A new array design using tunable loop microstrip (TLM) coil

Introduction: Mutual decoupling is an important issue for coil array design. Generally, element overlap, decoupling network and isolating preamplifier are normal approaches for maximizing the decoupling. Recently, parallel planar straight-line strip array, an inherently decoupling design, has been introduced [1, 2]. However, its geometry critically depends on substrate dielectric properties thus its applications are limited. To solve this problem, we have developed a novel tunable loop microstrip (TLM) coil [3], which can achieve higher Q factor and less frequency shift with loading than microstrip straight-line resonator. Capacitance tunning and matching are allowed which makes it much easier to design and fabricate than microstrip straight-line coil, and coil geometry can be selected much more flexible. With the advantage of intrinsical decoupling performance, the TLM arrays have great potential in MRI applications. Method: Schematic diagram of 2-element TLM array is shown in Fig.1. CT and CM are tuning and matching capacitor respectively. The dimension of each element is 12.0cm × 8.8cm with 1.25cm width copper tape. Teflon with thickness of 6mm is used for the substrate. (a). Coupling Theory: For 2-element TLM coil, coupling of nonadjacent parallel lines are very weak (<-40dB) that can be ignored. Microstrip coupling between two parallel microstrip lines can be analyzed with quasistatic even/odd mode theory [4]. The scattering parameter, S21 represented their mutual coupling can be expressed as the follows,