Toroid cavity detectors for high-resolution NMR spectroscopy and rotating frame imaging: capabilities and limitations

The capabilities of toroid cavity detectors for simultaneous rotating frame imaging and NMR spectroscopy have been investigated by means of experiments and computer simulations. The following problems are described: (a) magnetic field inhomogeneity and subsequent loss of chemical shift resolution resulting from bulk magnetic susceptibility effects, (b) image distortions resulting from off-resonance excitation and saturation effects, and (c) distortion of lineshapes and images resulting from radiation damping. Also, special features of signal analysis including truncation effects and the propagation of noise are discussed. B(0) inhomogeneity resulting from susceptibility mismatch is a serious problem for applications requiring high spectral resolution. Image distortions resulting from off-resonance excitation are not serious within the rather narrow spectral range permitted by the RF pulse lengths required to read out the image. Incomplete relaxation effects are easily recognized and can be avoided. Also, radiation damping produces unexpectedly small effects because of self-cancellation of magnetization and short free induction decay times. The results are encouraging, but with present designs only modest spectral resolution can be achieved.