Fast, accurate, and precise mapping of the RF field in vivo using the 180° signal null

RF field B(1) nonuniformity is the largest cause of error in the quantitative measurement of many clinically relevant parameters in MR images and spectra. Knowledge of the absolute flip angles at every region will improve the accuracy and precision of such parameters. This method uses the 180 degrees signal null to construct a flip angle map of the entire brain in less than 4 min, independent of T(1), T(2), and proton density. Three spoiled gradient echo volume acquisitions of the whole brain were made with three different flip angles. The optimum choice of flip angles was determined to be 145 degrees, 180 degrees, and 215 degrees. Linear regression analysis was used to determine the nominal (system calibrated) flip angle required for a signal null at every pixel and thence determine the absolute flip angle at that location. The experiment utilizes an existing MR sequence supplied by the scanner manufacturer. The technique is validated experimentally and a theoretical investigation into the optimum experimental parameters is presented.