychology. An effective scheme of removing closure noise using iterative disk expansion
method has been successfully demonstrated in our previous works. The boundary detection of ultrasonic breast lesions
can be further equivalent to the removal of speckle noise. By applying the disk expansion method to the binary image,
we can obtain a significant radius-based image where the radius for each pixel represents the corresponding disk
covering the specific object information. Finally, a signal transmission process is used for searching the complete breast
lesion region and thus the desired lesion boundary can be effectively and automatically determined. Our algorithm can
be performed iteratively until all desired objects are detected. Simulations and clinical images were introduced to
evaluate the performance of our approach. Several types of cysts with different contours and contrast resolutions images
were simulated with speckle characteristics. Four thousand sub-images of tumor objects and speckle-noised background
were used for SVM training. Comparison with conventional algorithms such as active contouring, the proposed
algorithm does not need to position any initial seed point within the lesion and is able to detect simultaneously multiple
irregular shape lesions in a single image, thus it can be regarded as a fully automatic process. The results show that the
mean normalized true positive area overlap between true contour and contour obtained by the proposed approach is
90%.
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