Calibration of fibre-optic RF E/H-field probes using a magnetic resonance (MR) compatible TEM cell and dedicated MR measurement techniques

Abstract

Supervision of specific absorption rate (SAR) during Magnetic Resonance Imaging (MRI) is crucial for patient safety. Electromagnetic field (EMF) simulations have proven to be essential tools in design and evaluation of dedicated MR coil setups. However, such simulations can only be validated comprehensively by physical measurements of EM field distribution using radio frequency (RF) electric and magnetic field probes. The reliability of those measurements strongly depends on the quality of the calibration procedure. This work presents a fully MR compatible Transverse Electromagnetic Cell (TEM cell) which was used to calibrate two different fibre coupled field probes by common MR measurement techniques only. In a water filled capillary mounted inside the TEM cell the B1(+) magnetic field component produced by the cell was measured by a flip angle mapping sequence. In a TEM cell the electric field and the magnetic field are related via E/H = 376.73 Ω. Therefore, the E-field can be related exactly to B1(+) by E = 2 c0 B1(+) (c0: velocity of light). For the acquisition of the sensor signals the receiver unit of a clinical 3T MR scanner and a 7T whole-body research scanner was used; as a result, no additional hardware was necessary. The estimated accuracy of the calibration procedure is 5 % or 0.4 dB and the linear behaviour of the calibrated sensor signals was verified. For further evaluation of uncertainties and homogeneity of the generated RF fields an improved design of the TEM cell with a characteristic impedance of 50 Ω is desired. © 2012 by Walter de Gruyter Berlin Boston.