Acoustic-resolution photoacoustic microscopy (ARPAM) plays an important role in studying the microcirculation system of biological tissues with deep penetration. High lateral resolution of ARPAM is achieved by using a high numerical aperture acoustic transducer. The deteriorated lateral resolution in the out-of-focus region can be alleviated by synthetic aperture focusing technique (SAFT). Previously, we reported a three-dimensional (3D) deconvolution ARPAM to improve both lateral and axial resolutions in the focus region. In this study, we present our extension of resolution enhancement to the out-of-focus region based on two-dimensional SAFT combined with the 3D deconvolution (SAFT+Deconv). In both the focus and out-of-focus regions, depth-independent lateral resolution provided by SAFT, together with inherently depth-independent axial resolution, ensures a depth-independent point spread function for 3D deconvolution algorithm. Imaging of 10 μm polymer beads shows that SAFT+Deconv ARPAM improves the -6 dB lateral resolutions from 65-700 μm to 20-29 μm, and the -6 dB axial resolutions from 35-42 μm to 12-19 μm in an extended depth of focus (DOF) of ∼2 mm. The signal-to-noise ratio is also increased by 6-30 dB. The resolution enhancement in three dimensions is validated by in vivo imaging of a mouse's dorsal subcutaneous microvasculature. Our results suggest that SAFT+Deconv ARPAM may allow fine spatial resolution with deep penetration and extended DOF for biomedical photoacoustic applications.

A photoacoustic correlation technique, inspired by its optical counterpart-the fluorescence correlation spectroscopy (FCS), was tested for the first time to demonstrate the feasibility of lowspeed flow measurement based on photoacoustic signal detection. A pulsed laser was used to probe the flow of light-absorbing beads. A photoacoustic correlation system of 0.8 sec temporal resolution was built and flow speeds ranging from 249 to 14.9 µm/s with corresponding flow time from 4.42 to 74.1 sec were measured. The experiment serves as a proof of concept for photoacoustic correlation spectroscopy, which may have many potential applications similar to FCS.Fluorescence correlation spectroscopy (FCS) is a powerful technique widely used in analytical chemistry and biological research [1]. In FCS, fluctuation of fluorescence intensity of a small number of fluorescent molecules is analyzed using temporal correlation. FCS has found a wide range of applications [2,3]. We propose a technology, photoacoustic correlation spectroscopy (PACS), by extending the fluorescence detection in FCS to the acoustic signal domain. The PACS is based on pulsed laser excitation. Autocorrelation is performed using measured photoacoustic signals, and the term "spectroscopy" refers to time-spectrum rather than in common usage as a frequency spectrum. PACS is different from other techniques using PA effects in correlation measurements, such as photoacoustic spectroscopy (PAS) [4] and correlation photoacoustic spectroscopy (CPAS) [5]. PAS was to analyze the absorbing chemical groups of samples from the measured IR spectrum. CPAS measures the cross correlation between excitation source and detected acoustic response. CPAS was mainly used to study static properties such as depth-profiling and thermal imaging. In contrast, the specific purpose of the PACS technique is to study functional dynamics of PA species.The PACS could open up a range of applications in medical diagnosis. As one example, the analysis of microcirculation system provides unique comprehension of disease processes [6]. Current used methods include Doppler related techniques and histological sectioning. However, these techniques in studying microcirculation have been restricted by issues of invasiveness, low resolution, limited imaging depth, and minimum measurable flow speed [7]. Take Doppler techniques for example. Doppler ultrasound method is not easy to detect the flow speed less than 1 mm/s [8]. Doppler optical coherence tomography has difficulties in flow measurement at depth greater than 1 mm [9]. One way to overcome these limitations aguo@umich.edu. Light-absorbing beads generate PA waves when they absorb laser energy and undergo an instantaneous thermal expansion. In PACS, we name the counterpart of fluorescence intensity in FCS as PACS strength. It can be expressed as NIH Public Access( 1) where I(r) is the normalized spatial fluence distribution of the laser beam, and n(r,t) is the bead concentration at position r and time t. The laser beam used in our PACS setup de...

Polymer microring resonators have been exploited for high-sensitivity and wideband photoacoustic imaging. To demonstrate high-sensitivity ultrasound detection, high-frequency photoacoustic imaging of a 49-μm-diameter black bead at an imaging depth of 5 mm was imaged photoacoustically using a synthetic 2-D array with 249 elements and a low laser fluence of 0.35 mJ/cm2. A bandpass filter with a center frequency of 28 MHz and a bandwidth of 16 MHz was applied to all element data but without signal averaging, and a signal-to-noise ratio of 16.4 dB was obtained. A wideband detector response is essential for imaging reconstruction of multiscale objects, e.g., various sizes of tissues, by using a range of characteristic acoustic wavelengths. A simulation of photoacoustic tomography of beads shows that objects with their boundaries characteristic of high spatial frequencies and the inner structure primarily of low spatial frequency components can be faithfully reconstructed using such a detector. Photoacoustic tomography experiments of 49- and 301-μm-diameter beads were presented. A high resolution of 12.5 μm was obtained. The boundary of a 301-μm bead was imaged clearly. The results demonstrated that the high sensitivity and broadband response of polymer microring resonators have potential for high resolution and high-fidelity photoacoustic imaging.

Abstract:Medical ultrasound is an imaging technique that utilizes ultrasonic signals as information carriers, and has wide applications such as seeing internal body structures, finding a source of a disease, and examining pregnant women. The most commonly used ultrasonic transducer today is based on piezoelectricity. The piezoelectric transducer, however, may have a limited bandwidth and insufficient sensitivity for reduced element size. Laser-generated ultrasound (LGUS) technique is an effective way to resolve these issues. The LGUS approach based on photoacoustic effect is able to greatly enhance the bandwidth of ultrasound signals and has the potential for high-resolution imaging. High-amplitude LGUS could also be used for therapy to accomplish high precision surgery without an incision. Furthermore, LGUS in conjunction with optical detection of ultrasound allows all-optical ultrasound imaging (i.e., ultrasound is generated and received optically). The all-optical platform offers unique advantages in providing high-resolution information and in facilitating the construction of miniature probes for endoscopic ultrasound. In this article, a detailed review of the recent development of various LGUS transmitters is presented. In addition, a recent research interest in all-optical ultrasound imaging, as well as its applications, is also discussed.

We demonstrate a novel optomechanical synchronization method to achieve ultrahigh-contrast time-gated fluorescence imaging using live zebrafish as models. Silicon quantum dot nanoparticles (SiQDNPs) with photoluminescence lifetime of about 16 μs were used as the long-lived probes to enable background autofluorescence removal and multiplexing through time-gating. A continuous-wave 405 nm laser as the excitation source was focused on a rotating optical chopper on which the emission light beam obtained from an inverted fluorescence microscope was also focused but with a phase difference such that in a short delay after the excitation laser is blocked, the emission light beam passes through the optical chopper, initiating the image acquisition by a conventional sensor. Both excitation and detection time windows were synchronized by one optical chopper, eliminating the need for pulsed light source and image intensifier which is often used as ultrafast optical shutter. Through use of the cost-effective time-gating method, nearly all background autofluorescence emitted from the yolk sac of a zebrafish embryo microinjected with the SiQDNPs was removed, leading to a 45-fold increase in signal-to-background ratio. Furthermore, two kinds of fluorescence signals emitted from the microinjected SiQDNPs and the intrinsic green fluorescent protein of transgenic zebrafish larvae can be clearly separated through time-gating.

Acoustic-resolution photoacoustic microscopy (ARPAM) provides a spatial resolution on the order of tens of micrometers, and is becoming an essential tool for imaging fine structures, such as the subcutaneous microvasculature. High lateral resolution of ARPAM is achieved using high numerical aperture (NA) of acoustic transducer; however, the depth of focus and working distance will be deteriorated correspondingly, thus sacrificing the imaging range and accessible depth. The axial resolution of ARPAM is limited by the transducer's bandwidth. In this work, we develop deconvolution ARPAM (D-ARPAM) in three dimensions that can improve the lateral resolution by 1.8 and 3.7 times and the axial resolution by 1.7 and 2.7 times, depending on the adopted criteria, using a 20-MHz focused transducer without physically increasing its NA and bandwidth. The resolution enhancement in three dimensions by D-ARPAM is also demonstrated by in vivo imaging of the microvasculature of a chick embryo. The proposed D-ARPAM has potential for biomedical imaging that simultaneously requires high spatial resolution, extended imaging range, and long accessible depth.

A miniature all-optical probe for high-resolution photoacoustic (PA)-ultrasound (US) imaging using a large synthetic aperture is developed. The probe consists of three optical fibers for PA excitation, US generation, and detection of acoustic waves, respectively. The fiber for PA excitation has a large numerical aperture (NA) for wide-angle laser illumination. On the other hand, the fiber with a carbon black-polydimethylsiloxane composite coated on the end face of the optical fiber is used for wide-angle US transmission through laser-US conversion. Both the excited PA and backscattered US signals are detected by a fiber-tip Fabry-Perot cavity for wide-angle acoustic detection. The probe outer diameter is only ~2 mm. The synergy of the three optical fibers makes a large-NA synthetic aperture focusing technique for high-resolution PA and US imaging possible. High PA lateral resolutions of 104-154 μm and high US lateral resolutions of 64-112 μm over a depth range of > 4 mm are obtained. Compared with other existing miniature PA-US probes, to our knowledge, our probe achieves by far the best performance in terms of lateral resolutions and imaging depth range. The constructed probe has potential for endoscopic and intravascular imaging applications that require PA and US contrasts with high resolutions over a large depth range.

We present a miniature probe capable of both optical-resolution (OR) and acoustic-resolution (AR) photoacoustic microscopy. A gradient-index-lens fiber and a multimode fiber are used to deliver light for OR and AR illumination, respectively. The probe achieves lateral resolution of 3.1 μm for OR mode and 46-249 μm (at depth of 1.2-4.3 mm) for AR mode, respectively. The size of the probe attains 3.7 mm in diameter, which can be used for endoscopic applications. In vivo imaging of several different parts of a mouse demonstrates the excellent imaging ability of the probe.