Speaker: Rock Hadley

Title: Microstrip RF Coils

Summary: Introduction: In some multiple coil MRI applications, RF coils are necessarily strongly coupled. One such example can be configured for breast MRI where a possible coil geometry would use two coils on parallel surfaces surrounding each breast (Figure 1). In this geometry, the medial coils tend to be highly coupled as their separation, D, may be small for some patients. A number of coil decoupling methods have been proposed, such as capacitive decoupling, transformer decoupling, low impedance pre-amplifier decoupling and active decoupling [1]. However, these systems either require hardware modifications to the RF section of the MR system, or are dependent on the coil geometry and thus require retuning for each patient. In order to overcome this problem, Wang [2] proposed a position-independent decoupling method by using a shielding coil composed of a multi-turn, open loop floating coil mounted on the back of the surface coil. When the surface coil detects the NMR signal, currents are set up in the shield which oppose this in the surface coil and serve to cancel out the magnetic field remote from the shielded side of the coil. However, due to the energy loss in the shielding coil, the SNR was found to be reduced by ~20% compared to an unshielded coil.

In this work, we present a position-independent decoupling method using micro-strip transmission line coils. Micro-strip transmission line receiving coils have been reported to have good SNR and stability [3-6]. They consist of a grounded back plane together with the conductive strip bonded to either side of a low-loss, high permittivity material such as Teflon. This two-part structure is very similar to that of the virtual shielding coil proposed by [2] and therefore should exhibit similar decoupling properties. Furthermore, the signal receiving strip and the grounded plane together form the resonance coil structure. Unlike the virtual shielding coil, in which the energy stored in the shielding coil is lossy, there is no signal loss for the micro-strip coil case. The results of our work showed that pairs of micro-strip coils placed back-to-back exhibit excellent decoupling over a range of geometries without need for individual adjustment, as well as good isolation properties. Due to this shielding affect, a micro-strip coil should exhibit asymmetric signal response resulting in a large signal from spins adjacent to the strip side compared to spins adjacent to the shielded side of the coil. We use this feature to provide decoupling from closely mounted medial coils in the four coil geometry of Figure 1. This allows the signals from each individual breast to be detected primarily from the coil facing the breast. Thus bilateral imaging based on TR interleaving can be achieved without the need for active coil decoupling [1].