IMPROVED AUTOMATIC OFF-RESONANCE CORRECTION WITHOUT A FIELD MAP IN SPIRAL IMAGING

Non-2DFT k-space readout strategies are useful in fast imaging but pro ne to blurring when reconstructed off resonance. Field inhomogeneities or susceptibility variations, coupled with a long readout time, are t he major sources of this artifact. Correction methods based on a prior i off-resonance information such as an acquired field map have been pr oposed in the literature. An alternative approach estimates the spatia lly varying off-resonance frequency from the data itself before applyi ng a correction. In this latter approach there is a tradeoff between t he extent of correction and the chance of increased artifact due to es timation error. This paper introduces an improved algorithm for field map estimation which is both faster and more robust than the existing method. It uses a multi-stage estimation of the field map, starting fr om a coarse estimate both in frequency and space and proceeds towards higher resolution. The new algorithm is applied to phantom and in vivo images acquired with radial and spiral sequences to give sharper imag es.