Low-Cost Scalable PCB-Based 2-D Transducer Arrays for Volumetric Photoacoustic Imaging

Abstract

Photoacoustic (PA) imaging provides deep tissue molecular imaging of chromophores with optical absorption contrast and ultrasonic resolution. Present PA imaging techniques are predominantly limited to one 2D plane per acquisition. 2D ultrasound transducers, required for real-time 3D PA imaging, are high-cost, complex to fabricate and have limited scalability in design. We present novel PCB-based 2D matrix ultrasound transducer arrays that are capable of being bulk manufactured at low-cost without using laborious ultrasound fabrication tools. The 2D ultrasound array specifications are easily scalable with respect to widely available PCB design and fabrication tools at low cost. To demonstrate scalability, we fabricated low (11 MHz) frequency 8x8 matrix array and high (40 MHz) frequency 4x4 matrix array by directly bonding an undiced polyvinylidene fluoride (PVDF) piezoelectric material of desired thickness to the custom designed PCB substrate. Characterization results demonstrate wideband PA receive sensitivity for both low (87%) and high (188%) frequency arrays. Volumetric PA imaging results of light absorbing targets inside optical scattering medium demonstrate improved spatial resolution and field of view with increase in aperture size.

Keywords: 2D Matrix Transducer Array; Beamforming; Photoacoustic Imaging; Polyvinylidene fluoride (PVDF); Printed Circuit Board (PCB); Ultrasound Transducer Fabrication.

Fig. 1.. Comparative photoacoustic (PA) imaging simulations to study volumetric imaging capabilities of 2D arrays.

Schematic of imaging for (a) 4x4. Volumetric PA image from 4x4 array with (b) $f_c=10MHz$ and (c) $f_c=25MHz$. Schematic of imaging for (d) 8x8. Volumetric PA image from 8x8 array with (e) $f_c=10MHz$ and (f) $f_c=25MHz$ Depth of light absorbing PA targets (0.4 mm diameter) from 2D PCB Arrays: T1: 1.5 mm T2: 3 mm. Lateral distance between targets: 3 mm.

Fig. 2.. PCB-based 2D ultrasound transducer array fabrication:

(a) Cross-sectional view highlighting different layers of a single element of the PCB transducer array. (b) Fabrication steps; (1) PCB with unfilled vias (2) Silver Epoxy filling (3) Z-Axis tape put over all elements (4) Undiced PVDF-TrFe bonded to Z-Axis tape (5) Front electrode connection as required (6) Insulating Parylene coating

Fig. 3.. PCB-Based 2D Matrix Arrays.

(a) Front and back side schematic of 4x4 PCB array, with element size 1 mm² with pitch of 1.2 mm, with header pins for elements and ground vias circled in red and black, respectively. (b) Fabricated 4x4 Array with backend interfacing connections for real time volumetric PA data acquisition. (c) Front and back side schematic of 8x8 PCB array, with element size 1 mm² with pitch of 1.5 mm, with peripheral solder pads for elements (connected by routing traces as shown by red arrows) and ground vias circled in red and black, respectively. (d) Fabricated 8x8 Array with backend interfacing connections.

Fig. 4.. Typical characterization results of 2D PCB-based ultrasound transducer arrays.

Electrical Impedance (red: $|z|$, black: $\angle z$) for (a) 4x4 and (b) 8x8 arrays. Pulse Echo US A-line and 6dB FWHM US BW for (c) 4x4 and (d) 8x8 arrays. PA A-line and 6dB FWHM PA BW for (e) 4x4 and (f) 8x8 2D arrays. Pulse-echo US peak-to-peak amplitude of individual elements (black: peak-to-peak mean, red: single standard deviation above and below peak-to-peak mean) for (g) 4x4 array using 5073 PR and (h) 8x8 array using Verasonics

Fig. 5.. Experimental setup for volumetric PA data acquisition using 2D PCB matrix arrays.

(a) Schematic of the experimental setup for acquiring PA data from 1.5% intralipid phantom embedded with two pencil lead targets and image Reconstruction process flow. (b) Picture of experimental setup for 1.5% intralipid phantom imaging using PCB Matrix Arrays and oblique angle of laser firing bundle.

Fig. 6.. Volumetric photoacoustic imaging of two pencil lead targets (T1 and T2) inside 1.5% intralipid phantom using 4x4 and 8x8 2D PCB Arrays.

(a) 4x4 Volumetric PA Image and corresponding (b) XZ and (c) YZ maximum intensity projection (MIP) images. (d) 8x8 Volumetric PA Image and corresponding (e) XZ and (f) YZ MIP images. Regions highlighted by white arrows denote regions of non-uniform receive sensitivity in the 8x8 array. (g) 4x4 and (h) 8x8 FWHM Axial and Lateral Spatial Resolution profiles of T1 and T2 highlighting improved axial and lateral resolution observed with 8x8 PCB Array. Depth of light absorbing PA targets (0.4 mm diameter) from surface of 4x4 and 8x8 2D PCB arrays: T1: 7 mm. T2: 9 mm. Lateral distance between T1 and T2 is 2 mm.