Development of a Low-Frequency Piezoelectric Ultrasonic Transducer for Biological Tissue Sonication

Abstract

The safety of ultrasound exposure is very important for a patient's well-being. High-frequency (1-10 MHz) ultrasound waves are highly absorbed by biological tissue and have limited therapeutic effects on internal organs. This article presents the results of the development and application of a low-frequency (20-100 kHz) ultrasonic transducer for sonication of biological tissues. Using the methodology of digital twins, consisting of virtual and physical twins, an ultrasonic transducer has been developed that emits a focused ultrasound signal that penetrates into deeper biological tissues. For this purpose, the ring-shaped end surface of this transducer is excited not only by the main longitudinal vibrational mode, which is typical of the flat end surface transducers used to date, but also by higher mode radial vibrations. The virtual twin simulation shows that the acoustic signal emitted by the ring-shaped transducer, which is excited by a higher vibrational mode, is concentrated into a narrower and more precise acoustic wave that penetrates deeper into the biological tissue and affects only the part of the body to be treated, but not the whole body.

Keywords: FEM; acoustic intensity; deep penetration; higher vibration mode; targeted acoustic wave.

Figure 1

Geometric dimensions of the piezo transducer in mm with flat surface (a) and ring-shaped surface (b) of the front mass 3: 1—back mass; 2—two ring-shaped piezoelectric elements; 3—front mass.

Figure 2

Set-up with Polytec PSV-500-3D scanning laser vibrometer: 1—transducer; 2—linear amplifier; 3—Polytec scanning laser head; 4—Polytec signal generator/data acquisition system.

Figure 3

Experimental setup with impedance analyzer 6500 B (Wayne Kerr Electronics Ltd., Bognor Regis, UK) and the two tested transducers: 1—transducer with flat surface; 2—transducer with ring-shaped surface.

Figure 4

Theoretical amplitude–frequency characteristics of piezoelectric transducers using Comsol multiphysics: (a) with flat surface; (b) with ring-shaped surface with 59 mm outer diameter; (c) with ring-shaped surface with 100 mm outer diameter.

Figure 5

Modal shapes of the piezoelectric transducer at the first natural frequency: (a) with flat surface; (b) with 58 mm diameter ring-shaped surface; (c) with 100 mm diameter ring-shaped surface.

Figure 5

Modal shapes of the piezoelectric transducer at the first natural frequency: (a) with flat surface; (b) with 58 mm diameter ring-shaped surface; (c) with 100 mm diameter ring-shaped surface.

Figure 6

Modal shapes of the piezoelectric transducer at the second natural frequency vibrations: (a) with flat surface; (b) with 58 mm diameter ring-shaped surface; (c) with 100 mm diameter ring-shaped surface.

Figure 7

Propagation of an acoustic field generated by piezoelectric transducer at the first mode in the muscle: (a) with flat surface; (b) with 58 mm diameter ring-shaped surface; (c) with 100 mm diameter ring-shaped surface.

Figure 8

Propagation of acoustic field generated by piezoelectric transducer at the second mode in the muscle: (a) with flat surface, (b) with 58 mm diameter ring-shaped surface, (c) with 100 mm diameter ring-shaped surface.

Figure 8

Propagation of acoustic field generated by piezoelectric transducer at the second mode in the muscle: (a) with flat surface, (b) with 58 mm diameter ring-shaped surface, (c) with 100 mm diameter ring-shaped surface.

Figure 9

Distribution of the acoustic pressure level of an ultrasound wave propagating in a muscle medium, simulated by piezoelectric transducers, in the second natural mode: (a) with a flat surface at a resonant frequency of 48 kHz; (b) with a ring-shaped surface and the different diameters of: 58 mm at a resonant frequency of 38 kHz (solid line) and with a diameter of 100 mm at resonant frequency of 47 kHz (dashed line).

Figure 10

Experimental impedance–phase vs. frequency characteristics: (a) the electric impedance of the transducer with flat surface; (b) the electric impedance of the transducer with ring-shaped surface.

Figure 11

Experimental amplitude–frequency characteristics of the two developed transducers measured with Polytec 3D scanning vibrometer: (a) with flat surface; (b) with ring-shaped surface.