On the performance and accuracy of 2D navigator pulses

The purpose of this study was to investigate and to optimize the performance of two-dimensional spatially selective excitation pulses used for navigator applications on a clinical scanner. The influence of gradient imperfections, off-resonance effects, and incomplete k-space covering on the pencil beam-shaped spatial excitation profile of the 2D RF pulse was studied. The studies involved experiments performed on phantoms and in vivo. In addition, simulations were carried out by numerical integration of the Bloch equations. The accuracy of positioning of the pencil beam was increased by a factor of three by employing a simple correction scheme for the compensation of gradient distortions. The spatial selectivity of the 2D RF pulse was improved by taking sampling density corrections into account. The 2D RF pulse performance was found to be sufficient to monitor the diaphragm motion even at moderate gradient strength. For applications, where a high spatial resolution is required or a less characteristic contrast is present a strong gradient system is recommended.