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Although a number of methods have been proposed for image-based RF field measurements (1–17), many of them are difficult or impossible to use in vivo because they require serial scans at a variable transmitter power or pulse duration (1–7), have a high sensitivity to static magnetic field inhomogeneity (8,9), and result in high RF power deposition (1,3). These difficulties have been overcome by several B1 mapping techniques that are more suitable for in vivo applications (10 –17); however, further improvements in time efficiency, anatomical coverage, and accuracy are needed to introduce B1 mapping into routine practice. A common method for B1 mapping is based on two scans acquired with a spin-echo (SE) or gradient-echo (GRE) imaging sequence at two nominal flip angles (FAs, usually alpha and 2 alpha) chosen in a range of optimal sensitivities to B1 variations for a particular sequence (10).
To avoid the dependence of the signal on T1, the repetition time (TR) of the sequence should be sufficiently long to achieve complete relaxation of magnetization (10). More time-efficient variants of the double-angle method (11–15) include driven recovery (11) or presaturation (15) of magnetization to reduce the TR, and its combination with fast acquisition sequences, such as fast SE (FSE) (12,13), echo-planar (14), and spiral (15). As an alternative to the double-angle approach, the RF distribution can be determined from a single scan with the use of specially designed pulse sequences that simultaneously acquire two signals characterized by different dependencies on the FA (16,17). A general advantage of such methods for in vivo measurements is the absence of possible misregistration between the images used to calculate the FA distribution. One of these techniques (16) observes stimulated echo and SE signals generated by the three-pulse sequence with FAs –2-, where the ratio of signals depends on the FA as cos(). Another method (17) employs a two-pulse sequence with identical pulses that generate two free induction decay (FID) signals in rapid succession, which scan the equilibrium and the subsequent state of the longitudinal magnetization.
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