Abstract
Photoacoustic imaging (PAI) is drawing extensive attention and gaining rapid development as an emerging biomedical imaging technology because of its high spatial resolution, large imaging depth, and rich optical contrast. PAI has great potential applications in endoscopy, but the progress of endoscopic PAI was hindered by the challenges of manufacturing and assembling miniature imaging components. Over the last decade, microelectromechanical systems (MEMS) technology has greatly facilitated the development of photoacoustic endoscopes and extended the realm of applicability of the PAI. As the key component of photoacoustic endoscopes, micromachined ultrasound transducers (MUTs), including piezoelectric MUTs (pMUTs) and capacitive MUTs (cMUTs), have been developed and explored for endoscopic PAI applications. In this article, the recent progress of pMUTs (thickness extension mode and flexural vibration mode) and cMUTs are reviewed and discussed with their applications in endoscopic PAI. Current PAI endoscopes based on pMUTs and cMUTs are also introduced and compared. Finally, the remaining challenges and future directions of MEMS ultrasound transducers for endoscopic PAI applications are given.
Highlights
Photoacoustic imaging (PAI), called optoacoustic imaging, is an emerging imaging technique that has been developed rapidly and has evolved from the laboratory to preclinical and clinical applications in the last decade [1,2,3]
The typical structure of a thickness extension mode piezoelectric MUTs (pMUTs) receiver is shown in Figure 3a, which consists of a piezoelectric layer sandwiched between a top and bottom electrode, a backing layer, and a front acoustic matching layer
Sputtered AlN and sol–gel PZT have been widely used as piezoelectric thin films for fabricating Flexural Vibration Mode (FVM) pMUTs
Summary
Photoacoustic imaging (PAI), called optoacoustic imaging, is an emerging imaging technique that has been developed rapidly and has evolved from the laboratory to preclinical and clinical applications in the last decade [1,2,3]. The imaging depth is affected by multiple factors, including the incident laser pulse energy, the optical and acoustical properties of the tissue and tumor, and the performance of the ultrasound transducer. Based on Equations (3)–(5), it can be seen that ultrasound transducers are the most critical components in PAI systems, as their NEP, bandwidth, and center frequency will directly determine the signal-to-noise ratio (SNR), spatial resolution, and imaging depth of a PAI system. Probe diameter and imaging resolution, depth, and speed are the key parameters of an endoscopic PAI system, which are largely dependent on the form factor and characteristics of the employed MUT. In order to better leverage the existing achievements, it is necessary to review and organize the MUTs developed up to date and their applications in endoscopic PAI systems, compare their advantages and limitations, and point out future directions with respect to these challenges. These two types of pMUTs and their applications in endoscopic PAI will be introduced
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