Objective:
In this paper, we introduce Global Maxwell Tomography (GMT), a novel, volumetric technique that estimates electric conductivity and permittivity by solving an inverse scattering problem based on magnetic resonance measurements.
Methods:
GMT relies on a fast volume integral equation solver, MARIE, for the forward path and a novel regularization method, Match Regularization, designed specifically for electrical properties estimation from noisy measurements. We performed simulations with three different tissue-mimicking numerical phantoms of different complexity, using synthetic transmit sensitivity maps with realistic noise levels as the measurements. We performed an experiment at 7T using an 8-channel coil and a uniform phantom.
Results:
We showed that GMT could estimate relative permittivity and conductivity from noisy magnetic resonance measurements with an average error as low as 0.3% and 0.2%, respectively, over the entire volume of the numerical phantom. Voxel resolution did not affect GMT performance and is currently limited only by the memory of the Graphics Processing Unit. In the experiment, GMT could estimate electrical properties within 5% of the values measured with a dielectric probe.
Conclusion:
This work demonstrated the feasibility of GMT with Match Regularization, suggesting that it could be effective for accurate in vivo electrical property estimation. GMT does not rely on any symmetry assumption for the electromagnetic field and can be generalized to estimate also the spin magnetization, at the expenses of increased computational complexity.
Significance:
GMT could provide insight into the distribution of electromagnetic fields inside the body, which represents one of the key ongoing challenges for various diagnostic and therapeutic applications.
Magnetic-resonance-based electrical property mapping using Global Maxwell Tomography with an 8-channel head coil at 7 Tesla: a simulation studyThe MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters.
CitationGiannakopoulos, Ilias I. et al. "Magnetic-resonance-based electrical property mapping using Global Maxwell Tomography with an 8channel head coil at 7 Tesla: a simulation study.
The use of electromagnetic fields applied to human tissues has proven to be beneficial in several applications, such as monitoring physiological parameters and delivering medical treatments. Often applications rely on targeted energy deposition into the tissue, or rely on wireless powering of implanted devices. In such cases, the system energy efficiency, the stability of the field, and ultimately the process safety could all benefit from minimizing the mismatch at the air-skin interface. In this paper, the maximization of the electric field transmitted into the muscle tissue is initially addressed by optimizing a dielectric-only matching layer in terms of thickness and relative dielectric permittivity, and under realistic constraints on low-cost available materials. The propagation of the electromagnetic field inside a multilayered medium that represents the body is evaluated by using the wave-transmission chain matrix approach. Furthermore, an innovative solution, based on the application of a metasurface matching layer (MML), is proposed to significantly improve the performance of the matching, thus enhancing the electromagnetic fields reaching the targeted muscle tissue. A thorough assessment of the performance is carried out considering both the presence of an air gap, and the case of plane waves impinging at oblique incidence.INDEX TERMS Biomedical applications, deep tissue coupling, metamaterials, plane wave excitation.
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