Systems and methods for multiband sweep imaging with fourier transformation
Systems and methods for magnetic resonance imaging (“MRI”) using a frequency swept excitation that utilizes multiple sidebands to achieve significant increases in excitation and acquisition bandwidth are provided. The imaging sequence efficiently uses transmitter power and has increased sensitivity as compared to other techniques used for imaging of fast relaxing spins. Additionally, the imaging sequence can provide information about both fast and slow relaxing spins in a single scan. These features are advantageous for numerous MRI applications, including musculoskeletal imaging, other medical imaging applications, and imaging materials.
1. A method of producing a magnetic resonance (MR) image of a subject with a magnetic resonance imaging (MRI) system, the steps of the method comprising:
(a) directing the MRI system to perform a pulse sequence that includes:
applying a sweeping frequency excitation having a duration, the sweeping frequency excitation comprising a plurality of spaced apart radio frequency (RF) excitation pulses each having a pulse width and each of the RF excitation pulses being spaced apart by a same inter-pulse spacing that is selected such that the sweeping frequency excitation simultaneously excites multiple different resonance frequencies at sidebands determined by the selected inter-pulse spacing;
acquiring a time domain signal during the duration, the time domain signal being based on evolution of the spins; and
(b) producing an MR image from the acquired time domain signal,
wherein at least one dimension of a matrix size of the MR image produced in step (b) is equal to a number of the sidebands covering a field-of-view depicted in the produced MR image and
wherein the number of sidebands covering the field-of-view depicted in the produced MR image is at least two.
2. The method as recited in claim 1 , wherein the selected inter-pulse spacing is selected such that a linear width of a voxel in the MR image produced in step (b) is matched with a width of at least one of the sidebands that is utilized in covering the field-of-view depicted in the produced MR image.
3. The method as recited in claim 1 , wherein step (b) includes Fourier transforming the acquired time domain signal in order to generate a frequency domain signal, correlating the generated frequency domain signal with a pulse function in a frequency domain in order to generate correlated signals, and producing, using a computer processor, the MR image of step (b) from the generated correlated signals.
4. The method as recited in claim 1 , wherein at least one dimension of the matrix size is equal to a number of samples acquired between each spaced apart RF excitation pulse that occurs in the applied sweeping frequency excitation.
5. The method as recited in claim 1 , wherein a spatial resolution of the MR image produced in step (b) is increased by sequentially exciting a number of sidebands in each voxel of the image using one of amplitude modulation or phase modulation of the sweeping frequency excitation.
6. The method as recited in claim 5 , wherein step (b) includes Fourier transforming time domain signals associated with at least one of the sidebands and producing the MR image of the subject from a combination of the Fourier transformed time domain signals.
7. The method as recited in claim 1 , wherein a spatial resolution of the image produced in step (b) is increased by increasing a number of frequency domain samples associated with each sideband by using a spatial phase modulation during the sweeping frequency excitation.
8. The method as recited in claim 1 , wherein the sweeping frequency excitation is a continuous frequency modulation.
9. The method as recited in claim 8 , wherein the continuous frequency modulation is at least one of a linear and a hyperbolic secant modulation.
10. The method as recited in claim 1 , wherein the sweeping frequency modulation is a stepped frequency modulation including a number of different discrete frequency steps.