MR ऀð in ऀð the ऀð Far ऀð Field: ऀð From ऀð Mode ऀð Transformation ऀð and ऀð Holography ऀð to ऀð Quasi ⴀ Optics

Since its inception, both MRI and NMR have operated in the long wavelength regime where far field electrodynamic effects are negligible. With the use of high field strength imagers (>4T), on large samples such as a human head, and coupled with the high dielectric constant of tissue (>80), the excitation wavelength becomes on the order of or smaller than the imaging sample. Under these conditions, new phenomena previously unknown in MR such as holography, arising from the spatial and spectral interference of the traveling excitation fields become possible. The spectral grating component has always been part of conventional MR. It arises naturally from either the intrinsic chemical shift anisotropy of the spin system or the field inhomogeneity due to the applied spatial encoding gradients. The spatial grating component is new and is due to the emergence of the propagating wave vector with traveling wave excitation fields. Farfield interference phenomena such as spatial-spectral holographic properties of storage, programmable time-delay, phase conjugation or time-reversal and Bragg selectivity are experimentally demonstrated for the first time in an MR sample. These ideas are shown to be extendable to complex holographic signal processing functions such as recognition, correlations and triple products. This approach has potential for new spatial localization techniques using quasi-optical techniques for focusing excitation fields, slice selection through volume Bragg selectivity, phase conjugate distortion free imaging as well as higher resolution encoding limited only by the spacing of spatial interference fringes and T2. EM mode Propagation and Transformation in MRI: At ultra-high field strength (>4T), the cut-off condition for the lowest waveguide mode propagation can be fulfilled provided that the bore is sufficiently large. Since MR scanners have cylindrical symmetry, the waveguides are ideally suited for far-field traveling waves MR experiments. For MR scanners, we can use both types of modes of cylindrical waveguide: TE and TM, however, only the lowest TE mode of cylindrical waveguide can propagate in a typical whole body 7.0 T scanner (60 cm bore) in empty bore due to stringent cut-off wavelength requirements. For practical MR imaging, this may not be the best mode because most of the B-field is wasted into the longitudinal component that does not result in spin flips. Fortunately for existing MR scanners, TM as well as higher order TE modes are allowed in high permittivity dielectrics, but B1 field map becomes complicated, which affects MR images.