Current Balancing Coaxial-Coils For Magnetic Resonance Imaging

Abstract

MRI surface coil arrays extend the field-of-view and improve the signal-to-noise ratio (SNR) compared to individual coils. However, inductive coupling between coils will degrade the SNR and can introduce image artefacts. The most common approach to array-coil decoupling combines coil-overlapping (geometric decoupling) and resonant high-impedance blocking circuits (preamplifier decoupling). Although this is effective, it requires tuning, making it narrowband and susceptible to detuning under varying patient loads.
This thesis proposes an active, electronically-controlled decoupling method. A coaxial-coil consisting of two tightly coupled sub-coils is driven by a current-balancing low-noise amplifier (LNA). By maintaining equal and opposite currents on the two sub-coils, the driver suppresses the external fields responsible for coupling. The proposed approach is robust to patient loading and is inherently broadband, eliminating the need for tuning. A noise-shaping filter separates the concerns of coil decoupling and noise matching, giving additional design flexibility.
Using analytical and finite-element (FEM) modelling, basic surface-coil performance was first established across key parameters and conventional decoupling effectiveness quantified using FEM-circuit co-simulation. Lumped-parameter models of the coaxial current-balancing design for the low- and high-frequency regimes were developed and validated against FEM. The proposed approach was evaluated over 50–350 MHz in two main studies: (1) a single 60 mm loop coil under external excitation to quantify current and hence field suppression, and (2) a 3 × 1 array of 60 mm coils to quantify the nearest/next-nearest neighbour (S12/S13) coupling as the coil-to-coil overlap was swept 0–50% of coil diameter. The scheme achieved an effective (equivalent) decoupling impedance of (approx.) 16 kΩ at 50 MHz and 1.6 kΩ at 350 MHz without retuning. Compared to resonant methods, this gives equal or modestly better isolation for typical input preamplifier impedances (0.5–5 Ω). This work also included a comprehensive analysis of the LNA/driver topologies best suited for the current balancing approach, identifying the series-driven, voltage-sensed designs as the most capable, lowest-noise option.
This simulation-based study shows that active, electronically controlled decoupling is a feasible, inherently broadband alternative to conventional resonant methods. It demonstrated comparable isolation without the need for tuning or retuning across 50–350 MHz, which has potential advantages for multi-nuclear imaging. The study is limited to receive-only surface coil arrays. Future work will address a compatible transmit-detuning strategy, practical prototyping, and in-scanner evaluation of performance.

Date of Award	20 Jan 2026
Original language	English
Awarding Institution	University of Bristol
Supervisor	Paul A Warr (Supervisor)