Fast multi-channel excitation field mapping

Spatial inhomogeneities of the radiofrequency excitation field are problematic in MRI, especially at ultra-high field strengths (> 3T). Quantitative spatially encoded measurement of these inhomogeneities, also called B1 or flip angle mapping, provides the means to correct for intensity variations in the images or allow the targeted design of excitation patterns to improve image quality. The application of flip angle mapping methods in the clinical environment requires short acquisition times and high accuracy. Current research focuses primarily on fast calibration methods for high (3T) and ultra-high-field strength (9.4T) MRI scanners with single or multi-channel (pTX) excitation systems based on the actual flip angle imaging (AFI) sequence (Yarnykh, 2007). Proprietary modifications allow faster and more accurate acquisitions compared to the traditional sequence.

Efficient spoiling is essential in order to get good results with AFI (accuracy). Better spoiling results are obtained by varying the changing spoiler gradients after each repetition. The figure shows a comparison of results obtained with gradient cycled AFI (a) and conventional AFI with short (b) and long repetition times. The gradient cycled AFI gives slightly better results than the conventional AFI with a long TR in half of the acquisition time.