Multi-wavelength photo-acoustic microscopy in the frequency domain for simultaneous excitation and detection of dyes

Bimodal optical and optoacoustic multiview microscope in the frequency-domain

Hybrid fluorescence and optoacoustic microscopy systems have recently emerged as powerful imaging modalities concurrently capturing both radiative and non-radiative molecular relaxations in biological tissues. Nevertheless, such approaches provide limited information as specimens are imaged exclusively from one side, not permitting the acquisition of their full anatomical, structural, or functional features in multiple views of interest. Herein we present a bimodal optical and optoacoustic multiview (BOOM) cost-efficient microscope operating in the frequency-domain for the comprehensive label-free imaging of established and emerging model organisms. Thus, the capabilities of BOOM microscopy have been proven suitable for highly demanding observations in developmental biology and embryology.

Deep learning-assisted frequency-domain photoacoustic microscopy

Frequency-domain photoacoustic microscopy (FD-PAM) constitutes a powerful cost-efficient imaging method integrating intensity-modulated laser beams for the excitation of single-frequency photoacoustic waves. Nevertheless, FD-PAM provides an extremely small signal-to-noise ratio (SNR), which can be up to two orders of magnitude lower than the conventional time-domain (TD) systems. To overcome this inherent SNR limitation of FD-PAM, we utilize a U-Net neural network aiming at image augmentation without the need for excessive averaging or the application of high optical power. In this context, we improve the accessibility of PAM as the system's cost is dramatically reduced, and we expand its applicability to demanding observations while retaining sufficiently high image quality standards.

Imaging Parhyale hawaiensis embryogenesis with frequency domain photoacoustic microscopy: A novel tool in developmental biology

We present the application of a low-cost frequency domain photoacoustic (FDPA) microscope for the label-free imaging of live developing embryos of the crustacean model organism Parhyale hawaiensis. By modulating the intensity of a continuous wave laser source at 9.5 MHz, we achieve the excitation of monochromatic PA waves, which are detected to provide amplitude and phase recordings. The data are subsequently processed to generate accurate maximum amplitude projection and surface reconstructions, delineating the morphological features of the embryos with high resolution and contrast. The findings of this study pave the way for the broader adoption of inexpensive PA diagnostic techniques in developmental biology, shedding light on various fundamental processes in established and emerging model organisms.

Low-Cost Multi-Wavelength Photoacoustic Imaging Based on Portable Continuous-Wave Laser Diode Module

Photoacoustic imaging (PAI), an emerging imaging technique, exploits the merits of both optical and ultrasound imaging, equipped with optical contrast and deep penetration. Typical linear PAI relies on a nanosecond laser pulse to induce photoacoustic signals. To construct a multi-wavelength PAI system, a multi-wavelength nano-second laser source is required, which greatly increases the cost of the PAI system. However, according to the nonlinear photoacoustic effect, the amplitude of the photoacoustic signals will vary with different base temperatures of the tissue. Therefore, using continuous-wave lasers with different wavelengths to induce different temperature variations at the same point of the tissue, and then using a single-wavelength pulsed laser to induce photoacoustic signals has been an alternative method to achieve multi-wavelength PAI. In this paper, based on the nonlinear photoacoustic effect, we developed a continuous-wave multi-wavelength laser source to cut down the cost of the conventional multi-wavelength PAI system. The principle will be introduced firstly, followed by qualitative and quantitative experiments.

Vascular bifurcation mapping with photoacoustic microscopy

The early detection of microvascular changes in cancer diagnosis is needed in the clinic. A change in the vascular bifurcation density is a biomarker for the sprouting activity. Here, Optical-Resolution PhotoAcoustic Microscopy is used for quantitative vascular bifurcation mapping in 2D after the creation of Virtual Tubes out of Bifurcations. In stacks of OR-PAM images of the hemoglobin distribution, bifurcations become tubes and are selected by the 3D tubeness filter. These fast analyses will be compared to a classical approach and are easier to implement for functional analysis of the vascular bifurcation density in healthy and diseased tissues.

Toward Wearable Healthcare: A Miniaturized 3D Imager With Coherent Frequency-Domain Photoacoustics

Medical monitoring is undergoing a translation from the hospital-based system to the personalized home-based system. With the aim of wearable application of photoacoustic technique, we propose a miniaturized photoacoustic 3D imager for superficial medical imaging. By employing the compact continuous-wave laser diode based optical irradiation and an ultrathin 2D matrix array based photoacoustic detection in the coherent frequency domain, a wearable imaging probe with a size of about 80 × 25 × 24 mm³ and a weight of 21 g is developed. At the backend, an FPGA controlled Howland current source drives the laser diodes to excite linear frequency modulated optical irradiation. Recorded by a portable multichannel data acquisition system, the generated photoacoustic responses are firstly compressed with the coherent frequency domain photoacoustic method and then extrapolated in the wavenumber-frequency domain for fast image reconstruction. With three-wavelength (450 nm, 638 nm, and 808 nm) laser irradiation, photoacoustic imaging can be operated multispectrally, endowing the developed imager with functional imaging capability in 3D space. With the imager worn on the human forearm, hemoglobin oxygen saturation level in superficial arm vasculature can be long-term monitored with high stability. When the imager is applied for imaging in a relatively large area (e.g., early melanoma detection in the human breast), flexible scanning in a handheld manner can be performed. This work opens the application potential of photoacoustic technique in a broad range of areas, including personalized healthcare, home health monitoring, and long-term physiologic monitoring.

Photo-acoustic tomography based on laser optical feedback imaging of surface displacements

We present how a laser optical feedback imaging (LOFI) setup can be used for the optical detection of ultrasound in photo-acoustic tomography (PAT). A PAT image is reconstructed by an inversion algorithm using surface displacement measurements made at several locations with our LOFI setup and following the optical irradiation with a pulsed Nd:YAG laser of a sample with absorbing inclusions. The width of the reconstructed inclusions and the signal-to-noise ratio (SNR) of the reconstructed images are first studied on the numerical model of a sample with three absorbing inclusions (i.e., with three acoustic punctual sources). Finally, an experimental PAT image of a phantom composed of two polyamide tubes with an internal diameter of 800 μm filled with red ink and submerged at -3.5  mm depth in a tank filled with water is reconstructed. Experimentally, the water surface displacement measurements have been made with our LOFI vibrometer, which provides an amplitude sensitivity of 1 nm (for a single-shot measurement) in a detection bandwidth of roughly 1 MHz adapted to the detection of the polyamide tubes. Under our experimental conditions, the surface energy densities of the LOFI focalized beam for the detection and of the pulsed Nd:YAG laser used for the irradiation, are compatible with the maximum permissive exposure for future biomedical measurements. The SNR and the resolution of the reconstructed PAT images are in good agreement with the theoretical predictions.