Hemodynamic and oxygen-metabolic responses of the awake mouse brain to hypercapnia revealed by multi-parametric photoacoustic microscopy

Photoacoustic imaging of squirrel monkey cortical responses induced by peripheral mechanical stimulation

Non-human primates (NHPs) are crucial models for studies of neuronal activity. Emerging photoacoustic imaging modalities offer excellent tools for studying NHP brains with high sensitivity and high spatial resolution. In this research, a photoacoustic microscopy (PAM) device was used to provide a label-free quantitative characterization of cerebral hemodynamic changes due to peripheral mechanical stimulation. A 5 × 5 mm area within the somatosensory cortex region of an adult squirrel monkey was imaged. A deep, fully connected neural network was characterized and applied to the PAM images of the cortex to enhance the vessel structures after mechanical stimulation on the forelimb digits. The quality of the PAM images was improved significantly with a neural network while preserving the hemodynamic responses. The functional responses to the mechanical stimulation were characterized based on the improved PAM images. This study demonstrates capability of PAM combined with machine learning for functional imaging of the NHP brain.

Free-moving-state microscopic imaging of cerebral oxygenation and hemodynamics with a photoacoustic fiberscope

We report the development of a head-mounted photoacoustic fiberscope for cerebral imaging in a freely behaving mouse. The 4.5-gram imaging probe has a 9-µm lateral resolution and 0.2-Hz frame rate over a 1.2-mm wide area. The probe can continuously monitor cerebral oxygenation and hemodynamic responses at single-vessel resolution, showing significantly different cerebrovascular responses to external stimuli under anesthesia and in the freely moving state. For example, when subjected to high-concentration CO2 respiration, enhanced oxygenation to compensate for hypercapnia can be visualized due to cerebral regulation in the freely moving state. Comparative studies exhibit significantly weakened compensation capabilities in obese rodents. This new imaging modality can be used for investigating both normal and pathological cerebrovascular functions and shows great promise for studying cerebral activity, disorders and their treatments.

Effects of Acute and Endurance Exercise on Cerebrovascular Function and Oxygen Metabolism: A Photoacoustic Microscopy Study

Regular exercise improves the cerebrovascular function and has shown considerable therapeutic effects on a wide variety of brain diseases. However, the influence of exercise on different aspects of the cerebrovascular function remains to be comprehensively examined. In this study, we combined awake-brain photoacoustic microscopy (PAM) and a motorized treadmill to assess the effects of both acute exercise stimulation and endurance exercise training on the cerebrovascular function and cerebral oxygen metabolism under both physiological and pathological conditions. Acute exercise stimulation in nondiabetic mice resulted in robust vasodilation, increased cerebral blood flow (CBF), reduced oxygen extraction fraction (OEF), and unchanged cerebral metabolic rate of oxygen (CMRO2)-demonstrating the utility of this experimental setting to evaluate the cerebrovascular reactivity. Also, endurance exercise training for six weeks in diabetic mice reversed the diabetes-induced increases in the resting-state CBF and CMRO2 and maintained a stable OEF and CMRO2 under the acute exercise stimulation-shedding new light on how exercise protects the brain from diabetes-induced small vessel disease. In summary, we established an experimental approach to assess the effects of both acute exercise stimulation and endurance exercise training on the cerebrovascular function and tissue oxygen metabolism at the microscopic level and applied it to study the therapeutic benefits of endurance exercise training in diabetic mice.

Laminar differences in functional oxygen metabolism in monkey visual cortex measured with calibrated fMRI

Blood-oxygenation-level-dependent functional magnetic resonance imaging (BOLD fMRI) of cortical layers relies on the hemodynamic response and is biased toward large veins on the cortical surface. Functional changes in the cerebral metabolic rate of oxygen (ΔCMRO2) may reflect neural cortical function better than BOLD fMRI, but it is unknown whether the calibrated BOLD model for functional CMRO2 measurement remains valid at high resolution. Here, we measure laminar ΔCMRO2 elicited by visual stimulation in macaque primary visual cortex (V1) and find that ΔCMRO2 peaks in the middle of the cortex, in agreement with autoradiographic measures of metabolism. ΔCMRO2 values in gray matter are similar as found previously. Reductions in CMRO2 are associated with veins at the cortical surface, suggesting that techniques for vein removal may improve the accuracy of the model at very high resolution. However, our results show feasibility of laminar ΔCMRO2 measurement, providing a physiologically meaningful metric of laminar functional metabolism.

Cortical microvascular blood flow velocity mapping by combining dynamic light scattering optical coherence tomography and two-photon microscopy

Significance

The accurate large-scale mapping of cerebral microvascular blood flow velocity is crucial for a better understanding of cerebral blood flow (CBF) regulation. Although optical imaging techniques enable both high-resolution microvascular angiography and fast absolute CBF velocity measurements in the mouse cortex, they usually require different imaging techniques with independent system configurations to maximize their performances. Consequently, it is still a challenge to accurately combine functional and morphological measurements to co-register CBF speed distribution from hundreds of microvessels with high-resolution microvascular angiograms.

Aim

We propose a data acquisition and processing framework to co-register a large set of microvascular blood flow velocity measurements from dynamic light scattering optical coherence tomography (DLS-OCT) with the corresponding microvascular angiogram obtained using two-photon microscopy (2PM).

Approach

We used DLS-OCT to first rapidly acquire a large set of microvascular velocities through a sealed cranial window in mice and then to acquire high-resolution microvascular angiograms using 2PM. The acquired data were processed in three steps: (i) 2PM angiogram coregistration with the DLS-OCT angiogram, (ii) 2PM angiogram segmentation and graphing, and (iii) mapping of the CBF velocities to the graph representation of the 2PM angiogram.

Results

We implemented the developed framework on the three datasets acquired from the mice cortices to facilitate the coregistration of the large sets of DLS-OCT flow velocity measurements with 2PM angiograms. We retrieved the distributions of red blood cell velocities in arterioles, venules, and capillaries as a function of the branching order from precapillary arterioles and postcapillary venules from more than 1000 microvascular segments.

Conclusions

The proposed framework may serve as a useful tool for quantitative analysis of large microvascular datasets obtained by OCT and 2PM in studies involving normal brain functioning, progression of various diseases, and numerical modeling of the oxygen advection and diffusion in the realistic microvascular networks.

Visualizing the early-stage testicular torsion by dual-modal photoacoustic and ultrasound imaging

Delayed treatment of testicular torsion (TT) can lead to permanent loss of reproductive capacity. Photoacoustic imaging (PAI) and ultrasound imaging (USI) was tested for detecting TT at early stage in mice based on PAI-obtained oxygen saturation (sO₂), and USI-collected color pixel density (CPD), peak systolic velocity (PSV) and resistance index (RI). For complete TT, both CPD (9.08 % ± 3.084 to almost zero) and sO₂ data (70.09 % ± 1.656-59.84 % ± 1.427) showed an significant change 2 h post-torsion. For incomplete TT, sO₂ data exhibited a strong time relationship (Mean values: 6 h, 64.83 % ± 1.898; 12 h, 60.67 % ± 3.555; 24 h, 57.85 % ± 3.575; P < 0.05). However, USI-collected CPD, PSV or IR data from the same TT models showed no significant difference. This study indicated that USI and PAI could identify complete TT. Meanwhile, PAI has shown great potential in the diagnosis of incomplete TT within 24 h based on time-related sO₂ map.

Freehand scanning photoacoustic microscopy with simultaneous localization and mapping

Optical-resolution photoacoustic microscopy offers high-resolution, label-free hemodynamic and functional imaging to many biomedical applications. However, long-standing technical barriers, such as limited field of view, bulky scanning probes, and slow imaging speed, have limited the application of optical-resolution photoacoustic microscopy. Here, we present freehand scanning photoacoustic microscopy (FS-PAM) that can flexibly image various anatomical sites. We develop a compact handheld photoacoustic probe to acquire 3D images with high speed, and great flexibility. The high scanning speed not only enables video camera mode imaging but also allows for the first implementation of simultaneous localization and mapping (SLAM) in photoacoustic microscopy. We demonstrate fast in vivo imaging of some mouse organs, and human oral mucosa. The high imaging speed greatly reduces motion artifacts and distortions from tissue moving, breathing, and unintended handshaking. We demonstrate small-lesion localization in a large region of the brain. FS-PAM offers a flexible high-speed imaging tool with an extendable field of view, enabling more biomedical imaging applications.

Functional photoacoustic microscopy of hemodynamics: a review

Functional blood imaging can reflect tissue metabolism and organ viability, which is important for life science and biomedical studies. However, conventional imaging modalities either cannot provide sufficient contrast or cannot support simultaneous multi-functional imaging for hemodynamics. Photoacoustic imaging, as a hybrid imaging modality, can provide sufficient optical contrast and high spatial resolution, making it a powerful tool for in vivo vascular imaging. By using the optical-acoustic confocal alignment, photoacoustic imaging can even provide subcellular insight, referred as optical-resolution photoacoustic microscopy (OR-PAM). Based on a multi-wavelength laser source and developed the calculation methods, OR-PAM can provide multi-functional hemodynamic microscopic imaging of the total hemoglobin concentration (C_Hb), oxygen saturation (sO₂), blood flow (BF), partial oxygen pressure (pO₂), oxygen extraction fraction, and metabolic rate of oxygen (MRO₂). This concise review aims to systematically introduce the principles and methods to acquire various functional parameters for hemodynamics by photoacoustic microscopy in recent studies, with characteristics and advantages comparison, typical biomedical applications introduction, and future outlook discussion.