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Objective: Photoacoustics has been evolving rapidly in the recent several decades. With the aim of clinical translation, a photoacoustic imaging platform with a portable system size, a miniaturized imaging probe, and convenient handheld capability is of great demand. Methods: In this work, by adopting an ultrathin central-holed matrix array and a compact coaxial photoacoustic design, a water-free handheld photoacoustic imager is developed (weight: 44 g). Optical Monte Carlo simulation and acoustic k-wave simulation are performed to confirm a relatively homogenous photoacoustic sensitivity distribution within a wide field of view (larger than 10 × 10 × 10 mm3). Results: Imaging resolution characterized by imaging two hairs is estimated to be about 0.80 mm in the lateral direction and 0.73 mm in the axial direction. To demonstrate the handheld capability in real time, handheld imaging guided needle biopsy in the rat's abdomen is performed at a rate of about 10 Hz (CNR ∼ 14.3 dB). Furthermore, handheld imaging is also demonstrated on visualizing vasculature (mainly cephalic vein) in the human arm (CNR > 4.1 dB). Conclusion: Results demonstrate that the volumetric photoacoustic imaging system using a central-holed 2D matrix array is an attractive choice for achieving convenient handheld operation in real time. Significance: Such a compact handheld design may promote the clinical translation of photoacoustic technique. [ABSTRACT FROM AUTHOR]

A handheld approach to 3D photoacoustic imaging is essential in clinical applications. To this end, we develop a 3D handheld photoacoustic imager for dynamic (temporally and spatially) volumetric visualization. In this 3D imager, the optically transmitting part and the acoustically receiving part are integrated into a single handheld probe with a compact size about 160 mm $\times64$ mm $\times40$ mm. Besides, a dedicated imaging reconstruction algorithm for the heterogeneous medium is developed based on the phase-shift migration method in the frequency domain, which deals well with the stratified condition in the designed system. Dynamic 3D imaging supporting flexible handheld operation is demonstrated with needle biopsy and in vitro temperature measurement for photothermal therapy. The development of such a 3D handheld photoacoustic system paves the way for compact and handheld-operating implementations, and its further clinical exploration is promising. [ABSTRACT FROM AUTHOR]

Photoacoustic imaging (PAI) is a rapidly developing emerging non-invasive biomedical imaging technique that combines the strong contrast from optical absorption imaging and the high resolution from acoustic imaging. Abnormal biological tissues (such as tumors and inflammation) generate different levels of thermal expansion after absorbing optical energy, producing distinct acoustic signals from normal tissues. This technique can detect small tissue lesions in biological tissues and has demonstrated significant potential for applications in tumor research, melanoma detection, and cardiovascular disease diagnosis. During the process of collecting photoacoustic signals in a PAI system, various factors can influence the signals, such as absorption, scattering, and attenuation in biological tissues. A single ultrasound transducer cannot provide sufficient information to reconstruct high-precision photoacoustic images. To obtain more accurate and clear image reconstruction results, PAI systems typically use a large number of ultrasound transducers to collect multi-channel signals from different angles and positions, thereby acquiring more information about the photoacoustic signals. Therefore, to reconstruct high-quality photoacoustic images, PAI systems require a significant number of measurement signals, which can result in substantial hardware and time costs. Compressed sensing is an algorithm that breaks through the Nyquist sampling theorem and can reconstruct the original signal with a small number of measurement signals. PAI based on compressed sensing has made breakthroughs over the past decade, enabling the reconstruction of low artifacts and high-quality images with a small number of photoacoustic measurement signals, improving time efficiency, and reducing hardware costs. This article provides a detailed introduction to PAI based on compressed sensing, such as the physical transmission model-based compressed sensing method, two-stage reconstruction-based compressed sensing method, and single-pixel camera-based compressed sensing method. Challenges and future perspectives of compressed sensing-based PAI are also discussed. [ABSTRACT FROM AUTHOR]

Photoacoustic imaging (PAI) is susceptible to speed of sound (SOS) differences in heterogeneous media which greatly reduce the resolutions and qualities of the imaging results. Several reconstruction methods have been reported to adapt for heterogenous media, but they are limited by specific deficiencies such as efficiency, accuracy, and model limitation problems. Among them, the plane wave model based on wavefield reconstruction is the most efficient and promising one for high-efficiency three-dimensional PAI. However, the classic plane wave model only suits for planar layered media, severely limiting its applications in practice. To this end, we modify the plane wave model to apply for irregularly layered heterogeneous media and propose a corresponding wavefield extrapolation to reconstruct photoacoustic image. This method employs split-step Fourier to compensate the SOS differences, extrapolates wavefields and reconstructs the image depth by depth. Furthermore, a floating discretization strategy is introduced to control and balance the efficiency and accuracy with a hyperparameter. The simulation and experiment results demonstrate that the proposed method can reconstruct the image with an equivalent resolution to time reversal’s and even have higher efficiency and robustness. To reconstruct a three-dimensional image with 50 × 50 × 600 pixels, the proposed method takes only 5.5 seconds using a laptop loaded with Intel(R) Core (TM) i7-8550U CPU @1.8GHz. [ABSTRACT FROM AUTHOR]

Photoacoustic imaging reconstructions usually assume a known speed-of-sound (SOS) distribution; however, in most cases, the SOS distribution is not revealed and is difficult to estimate from photoacoustic signals. In this paper, we propose passive ultrasound aided acoustic resolution photoacoustic microscopy which simultaneously reconstructs SOS distributions and photoacoustic images for layered heterogeneous media. The passive ultrasound is a kind of laser-induced acoustic wave generated by a transducer absorbing the backscattered light. It can be used to measure the layer thicknesses due to its sensitivity to structural information and broad bandwidth and further determine the SOS distributions. After estimating the SOS distributions, a phase shift plus interpolation is employed to reconstruct the photoacoustic image for heterogeneous media. Without introducing additional hardware, this method can be conveniently incorporated into a conventional photoacoustic imaging system. A curved shell immersed in water is adopted as a layered heterogeneous phantom, and the proposed method reconstructs the targets (carbon rods) under this shell. Under the test of a 5 MHz focused transducer (NA 0.25), the maximum reconstruction deviation of 1.2 mm carbon rods is only 0.15 mm. [ABSTRACT FROM AUTHOR]