Electric Polarization of Soft Biological Tissues Induced by Ultrasound Waves

Ultrasound irradiation makes it possible to generate alternating electric polarization through the electromechanical coupling of materials. It follows that electromagnetic fields are often emitted to the surrounding environment when materials are acoustically stimulated. We investigate the acoustically stimulated electromagnetic (ASEM) response of soft biological tissues. The ASEM signal is detected through a capacitive resonant antenna tuned to the MHz frequency of the irradiated ultrasound waves. The signal is well explained by the stress-induced polarization, which responds linearly to the applied acoustic stress. Induced polarization is clearly observed in the Achilles tendon, aortic wall, and aortic valve samples, whereas it is small in adipose tissue and myocardium samples, indicating that fibrous tissues exhibit electromechanical coupling.

Figure 1

(a) Photograph of the experimental setup. (b) Photograph of an aortic wall sample set in a plastic sample holder. (c) Block diagram of the ASEM measurement system. (d) Schematics of the geometric placement in the sample setting.

Figure 2

Typical time traces of (a) echo signals in the Achilles tendon and time traces of ASEM signals in (b) the Achilles tendon, (c) aortic wall (intima side), and (d) aortic valve. The data are taken for $h=21\mathrm{mm}$ ($h/R=1.4$). The insets show photographs of the samples. The circles in the insets indicate the positions of the measurements.

Figure 3

Typical time traces of ASEM signals of (a) adipose tissue (porcine) and (b) myocardium (porcine). The data are taken for $h=21\mathrm{mm}$ ($h/R=1.4$). The insets show photographs of the samples. The circles in the insets indicate the positions of the measurements. We checked the spatial distribution of the ASEM response by two-dimensional mapping (more than 200 positions) for individual tissues. The spatially averaged ASEM signal amplitude of both tissues is estimated to be about 20 nV, which corresponds to the detection limit in our measurement setup.

Figure 4

Imaging of the Achilles tendon sample. (a) Echo lateral image, (b) ASEM lateral image, (c) echo tomographic image (B mode), and (d) ASEM tomographic image. The dotted red arrows in (a) and (b) indicate the position of the tomographic images shown in (c) and (d), respectively. The dashed yellow curves show the surface form of the sample. The data are taken for $h=21\mathrm{mm}$ ($h/R=1.4$).

Figure 5

Imaging of the aortic wall sample. (a) Echo lateral image, (b) ASEM lateral image, (c) echo tomography (B mode), and (d) ASEM tomography. Ultrasound waves are irradiated from the adventitia side of the aortic wall. The dotted red arrows in (a) and (b) indicate the position of the tomographic images shown in (c) and (d), respectively. The dashed yellow curves show the surface form of the sample. The data are taken for $h=21\mathrm{mm}$ ($h/R=1.4$).

Figure 6

(a) Schematic of the dipole detection model. (b) Signal amplitude multiplied by $R$ as a function of $h/R$ for the Achilles tendon and aortic wall samples. The solid and dashed lines represent the best-fit curves based on the dipole detection model for the Achilles tendon and aortic wall, respectively. (b) Induced polarization charge $q_0$ as a function of acoustic force $F_{\mathrm{ac}}$ for the Achilles tendon and aortic wall samples. The data are taken for $h=21\mathrm{mm}$ ($h/R=1.4$).