Off-resonant Artifact Correction of RF Pulse Concatenation for Blood Flow Imaging Applications

Introduction: To obtain accurate quantitative perfusion data, continuous arterial spin labeling (CASL) requires balancing magnetization transfer (MT) effects in the control and tag experiments. Traditionally, this is done by applying RF pulses in control experiments that are placed at equal distance as the tag pulse to the imaging slice, but distal to the imaging slice of interest and far enough to prevent tagging of venous flow. More recent methods including RF modulation [1], they require that the tagging plane to be at some distance from the imaging slice in order to equilibrate MT effects between tag and control. However, in order to reduce transit time effects, it may be desirable to have the saturation or inversion plane very close to the imaging slice. Additionally, in newly developed flow imaging techniques such as flow enhancement of signal intensity (FENSI) [2], in which the tagging plane has to be within the imaging slice, the requirement for close or coincident tagging planes makes control of MT effects difficult. In this study, we investigate the use of concatenated RF pulses for labeling at the imaging slice. These pulses have been used in ASL to control magnetization transfer, in the method called magnetization transfer insensitive labeling technique (TILT) [3,4]. Tag composite RF pulses are obtained by applying the RF pulses in phase with each other, resulting in a net flip angle, and control composite RF pulses are obtained by applying the RF pulses with opposite phase, resulting in no net flip angle (Fig.1). However, the time delay between the centers of the RF pulses will lead to sensitivity to magnetic field inhomogeneity and off-resonance effects. In this abstract, we provide a framework for analyzing and compensating for the off-resonance effects in concatenated RF pulses.