2 mm Radius Loop Antenna and Linear Active Balun for Near Field Measurement of Magnetic Field in MRI-Conditional Testing of Medical Devices

This paper presents the design and construction of a magnetic field probe with an active balun for conditional testing of medical devices within a magnetic resonance imaging (MRI). The magnetic field probe was designed using a small loop antenna with 2 mm radius to have high spatial resolution and accuracy. It was tuned and matched at a center frequency of 128 MHz, which corresponds to 3 T MRI systems or equivalent radio frequency (RF) exposure systems for MRI-conditional testing of medical devices. An active balun with a differential transistor topology and a passive low-profile transformer were employed to boost the detected magnetic field signal level lead to higher sensitivity. It also has high input impedance that improves the decoupling of the probe to nearby medical devices. The designed magnetic field probe and active balun have been fabricated on a double-sided printed circuit board, FR4 thickness of 1.57 mm and a copper thickness of 35 μm, with overall footprint of 22 mm × 11 mm. A verification test setup was developed to generate a known field and calibrate the probe based upon an analytic calculation of field, FDTD simulation and a 10-mm radius passive tuned/matched loop antenna.