Mapping of the radio frequency magnetic field with a MR snapshot FLASH technique

A fast method to measure the radio frequency magnetic field intensity distribution during magnetic resonance examinations within objects or patients is proposed. Using a snapshot FLASH sequence, several images are obtained with different transmitter amplitudes for the single prepulse prior to the imaging sequence. The signal intensity at each pixel within a selected slice is diminished by this prepulse, unless the pulse angle is 0 degrees or a multiple of 360 degrees. This condition can be used to measure that transmitter amplitude Umax for each pixel which causes an excitation of 360 degrees by the prepulse. The values of Umax can be visualized in an image and are inversely proportional to the radio frequency magnetic field strength at each position within the image. The method can also be used for in-vivo measurements, where it may be difficult to realize prepulses with sufficient high flip angles. In these cases, signal corresponding to a 180 degree prepulse can be used for calculating Umax.