Imaging electric properties of biological tissues by RF field mapping in MRI

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 calculation, a major concern in high-field MRI, as well as in nonmedical areas such as wireless telecommunications. The high-field MRI system is accompanied by significant wave propagation effects, and the RF radiation is dependent on the EPs of biological tissue. On the basis of 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 EPs of biological tissues. The finite-element method was utilized in 3-D modeling and B₁ field calculation. A series of computer simulations were conducted to evaluate the feasibility and performance of the proposed method on a 3-D head model within a TEM coil and a birdcage 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 EPs from B1 mapping can be achieved.