Effect of an annular sleeve of high permittivity material on resonant modes and homogeneity of a birdcage coil

We investigated how high permittivity materials affect the S-parameters and transmit efficiency of a head-sized birdcage coil. We also studied whether re-tuning and matching are necessary for improved performance. While adjusting the S-parameters is possible during coil development and for prototype coils, it is often unfeasible to re-tune and match commercially manufactured coils, and poses a challenge after adding dielectric material to such coils. Our work demonstrates the change in the S-parameters and the transmit efficiency after placing a dielectric sleeve within a head-sized birdcage coil, and reports that coil performance can be improved after re-tuning and matching the coil in the presence of the dielectric material.