While photoacoustic imaging (PAI) continues to undergo clinical translation, evaluation of PAI devices remains challenging. Computational modeling offers an inexpensive and convenient approach to study fundamentals of device performance. Our objective was to develop a model to accurately predict PAI out-of-plane sensitivity effects. We combined 3D optical Monte Carlo simulations with the 2D and 3D acoustic propagation models in the k-Wave toolbox. Reconstructed images of an in silico phantom using 3D data showed that longer targets had higher contrast than shorter targets due to out-of-plane signal contributions. 3D computation modeling provided insights into PAI working mechanisms and performance limitations.
Photoacoustic imaging (PAI) and photoacoustic microscopy (PAM) continue to undergo advancement, but standardized performance test methods are needed to facilitate development and translation. Most tissue-mimicking materials (TMMs) have not been adequately characterized at high acoustic frequencies relevant to PAM systems (>20 MHz). We characterized acoustic properties of various polyacrylamide TMM formulations over 10-60 MHz using a pulse-echo method, while optical properties were characterized over 400-1000 nm. We evaluated performance of a custom PAM system using phantoms containing gold nanoparticles. Polyacrylamide had highly tunable acoustic properties similar to human skin, and performance tests provided key insights into PAM system performance.
To date, most optical-resolution photoacoustic microscopy (OR-PAM) systems rely on mechanical scanning with confocally aligned optical excitation and ultrasonic detection. As a result, the imaging speed of these systems is limited by the scanning speed. Although several multifocal OR-PA computed tomography (MFOR-PACT) systems had been developed to address this limitation, they were hindered by the complex design in a constrained physical space. Here, we present a two-dimensional (2D) MFOR-PAM system based on a 2D microlens array and an acoustic ergodic relay. This system is able to detect PA signals generated from 400 optical foci in parallel with a single-element transducer, and then raster scan the optical foci patterns to form an image. This system has improved the imaging resolution of a conventional photoacoustic ergodic relay system from 220 μm to 13 μm. Moreover, this system has reduced the imaging time of a conventional OR-PAM system at the same resolution and laser repetition rate by 400 times. We demonstrated the ability of the system with both in vitro and in vivo experiments.
Optical-resolution photoacoustic microscopy (OR-PAM) has demonstrated fast, label-free volumetric imaging of optical-absorption contrast within the quasiballistic regime of photon scattering. However, the limited numerical aperture of the ultrasonic transducer restricts the detectability of the photoacoustic waves, thus resulting in incomplete reconstructed features. To tackle the limited-view problem, we added an oblique illumination beam to the original coaxial optical-acoustic scheme to provide a complementary detection view. The virtual augmentation of the detection view was validated through numerical simulations and tissue-phantom experiments. More importantly, the combination of top and oblique illumination successfully imaged a mouse brain in vivo down to 1 mm in depth, showing detailed brain vasculature. Of special note, it clearly revealed the diving vessels that were long missing in images from original OR-PAM.
In biomedical imaging, all optical techniques face a fundamental trade-off between spatial resolution and tissue penetration. Therefore, obtaining an organelle-level resolution image of a whole organ has remained a challenging and yet appealing scientific pursuit. Over the past decade, optical microscopy assisted by mechanical sectioning or chemical clearing of tissue has been demonstrated as a powerful technique to overcome this dilemma, one of particular use in imaging the neural network. However, this type of techniques needs lengthy special preparation of the tissue specimen, which hinders broad application in life sciences. Here, we propose a new label-free three-dimensional imaging technique, named microtomy-assisted photoacoustic microscopy (mPAM), for potentially imaging all biomolecules with 100% endogenous natural staining in whole organs with high fidelity. We demonstrate the first label-free mPAM, using UV light for label-free histology-like imaging, in whole organs (e.g., mouse brains), most of them formalin-fixed and paraffin- or agarose-embedded for minimal morphological deformation. Furthermore, mPAM with dual wavelength illuminations is also employed to image a mouse brain slice, demonstrating the potential for imaging of multiple biomolecules without staining. With visible light illumination, mPAM also shows its deep tissue imaging capability, which enables less slicing and hence reduces sectioning artifacts. mPAM could potentially provide a new insight for understanding complex biological organs.
As a window on the microcirculation, human cuticle capillaries provide rich information about the microvasculature, such as its morphology, density, dimensions, or even blood flow speed. Many imaging technologies have been employed to image human cuticle microvasculature. However, almost none of these techniques can noninvasively observe the process of oxygen release from single red blood cells (RBCs), an observation which can be used to study healthy tissue functionalities or to diagnose, stage, or monitor diseases. For the first time, we adapted single-cell resolution photoacoustic (PA) microscopy (PA flowoxigraphy) to image cuticle capillaries and quantified multiple functional parameters. Our results show more oxygen release in the curved cuticle tip region than in other regions of a cuticle capillary loop, associated with a low of RBC flow speed in the tip region. Further analysis suggests that in addition to the RBC flow speed, other factors, such as the drop of the partial oxygen pressure in the tip region, drive RBCs to release more oxygen in the tip region.
Lipid-correlated disease such as atherosclerosis has been an important medical research topic for decades. Many new microscopic imaging techniques such as coherent anti-Stokes Raman scattering and third-harmonic generation (THG) microscopy were verified to have the capability to target lipids in vivo. In the case of THG microscopy, biological cell membranes and lipid bodies in cells and tissues have been shown as good sources of contrast with a laser excitation wavelength around 1200 nm. We report the THG excitation spectroscopy study of two pure free fatty acids including oleic acid and linoleic acid from 1090 to 1330 nm. Different pure fatty acids presented slightly-different THG χ(3) spectra. The measured peak values of THG third-order susceptibility χ(3) in both fatty acids were surprisingly found not to match completely with the resonant absorption wavelengths around 1190 to 1210 nm, suggesting possible wavelengths selection for enhanced THG imaging of lipids while avoiding laser light absorption. Along with the recent advancement in THG imaging, this new window between 1240 to 1290 nm may offer tremendous new opportunities for sensitive label-free lipid imaging in biological tissues.
Transdermal drug delivery, transporting the drug molecules through epidermis to dermis, has been extensively investigated but not studied in dynamic detail. The objective of this study is to monitor the dynamical changes of drug permeation and that of polarization of skin tissue with oleic acid treatment. We utilize two-photon fluorescence microscopy (TPM) to investigate the dynamics of transdermal drug delivery in skin excised from the abdominal region of euthenized nude mice with sulforhodamine B (SRB) modeling as a drug and Laurdan serving as a polarity indicator. The treatment of oleic acid increases the permeation rate of SRB, quickly reaching to the steady state of permeation. Increases in polarity within the skin tissue (in both intercellular and intracellular region of stratum corneum) are observed in SRB permeation enhanced by oleic acid treatment. TPM has successfully demonstrated the ability to study spatial distribution of transport dynamics in oleic acid-enhanced transdermal delivery.
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