Phase sensitive radio frequency mapping for magnetic resonance imaging
The disclosure provides phase-sensitive methods of radio frequency field mapping (e.g., B 1 mapping), in which flip angle is encoded in the phase of a transverse magnetization produced by a series of RF pulses. The phase-sensitive methods can measure flip angle over a wider range than other methods such as double-angle methods. In one aspect, a phase-sensitive method of radio frequency mapping includes acquiring a first transverse magnetization phase measurement using an RF excitation pulse with flip angle 2α and acquiring a second transverse magnetization phase measurement using an RF excitation pulse with flip angle −2α. The method also includes computing a phase difference between the two phase measurements and generating a radio frequency map based on the phase difference. The radio frequency map may be used, for example, to correct an image for variation in image intensity caused by B 1 field inhomogeneity and correct T 1 measurements for variations caused by B 1 field inhomogeneity.
1. A phase-sensitive method of radio frequency field mapping for magnetic resonance imaging, the method comprising:
acquiring a first transverse magnetization phase measurement of a subject from an MRI machine using an RF pulse with flip angle 2α about one axis followed by an RF pulse with a flip angle of α about an axis orthogonal to the one axis;
acquiring a second transverse magnetization phase measurement using an RF pulse with flip angle −2α about the one axis followed by a second RF pulse with a flip angle of α about the axis orthogonal to the one axis;
computing a phase difference between the two phase measurements; and
generating a radio frequency map based on the phase difference.
2. The method of claim 1 , wherein the radio frequency field is approximately transverse to a main permanent magnetic field of the MRI machine.
3. The method of claim 1 , further comprising applying the radio frequency map to a magnetic resonance image to correct variations in image intensity caused by B 1 field inhomogeneity.
4. The method of claim 1 , further comprising apply the radio frequency map to T 1 measurements to correct variations in the T 1 measurements caused by B 1 field inhomogeneity.
5. A system for performing radio frequency mapping for magnetic resonance imaging, the method comprising:
a processor configured to acquire a first transverse magnetization phase measurement of a subject from an MRI machine using an RF pulse with flip angle 2α about one axis followed by an RF pulse with a flip angle of α about an axis orthogonal to the one axis, to acquire a second transverse magnetization phase measurement from the MRI machine using an RF pulse with flip angle −2α about the one axis followed by a second RF pulse with a flip angle of α about the axis orthogonal to the one axis, to compute a phase difference between the two phase measurements, and to generate a radio frequency map based on the phase difference.
6. The system of claim 5 , wherein the radio frequency field is approximately transverse to a main permanent magnetic field of the MRI machine.
7. The system of claim 5 , wherein the processor is configured to apply the radio frequency map to a magnetic resonance image to correct variations in image intensity caused by B 1 field inhomogeneity.
8. The system of claim 7 , wherein the processor is configured to output the corrected magnetic resonance image to a display.
9. The system of claim 5 , wherein the processor is configured to apply the radio frequency map to T 1 measurements to correct variations in the T 1 measurements caused by B 1 field inhomogeneity.