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.

Keywords: capacitive micromachined ultrasound transducer; photoacoustic computed tomography; photoacoustic endoscopy; photoacoustic microscopy; photoacoustic tomography.

Figure 1

(a) Diagram of the working principle of Fabry–Perot interferometers (FPI). An incoming ultrasound wave causes a variation in thickness which in turn results in a phase modulation (Reproduced from [88], with the permission of AIP Publishing.); (b) schematic diagram of an micro ring resonators (MRR); (c) schematic diagram of a forward-viewing photoacoustic probe for endoscopy imaging used in [69]; (d) photoacoustic endoscopy with a MRR detector used in (Adapted with permission from ref [82], [The Optical Society]).

Figure 2

(a) Capacitive micromachined ultrasound transducers (CMUT) transmission mode; (b) CMUT receiving mode.

Figure 3

(a) Model of the chicken breast phantom, (b) ultrasonic imaging, (c) PAI, and (d) a combination of photoacoustic and ultrasonic imaging (© [2009] IEEE. Reprinted, with permission, from [117]); (e) working principle of top orthogonal to bottom electrode (TOBE) (© [2014] IEEE. Reprinted, with permission, from [124]).

Figure 4

(a) Optical absorption of silicon under different wavelength (© [2010] IEEE. Reprinted, with permission, from [118]) and (b) structure of optically transparent CMUT (© [2018] IEEE. Reprinted, with permission, from [126]), (c) imaging of mouse brain using the different frequencies of the CMUT (© [2018] IEEE. Reprinted, with permission, from [130]), (d) interlaced CMUT (© [2017] IEEE. Reprinted, with permission, from [128]), (e) multi-band CMUT (Adapted with permission from ref [129], [The Optical Society]), (f) monolithic multiband CMUT with five frequencies (© [2018] IEEE. Reprinted, with permission, from [130]).

Figure 5

Timeline of CMUT designs for PAI endoscopes [118,119,120,121]; (a) inward-looking cylindrical transducer (© [2006] IEEE. Reprinted, with permission, from [120]); (b) 9F MicroLinear CMUT ICE catheter (© [2012] IEEE. Reprinted, with permission, from [119]); (c) miniature needle-shaped CMUT (© [2010] IEEE. Reprinted, with permission, from [118]); (d) integrated ring CMUT array (© [2013] IEEE. Reprinted, with permission, from [121]).