Reconfigurable dipole receive array
Accelerating imaging speed has been one of the most important and challenging goals in MRI over the past three decades. About two decades ago, the landscape for rapid MRI changed dramatically with the invention of parallel imaging (PI) techniques, which allow significantly accelerated acquisitions. A fundamental problem of all PI techniques is the decrease in signal-to-noise ratio with increasing imaging acceleration.
In previous work, we presented a novel approach to dynamically modulate the receive sensitivity profiles of surface loop elements during MR signal acquisition to improve PI performance. The initial study focused on 2D imaging with in-plane sensitivity variation. The objective of the present work is to extend the approach to 3D imaging with sensitivity variation along the z-axis (head-foot direction).
To this end, we demonstrate a novel RF Rx array coil based on eight reconfigurable dipole elements, rather than on receive loops, which allows dynamic sensitivity variation along the dipole axis. This is achieved by using PIN diodes to switch between capacitive and inductive impedance in the dipole arms, resulting in the formation of spatially distinct sensitivity profiles that can be rapidly modulated. Applying dynamic sensitivity modulation during image acquisition emulates two virtual rows of receive elements along the z-direction, which is expected to improve PI performance for 3D acquisitions.
We investigated this concept in detail using electromagnetic simulations. Based on these findings, a prototype coil array was built and tested on the 9.4T MR scanner in both phantom and human subject. In the in vivo experiment, for 3 × 2 (Ry × Rz) acceleration, we obtained up to 220% improvement compared to the static configuration case in terms of maximum noise amplification, i.e., g-factor, and up to 54% improvement in terms of mean g-factor.
Future work will focus on improving the scalability of the approach to a larger number of elements by reducing the number of DC wires, which are prone to detrimental interactions.