Thermoacoustic tomography from magnetic nanoparticles by single-pulse magnetic field

A mechanism of single-pulse magnetic field (SMF) inducing magnetic nanoparticles (MNPs) to generate the thermoacoustic (TA) wave is proposed, and its feasibility is proved by simulation and experiment.

According to the principle of dimensional consistency, it is proposed that the internal energy variation of MNPs under the adiabatic condition mainly stems from the accumulation of magnetization energy, which leads to the magnetothermal effect, and then the TA wave is excited by thermal expansion. The analytical model of the forward problem is derived based on the method of space-time separation. The magnetization curve of MNPs is obtained from Langevin theory, and a three-dimension simulation model based on the magnetization curve is established to analyze the generation process of the TA wave. In the Experimental section, a gel phantom with a 0.5 mm gap is prepared with the magnetic fluid injecting into the gap, and the cross-sectional image of the gel phantom is reconstructed by the image fusion algorithm based on B-scan imaging.

The simulation analysis shows that the generated TA signal can reflect the boundary information of the MNPs region, and when the MNPs are in the unsaturated magnetized region, the intensity of the TA signal is positively correlated with the concentration of MNPs. The B-scan imaging along the X-axis and Y-axis directions are obtained through the experimental data. After that, the phantom with 0.5 mm gap labeled by MNPs is faithfully reconstructed by combining image morphology processing and image fusion technology based on wavelet transform.

The results show that the TA tomography from MNPs by SMF uses MNPs as a contrast agent to reconstruct the size and shape of the marked phantom with submillimeter resolution, which is expected to reconstruct the image of the tumor labeled by MNPs in the future. However, it is also a certain challenge to use low-concentration MNPs to image in vivo.