1.

INTRODUCTION

Photoacoustic (PA) imaging is a technology that combines rich optical contrast with higher ultrasonic spatial resolution, and greater penetration.⁷ PA imaging can be performed by illuminating a large tissue area with a pulsed laser beam and receiving the ultrasound signals on a linear or 2D ultrasound array. Alternatively, a focused laser beam can be rastered on a tissue and receive the signals with a single-element ultrasound transducer, known as optical resolution. However, most photoacoustic imaging techniques use non-optimal light delivery methods where optical and acoustic paths are separated because of opaque ultrasound transducers.⁸ Transparent transducers would enable light delivery through the transducer rather than around it. Hence, transparent ultrasound transducers could improve light delivery, leading to high signal-to-noise (SNR) ratios and compact imaging probes. Transparent ultrasound transducers have been developed and fabricated based on lithium niobate piezoelectric and capacitive micromachined ultrasound transducers (CMUT) in the last few years. Nevertheless, they are mostly fabricated in a single-element form or a 1D linear array with a limited aperture size. Here, we present the fabrication and preliminary PA imaging results of an unprecedented 128×128 transparent TOBE array made of electrostrictive lead magnesium niobate - lead titanate (PMN-PT). The PA imaging on the array was performed using a Hadamard Matrix coding/decoding as described in.⁹ Therefore, 128 laser shots are needed to decode the aperture and read the ultrasound data from each individual element of the array to form a 3D PA image.

2.

FABRICATION

The fabrication of the proposed array starts with mounting bulk PMN-PT on a carrier wafer using a crystal bond, followed by lapping and polishing to achieve a flat and optically transparent surface. Indium tin oxide (ITO) is sputtered and patterned using lithography to form the top electrodes. To improve the electrical conductivity, thin Cr/Au strips are patterned over the ITO at the center of each electrode. To maintain optical transparency, these thin strips are only 5 μm in width. Alternatively, A thin layer of silver can be used, sandwiched between two ITO layers to simplify the fabrication processes. Then, the array is flipped, and the same processes are performed on the backside. The back electrodes are patterned orthogonal to the top electrodes to form a TOBE array. The final array is wire bonded to a PCB, and a transparent epoxy backing and matching layers are used. The array is interfaced with custom biasing electronics and a programmable ultrasound platform. The prototype fabricated TOBE array is shown in Fig. 1.

Figure 1.

The fabricated transparent 128×128 PMN-PT TOBE array; (a) the schematic of the fabricated array on a custom PCB (b) the Cr/Au stripes on the ITO layer to improve the electrical conductivity (c) the fabricated transparent array. The logo of the University of Alberta can be seen through the array.

A 532 nm pulsed laser is used to perform the through-illumination PA imaging using Hadamard bias-encoding to read out signals from every element of the array in the same way as described in.⁹

3.

RESULTS

The fabricated array demonstrates good transparency in the visible light range, reaching as high as 65% around 532 nm. The preliminary PA imaging is done on 25 μm crossed gold wires. The Hadamard DC biasing patterns are applied on rows, while the columns are used to receive the ultrasound signals with zero DC voltages. For the fabricated array, 128 laser shots with 128 different DC biasing patterns are used. Transparent epoxy as the backing layer provides good optical transparency, while the acoustic impedance is less than ideal, which may lead to a poor axial resolution for some applications. However, this is less important for C-scan projection imaging. In contrast, further improvements are possible by using a bulk glass delay line as a backing layer. The center frequency of the fabricated array was measured to be 13.6 MHz. The PA images of the crossed wires are shown in Fig. 2.

Figure 2.

The PA images of 25 μm crossed gold wires: (a) the maximum PA projection in XY plane (b) cross-plane PA image of the crossed wires.