B₁⁺ phase mapping at 7 T and its application for in vivo electrical conductivity mapping

In this study, a new approach to measure local electrical conductivity in tissue is presented, which is based on the propagating B₁⁺ phase and the homogeneous Helmholtz equation. This new MRI technique might open future opportunities for tumor and lesion characterization based on conductivity differences, while it may also find application in radio frequency safety assessment. Prerequisites for conductivity mapping using only the B ₁⁺ phase (instead of the complex B₁⁺ field) are addressed. Furthermore it was found that the B₁ ⁺ phase can be derived directly from the measurable transceive phase arg(B₁⁺ B₁^-) in the head. Validation for a human head excited by a 7 T-birdcage coil using simulations and measurements showed that it is possible to measure in vivo conductivity patterns in the brain using B₁⁺ phase information only. Conductivity contrast between different brain tissues is clearly observed. The measured mean values for white matter, gray matter and cerebrospinal fluid differed 54%, 26%, and -13% respectively from literature values. The proposed method for B₁⁺ phase measurements is very suited for in vivo applications, as the measurement is short (less than a minute per imaged slice) and exposes the patient to low RF power, contrary to earlier proposed approaches. Magn Reson Med, 2012.