A non-linear triangular split-ring based metaresonator for targeted scanning at 1.5T MRI

In magnetic resonance imaging (MRI), RF signals are initially transmitted to stimulate the body protons which eventually release the electromagnetic energy while returning back to their original states. The image resolution and scanning efficiency of MRI can be improved by enhancing the magnetic fields received from the patient's body using metamaterials. The major limitation of linear metamaterials is that they amplify RF magnetic fields both during transmission and reception phases. This requires modification of the RF excitation pulses during the transmission phase. Further, local increase of transmitted power poses a potential threat of tissue-heating and high specific absorption rate (SAR) values in addition to perturbing the transmit field homogeneity. In order to circumvent these problems, we propose a self-adaptive metaresonator which has the capability of self-detuning itself during transmission of RF pulses during MRI scans. A triangular split-ring based metaresonator is designed for maximum thirty-fold SNR improvement in 1.5T MRI. Switching diodes have been employed for switching on and off the magnetic field enhancement by the metaresonator. During transmission phase when the switching diodes get turned on, the metaresonator is detuned. During reception phase when the switching diodes get turned off, the metaresonator is tuned to 63.8 MHz which is the Larmor frequency of 1.5T MRI. The proposed metaresonator is thin and compact which enables its easy placement in the multi-element arrays of clinical MRI.