A TiO(2) dielectric filled toroidal radio frequency cavity resonator for high-field NMR.
R(F) performance in high-field MRI applications is improved by filling the resonator with material of relative dielectric constant approximating that of human soft tissue. We demonstrate this by filling a toroidal cavity resonator operating in TEM(00) (cyclotron) mode with titanium dioxide (TiO(2)) in powdered rutile form, and acquiring phantom, human lower leg and human breast images of good quality at 4.1 T. Images made with this resonator had unusually high SNR, while the level of R(f) power required to produce a 90 degrees flip angle pulse was about a quartes as high for the filled resonator as for the same resonator before filling. Phantom images obtained with the filled resonator had an SNR of nearly 300 at a resolution of 256 x 256 voxels, nearly three times that of images of the same phantom obtained using a standard volume R(f) coil in frequent use at this laboratory. Breast images made at 256 x 256 voxels resolution had an SNR of 174, also unusually high for a volume coil. High-resolution (512 x 512 voxels) were also obtained, with SNR = 60. Preliminary phantom and in vivo human images are presented in this article. Acquiring the phantom and leg images required significantly less R(f) power than did comparable imaging using a conventional coil. In addition, the field lines were focused as they penetrated into the sample, and this resulted in a more homogeneous B(1)-field. We believe that these improvements occurred because the dielectric presence minimizes the large dielectric mismatch between air and sample.