Non-invasively focusing light into strongly scattering media, such as biological tissue, is highly desirable but challenging. Recently, ultrasonically guided wavefront shaping technologies have been ...developed to address this limitation. So far, the focusing resolution of most implementations has been limited by acoustic diffraction. Here, we introduce nonlinear photoacoustically guided wavefront shaping (PAWS), which achieves optical diffraction-limited focusing in scattering media. We develop an efficient dual-pulse excitation approach to generate strong nonlinear photoacoustic (PA) signals based on the Grueneisen relaxation effect. These nonlinear PA signals are used as feedback to guide iterative wavefront optimization. As a result, light is effectively focused to a single optical speckle grain on the scale of 5-7 µm, which is ~10 times smaller than the acoustic focus with an enhancement factor of ~6,000 in peak fluence. This technology has the potential to benefit many applications that desire highly confined strong optical focus in tissue.
The temperature-dependent property of the Grueneisen parameter has been employed in photoacoustic imaging mainly to measure tissue temperature. Here we explore this property using a different ...approach and develop Grueneisen relaxation photoacoustic microscopy (GR-PAM), a technique that images nonradiative absorption with confocal optical resolution. GR-PAM sequentially delivers two identical laser pulses with a microsecond-scale time delay. The first laser pulse generates a photoacoustic signal and thermally tags the in-focus absorbers. When the second laser pulse excites the tagged absorbers within the thermal relaxation time, a photoacoustic signal stronger than the first one is produced, owing to the temperature dependence of the Grueneisen parameter. GR-PAM detects the amplitude difference between the two colocated photoacoustic signals, confocally imaging the nonradiative absorption. We greatly improved axial resolution from 45 μm to 2.3 μm and, at the same time, slightly improved lateral resolution from 0.63 μm to 0.41 μm. In addition, the optical sectioning capability facilitates the measurement of the absolute absorption coefficient without fluence calibration.
Wavefront shaping (WFS) is emerging as a promising tool for controlling and focusing light in complex scattering media. The shaping system's speed, the energy gain of the corrected wavefronts, and ...the control degrees of freedom (DOF) are the most important metrics for WFS, especially for highly scattering and dynamic samples. Despite recent advances, current methods suffer from trade-offs that limit satisfactory performance to only one or two of these metrics. Here, we report a WFS technique that simultaneously achieves high speed, high energy gain, and high control DOF. By combining photorefractive crystal-based analog optical phase conjugation (AOPC) and stimulated emission light amplification, our technique achieves an energy gain approaching unity, more than three orders of magnitude larger than conventional AOPC. The response time of ~10
s with about 10
control modes corresponds to an average mode time of about 0.01 ns/mode, which is more than 50 times lower than some of the fastest WFS systems to date. We anticipate that this technique will be instrumental in overcoming the optical diffusion limit in photonics and translate WFS techniques to real-world applications.
Single-shot ultrafast optical imaging can capture two-dimensional transient scenes in the optical spectral range at ≥100 million frames per second. This rapidly evolving field surpasses conventional ...pump-probe methods by possessing the real-time imaging capability, which is indispensable for recording non-repeatable and difficult-to-reproduce events and for understanding physical, chemical, and biological mechanisms. In this mini-review, we survey comprehensively the state-of-the-art single-shot ultrafast optical imaging. Based on the illumination requirement, we categorized the field into active-detection and passive-detection domains. Depending on the specific image acquisition and reconstruction strategies, these two categories are further divided into a total of six sub-categories. Under each sub-category, we describe operating principles, present representative cutting-edge techniques with a particular emphasis on their methodology and applications, and discuss their advantages and challenges. Finally, we envision prospects of technical advancement in this field.
Label-free functional imaging of single red blood cells (RBCs) in vivo holds the key to uncovering the fundamental mechanism of oxygen metabolism in cells. To this end, we developed single-RBC ...photoacoustic flowoxigraphy (FOG), which can image oxygen delivery from single flowing RBCs in vivo with millisecond-scale temporal resolution and micrometer-scale spatial resolution. Using intrinsic optical absorption contrast from oxyhemoglobin (HbO ₂) and deoxyhemoglobin (HbR), FOG allows label-free imaging. Multiple single-RBC functional parameters, including total hemoglobin concentration (C Hb), oxygen saturation (sO ₂), sO ₂ gradient (Formula), flow speed (v f), and oxygen release rate (rO ₂), have been quantified simultaneously in real time. Working in reflection instead of transmission mode, the system allows minimally invasive imaging at more anatomical sites. We showed the capability to measure relationships among sO ₂, Formula, v f, and rO ₂ in a living mouse brain. We also demonstrated that single-RBC oxygen delivery was modulated by changing either the inhalation gas or blood glucose. Furthermore, we showed that the coupling between neural activity and oxygen delivery could be imaged at the single-RBC level in the brain. The single-RBC functional imaging capability of FOG enables numerous biomedical studies and clinical applications.
Photoacoustic (PA) computed tomography (PACT) is a noninvasive hybrid imaging technique that combines optical excitation and acoustic detection to realize high contrast, high resolution, and deep ...penetration in biological tissues. However, the spatial resolution of PACT is limited by acoustic diffraction. Here, we report in vivo superresolution PACT, which breaks the acoustic diffraction limit by localizing the centers of single dyed droplets that are flowing in blood vessels. The droplets were prepared by dissolving hydrophobic absorbing dye in oil, followed by mixing with water. The dyed droplets generate much higher-amplitude PA signals than blood and can flow smoothly in vessels; thus, they are excellent tracers for localization-based superresolution imaging. The in vivo resolution enhancement was demonstrated by continuously imaging the cortical layer of a mouse brain during droplet injection. The droplets that were flowing in the vessels were localized, and their center positions were used to construct a superresolution image that exhibits sharper features and more finely resolved vascular details. An improvement in spatial resolution by a factor of 6 has been realized in vivo by the droplet localization technique.
Photoacoustic (optoacoustic) tomography, combining optical absorption contrast and highly scalable spatial resolution (from micrometer optical resolution to millimeter acoustic resolution), has ...broken through the fundamental penetration limit of optical ballistic imaging modalities-including confocal microscopy, two-photon microscopy, and optical coherence tomography-and has achieved high spatial resolution at depths down to the diffusive regime. Optical absorption contrast is highly desirable for microvascular imaging and characterization because of the presence of endogenous strongly light-absorbing hemoglobin. We focus on the current state of microvascular imaging and characterization based on photoacoustics. We first review the three major embodiments of photoacoustic tomography: microscopy, computed tomography, and endoscopy. We then discuss the methods used to characterize important functional parameters, such as total hemoglobin concentration, hemoglobin oxygen saturation, and blood flow. Next, we highlight a few representative applications in microvascular-related physiological and pathophysiological research, including hemodynamic monitoring, chronic imaging, tumor-vascular interaction, and neurovascular coupling. Finally, several potential technical advances toward clinical applications are suggested, and a few technical challenges in contrast enhancement and fluence compensation are summarized.
Photoacoustic computed tomography (PACT) images optical absorption contrast by detecting ultrasonic waves induced by optical energy deposition in materials such as biological tissues. An ultrasonic ...transducer array or its scanning equivalent is used to detect ultrasonic waves. The spatial distribution of the transducer elements must satisfy the spatial Nyquist criterion; otherwise, spatial aliasing occurs and causes artifacts in reconstructed images. The spatial Nyquist criterion poses different requirements on the transducer elements' distributions for different locations in the image domain, which has not been studied previously. In this research, we elaborate on the location dependency through spatiotemporal analysis and propose a location-dependent spatiotemporal antialiasing method. By applying this method to PACT in full-ring array geometry, we effectively mitigate aliasing artifacts with minimal effects on image resolution in both numerical simulations and in vivo experiments.
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