Method for mapping of the radio frequency field amplitude in a magnetic resonance imaging system using adiabatic excitation pulses
A method for determining the spatial distribution of the magnitude of the radio frequency transmission field B1 in a magnetic resonance imaging apparatus, wherein the method comprises performing an MRI experiment in which a B1-sensitive complex image (SI) of a sample is obtained, wherein the phase distribution within the B1-sensitive complex image (SI) depends on the spatial distribution of the magnitude of the field B1. For establishing the dependency of the phase distribution within the B1-sensitive complex image (SI) on the spatial distribution of the field B1, one or more adiabatic RF pulses are applied. The method provides a simple procedure for mapping the B1 field of a magnetic resonance imaging apparatus with an improved accuracy and a wider measurement range.
1. A method for determining a spatial distribution of a magnitude of a radio frequency transmission B1 field in a magnetic resonance imaging apparatus by performing an MRI experiment in which a B1-sensitive complex image of a sample is obtained, with a real part and an imaginary part, wherein a phase distribution within the B1-sensitive complex image depends on the spatial distribution of the magnitude of the B1 field, the phase being determined by an arc tangent function, the method comprising the steps of:
a) applying one or more adiabatic RF pulses with a frequency sweep ending off-resonance for establishing a dependency of the phase distribution within the B1-sensitive complex image on the spatial distribution of the B1 field;
b) obtaining a reference complex image of the sample for eliminating any superimposed phase distribution components of the B1-sensitive complex image which are independent of the B1 field, the reference complex image of the sample having a same superimposed phase distribution components;
c) generating a phase difference image of the B1-sensitive complex image and the reference complex image; and
d) calculating a B1 magnitude image from the phase difference image of step c).
2. The method of claim 1 , wherein the B1-sensitive complex image is acquired with a gradient-echo imaging method using an adiabatic half-passage RF excitation pulse with a frequency sweep starting on-resonance, and the reference complex image is acquired with a same gradient-echo imaging method using a non-adiabatic RF pulse.
3. The method of claim 1 , wherein the MRI experiment comprises an imaging sequence which contains an excitation RF pulse of any type, followed by an adiabatic full-passage refocusing pulse to generate a first spin-echo signal, which, in turn, is followed by a second adiabatic full-passage refocusing pulse to produce a second spin-echo signal, wherein the first spin echo signal is used to produce the B1-sensitive complex image and the second spin-echo signal is used to produce the reference complex image.
4. The method of claim 1 , wherein the MRI experiment comprises an imaging sequence which contains an excitation RF pulse of any type followed by a periodic train of adiabatic full-passage RF pulses to produce a train of spin-echoes, in which, after every two spin-echoes, an encoding changes so that consecutive pairs of spin-echoes are differently encoded, wherein odd spin-echoes are used to produce the B1-sensitive complex image and even spin-echoes are used to produce the reference complex image.
5. The method of claim 1 , wherein the B1 magnitude image is saved digitally and used for image postprocessing.
6. The method of claim 1 , wherein the B1 magnitude image is used to determine parameters controlling RF transmission in an MRI apparatus.