Thin-film optical-acoustic combiner enables high-speed wide-field multi-parametric photoacoustic microscopy in reflection mode

Wavelength-time-division multiplexed fiber-optic sensor array for wide-field photoacoustic microscopy

Photoacoustic microscopy (PAM) faces a fundamental trade-off between detection sensitivity and field of view (FOV). While optical ultrasound sensors offer high-sensitivity unfocused detection, implementing multichannel detection remains challenging. Here, we present a wavelength-time-division multiplexed (WTDM) fiber-optic sensor array that assigns distinct wavelengths to individual sensors and employs varying-length delay fibers for temporal separation, enabling efficient multichannel detection through a single photodetector. Using a 4-element sensor array, we achieved an expanded FOV of 5 × 8 mm² while maintaining high temporal resolution (160 kHz A-line rate, 0.25 Hz frame rate) and microscopic spatial resolution (10.7 μm). The system's capabilities were validated through comparative monitoring of cerebral and intestinal hemodynamics in mice during hypercapnia challenge, revealing distinct temporal patterns with notably delayed recovery in cerebral vascular response compared to intestinal vasculature. This WTDM approach establishes a promising platform for large-field, high-speed photoacoustic imaging in biomedical applications.

Mid-Infrared Photoacoustic Stimulation of Neurons through Vibrational Excitation in Polydimethylsiloxane

Photoacoustic (PA) emitters are emerging ultrasound sources offering high spatial resolution and ease of miniaturization. Thus far, PA emitters rely on electronic transitions of absorbers embedded in an expansion matrix such as polydimethylsiloxane (PDMS). Here, it is shown that mid-infrared vibrational excitation of C─H bonds in a transparent PDMS film can lead to efficient mid-infrared photoacoustic conversion (MIPA). MIPA shows 37.5 times more efficient than the commonly used PA emitters based on carbon nanotubes embedded in PDMS. Successful neural stimulation through MIPA both in a wide field with a size up to a 100 µm radius and in single-cell precision is achieved. Owing to the low heat conductivity of PDMS, less than a 0.5 °C temperature increase is found on the surface of a PDMS film during successful neural stimulation, suggesting a non-thermal mechanism. MIPA emitters allow repetitive wide-field neural stimulation, opening up opportunities for high-throughput screening of mechano-sensitive ion channels and regulators.

Perspectives on endoscopic functional photoacoustic microscopy

Endoscopy, enabling high-resolution imaging of deep tissues and internal organs, plays an important role in basic research and clinical practice. Recent advances in photoacoustic microscopy (PAM), demonstrating excellent capabilities in high-resolution functional imaging, have sparked significant interest in its integration into the field of endoscopy. However, there are challenges in achieving functional PAM in the endoscopic setting. This Perspective article discusses current progress in the development of endoscopic PAM and the challenges related to functional measurements. Then, it points out potential directions to advance endoscopic PAM for functional imaging by leveraging fiber optics, microfabrication, optical engineering, and computational approaches. Finally, it highlights emerging opportunities for functional endoscopic PAM in basic and translational biomedicine.

Cortex-wide transcranial localization microscopy with fluorescently labeled red blood cells

Large-scale imaging of brain activity with high spatio-temporal resolution is crucial for advancing our understanding of brain function. The existing neuroimaging techniques are largely limited by restricted field of view, slow imaging speed, or otherwise do not have the adequate spatial resolution to capture brain activities on a capillary and cellular level. To address these limitations, we introduce fluorescence localization microscopy aided with sparsely-labeled red blood cells for cortex-wide morphological and functional cerebral angiography with 4.9 µm spatial resolution and 1 s temporal resolution. When combined with fluorescence calcium imaging, the proposed method enables extended recordings of stimulus-evoked neuro-vascular changes in the murine brain while providing simultaneous multiparametric readings of intracellular neuronal activity, blood flow velocity/direction/volume, and vessel diameter. Owing to its simplicity and versatility, the proposed approach will become an invaluable tool for deciphering the regulation of cortical microcirculation and neurovascular coupling in health and disease.

High spatiotemporal mapping of cortical blood flow velocity with an enhanced accuracy

Cerebral blood flow velocity is one of the most essential parameters related to brain functions and diseases. However, most existing mapping methods suffer from either inaccuracy or lengthy sampling time. In this study, we propose a particle-size-related calibration method to improve the measurement accuracy and a random-access strategy to suppress the sampling time. Based on the proposed methods, we study the long-term progress of cortical vasculopathy and abnormal blood flow caused by glioma, short-term variations of blood flow velocity under different anesthetic depths, and cortex-wide connectivity of the rapid fluctuation of blood flow velocities during seizure onset. The experimental results demonstrate that the proposed calibration method and the random-access strategy can improve both the qualitative and quantitative performance of velocimetry techniques and are also beneficial for understanding brain functions and diseases from the perspective of cerebral blood flow.

In vivo structural and functional imaging of human nailbed microvasculature using photoacoustic microscopy

Monitoring microvascular structure and function is of great significance for the diagnosis of many diseases. In this study, we demonstrate the feasibility of OR-PAM to nailbed microcirculation detection as a new, to the best of our knowledge, application scenario in humans. We propose a dual-wavelength optical-resolution photoacoustic microscopy (OR-PAM) with improved local-flexible coupling to image human nailbed microvasculature. Microchip lasers with 532 nm wavelength are employed as the pump sources. The 558 nm laser is generated from the 532 nm laser through the stimulated Raman scattering effect. The flowing water, circulated by a peristaltic pump, maintains the acoustic coupling between the ultrasonic transducer and the sample. These designs improve the sensitivity, practicality, and stability of the OR-PAM system for human in vivo experiments. The imaging of the mouse ear demonstrates the ability of our system to acquire structural and functional information. Then, the system is applied to image human nailbed microvasculature. The imaging results reveal that the superficial capillaries are arranged in a straight sagittal pattern, approximately parallel to the long axis of the finger. The arterial and venular limbs are distinguished according to their oxygen saturation differences. Additionally, the images successfully discover the capillary loops with single or multiple twists, the oxygen release at the end of the capillary loop, and the changes when the nailbed is abnormal.