Transcorneal electrical stimulation (TES), which as a noninvasive approach of retinal electrical stimulation, can activate releasing of neurotrophic factors and regenerate injured retinal neurons, has become a potential therapeutic method for retinal diseases. However, the mechanism of TES on the regeneration of retinal neurons has not been completely determined. The purpose of this study was to explore how the depth-resolved intrinsic optical signals (IOSs) and blood flow in cat retina change with TES. In the experiments, the cat retinae were imaged by our custom-designed spectraldomain OCT with a central wavelength of 840nm. OCT scanning and TES were synchronized so as to record images of retina at pre-, during and post-stimulation period, respectively. In each period, the IOSs were then extracted in structure images by registration and segmentation algorithms. And the blood flow was extracted in phase contrast images derived by phase-resolved Doppler OCT method including bulk motion compensation and phase unwrapping. Based on our preliminary results on 5 eyes of 4 cats, we found a significant increase of both positive and negative IOSs in each layer during and after TES compared to those of pre-stimulation and sham stimulation, while the changes of average blood flow before, during and 6 seconds after TES were not obvious. The preliminary results provide experimental data of neurovascular alterations under TES, which will benefit the study of the therapeutic mechanism of TES. But complete understanding of neurovascular response to TES should be further investigated.
Heart pumps blood through the blood vessels to provide body with oxygen and nutrients. As the result, the blood flow, volume and oxygenation in arteries has a pulsatile nature. Measuring these pulsatile parameters enables more precise monitoring of oxygen metabolic rate and is thus valuable for researches and clinical applications. Photoacoustic microscopy (PAM) is a proven label-free method for in vivo measuring blood oxygenation at single blood vessel level. However, studies using PAM to observe the pulsatile nature of blood oxygenation in arteries were not reported. In this paper, we use optical-resolution PAM (OR-PAM) technology to study the blood oxygenation dynamics of pulsatile arteries. First, the ability of our OR-PAM system to accurately reflect the change of optical absorption in imaged objects is demonstrated in a phantom study. Then the system is used to image exposed cortical blood vessels of cat. The pulsatile nature of blood volume and oxygenation in arteries is clearly reflected in photoacoustic (PA) signals, whereas it’s not observable in veins. By using a multi-wavelength laser, the dynamics of the blood oxygenation of pulsatile arteries in cardiac cycles can be measured, based on the spectroscopic method.
KEYWORDS: Blood, Photoacoustic microscopy, Blood vessels, Oxygen, Monte Carlo methods, Absorption, Tissues, Photoacoustic spectroscopy, Sensors, In vivo imaging
Accuracy of photoacoustic signal is the crux on measurement of oxygen saturation in functional photoacoustic imaging, which is influenced by factors such as defocus of laser beam, curve shape of large vessels and nonlinear saturation effect of optical absorption in biological tissues. We apply Monte Carlo model to simulate energy deposition in tissues and obtain photoacoustic signals reaching a simulated focused surface detector to investigate corresponding influence of these factors. We also apply compensation on photoacoustic imaging of in vivo cat cerebral cortex blood vessels, in which signals from different lateral positions of vessels are corrected based on simulation results. And this process on photoacoustic images can improve the smoothness and accuracy of oxygen saturation results.
Laser-scanning optical-resolution photoacoustic microscopy (LSOR-PAM) has a high application potential in ophthalmology and other clinical fields because of its high resolution and imaging speed. The stationary unfocused ultrasonic transducer of this system decides the efficiency and field of view (FOV) of photoacoustic signal detection, but the refraction and attenuation of laser generated photoacoustic signal in different tissue mediums will cause signal strength and direction distribution uneven. In this study, we simulated the photoacoustic signal propagation and detection in compound medium models with different tissue parameters using k-space method based on LSOR-PAM imaging principle. The results show a distance related signal strength attenuation and FOV changes related to transducer angle. Our study provides a method for photoacoustic signal detection optimization for different complex tissue structure with LSOR-PAM.
Quantitative analysis of optical clearing effects (OCE) induced by hyperosmotic agents is very important to optical tissue clearing applications in biomedical diagnostic imaging and therapeutics. This study aims at investigating the effect of glycerol concentration on the laser-scanning optical-resolution photoacoustic microscopy (LSOR-PAM) imaging contrast and light penetration depth. The photoacoustic (PA) signal amplitude changes are evaluated as a function of the concentration of glycerol. The results reveal that the PA signal amplitudes are enhanced with the glycerol concentration increasing, and also show that higher concentration of glycerol produces better light penetration and OCE on a phantom. The PA signal amplitude increases only 8.1% for 20% glycerol, but for higher concentrations, the increases are 76% and 165% for 40% and 60% glycerol, respectively. This preliminary study demonstrates that application of glycerol as an optical contrast agent reduces the tissue scattering and is beneficial to PAM imaging and optical diagnosis in clinical dermatology.
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