Three-dimensional photoacoustic imaging using a two-dimensional CMUT array

In this paper, we describe using a 2-D array of capacitive micromachined ultrasonic transducers (CMUTs) to perform 3-D photoacoustic and acoustic imaging. A tunable optical parametric oscillator laser system that generates nanosecond laser pulses was used to induce the photoacoustic signals. To demonstrate the feasibility of the system, 2 different phantoms were imaged. The first phantom consisted of alternating black and transparent fishing lines of 180 μm and 150 μm diameter, respectively. The second phantom comprised polyethylene tubes, embedded in chicken breast tissue, filled with liquids such as the dye indocyanine green, pig blood, and a mixture of the 2. The tubes were embedded at a depth of 0.8 cm inside the tissue and were at an overall distance of 1.8 cm from the CMUT array. Two-dimensional cross-sectional slices and 3-D volume rendered images of pulse-echo data as well as photoacoustic data are presented. The profile and beamwidths of the fishing line are analyzed and compared with a numerical simulation carried out using the Field II ultrasound simulation software. We investigated using a large aperture (64 x 64 element array) to perform photoacoustic and acoustic imaging by mechanically scanning a smaller CMUT array (16 x 16 elements). Two-dimensional transducer arrays overcome many of the limitations of a mechanically scanned system and enable volumetric imaging. Advantages of CMUT technology for photoacoustic imaging include the ease of integration with electronics, ability to fabricate large, fully populated 2-D arrays with arbitrary geometries, wide-bandwidth arrays and high-frequency arrays. A CMUT based photoacoustic system is proposed as a viable alternative to a piezoelectric transducer based photoacoustic systems.

I. Introduction

Photoacoustic imaging (PAI), also called optoacoustic imaging, is a promising medical imaging technology because it combines the contrast information of optical imaging with the spatial resolution of acoustic imaging [1]. It is a non-ionizing and non-invasive imaging modality. Potential clinical applications for PAI include functional and molecular imaging as well as therapy monitoring [2]–[4]. In PAI, the target tissue is illuminated with short laser pulses that cause brief heating of absorbing structures such as blood vessels. The induced temperature increase generates acoustic pressure waves because of the thermoelastic effect. These pressure waves propagate to the surface of the tissue where they can be detected with ultrasound transducers. Those regions that are more optically absorbent than others will generate a stronger ultrasound signal. Using the ultrasound data, an image of the optical absorption properties of the tissue can be reconstructed.