Complex B₁ mapping and electrical properties imaging of the human brain using a 16-channel transceiver coil at 7T

The electric properties of biological tissue provide important diagnostic information within radio and microwave frequencies, and also play an important role in specific absorption rate calculation which is a major safety concern at ultrahigh field. The recently proposed electrical properties tomography (EPT) technique aims to reconstruct electric properties in biological tissues based on B₁ measurement. However, for individual coil element in multichannel transceiver coil which is increasingly utilized at ultrahigh field, current B₁-mapping techniques could not provide adequate information (magnitude and absolute phase) of complex transmit and receive B₁ which are essential for electrical properties tomography, electric field, and quantitative specific absorption rate assessment. In this study, using a 16-channel transceiver coil at 7T, based on hybrid B₁-mapping techniques within the human brain, a complex B₁-mapping method has been developed, and in vivo electric properties imaging of the human brain has been demonstrated by applying a logarithm-based inverse algorithm. Computer simulation studies as well as phantom and human experiments have been conducted at 7T. The average bias and standard deviation for reconstructed conductivity in vivo were 28% and 67%, and 10% and 43% for relative permittivity, respectively. The present results suggest the feasibility and reliability of proposed complex B₁-mapping technique and electric properties reconstruction method.