Four-row MRI receive array with remote circuitry for improved parallel imaging in radiation therapy systems

Objective.Previous MR guided radiation therapy (MRgRT) radiofrequency coil arrays have been limited to one to two rows of coils in the head-foot direction because of the desire to place radiation-opaque coil circuitry outside the window through which the radiation beam travels. However, such layouts limit parallel imaging undersampling in the head-foot direction. We recently demonstrated a three-row array with a remote coil circuit that improved parallel imaging performance, while preserving the signal-to-noise ratio (SNR) and the radiolucent window. Here we evaluate a four-row prototype design to determine if further parallel imaging advantages could be realized.Approach.We built remote circuits that allowed radio-opaque components to be placed outside the field of view through which the radiation beam is expected to travel. The circuit consisted of a phase shifter to cancel the phase introduced by the radiolucent coaxial link between the circuit and coil, followed by standard components for tuning, matching, detuning, and preamplifier decoupling. Measurements were performed on an abdominal phantom to compare single-channel coils with remote or local circuits, followed by tests on a 16-channel four-row array.Main results.The four-row array maintained SNR comparable to two- and three-row designs while supporting 3× head-foot acceleration (minimum reciprocalg-factor = 0.74) and 2 × 3 multi-directional acceleration (minimum reciprocalg-factor = 0.72), capabilities which were not achievable with previous designs.Significance.These results demonstrate the technical feasibility of four-row designs, which may benefit MRgRT applications that require high SNR and temporal-resolution.