Photoacoustic Imaging with Capacitive Micromachined Ultrasound Transducers: Principles and Developments

Abstract

Photoacoustic imaging (PAI) is an emerging imaging technique that bridges the gap between pure optical and acoustic techniques to provide images with optical contrast at the acoustic penetration depth. The two key components that have allowed PAI to attain high-resolution images at deeper penetration depths are the photoacoustic signal generator, which is typically implemented as a pulsed laser and the detector to receive the generated acoustic signals. Many types of acoustic sensors have been explored as a detector for the PAI including Fabry–Perot interferometers (FPIs), micro ring resonators (MRRs), piezoelectric transducers, and capacitive micromachined ultrasound transducers (CMUTs). The fabrication technique of CMUTs has given it an edge over the other detectors. First, CMUTs can be easily fabricated into given shapes and sizes to fit the design specifications. Moreover, they can be made into an array to increase the imaging speed and reduce motion artifacts. With a fabrication technique that is similar to complementary metal-oxide-semiconductor (CMOS), CMUTs can be integrated with electronics to reduce the parasitic capacitance and improve the signal to noise ratio. The numerous benefits of CMUTs have enticed researchers to develop it for various PAI purposes such as photoacoustic computed tomography (PACT) and photoacoustic endoscopy applications. For PACT applications, the main areas of research are in designing two-dimensional array, transparent, and multi-frequency CMUTs. Moving from the table top approach to endoscopes, some of the different configurations that are being investigated are phased and ring arrays. In this paper, an overview of the development of CMUTs for PAI is presented.

Overview of Ultrasound Detection Technologies for Photoacoustic Imaging

“…The physical ultrasound transducer technologies including PZT, CMUT, and PMUT are compared in terms of sensitivity, bandwidth, energy conversion and some other technical specifications in Table 5. Quantitative measurements of piezoelectric and CMUT are based on 2.43 and 2.63 MHz transducers, respectively, presented in [146,147] and that of PMUT are based on a 7~9 MHz transducer presented in [134,135].…”

Towards non-contact photoacoustic imaging [review]

“…In PAT, an unfocused optical beam excites the region of interest, and an array of ultrasonic transducers measures the generated ultrasound waves in multiple positions [ [37] , [38] , [39] , [40] ]. Common detection geometries used in PAT are planar, cylindrical, and spherical.…”

Endoscopic Optical Imaging Technologies and Devices for Medical Purposes: State of the Art

“…Thus, there is a trade-off between the resolution and imaging depth, with the penetration depth:resolution ratio being about 200 [45]. The penetration depth also depends on the PA modality used for imaging and on the nature of the transducer used for signal detection [46]. Using NIR light, the penetration depth can reach up to 7 cm [45].…”