Electric current generated by ultrasonically induced Lorentz force in biological media

Abstract

The ions of solutions exposed to the propagation of ultrasound in the presence of a magnetic field experience Lorentz force. Their movement gives rise to a local electric current density, which is proportional to the electric conductivity of the medium. In vitro assessment of this current is performed using simple models of biological media. A constant magnetic field of 0.35 T and 500 kHz pulsed ultrasound are used. The sensing electrodes are exposed to neither the pressure wave nor the magnetic field, thus ensuring that the signal is not due to any undesirable electrode effect. The experimental results confirm that the current is proportional to the electrical conductivity of the medium. The changes in the measured current against the width of the measurement chamber show that the electrodes only collect a fraction of the current created within the medium. The magnitude of the measured current is 50nA in a saline solution of 0.5 S/m conductivity. The technique enabled the determination of the conductivity of a porcine blood sample against haematocrit. It is concluded that this type of measurement has the potential to allow the electrical conductivity of a medium to be determined using ultrasound.

Mathematical models and reconstruction methods in magneto-acoustic imaging

“…This gives rise to a localized current density within the medium. The current density is proportional to the local electrical conductivity [14]. In practice, the ultrasounds impact a spherical or ellipsoidal zone, of a few millimeters in diameter.…”

“…This in turn creates a pressure wave that can be detected using an ultrasound hydrophone [14]. The MAT-MI uses this acoustic pressure wave to reconstruct the conductivity distribution of the sample as the focus of the ultrasound beam scans the entire domain.…”

“…The probe signal produces by the Lorentz force an induced signal that is a function of the local electrical conductivity of the biological tissue [14]. If the probe signal is an acoustic wave, then the induced signal is an electric current and the Lorentz force causes a local current density.…”

“…We refer the reader to [14,12,13,17,18,8,15,16] for physical basic principles of vibration potential tomography, magneto-acoustic tomography with magnetic induction, and magneto-acoustic current imaging.…”

Three-dimensional multiexcitation magnetoacoustic tomography with magnetic induction

“…On the other hand, one can also apply ultrasonic energy to a sample sitting in a static magnetic field and record voltage/current signals to obtain the sample's conductivity information. 14,16,17 However, the use of current injection or voltage/current measurement through surface electrodes still make these methods limited by the "shielding effect" 15 which suggests a degraded sensitivity in those areas surrounded by low conductive layers such as the brain and breast in human body.…”