Imaging Electric Properties of Biological Tissues by RF Field Mapping in MRI

Abstract

The electric properties (EPs) of biological tissue, i.e., the electric conductivity and permittivity, can provide important information in the diagnosis of various diseases. The EPs also play an important role in specific absorption rate (SAR) calculation, a major concern in high-field Magnetic Resonance Imaging (MRI), as well as in non-medical areas such as wireless-telecommunications. The high-field MRI system is accompanied by significant wave propagation effects, and the radio frequency (RF) radiation is dependent on the EPs of biological tissue. Based on the measurement of the active transverse magnetic component of the applied RF field (known as B1-mapping technique), we propose a dual-excitation algorithm, which uses two sets of measured B1 data to noninvasively reconstruct the electric properties of biological tissues. The Finite Element Method (FEM) was utilized in three-dimensional (3D) modeling and B1 field calculation. A series of computer simulations were conducted to evaluate the feasibility and performance of the proposed method on a 3D head model within a transverse electromagnetic (TEM) coil and a birdcage (BC) coil. Using a TEM coil, when noise free, the reconstructed EP distribution of tissues in the brain has relative errors of 12% ∼ 28% and correlated coefficients of greater than 0.91. Compared with other B1-mapping based reconstruction algorithms, our approach provides superior performance without the need for iterative computations. The present simulation results suggest that good reconstruction of electric properties from B1 mapping can be achieved.

Tissue Electrical Property Mapping From Zero Echo-Time Magnetic Resonance Imaging

“…The quality of the conductivity maps compares favorably with previously reported results obtained with an ultra-short TE sequence [15]. Whereas no attempt was made to segment the brain into different tissue types, the observed average conductivity is compatible with the brain tissue conductivity values in literature: white matter (0.34 S/m), grey matter (0.59 S/m), and cerebrospinal fluid (2.14 S/m) [34, 35]. …”

From Complex <formula formulatype="inline"><tex Notation="TeX">${\rm B}_{1}$</tex></formula> Mapping to Local SAR Estimation for Human Brain MR Imaging Using Multi-Channel Transceiver Coil at 7T

“…Recently, B 1 -mapping based electric field and SAR estimation approaches have been developed by post-processing the measured complex B 1 fields (rotating RF field induced by MRI RF coils, consisting of transmit B 1 and receive B 1 [20]) through a series of mathematical deduction, which involves the retrieval of complex B 1 information and the reconstruction of electrical properties (EPs) of the subject (known as Electrical Properties Tomography, EPT) [21–24]. This approach overcomes the limitation of conventional simulation methods relying on a generic human body model, and allows for real-time calculation [23],[25].…”

“…It can be anticipated that, when using this assumption in biological tissues with complex anatomical structures, artifacts may arise in the vicinity of tissue boundaries in reconstructed EPs maps, resulting in an inaccurate SAR estimation. Nevertheless, the Dual-Excitation EPT algorithm, that has been previously proposed [24], takes the spatial variation of EPs distribution into account and produced reasonable simulation results for head tissues, indicating that this method could improve EPs reconstruction accuracy.…”

“…As described in our recent studies [24],[26],[27], three assumptions have been proposed and employed throughout the mathematical deduction and data analysis, from complex B 1 -mapping to EPs reconstruction: (1) a fairly left-right symmetric structure of the human brain about coronal plane through the brain center, (2) the 16-fold elliptical symmetric head coil structure about the long axis, and (3) negligible spatial variations of the z-axis component of induced RF magnetic fields. Besides quantitatively and separately evaluating the performance of above assumptions in the calculated intermediate quantities (e.g., reconstructed EPs in [24], and extracted proton density in [27]), it is necessary to rigorously investigate the resulted error propagation altogether along the whole data post-processing towards the ultimate local SAR estimation.…”

Magneto acoustic tomography with short pulsed magnetic field for in-vivo imaging of magnetic iron oxide nanoparticles

“…Therefore, to further obtain a quantitative map of the IONP distribution, this z dependence of the acoustic source would have to be taken into account. This can be achieved by varying the z profile of the magnetic field and performing multiple measurements and combining them as used in multi excitation methods proposed in electrical properties imaging [51][52]. This could give a quantitative distribution which could be useful in designing therapeutic applications.…”