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

Abstract

A double-loop H-field probe is used to measure current on RF coils for MRI. Direct S parameter measurement, denoted by 𝑆𝑆 21,𝑏𝑏 , is not proportional to coil current. After subtracting the coil-free S parameter, 𝑆𝑆 21,𝑎𝑎 , the quantity 𝑆𝑆 21,cal = 𝑆𝑆 21,𝑏𝑏 − 𝑆𝑆 21,𝑎𝑎 is proportional to coil current. The ratio �𝑆𝑆 21,cal /𝐼𝐼 coil � is plotted for several setups of different probe size, different coil size and different coil-to-probe distance. For double-loop H-field probes of ⌀1.3 cm probes and coil-to-probe distance no less than 5 cm, there is 𝑆𝑆 21,cal /𝐼𝐼 coil ≈ j𝜔𝜔𝜔𝜔 exp(j𝜔𝜔𝜔𝜔) up to 300 MHz, where 𝜔𝜔 < 0 and 𝜔𝜔 < 0 are setup-specific constants.