Deep tissue photoacoustic imaging using a miniaturized 2-D capacitive micromachined ultrasonic transducer array

Abstract

In this paper, we demonstrate 3-D photoacoustic imaging (PAI) of light absorbing objects embedded as deep as 5 cm inside strong optically scattering phantoms using a miniaturized (4 mm × 4 mm × 500 μm), 2-D capacitive micromachined ultrasonic transducer (CMUT) array of 16 × 16 elements with a center frequency of 5.5 MHz. Two-dimensional tomographic images and 3-D volumetric images of the objects placed at different depths are presented. In addition, we studied the sensitivity of CMUT-based PAI to the concentration of indocyanine green dye at 5 cm depth inside the phantom. Under optimized experimental conditions, the objects at 5 cm depth can be imaged with SNR of about 35 dB and a spatial resolution of approximately 500 μm. Results demonstrate that CMUTs with integrated front-end amplifier circuits are an attractive choice for achieving relatively high depth sensitivity for PAI.

Fig. 1

Typical impulse response from one of the elements of the 2-D CMUT array.

Fig. 2

Schematic photoacoustic experimental setup. Laser beam is delivered along the X-axis with a free-space optics setup. The CMUT along with interface electronics is fixed to X–Y micrometer for manual scanning. CMUT array is looking forward along the +Z-axis. The phantom holder was suspended stationary inside an oil tank and dots represent horse hair pieces placed diagonally (with respect to both light and ultrasound positions) at different depths inside the phantom.

Fig. 3

Photoacoustic imaging of a 2.5-cm-thick chicken breast tissue phantom. (a) Open (top view) picture of an arrow-shaped object made of three horse black hair pieces (diameter of each hair piece is about 100 µm) placed about 2 cm deep inside the chicken breast tissue phantom. (b) Side view of the phantom. As shown in Fig. 2, light is delivered on the left side of the phantom, along the X-axis, with a free-space optics setup and the CMUT array is looking forward along the +Z-axis. (c) Reconstructed volumetric photoacoustic image of the phantom.

Fig. 4

Deep tissue photoacoustic imaging of four horse hair objects placed 2.2, 3.1, 4.1, and 5.3 cm deep inside the chicken breast tissue phantom. (a)–(c) show objects with different orientations placed 3.1, 4.1, and 5.3 cm deep inside the chicken breast tissue phantom, respectively. (d) Side view of the phantom. The diagonal arrow indicates the direction of the objects being placed inside the phantom with respect light (−X) and CMUT array (−Z) positions. (e) and (f) are different volumetric views of the reconstructed photoacoustic image. (g) ESF of all objects.

Fig. 5

ICG sensitivity study at 5 cm depth inside an optically scattering phantom made of intralipid–gelatin mixture. The optical reduced scattering and absorption coefficients of the background are µ′_s = 8 cm⁻¹ and µ_a = 0.1 cm⁻¹, respectively. (a) Side view of the phantom. (b) Experimental setup of the phantom with respect to light and CMUT array. (c)–(e) show volumetric photoacoustic images of 0.38 mm inner diameter polyethylene tube, filled with ICG concentrations of 10 µM, 1 µM, and 100 nM, respectively, placed 5 cm deep inside the phantom with respect to both light and CMUT array positions.