Wearable optical resolution photoacoustic microscopy

Optical resolution photoacoustic microscopy (ORPAM) is an emerging imaging technique, which has been extensively used to study various brain activities and disorders of the anesthetized/restricted rodents with a special focus on the morphological and functional visualization of cerebral cortex. However, it is challenging to develop a wearable photoacoustic microscope, which enables the investigation of brain activities/disorders on freely moving rodents. Here, we report a wearable and robust optical resolution photoacoustic microscope (W‐ORPAM), which utilizes a small, light, stable and fast optical scanner. This wearable imaging probe features high spatiotemporal resolution, large field of view (FOV) and easy assembly as well as adjustable optical focus during the in vivo experiment, which makes it accessible to image cerebral cortex activities of freely moving rodents. To demonstrate the advantages of this technique, we used W‐ORPAM to monitor both morphological and functional variations of vasculature in cerebral cortex during the induction of ischemia and reperfusion of a freely moving rat.     

Three‐dimensional interventional photoacoustic imaging for biopsy needle guidance with a linear array transducer

Needle placement is important for many clinical interventions, such as tissue biopsy, regional anesthesia and drug delivery. It is essential to visualize the spatial position of the needle and the target tissue during the interventions using appropriate imaging techniques. Based on the contrast of optical absorption, photoacoustic imaging is well suited for the guidance of interventional procedures. However, conventional photoacoustic imaging typically provides two‐dimensional (2D) slices of the region of interest and could only visualize the needle and the target when they are within the imaging plane of the probe at the same time. This requires great alignment skill and effort. To ease this problem, we developed a 3D interventional photoacoustic imaging technique by fast scanning a linear array ultrasound probe and stitching acquired image slices. in vivo sentinel lymph node biopsy experiment shows that the technique could precisely locate a needle and a sentinel lymph node in a tissue volume while a perfusion experiment demonstrates that the technique could visualize the 3D distribution of injected methylene blue dye underneath the skin at high temporal and spatial resolution. The proposed technique provides a practical way for photoacoustic image‐guided interventions.      

Correcting the limited view in optical‐resolution photoacoustic microscopy

Optical‐resolution photoacoustic microscopy (OR‐PAM) has proven useful for anatomical and functional imaging with high spatial resolutions. However, the coherent signal generation and the desired reflection‐mode detection in OR‐PAM can result in a limited detectability of features aligned with the acoustic axis (ie, vertical structures). Here, we investigated the limited‐view phenomenon in OR‐PAM by simulating the generation and propagation of the acoustic pressure waves and determined the key optical parameters affecting the visibility of vertical structures. Proof‐of‐concept numerical experiments were performed with different illumination angles, optical foci and numerical apertures (NA) of the objective lens. The results collectively show that an NA of 0.3 can readily improve the visibility of vertical structures in a typical reflection‐mode OR‐PAM system. This conclusion was confirmed by numerical simulations on the cortical blood vessels in a mouse brain and by experiments in a suture‐cross phantom and in a mouse brain in vivo.      

Low‐photobleaching line‐scanning confocal microscopy using dual inclined beams

Confocal microscopy is an indispensable tool for biological imaging due to its high resolution and optical sectioning capability. However, its slow imaging speed and severe photobleaching have largely prevented further applications. Here, we present dual inclined beam line‐scanning (LS) confocal microscopy. The reduced excitation intensity of our imaging method enabled a 2‐fold longer observation time of fluorescence compared to traditional LS microscopy while maintaining a good sectioning capability and single‐molecule sensitivity. We characterized the performance of our method and applied it to subcellular imaging and three‐dimensional single‐molecule RNA imaging in mammalian cells.      

Self‐referenced axial chromatic dispersion measurement in multiphoton microscopy through 2‐color third‐harmonic generation imaging

With tunable excitation light, multiphoton microscopy is widely used for imaging biological structures at subcellular resolution. Axial chromatic dispersion, present in virtually every transmissive optical system including the multiphoton microscope, leads to focal (and the resultant image) plane separation. Here, we experimentally demonstrate a technique to measure the axial chromatic dispersion in a multiphoton microscope, using simultaneous 2‐color third‐harmonic generation imaging excited by a 2‐color soliton source with tunable wavelength separation. Our technique is self‐referenced, eliminating potential measurement error when 1‐color tunable excitation light is used which necessitates reciprocating motion of the mechanical translation stage. Using this technique, we demonstrate measured axial chromatic dispersion with 2 different objective lenses in a multiphoton microscope. Further measurement in a biological sample also indicates that this axial chromatic dispersion, in combination with 2‐color imaging, may open up opportunity for simultaneous imaging of 2 different axial planes.      

Real‐time functional optical‐resolution photoacoustic microscopy using high‐speed alternating illumination at 532 and 1064 nm

Optical‐resolution photoacoustic microscopy (OR‐PAM), which has been widely used and studied as a noninvasive and in vivo imaging technique, can yield high‐resolution and absorption contrast images. Recently, metallic nanoparticles and dyes, such as gold nanoparticles, methylene blue, and indocyanine green, have been used as contrast agents of OR‐PAM. This study demonstrates real‐time functional OR‐PAM images with high‐speed alternating illumination at 2 wavelengths. To generate 2 wavelengths, second harmonic generation at 532 nm with an LBO crystal and a pump wavelength of 1064 nm is applied at a pulse repetition rate of 300 kHz. For alternating illumination, an electro‐optical modulator is used as an optical switch. Therefore, the A‐line rate for the functional image is 150 kHz, which is half of the laser repetition rate. To enable fast signal processing and real‐time displays, parallel signal processing using a graphics processing unit (GPU) is performed. OR‐PAM images of the distribution of blood vessels and gold nanorods in a BALB/c‐nude mouse's ear can be simultaneously obtained with 500 × 500 pixels and real‐time display at 0.49 fps.      

Quantitative investigation of vascular response to mesenteric venous thrombosis using large‐field‐of‐view photoacoustic microscopy

Mesenteric venous thrombosis (MVT) is one of major causes leading to severe mesenteric ischemia. Vascular network plays an important role during the occurrence and development of MVT. However, there lacks an appropriate imaging method, which features advanced volumetric resolving capability, superior sensitivity to hemoglobin, and ultra‐large field‐of‐view (FOV), to investigate vascular response of MVT. In this study, we developed and applied a large‐FOV optical resolution photoacoustic microscopy to quantify the vascular response during the entire course of two different MVT models in which we ligated the superior mesenteric vein and inferior mesenteric vein, respectively. Furthermore, we developed a quantitative algorithm to derive total vascular length, relative concentration of total hemoglobin and vascular density over the FOV to reveal different vascular responses in different MVT models.     

In vivo label‐free functional photoacoustic monitoring of ischemic reperfusion

Pressure ulcer formation is a common problem among patients confined to bed or restricted to wheelchairs. The ulcer forms when the affected skin and underlying tissues go through repeated cycles of ischemia and reperfusion, leading to inflammation. This theory is evident by intravital imaging studies performed in immune cell–specific, fluorescent reporter mouse skin with induced ischemia‐reperfusion (I‐R) injuries. However, traditional confocal or multiphoton microscopy cannot accurately monitor the progression of vascular reperfusion by contrast agents, which leaks into the interstitium under inflammatory conditions. Here, we develop a dual‐wavelength micro electro mechanical system (MEMS) scanning–based optical resolution photoacoustic microscopy (OR‐PAM) system for continuous label‐free functional imaging of vascular reperfusion in an IR mouse model. This MEMS‐OR‐PAM system provides fast scanning speed for concurrent dual‐wavelength imaging, which enables continuous monitoring of the reperfusion process. During reperfusion, the revascularization of blood vessels and the oxygen saturation (sO₂) changes in both arteries and veins are recorded, from which the local oxygen extraction ratios of the ischemic tissue and the unaffected tissue can be quantified. Our MEMS‐OR‐PAM system provides novel perspectives to understand the I‐R injuries. It solves the problem of dynamic label‐free functional monitoring of the vascular reperfusion at high spatial resolution.      

Portable optical‐resolution photoacoustic microscopy for volumetric imaging of multiscale organisms

Photoacoustic microscopy (PAM) provides a fundamentally new tool for a broad range of studies of biological structures and functions. However, the use of PAM has been largely limited to small vertebrates due to the large size/weight and the inconvenience of the equipment. Here, we describe a portable optical‐resolution photoacoustic microscopy (pORPAM) system for 3‐dimensional (3D) imaging of small‐to‐large rodents and humans with a high spatiotemporal resolution and a large field of view. We show extensive applications of pORPAM to multiscale animals including mice and rabbits. In addition, we image the 3D vascular networks of human lips, and demonstrate the feasibility of pORPAM to observe the recovery process of oral ulcer and cancer‐associated capillary loops in human oral cavities. This technology is promising for broad biomedical studies from fundamental biology to clinical diseases.      

Three‐dimensional cellular imaging in thick biological tissue with confocal detection of one‐photon fluorescence in the near‐infrared II window

Fluorescence imaging in the second near‐infrared optical window (NIR‐II, 900‐1700 nm) has become a technique of choice for noninvasive in vivo imaging in recent years. Greater penetration depths with high spatial resolution and low background can be achieved with this NIR‐II window, owing to low autofluorescence within this optical range and reduced scattering of long wavelength photons. Here, we present a novel design of confocal laser scanning microscope tailored for imaging in the NIR‐II window. We showcase the outstanding penetration depth of our confocal setup with a series of imaging experiments. HeLa cells labeled with PbS quantum dots with a peak emission wavelength of 1276 nm can be visualized through a 3.5‐mm‐thick layer of scattering medium, which is a 0.8% Lipofundin solution. A commercially available organic dye IR‐1061 (emission peak at 1132 nm), in its native form, is used for the first time, as a NIR‐II fluorescence label in cellular imaging. Our confocal setup is capable of capturing optically sectioned images of IR‐1061 labeled chondrocytes in fixed animal cartilage at a depth up to 800 μm, with a superb spatial resolution of around 2 μm.      

A biopsy‐needle compatible varifocal multiphoton rigid probe for depth‐resolved optical biopsy

In this work, we report a biopsy‐needle compatible rigid probe, capable of performing three‐dimensional (3D) two‐photon optical biopsy. The probe has a small outer diameter of 1.75 mm and fits inside a gauge‐14 biopsy needle to reach internal organs. A carefully designed focus scanning mechanism has been implemented in the rigid probe, which, along with a rapid two‐dimensional MEMS scanner, enables 3D imaging. Fast image acquisition up to 10 frames per second is possible, dramatically reducing motion artifacts during in vivo imaging. Equipped with a high‐numerical aperture micro‐objective, the miniature rigid probe offers a high two‐photon resolution (0.833 × 6.11 μm, lateral × axial), a lateral field of view of 120 μm, and an axial focus tuning range of 200 μm. In addition to imaging of mouse internal organs and subcutaneous tumor in vivo, first‐of‐its‐kind depth‐resolved two‐photon optical biopsy of an internal organ has been successfully demonstrated on mouse kidney in vivo and in situ.      

Reflection‐mode switchable subwavelength Bessel‐beam and Gaussian‐beam photoacoustic microscopy in vivo

We have developed a reflection‐mode switchable subwavelength Bessel‐beam (BB) and Gaussian‐beam (GB) photoacoustic microscopy (PAM) system. To achieve both reflection‐mode and high resolution, we tightly attached a very small ultrasound transducer to an optical objective lens with numerical aperture of 1.0 and working distance of 2.5 mm. We used axicon and an achromatic doublet in our system to obtain the extended depth of field (DOF) of the BB. To compare the DOF performance achieved with our BB‐PAM system against GB‐PAM system, we designed our system so that the GB can be easily generated by simply removing the lenses. Using a 532 nm pulse laser, we achieved the lateral resolutions of 300 and 270 nm for BB‐PAM and GB‐PAM, respectively. The measured DOF of BB‐PAM was approximately 229 μm, which was about 7× better than that of GB‐PAM. We imaged the vasculature of a mouse ear using BB‐PAM and GB‐PAM and confirmed that the DOF of BB‐PAM is much better than the DOF of GB‐PAM. Thus, we believe that the high resolution achieved at the extended DOF by our system is very practical for wide range of biomedical research including red blood cell (RBC) migration in blood vessels at various depths and observation of cell migration or cell culture.      

Contact‐free endoscopic photoacoustic sensing using speckle analysis

Photoacoustic endoscopy (PAE) is an emerging imaging modality, which offers a high imaging penetration and a high optical contrast in soft tissue. Most of the developed endoscopic photoacoustic sensing systems use miniaturized contact ultrasound transducers or complex optical approaches. In this work, a new fiber‐based detection technique using speckle analysis for contact‐free signal detection is presented. Phantom and ex vivo experiments are performed in transmission and reflection mode for proof of concept. In summary, the potential of the technique for endoscopic photoacoustic signal detection is demonstrated. The new technique might help in future to broaden the applications of PAE in imaging or guiding minimally invasive laser procedures.      

Optical‐resolution photoacoustic microscopy with ultrafast dual‐wavelength excitation

Fast functional and molecular photoacoustic microscopy requires pulsed laser excitations at multiple wavelengths with enough pulse energy and short wavelength‐switching time. Recent development of stimulated Raman scattering in optical fiber offers a low‐cost laser source for multiwavelength photoacoustic imaging. In this approach, long fibers temporally separate different wavelengths via optical delay. The time delay between adjacent wavelengths may eventually limits the highest A‐line rate. In addition, a long‐time delay in fiber may limit the highest pulse energy, leading to poor image quality. In order to achieve high pulse energy and ultrafast dual‐wavelength excitation, we present optical‐resolution photoacoustic microscopy with ultrafast dual‐wavelength excitation and a signal separation method. The signal separation method is validated in numerical simulation and phantom experiments. We show that when two photoacoustic signals are partially overlapped with a 50‐ns delay, they can be recovered with 98% accuracy. We apply this ultrafast dual‐wavelength excitation technique to in vivo OR‐PAM. Results demonstrate that A‐lines at two wavelengths can be successfully separated, and sO₂ values can be reliably computed from the separated data. The ultrafast dual‐wavelength excitation enables fast functional photoacoustic microscopy with negligible misalignment among different wavelengths and high pulse energy, which is important for in vivo imaging of microvascular dynamics.     

Fingerprint‐to‐CH stretch continuously tunable high spectral resolution stimulated Raman scattering microscope

Stimulated Raman scattering (SRS) microscopy is a label‐free method generating images based on chemical contrast within samples, and has already shown its great potential for high‐sensitivity and fast imaging of biological specimens. The capability of SRS to collect molecular vibrational signatures in bio‐samples, coupled with the availability of powerful statistical analysis methods, allows quantitative chemical imaging of live cells with sub‐cellular resolution. This application has substantially driven the development of new SRS microscopy platforms. Indeed, in recent years, there has been a constant effort on devising configurations able to rapidly collect Raman spectra from samples over a wide vibrational spectral range, as needed for quantitative analysis by using chemometric methods. In this paper, an SRS microscope which exploits spectral shaping by a narrowband and rapidly tunable acousto‐optical tunable filter (AOTF) is presented. This microscope enables spectral scanning from the Raman fingerprint region to the Carbon‐Hydrogen (CH)‐stretch region without any modification of the optical setup. Moreover, it features also a high enough spectral resolution to allow resolving Raman peaks in the crowded fingerprint region. Finally, application of the developed SRS microscope to broadband hyperspectral imaging of biological samples over a large spectral range from 800 to 3600 cm^?1, is demonstrated.     

Cuffing‐based photoacoustic flowmetry in humans in the optical diffusive regime

Measuring blood flow speed in the optical diffusive regime in humans has been a long standing challenge for photoacoustic tomography. In this work, we proposed a cuffing‐based method to quantify blood flow speed in humans with a handheld photoacoustic probe. By cuffing and releasing the blood vessel, we can measure the blood flow speed downstream. In phantom experiments, we demonstrated that the minimum and maximum measurable flow speeds were 0.035 mm/s and 42 mm/s, respectively. In human experiments, flow speeds were measured in three different blood vessels: a radial artery in the right forearm, a radial artery in the index finger of the right hand, and a radial vein in the right forearm. Taking advantage of the handheld probe, our method can potentially be used to monitor blood flow speed in the clinic and at the bedside.

     

Non‐invasive sentinel lymph node mapping and needle guidance using clinical handheld photoacoustic imaging system in small animal

Translating photoacoustic imaging (PAI) into clinical setup is a challenge. Handheld clinical real‐time PAI systems are not common. In this work, we report an integrated photoacoustic (PA) and clinical ultrasound imaging system by combining light delivery with the ultrasound probe for sentinel lymph node imaging and needle guidance in small animal. The open access clinical ultrasound platform allows seamless integration of PAI resulting in the development of handheld real‐time PAI probe. Both methylene blue and indocyanine green were used for mapping the sentinel lymph node using 675 and 690 nm wavelength illuminations, respectively. Additionally, needle guidance with combined ultrasound and PAI was demonstrated using this imaging system. Up to 1.5 cm imaging depth was observed with a 10 Hz laser at an imaging frame rate of 5 frames per second, which is sufficient for future translation into human sentinel lymph node imaging and needle guidance for fine needle aspiration biopsy.      

Nonlinear‐optical stain‐free stereoimaging of astrocytes and gliovascular interfaces

Methods of nonlinear optics provide a vast arsenal of tools for label‐free brain imaging, offering a unique combination of chemical specificity, the ability to detect fine morphological features, and an unprecedentedly high, subdiffraction spatial resolution. While these techniques provide a rapidly growing platform for the microscopy of neurons and fine intraneural structures, optical imaging of astroglia still largely relies on filament‐protein‐antibody staining, subject to limitations and difficulties especially severe in live‐brain studies. Once viewed as an ancillary, inert brain scaffold, astroglia are being promoted, as a part of an ongoing paradigm shift in neurosciences, into the role of a key active agent of intercellular communication and information processing, playing a significant role in brain functioning under normal and pathological conditions. Here, we show that methods of nonlinear optics provide a unique resource to address long‐standing challenges in label‐free astroglia imaging. We demonstrate that, with a suitable beam‐focusing geometry and careful driver‐pulse compression, microscopy of second‐harmonic generation (SHG) can enable a high‐resolution label‐free imaging of fibrillar structures of astrocytes, most notably astrocyte processes and their endfeet. SHG microscopy of astrocytes is integrated in our approach with nonlinear‐optical imaging of red blood cells based on third‐harmonic generation (THG) enhanced by a three‐photon resonance with the Soret band of hemoglobin. With astroglia and red blood cells providing two physically distinct imaging contrasts in SHG and THG channels, a parallel detection of the second and third harmonics enables a high‐contrast, high‐resolution, stain‐free stereoimaging of gliovascular interfaces in the central nervous system. Transverse scans of the second and third harmonics are shown to resolve an ultrafine texture of blood‐vessel walls and astrocyte‐process endfeet on gliovascular interfaces with a spatial resolution within 1 μm at focusing depths up to 20 μm inside a brain.     

Isotropic high resolution optoacoustic imaging with linear detector arrays in bi‐directional scanning

Optoacoustic (photoacoustic) imaging is often performed with one‐dimensional transducer arrays, in analogy to ultrasound imaging. Optoacoustic imaging using linear arrays offers ease of implementation but comes with several performance drawbacks, in particular poor elevation resolution, i.e. the resolution along the axis perpendicular to the focal plane. Herein, we introduce and investigate a bi‐directional scanning approach using linear arrays that can improve the imaging performance to quasi‐isotropic transverse resolution. We study the approach theoretically and perform numerical simulations and phantom measurements to evaluate its performance under defined conditions. Finally, we discuss the features and the limitations of the proposed method.

The poor elevation resolution in a linear scan (left image) is overcome by the proposed bi‐directional scanning approach that yields isotropic transverse resolution (right).     

Miniature probe for dual‐modality photoacoustic microscopy and white‐light microscopy for image guidance: A prototype toward an endoscope

In this study, a novel photoacoustic microscopy (PAM) probe integrating white‐light microscopy (WLM) modality that provides guidance for PAM imaging and complementary information is implemented. One single core of an imaging fiber bundle is employed to deliver a pulsed laser for photoacoustic excitation for PAM mode, which provides high resolution with deep penetration. Meanwhile, for WLM mode, the imaging fiber bundle is used to transmit two‐dimensional superficial images. Lateral resolution of 7.2 μm for PAM is achieved. Since miniature components are used, the probe diameter is only 1.7 mm. Imaging of phantom and animals in vivo is conducted to show the imaging capability of the probe. The probe has several advantages by introducing the WLM mode, such as being able to conveniently identify regions of interest and align the focus for PAM mode. The prototype of an endoscope shows potential to facilitate clinical photoacoustic endoscopic applications.