MR in the far field: From mode transformation and holography to quasi-optics

Abstract

Fundamental properties of holography such as storage, recall and matched filtering, arising out of momentum matching considerations of the propagating excitation fields, have been experimentally demonstrated for the first time in MR. As the wavelength of the MR scanner becomes commensurate or smaller than the geometrical thickness of the sample, new phenomena common in quantum optics but hitherto unknown in MR. Far-field concept such as interference and diffraction, will become prevalent with the use of propagating excitation fields in MRI. This realization of holographic principles in MR can be fruitful in designing MR imaging and spectroscopic techniques such as phase conjugate imaging for correcting image distortions caused by field inhomogeneities, as well as new spatial encoding schemes based on a holographic grating encoding. In addition, it has potential to lead to new concepts for information storage and processing at MR frequencies. For example, the use of convolution operations opens the possibility of applying spectral filters directly to the hologram as part of the readout while holographic recording has the potential to increase resolution in MR limited only by the fringe spacing and T2 of the sample. Our analysis shows that for a Larmor frequency of 300 MHz in a 7.0 T whole-body scanner, traveling wave modes in dielectric samples within the range of biological tissues can be sufficient to support imaging of the body parts. The modes diversity depends on the tissue efficient diameter, relative permittivity, conductivity, and the Larmor frequency. The imaging contrast will depend on the particular modes that have been excited in the tissue. A more complicated case of heterogeneous axial symmetric dielectric can be also analyzed using effective permittivity with our approach of mode transformation.