Calibrating Double-Loop H-Field Probe Measurements of RF Coil Current for MRI

Objective: Double-loop H-field probes are often used to measure current on loop antennae for magnetic resonance imaging (MRI). Loop crosstalk limits the dynamic range of direct measurements with such probes. The crosstalk can be removed by simple calibration. This work analyses the quantitative relation of a probe's calibrated ${S}_{21}$ with the RF coil current. Method: The analytical relation between RF coil current and calibrated ${S}_{21}$ measurements of a probe is established with the multi-port network theory, and verified by full-wave simulation and benchtop measurements. The effect of calibration is demonstrated by measuring the ¹H trap frequency, the active detuning, and the preamplifier decoupling. Results: The calibration removes the effect of crosstalk in a probe and improves the lower bound of $| {{S}_{21}} |$. The calibrated ${S}_{21}$ is proportional to coil current. In the lower frequency range, the ratio of calibrated ${S}_{21}$ to coil current changes almost linearly with frequency. Impact: The calibration method improves the sensitivity of probe measurements and facilitates fine-tuning current-suppressing circuits like active detuning circuits, traps, preamplifier decoupling. The linear frequency dependency between ${S}_{21}$ measurements and coil current allows easy, fair comparison of coil current up to 128 MHz, and in some cases 298 MHz, helping build multi-nucleus coils.