Magnetic Induction Magnetic Sound Imaging And Its Application In HIFU Noninvasive Temperature Measurement

Magnetoacoustic tomography with magnetic induction (MAT-MI) is a recently proposed bio-tissue electrical impedance tomography modality, combining the good contrast of electrical impedance tomography and high spatial resolution of ultrasonic imaging. Due to the obvious correspondence between bio-tissue conductivity and temperature, MAT-MI can be used as a new auxiliary mean for noncontact and noninvasive temperature estimation of ultrasonic hyperthermia, such as high intensity focused ultrasound (HIFU). In this paper, based on the acoustic dipole radiation theory, several MAT-MI related problems, such as layer effect, transducer reception pattern and acoustic source distribution, are discussed in detail with theoretical derivation, numerical simulation and experimental study. Meanwhile, the relationship between MAT-MI signals and temperature rise of HIFU focal region is studied for the feasibility analysis of HIFU noninvasive temperature estimation with MAT-MI.The results show that the acoustic dipole radiation theory can well explain the generation mechanism of Lorentz force induced acoustic sources. Different amplitudes and vibration phases of the wave clusters in the collected waveforms reflect the conductivity boundary information along the normal of the transducer surface. With the back projection algorithm for diffraction acoustic sources, reconstructed conductivity contrast image agrees well with the conductivity configuration on the scanned layer. Strong directional transducer with a large surface radius can effectively reduce the influence of equivalent sources, projection sources and layer effects, improve the normal signal detection ability of the transducer and the quality of reconstructed image. MAT-MI sources consist of inner and outer sources. For models with abrupt conductivity changes, acoustic strength of outer sources is much stronger than that of inner ones, and the contribution of inner sources is neglectable. But for models with gradual conductivity changes, the ratio of maximum strengths of inner and outer sources grows linearly with the increase of size of transition region, so both the inner and outer sources should be considered. The conductivity distribution of HIFU focal region has the same characteristics as the models with gradual conductivity changes. The maximum amplitudes of MAT-MI signals has the same time-varying trend as that of the temperature rise in HIFU focal region, and this feature can be used to realize the HIFU noninvasive temperature estimation with MAT-MI.