A Concentrically Shielded Transceive Array Coil Optimized for B1+ Inhomogeneity Correction at 7T

Introduction: A multiple channel transceive array offers the possibility for flexible mitigation of the transmit inhomogeneity at high fields through either B1 + shimming or by the use of spatially tailored RF excitation pulses accelerated to useful durations using parallel transmit (pTx). Accelerating the transmit RF pulses requires a distinct spatial sensitivity profile. One of the major problem in the design of transmit array coil at high fields is the coil coupling and the radiation losses which increases as the square of the coil size and to the fourth power of the frequency. The conventional decoupling either by overlapping or by a shared capacitor requires the array coil to be large. In this sense, the size of the array coil depends on the decoupling method used. By placing an in-plane concentric shield around the coil, as proposed by Lanz et al. (1), the return flux from the loop is concentrated between the coil conductor and the ground loop, thereby reducing the flux threading the adjacent elements in an array. This decoupling method provides extra freedom in Tx array design since it does not impose a restriction on the size of the transmit elements. We utilize this degree of freedom to develop a loop coil array with a loop size intermediate between that needed for an overlapped or capacitively decoupled loop array and a narrow loop array where the loops are far enough apart to have minimal coupling. We validate this principle in an optimized 8 channel concentrically shielded loop array coil at 7T and evaluated its performance for B1 shimming.