Two-step proximal gradient descent algorithm for photoacoustic signal unmixing

All-fiber three-wavelength laser for functional photoacoustic microscopy

Advanced multi-wavelength pulsed laser is a key technique for functional optical-resolution photoacoustic microscopy (OR-PAM). By utilizing the stimulated Raman scattering (SRS) effect, we can generate various wavelengths from a single-wavelength pump laser, offering a simple and cost-effective solution for OR-PAM. However, existing multi-wavelength SRS lasers typically require fine alignment of many free-space optical components with single-mode fibers, which are susceptible to mechanical disturbances and temperature fluctuations, leading to high maintenance costs. To address this challenge, we develop an all-fiber three-wavelength SRS laser source for functional OR-PAM. A pump laser beam is launched into an optical fiber network, which splits and delays these laser pulses and generates different optical wavelengths in different fiber branches, and then merges them at the output end of the fiber network. This approach requires only one instance of fiber launching, dramatically simplifying the alignment and improving the laser stability. Using a decoding algorithm, we can separate the PA signals from different optical wavelengths and then calculate oxygen saturation (sO2) and flow speed. The SRS fiber network provides stable energy ratios among different optical wavelengths during long-time operation. We use the all-fiber OR-PAM system to monitor the brain function for four hours, demonstrating exceptional stability in functional imaging. The small size, simple structure, and low cost make it suitable for many preclinical and clinical applications.

Light on Alzheimer's disease: from basic insights to preclinical studies

Alzheimer's disease (AD), referring to a gradual deterioration in cognitive function, including memory loss and impaired thinking skills, has emerged as a substantial worldwide challenge with profound social and economic implications. As the prevalence of AD continues to rise and the population ages, there is an imperative demand for innovative imaging techniques to help improve our understanding of these complex conditions. Photoacoustic (PA) imaging forms a hybrid imaging modality by integrating the high-contrast of optical imaging and deep-penetration of ultrasound imaging. PA imaging enables the visualization and characterization of tissue structures and multifunctional information at high resolution and, has demonstrated promising preliminary results in the study and diagnosis of AD. This review endeavors to offer a thorough overview of the current applications and potential of PA imaging on AD diagnosis and treatment. Firstly, the structural, functional, molecular parameter changes associated with AD-related brain imaging captured by PA imaging will be summarized, shaping the diagnostic standpoint of this review. Then, the therapeutic methods aimed at AD is discussed further. Lastly, the potential solutions and clinical applications to expand the extent of PA imaging into deeper AD scenarios is proposed. While certain aspects might not be fully covered, this mini-review provides valuable insights into AD diagnosis and treatment through the utilization of innovative tissue photothermal effects. We hope that it will spark further exploration in this field, fostering improved and earlier theranostics for AD.

Super-Low-Dose Functional and Molecular Photoacoustic Microscopy

Photoacoustic microscopy can image many biological molecules and nano-agents in vivo via low-scattering ultrasonic sensing. Insufficient sensitivity is a long-standing obstacle for imaging low-absorbing chromophores with less photobleaching or toxicity, reduced perturbation to delicate organs, and more choices of low-power lasers. Here, the photoacoustic probe design is collaboratively optimized and a spectral-spatial filter is implemented. A multi-spectral super-low-dose photoacoustic microscopy (SLD-PAM) is presented that improves the sensitivity by ≈33 times. SLD-PAM can visualize microvessels and quantify oxygen saturation in vivo with ≈1% of the maximum permissible exposure, dramatically reducing potential phototoxicity or perturbation to normal tissue function, especially in imaging of delicate tissues, such as the eye and the brain. Capitalizing on the high sensitivity, direct imaging of deoxyhemoglobin concentration is achieved without spectral unmixing, avoiding wavelength-dependent errors and computational noises. With reduced laser power, SLD-PAM can reduce photobleaching by ≈85%. It is also demonstrated that SLD-PAM achieves similar molecular imaging quality using 80% fewer contrast agents. Therefore, SLD-PAM enables the use of a broader range of low-absorbing nano-agents, small molecules, and genetically encoded biomarkers, as well as more types of low-power light sources in wide spectra. It is believed that SLD-PAM offers a powerful tool for anatomical, functional, and molecular imaging.

In vivo photoacoustic monitoring of vasoconstriction induced by acute hyperglycemia

Postprandial hyperglycemia, blood glucose spikes, induces endothelial dysfunction, increasing cardiovascular risks. Endothelial dysfunction leads to vasoconstriction, and observation of this phenomenon is important for understanding acute hyperglycemia. However, high-resolution imaging of microvessels during acute hyperglycemia has not been fully developed. Here, we demonstrate that photoacoustic microscopy can noninvasively monitor morphological changes in blood vessels of live animals' extremities when blood glucose rises rapidly. As blood glucose level rose from 100 to 400 mg/dL following intraperitoneal glucose injection, heart/breath rate, and body temperature remained constant, but arterioles constricted by approximately -5.7 ± 1.1% within 20 min, and gradually recovered for another 40 min. In contrast, venular diameters remained within about 0.6 ± 1.5% during arteriolar constriction. Our results experimentally and statistically demonstrate that acute hyperglycemia produces transitory vasoconstriction in arterioles, with an opposite trend of change in blood glucose. These findings could help understanding vascular glucose homeostasis and the relationship between diabetes and cardiovascular diseases.

Simultaneous dual-modal photoacoustic and harmonic ultrasound microscopy with an optimized acoustic combiner

Simultaneous photoacoustic (PA) and ultrasound (US) imaging provides rich optical and acoustic contrasts with high sensitivity, specificity, and resolution, making it a promising tool for diagnosing and assessing various diseases. However, the resolution and penetration depth tend to be contradictory due to the increased attenuation of high-frequency ultrasound. To address this issue, we present simultaneous dual-modal PA/US microscopy with an optimized acoustic combiner that can maintain high resolution while improving the penetration of ultrasound imaging. A low-frequency ultrasound transducer is used for acoustic transmission, and a high-frequency transducer is used for PA and US detection. An acoustic beam combiner is utilized to merge the transmitting and receiving acoustic beams with a predetermined ratio. By combining the two different transducers, harmonic US imaging and high-frequency photoacoustic microscopy are implemented. In vivo experiments on the mouse brain demonstrate the simultaneous PA and US imaging ability. The harmonic US imaging of the mouse eye reveals finer iris and lens boundary structures than conventional US imaging, providing a high-resolution anatomical reference for co-registered PA imaging.

Array-based high-intensity focused ultrasound therapy system integrated with real-time ultrasound and photoacoustic imaging

High-intensity focused ultrasound (HIFU) is a promising non-invasive therapeutic technique in clinical applications. Challenges in stimulation or ablation HIFU therapy are to accurately target the treatment spot, flexibly deliver or fast-move focus points in the treatment region, and monitor therapy progress in real-time. In this paper, we develop an array-based HIFU system integrated with real-time ultrasound (US) and photoacoustic (PA) imaging. The array-based HIFU transducer can be dynamically focused in a lateral range of ∼16 mm and an axial range of ∼40 mm via electronically adjusting the excitation phase map. To monitor the HIFU therapy progress in real-time, sequential HIFU transmission, PA imaging, PA thermometry, and US imaging are implemented to display the dual-modal images and record the local temperature changes. Co-registered dual-modal images show structural and functional information and thus can guide the HIFU therapy for precise positioning and dosage control. Besides therapy, the multi-element HIFU transducer can also be used to acquire US images to precisely align the imaging coordinates with the HIFU coordinates. Phantom experiments validate the precise and dynamic steering capability of HIFU ablation. We also show that dual-modal imaging can guide HIFU in the designated region and monitor the temperature in biological tissue in real-time.