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A novel RF coil: tunable loop microstrip (TLM) coil

Introduction: Microstrip coils terminated with open or short circuit have been presented to achieve better SNR than conventional surface coil in ultra-high field [1,2]. However, the coil dimension is critically limited by strip length and substrate permittivity. In addition, frequency tuning by changing strip length [1] is not convenient. Ring resonator has been widely used in microwave measurement and filter design [3-5]. Ring resonator can achieve higher Q and less frequency shift with loading than microstrip coils terminated with open or short circuit [6]. Taking advantage of the ring structure, we design a novel tunable loop microstrip (TLM) coil for MRI. Theory: The basic structure of TLM coil and its equivalent circuit based on odd mode analysis [6] are shown in Fig 1. θ and T C are electric length and tuning capacitance respectively. The resonant frequency and Q of TLM coil were analyzed from this equivalent circuit. Resonant frequency: The ABCD parameter of TLM coil can be expressed as the follows: