Radial Tagging of MR images: a continuous RF excitation approach

Introduction: Spatial modulation of magnetization, applied to tag the image object in MRI, is a well established method for noninvasive measurement of deformation and strain. In principle, many different tagging patterns can be generated through combinations of RF excitation and gradient pulses. The usefulness of the patterns as well as the stability and efficiency of the sequences, however, limits these choices in practice. The conventional tagging sequence is based on short alternating RF and gradient pulses that generate a pattern of parallel lines in one or two directions. Another pattern of interest is radial tagging that facilitates the measurement of angular information reflected in shear and twist of the left ventricle. This radial modulation of longitudinal magnetization has not been widely used due to lack of an efficient preparation sequence to generate the pattern robustly. In this work we introduce a continuous RF approach that acts on a rotating excitation plane. Its theoretical basis and practical details as well as initial results in phantoms and human hearts are presented.