Wireless power transfer by detuned birdcage coil in magnetic resonance imaging

Magnetic resonance imaging (MRI) scanners may accommodate additional in-bore devices requiring power supply. One of the relevant directions is the development of wireless power transfer systems for powering such devices. The present paper focuses on a method of wireless power transfer with a birdcage coil acting as a transmitting system and energy transmission performed at a frequency other than the Larmor frequency [8]. Such resonators are already incorporated in MRI scanners with 1.5 T magnetic field. The aim of the current work is to develop a system for detuning a birdcage coil from the Larmor frequency, which would conserve the RF field distributions of the high-order modes and the order of their frequencies. This is accomplished by incorporating a switched inductance in the segments of the birdcage coil, so that detuning would lead to an increase in their electrical length rather than complete disconnection. The aforementioned wireless power transfer method, only applicable during the spin excitation phase if a standard detuning system is used, could then be extended to the entire duration of the pulse sequence. We use numerical simulations to compare the scattering parameters of birdcage coils with the standard detuning system and the proposed one, ensure that the uniformity of the spin excitation RF field is conserved, and verify that the birdcage coil decouples from the local coils in the detuned mode. We simulate the specific absorption rate and evaluate the maximum safe power. Finally, we demonstrate wireless power transmission from a birdcage coil detuned from the Larmor frequency to a power receiver based on a loop antenna experimentally.