Cutoff-free traveling wave NMR

Recently, the concept of traveling wave NMR/MRI was introduced by Brunner et al. (Nature 2009;457:994-992), who demonstrated MR images acquired using radio frequency (RF) waves propagating down the bore of a MR scanner which acts as a waveguide. One of the significant limitations of this approach is that each bore has a specific cutoff frequency, which can be higher than most Larmor frequencies at the magnetic field strengths commonly in use for MR imaging and spectroscopy today. One can overcome this limitation by using a central conductor in the waveguide and thereby converting it in to a transmission line which has no cutoff frequency. Broadband propagation of waves through the sample thus becomes possible. NMR spectra and images with such an arrangement are presented and traveling wave behavior is demonstrated. In addition to facilitating NMR spectroscopy and imaging in smaller bores via traveling waves, this approach also allows one to perform multinuclear traveling wave experiments (an example of which is shown), and to study otherwise difficult-to-access samples in unusual geometries.