A Comprehensive Study of Ultrasound Transducer Characteristics in Microscopic Ultrasound Neuromodulation

Abstract

In order to improve the spatial resolution of transcranial focused ultrasound stimulation (tFUS), we have recently proposed microscopic ultrasound stimulation (μUS). In μUS, either an electronically phased array of ultrasound transducers or several millimeter-sized focused transducers are placed on the brain surface or sub-millimeter-sized transducers are implanted inside the brain tissue to steer and deliver a focused ultrasound pressure directly to the neural target. A key element in both tFUS and μUS is the ultrasound transducer that converts electrical power to acoustic pressure. The literature lacks a comprehensive study (in a quantitative manner) of the transducer characteristics, such as dimension, focusing, acoustic matching, backing material, and sonication frequency (f_p), in the μUS. This paper studies the impact of these design parameters on the acoustic beam profile of millimeter-sized transducers with the emphasis on the stimulation spatial resolution and energy efficiency, which is defined as the μUS figure-of-merit (FoM). For this purpose, disc-shaped focused and unfocused piezoelectric (PZT-5A) transducers with different dimension (diameter, thickness), backing material (PCB, air) and acoustic matching in the frequency range of 2.2-9.56 MHz were fabricated. Our experimental studies with both water and sheep brain phantom medium demonstrate that acoustically matched focused transducers with high quality factor are desirable for μUS, as they provide fine spatial resolution and high acoustic intensities with low input electrical power levels (i.e., high FoM).

Fig. 1.

(a) Comparison of different neuromodulation approaches in terms of their spatial coverage, spatial resolution and invasiveness. (b) Conceptual schematic of the microscopic ultrasound stimulation (μUS) system (the minimally invasive variation with large spatial coverage) [9].

Fig. 2.

Transmission line model (KLM) of a piezoelectric transducer [38].

Fig. 3.

The generated acoustic pressure beam by a disc-shaped ultrasound transducer and its acoustic intensity (magnitude variations) along the axial and lateral directions. It can be seen that even an unfocused transducer features a natural focus.

Fig. 4.

Some examples of fabricated transducers used in our measurements. (a) US₂: PCB-backed transducer with D_o = 6.8 mm and t = 0.75 mm, (b) US₁: air-backed transducer with D_o = 6.8 mm and t = 0.75 mm, (c) US₆: PCB-backed transducer with D_o = 2.8 mm and t = 0.3 mm, (d) and (e) US₈ and US₉: focused PCB-backed transducers (D_o = 5.8 mm, t = 1 mm) encapsulated with EPO-TEK and EPO-TEK + Alumina, respectively.

Fig. 5.

(a) Meaurement setup used to measure the acoustic pressure generated by each ulrasound transducer. (b) Simulation setup in COMSOL used to find electrical and acoustic characteristics of the ultrasound transducers (US).

Fig. 6.

Measured transient voltage of the hydrophone (after preamplifier) for driving US₂ at the axial distance of (a) 12.3 mm and (b) 6.9 mm.

Fig. 7.

Comparison between measured and simulated characteristics of US₁. (a) Electrical impedance, (b) simulated (COMSOL) acoustic beam profile at f_p = 2.85 MHz, (c) measured acoustic beam profile at f_p = 2.8 MHz, and (d) axial and lateral resolution. Acoutic intenstiry was found for 1 V sinusoidal input.

Fig. 8.

Measured acoustic beam profile of US₁ at the frequencies of (a) 2.6 MHz and (b) 2.7 MHz. (c) Axial and lateral resolution. Fig. 7c shows the measured beam profile of US₁ at 2.8 MHz.

Fig. 9.

Impact of the backing layer (air vs. PCB) on the acoustic beam profile of air-backed US₁ (Fig. 7c) and PCB-backed US₂. (a) US₂ measured electrical impedance, (b) US₂ measured acoustic beam profile at f_p = 2.8 MHz, and (c) axial and lateral resolution of US₁ and US₂.

Fig. 10.

Impact of t on the acoustic beam profile of US₄ (t = 0.73) and US₅ (t = 0.4) with similar D_o = 4.2 mm. (a) Measured electrical impedance, (b) US₄ measured intensity at f_p = 3.05 MHz, (c) US₅ measured acoustic intensity at f_p = 5.56 MHz, and (d) axial and lateral resolution of US₄ and US₅.

Fig. 11.

Impact of D_o on the acoustic beam profile (t = 0.75 mm) of US₂ (Fig. 7c) and US₃. (a) US₃ measured acoustic beam profile at f_p = 2.8 MHz (US₂ profile in Fig. 9b) and (b) axial and lateral resolution of US₂ and US₃.

Fig. 12.

US₆ measured acoustic beam profile at f_p = 9.56 MHz.

Fig. 13.

Impact of beam focusing and acoustic mathicng. (a) US_7–9 measured electrical impedance, (b), (c), (d) US₇, US₈ and US₉ measured acoustic intensity, respectively, and (e) axial and lateral resolution of US_7–9.

Fig. 14.

Experiment setup used to measure the impact of sheep brain phantom on the acoustic beam profile generated by US₁ and US₆.

Fig.15.

Comparison of measured normalzied acoustic beam profile with the introduction of a sheep brain phantom. (a) and (b) US₁ (2.8 MHz) measurements without and with the phantom, respectrively. (c) and (d) US₆ (9.56 MHz) measurements without and with the phantom, respectrively.

Fig. 16.

Our future plan for integration of the whole μUS system into a portable device for the proof-of-concept testing on rodents. Thanks to the transducer miniaturization in this work, multiple mm-sized transducers can be placed on the animal’s head to target multiple brain regions.