Photoacoustic imaging is a promising method for visualizing biological tissues with optical absorbers. This article provides an overview of the rapidly developing field of photoacoustic imaging. Photoacoustics, the physical basis of photoacoustic imaging, is analyzed briefly. The merits of photoacoustic technology, compared with optical imaging and ultrasonic imaging, are described. Various imaging techniques are also discussed, including scanning tomography, computed tomography and original detection of photoacoustic imaging. Finally, some biomedical applications of photoacoustic imaging are summarized.

Photoacoustic imaging in biomedicine

This paper deals with a multiphysics numerical modelling via finite element method (FEM) of an air-coupled array piezoelectric micromachined ultrasonic transducers (PMUTs). The proposed numerical model is fully 3D with the following features: the presence of the fabrication induced residual stresses, which determine a geometrically non-linear initial deformed configuration of the diaphragms and a remarkable shift of the fundamental frequency; the multiple coupling between different physics, namely electro-mechanical-coupling for the piezo-electric model, acoustic-structure interaction at the acoustic-structure interface and pressure acoustics in the surrounding air. The model takes into account the complete set of PMUTs belonging to the silicon die in a 4 x 4 array configuration and the protective package, as well. The results have been validated by experimental data, in terms of initial static pre-deflected configuration of the diaphragms and frequency response function of the PMUT. The numerical procedure was applied, to analyze different package configurations of the device, to study the influence of the holes on the acoustic transmission in terms of SPL and propagation pattern and consequently extract a set of design guidelines.

https://www.mdpi.com/2072-666X/11/3/307/pdf

On the Effects of Package on the PMUTs Performances-Multiphysics Model and Frequency Analyses.

The purpose of the present study is to numerically analyze and optimize structural parameters effecting on the transmitting performance of piezoelectric micromachined ultrasonic transducers (PMUTs) with a vibration layer. Using the finite elements method (FEM), a planar and two curved (dome-shaped and concave) structures of PMUTs based on aluminum nitride (A1N) were modeled separately. By applied a sine excitation working on a frequency band ranged from 20 kHz to 25 MHz, resultant transmitting performances with various structural parameters of each PMUT model were estimated numerically in water. The obtained results clearly show that there is a relative higher transmitting performance both of the dome-shaped and concave structures comparing with that of the planar one. Importantly, the transmitting performance would be optimized with a diameter of 64 μm for the top electrode of the planar element, which is consistent with previous theoretic works, and a radius of 200 μm would be suggested for the curvature of the curved structures so as to launch a larger acoustic pressure. In conclusion, both the physical parameters and geometric structures of a PMUT-based ultrasound sensor combining its work frequency could be optimized efficiently with a numerical method.

Piezoelectric micromachined ultrasonic transducers: Modeling and simulation of structural parameters on transmitting performance

This paper presents the modeling and characterization of piezoelectric micromachined ultrasound transducers (PMUT) based on a three-dimensional dome shaped structure fabricated by a lithographic process. The transducer array is based on a high performance PZNT film integrated in a 3D dome shaped structure, which forms the basic resonator cell of the transducer. Acoustic and electromechanics simulations have been performed in order to design transducers with the desired optimum behavior. The transducer having low operating voltage below 20V and high sensitivity can be implemented in a ultraportable type of ultrasound system. High sensitivity of the transducers allowed acquisition of images of a wire target phantom using a phased array algorithm demonstrating high axial and lateral resolution.

Modeling and characterization of a 3D-MEMS piezoelectric ultrasound transducer

Abstract One of the primary challenges in ring single‐element photoacoustic tomography systems is the low image quality in areas away from the center of the ring. This is mainly due to the limited field of view (FOV) of each transducer, which in turn reduces the imaging FOV. To address this shortcoming, we have put forward a practical and straightforward solution to enhance the FOV of circular scanning‐based photoacoustic tomography (CS‐PAT). This is accomplished by placing transducers at different angles instead of using a single transducer placed at a normal angle to the imaging target. We also modified the ring scanner inner wall surface to significantly reduce photoacoustic reverberation. By imaging several phantoms, we show a significant improvement in the images generated by our system imaging from 4.1 to over 7 for the signal‐to‐noise ratio and structural similarity index increased from 41% to 70%. 

A practical solution to improve the field of view in circular scanning‐based photoacoustic tomography

Piezoelectric Micromachined Ultrasonic Transducers for Photoacoustic Imaging: Modeling and Simulation of Structural Parameters on Receive Performance

Photoacoustic tomography (PAT) is a new modality with a high-resolution and a high contrast, the reconstruction process of which can result in improvements of the imaging quality. The aim of this study is to present an attempt to develop a novel reconstruction method with a time reversal algorithm (TR) for the PAT in an inhomogeneous media. By incorporating the finite difference time domain method (FDTD), the imaging reconstruction with the TR algorithm was formulated firstly. Then, two numerical simulations and an ex vivo experiment were both carried out to evaluate the capability of the proposed method in this work. The obtained results showed qualitatively the PAT images could be reconstructed well both in an inhomogeneous numerical model from different scanning ways and in a phantom model embedded with an ex vivo tumor of patients. Although the TR based algorithms may cost a bit more time than another in reconstruction process, it would be more practical approach for photoacoustic tomography with arbitrary scanning ways and better capability of imaging in inhomogeneous media, could also encourage us to further discuss its applicability of tumor detection in patients.

A Novel Reconstruction Method with Time Reversal Algorithm and Evaluations for Photoacoustic Tomography

Erectile dysfunction (ED) is a frequent disorder in aging men, affecting men's physical and mental health seriously. Generally, clinical diagnosis of the physical ED need image of penile vessels to distinguish vasculogenic ED disorders from psychological disorders so as to determine an appropriate treatment. The aim of this study is to present a novel photoacoustic microscopy (PAM) for imaging of vasculogenic ED with a high resolution and a high sensitivity. In this work, a vessel phantom using a carbon pencil and ten animals' penis models isolated from health rats both were prepared carefully. A home-made PAM platform was builted for rat's penile vessels, and the corresponding two dimensional (2D) PAM image was obtained for each model. Particularly, an imaging capability of a high spatial resolution of five hundreds microns with high contrast was obtained. The preliminary imaging results of isolated penis vessel clearly indicated that the method of photoacoustic imaging would be a useful diagnostic tool for assessing vasculogenic ED and could improve the diagnosis of venous leakage visualization by combining structural and functional imaging.

Photoacoustic imaging: A potential new tool for diagnosing vasculogenic erectile dysfunction

Compared with clinical and experimental approaches, numerical modeling of defibrillation offers a great opportunity to optimize the defibrillation strategy in a more individualized way. Through numerical simulation of the shock-induce electric field distribution, the outcome of a certain defibrillation shock could be predicted according to several different metrics. In this paper, we propose a novel evaluation method, in which four defibrillation criteria are assigned with separate weighting factors to quantitatively assess the efficiency of a certain defibrillation shock. Three anatomically realistic finite element models of swine were constructed for the evaluation study of 8 electrode pairs in different placements. In addition, corresponding animal experiments were performed to determine the defibrillation threshold of 8 electrode placements. Both computational and experimental results suggest that the clinical recommended anterior-lateral position is the most efficient electrode displacement for transthoracic defibrillation in swine. In conclusion, the good agreement between stimulations and experiments indicates that the present multi-criteria evaluation method would be potentially useful for optimizations of cardiac defibrillation outcome.

A multi-criteria evaluation method for assessing the defibrillation outcome of different electrode placements in swine

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