Approximate Fourier domain expression for Bloch-Siegert shift.
OBJECTIVE
In this study, a new simple Fourier domain-based analytical expression for the Bloch-Siegert (BS) shift-based B1 mapping method is proposed to obtain |B1+| more accurately while using short BS pulse durations and small off-resonance frequencies.


METHODS
A new simple analytical expression for the BS shift is derived by simplifying the Bloch equations. In this expression, the phase is calculated in terms of the Fourier transform of the radiofrequency pulse envelope, and thus making the off- and on-resonance effects more easily understandable. To verify the accuracy of the proposed expression, Bloch simulations and MR experiments are performed for the hard, Fermi, and Shinner-Le Roux pulse shapes.


RESULTS
Analyses of the BS phase shift-based B1 mapping method in terms of radiofrequency pulse shape, pulse duration, and off-resonance frequency show that |B1+| can be obtained more accurately with the aid of this new expression.


CONCLUSIONS
In this study, a new simple frequency domain analytical expression is proposed for the BS shift. Using this expression, |B1+| values can be predicted from the phase data using the frequency spectrum of the radiofrequency pulse. This method works well even for short pulse durations and small offset frequencies.
National Magnetic Resonance Research Center (UMRAM), Bilkent University, Bilkent, Ankara, Turkey.|Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey.
Esra Abaci
EA
Turk
Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey.
Yusuf Ziya
YZ
Ider
Department of Electrical and Electronics Engineering, TOBB-University of Economics and Technology, Ankara, Turkey.
Arif Sanli
AS
Ergun
National Magnetic Resonance Research Center (UMRAM), Bilkent University, Bilkent, Ankara, Turkey.|Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey.
Ergin
E
Atalar
D000465:Algorithms; D003198:Computer Simulation; D005583:Fourier Analysis; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008954:Models, Biological; D009716:Numerical Analysis, Computer-Assisted; D019047:Phantoms, Imaging; D012815:Signal Processing, Computer-Assisted
B1 mapping; Bloch-Siegert shift; magnetic resonance imaging