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Thesis by SUN Zhishen

Application of Lorentz force in ultrasound-electromagnetic-field-coupled electrical impedance tomography and elastography
Defended on 21 november 2019
The Lorentz force, as the linkage between the mechanical displacement and the electrical current, is investigated in this thesis in two fields of the medical acoustics, i.e. scanning electric conductivity gradients with ultrasonically induced Lorentz force (SECG-UILF) and elastography. The first part of the thesis studies the SECG-UILF. To reduce the instantaneous stimulation power to the transmitting transducer and at the same time the peak acoustic pressure from the transducer, this thesis proposes to apply the linearly frequency-modulated ultrasound pulse excitation or the sinusoidal step-frequency ultrasound pulse excitation in SECG-UILF. The second part of the thesis studies elastography which uses the cross-correlation approach of diffuse field to reconstruct the shear wave velocity. First, generation of shear wave sources on the soft medium surface is investigated by stimulating a non-ferromagnetic conductive ring or patch with a transient magnetic field. Due to the Faraday's law of induction, an eddy current is generated remotely in the conductive ring or patch by the transient magnetic field. The eddy current at the same time couples with the magnetic field, generating the Lorentz force, which constitutes a torque on the ring or patch and vibrates the ring or patch. The origin and the frequency and amplitude characteristics of the Lorentz force acting on the conductive ring are confirmed by the displacement measurement using an interferometric laser probe: (1) the eddy current induced by the transient magnetic field and the orthogonal component of the magnetic field constitute the two essential elements of the Lorentz force (2) the carrier frequency of the Lorentz force equals the sum of the carrier frequencies of the stimulating transient magnetic field and its time derivative and (3) the amplitude of the Lorentz force increases quadratically upon the amplitude of the stimulating current to the coil. Under a transient magnetic field of changing rate of 10.44 kTs-1, the patch generates a shear wave field source of amplitude of 100 μm at the surface of the sample of polyvinyl alcohol (PVA) phantom. The shear wave fields created and propagating in the PVA phantom by experiments agree qualitatively well with the theoretical shear wave fields calculated through the analytical Green function solution. The potential of the generated shear wave fields for the cross-correlation based shear wave velocity reconstruction is explored. The displacement fields in the multi-shaped soft biological tissue mimicking agar phantoms are recorded at a sampling rate of 1000 frames/s as the multiple shear wave sources propagate in the phantoms. Based on the cross-correlation approach, the shear wave velocity maps are reconstructed from 100 frames of the displacement fields, which show that: (1) the interfaces or boundaries between regions of different stiffness can be clearly recognized, which are completely concealed in the ultrasound images (2) cylindrical inclusions of diameter as small as 5 mm can be differentiated from the background, but at the same time, there are phony boundaries in the homogeneous areas which undermine the performance of the method and (3) using the cross-correlation approach, it is possible to qualitatively but not quantitatively reconstruct the shear wave velocity in the samples from 100 frames of the displacement fields

Soutenue le 21 November 2019