TiO2 dielectric filled toroidal radio frequency cavity resonator for high-field NMR

R-F performance in high-field MRI applications is improved by filling the r esonator with material of relative dielectric constant approximating that o f human soft tissue. We demonstrate this by filling a toroidal cavity reson ator operating in TEM00 (cyclotron) mode with titanium dioxide (TiO2) in po wdered 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 unusu ally high SNR, while the level of R-f power required to produce a 90 degree s 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 fil led 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 sta ndard volume R-f coil in frequent use at this laboratory. Breast images mad e 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 th is article. Acquiring the phantom and leg images required significantly les s R-f power than did comparable imaging using a conventional coil. In addit ion, the field Lines were focused as they penetrated into the sample, and t his resulted in a more homogeneous B-1-field. We believe that these improve ments occurred because the dielectric presence minimizes the large dielectr ic mismatch between air and sample. Copyright (C) 2001 John Wiley & Sons, L td.