Method for magnetic resonance imaging using slice quadratic phase for spatiotemporal encoding
Described here are systems and methods for volumetric excitation in magnetic resonance imaging (“MRI”) using frequency modulated radio frequency (“RF”) pulses. In general, quadratic phase modulation along the slice encoding direction is implemented for additional spatiotemporal encoding, which better distributes signal content in the slice direction and enables higher acceleration rates that are robust to slice-undersampling.
1. A method for imaging a subject with a magnetic resonance imaging (MRI) system, the steps of the method comprising:
(a) providing a radio frequency (RF) excitation waveform to the MRI system, wherein the RF excitation waveform defines a frequency-swept RF excitation with a quadratic phase profile;
(b) acquiring data from a subject using the MRI system and a pulse sequence that includes the provided RF excitation waveform; and
(c) reconstructing an image of the subject from the acquired data, wherein reconstructing the image comprises using a reconstruction algorithm in which the quadratic phase profile of the RF excitation waveform is used as a parameter during reconstruction of the image to remove slice-specific phase contributions along a slice encoding direction of the acquired data before performing at least one of phase correction or signal interpolation on the acquired data, and wherein the quadratic phase profile of the RF excitation waveform provides additional spatial encoding information.
2. The method as recited in claim 1 , wherein the RF excitation waveform is selected to have a time-bandwidth product that is matched based at least in part on a spatial resolution achieved by the pulse sequence along a slice encoding direction.
3. The method as recited in claim 2 , wherein the time-bandwidth product is selected as a number of slices being resolved along the slice encoding direction.
4. The method as recited in claim 1 , wherein the RF excitation waveform is a frequency-swept hyperbolic secant (Hsn) RF excitation waveform.
5. The method as recited in claim 1 , wherein step (c) includes:
removing the quadratic phase from the acquired data along the slice encoding direction;
synthesizing additional k-space lines along the slice encoding direction;
forming combined k-space data by combining the additional k-space lines with the acquired data;
adding the quadratic phase to the combined k-space data along the slice-encoding directions to form updated data; and
reconstructing the image of the subject from the updated data.
6. The method as recited in claim 5 , wherein the additional k-space lines are synthesized using a GRAPPA algorithm.
7. The method as recited in claim 5 , wherein the synthesized additional k-space lines are different for adjacent slice encoding planes.
8. The method as recited in claim 1 , wherein step (c) includes reconstructing the image using a SENSE algorithm that includes a convolution along the slice encoding direction using a Fourier transform of the quadratic phase.
9. The method as recited in claim 1 , wherein the RF excitation waveform comprises a multiband RF excitation that simultaneously excites spins in multiple different slices.
10. The method as recited in claim 9 , wherein the multiband RF excitation comprises a field-of-view shift applied along the phase-encoding direction in an alternating manner.
11. The method as recited in claim 1 , wherein the data are acquired in step (b) from a three-dimensional image volume using a three-dimensional slab acquisition.
12. The method as recited in claim 1 , wherein the pulse sequence used in step (b) is an echo planar imaging (EPI) pulse sequence, and step (c) includes spatially aligning data in the acquired data before reconstructing the image of the subject.
13. The method as recited in claim 1 , wherein the quadratic phase profile of the RF excitation waveform is a quadratic phase along a slice encoding direction.
14. The method as recited in claim 13 , wherein the quadratic phase profile of the RF excitation waveform also includes a quadratic phase along a phase encoding direction.
15. The method as recited in claim 1 , the pulse sequence also includes a frequency-swept RF refocusing pulse with the quadratic phase profile.
16. The method as recited in claim 1 , wherein the data acquired in step (b) are acquired by sampling k-space in a random pattern.
17. The method as recited in claim 1 , further comprising,
identifying motion corrupted data that has been corrupted by motion that occurred when the data were acquired in step (b);
generating corrected data by correcting the motion corrupted data based on a magnitude of the acquired data; and
reconstructing the image in step (c) from the corrected data.
18. The method as recited in claim 17 , wherein generating the corrected data comprises calculating a change in effective slice direction from the magnitude of the acquired data and reassigning a correct encoding for the motion corrupted data.
19. The method as recited in claim 1 , further comprising,
identifying motion corrupted data that has been corrupted by motion that occurred when the data were acquired in step (b);
calculating a change in effective slice direction from the magnitude of the acquired data; and
reconstructing the image in step (c) from the acquired data using an image reconstruction algorithm implemented with a hardware processor and a memory, wherein the image reconstruction algorithm incorporates a motion model that uses the calculated change in effective slice direction to correct the acquired data to be consistent.