Review in [1]:

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.




[1]V. L. Yarnykh, "Actual flip-angle imaging in the pulsed steady state: A method for rapid three-dimensional mapping of the transmitted radiofrequency field," Magn. Reson. Med., vol. 57, no. 1, pp. 192-200, Jan. 2007. [Online]. Available: http://dx.doi.org/10.1002/mrm.21120

References in [1]:

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Ser A 1993;103:82– 85.

11. Stollberger R, Wach P. Imaging of the active B1
field in vivo. Magn
Reson Med 1996;35:246 –251.

12. Sled JG, Pike GB. Correction for B1
and B0
variations in quantitative T2
measurements using MRI. Magn Reson Med 2000;43:589 –593.

13. Fernandez-Seara MA, Song HK, Wehrli FW. Trabecular bone volume
fraction mapping by low-resolution MRI. Magn Reson Med 2001;46:
103–113.

14. Wang J, Qiu M, Constable RT. In vivo method for correcting transmit/
receive nonuniformities with phased array coils. Magn Reson Med
2005;53:666 – 674.

15. Cunningham CH, Pauly JM, Nayak KS. Saturated double-angle method
for rapid B1	 mapping. Magn Reson Med 2006;55:1326 –1333.

16. Akoka S, Franconi F, Seguin F, Le Pape A. Radiofrequency map of an
NMR coil by imaging. Magn Reson Imaging 1993;11:437– 41.

17. Pan JW, Twieg DB, Hetherington HP. Quantitative spectroscopic imaging of the human brain. Magn Reson Med 1998;40:363–369.