Navigator flip angle optimization for free-breathing T1-weighted hepatobiliary phase imaging with gadoxetic acid

To characterize and optimize the navigator-flip angle (FA), and the influence of imaging-FA on optimizing liver/lung contrast of the navigator profile, while avoiding visible navigator saturation artifacts, for hepatobiliary phase gadoxetic acid-enhanced MRI.Ten volunteers; six men, four women; ages 37.1 ± 11.0 years underwent navigator-gated three-dimensional (3D) -spoiled-gradient-echo sequences in randomized combinations of imaging-FA (10°/30°) and navigator-FA (10-90°) before contrast and 20 min after injection of gadoxetic acid at 3 Tesla. The signal intensities of the liver and lung were measured from navigator profiles. Furthermore, the intensity of saturation artifacts for each navigator FA was quantified using measurements of relative contrast at the artifact location.For the postcontrast images, the optimal navigator FA was 90°. However, saturation artifacts were highly dependent on the imaging-FA and the presence of gadolinium contrast. A smaller imaging-FA of 10° led to greater saturation artifacts for both pre- and postcontrast, and saturation artifacts worsen with increasing navigator-FA. Using a higher imaging-FA of 30°, saturation artifacts are ignorable over the entire range of navigator-FA.For navigator-gated gadoxetic acid-enhanced hepatobiliary imaging, navigator-FA of 90° and imaging-FA of 30° provide an optimal balance with maximum navigator liver/lung contrast while avoiding visible imaging saturation artifacts.J. Magn. Reson. Imaging 2014;40:1129-1136. © 2013 Wiley Periodicals, Inc.