Revisiting Inductively Coupled Wireless Coils in MRI: Mitigating Over-Coupling With Preamplifiers

Inductively coupled coils enhance local MRI sensitivity, yet strong coupling with nearby primary coils typically causes resonance splitting and impedance mismatch, which are traditionally considered detrimental. This work investigates why inductively coupled coils can still function effectively even in the presence of severe coupling and clarifies the role of modern receive preamplifiers in mitigating coupling effects.

Bench experiments were performed using primary coils (10 and 15 cm) and secondary inductively coupled coils (3-9 cm) tuned to the same Larmor frequency at 1.5, 3, and 7 T. Resonance characteristics and primary-coil impedance variations were evaluated under open-circuit, 50-Ω, and low-input-impedance preamplifier terminations. MRI validation was conducted at 7 T without retuning the primary coil after introducing a closely positioned, inductively coupled coil, while intentionally varying preamplifier decoupling conditions.

Open-circuit and 50-Ω terminations produced pronounced resonance splitting and significant impedance distortion. In contrast, low-input-impedance preamplifier termination preserved the inductively coupled coil resonance despite strong coupling. Although the primary-coil impedance shifted substantially, it remained within acceptable noise-figure contours, resulting in negligible SNR penalty. Degraded preamplifier decoupling led to a 21%-23% SNR reduction.

Modern preamplifiers fundamentally alter coupled-coil behavior, enabling inductively coupled coils to operate near primary coils without significant SNR degradation and simplifying inductively coupled coil design.