Gap cycling for SWIFT

Broadband NMR with random-window noise excitation

Motivated by recent generations of compact portable and benchtop NMR instrumentation, we present a new method for the acquisition of broad bandwidth NMR spectra using stochastic excitation. The new method decouples the spectral width from the durations of the excitation pulses and ring-down delays, allowing the receiver bandwidth to be arbitrarily large. This allows low-power, low-flip-angle pulses to be used to acquire broad-band NMR spectra such as wideline powder patterns from solids. The new method divides experimental time into distinct "pulse" and "acquire" blocks that are interleaved according to a pseudo-random noise sequence and combined with a second sequence for phase modulation. The method is described mathematically, and is demonstrated with both liquids and wide-line solids experiments.

Multi-Band-SWIFT

A useful extension to SWIFT (SWeep Imaging with Fourier Transformation) utilizing sidebands of the excitation pulse is introduced. This MRI method, called Multi-Band-SWIFT, achieves much higher bandwidth than standard SWIFT by using multiple segmented excitations (bands) of the field of view. A description of the general idea and variants of the pulse sequence are presented. From simulations and semi-phenomenological theory, estimations of power deposition and signal-to-noise ratio are made. MB-SWIFT and ZTE (zero-TE) sequences are compared based on images of a phantom and human mandible. Multi-Band-SWIFT provides a bridge between SWIFT and ZTE sequences and allows greatly increased excitation and acquisition bandwidths relative to standard SWIFT for the same hardware switching parameters and requires less peak amplitude of the radiofrequency field (or greater flip angle at same peak amplitude) as compared to ZTE. Multi-Band-SWIFT appears to be an attractive extension of SWIFT for certain musculoskeletal and other medical imaging applications, as well as for imaging materials.