System and methods for ultra-fast multi-dimensional diffusion-relaxation MRI using time-division multiplexing sequences
Scan time in diffusion-relaxation magnetic resonance imaging (“MRI”) is reduced by implementing time-division multiplexing (“TDM”). In general, time-shifted radio frequency (“RF”) pulses are used to excite two or more imaging volumes. These RF pulses are applied to induce separate echoes for each slice. Diffusion MRI data can thus be acquired with different echo times, or alternatively with the same echo time, in significantly reduced overall scan time. Multidimensional correlations between diffusion and relaxation parameters can be estimated from the resulting data.
1. A method for magnetic resonance imaging, the method comprising:
(a) controlling a magnetic resonance imaging (MRI) system to perform a pulse sequence that includes:
at least a first radio frequency (RF) excitation pulse that excites spins in a first imaging volume and a second RF excitation pulse that excites spins in a second imaging volume, wherein the second RF excitation pulse is time-shifted relative to the first RF excitation pulse by a time duration;
at least a first RF refocusing pulse that refocuses spins in the first imaging volume at a first echo time relative to the first RF excitation pulse and a second RF refocusing pulse that refocuses spins in the second imaging volume at a second echo time relative to the second RF excitation pulse, wherein the second RF refocusing pulse is time-shifted relative to the first RF refocusing pulse by the time duration;
a first readout during which first magnetic resonance data are acquired from the first imaging volume;
a second readout during which second magnetic resonance data are acquired from the second imaging volume;
(b) reconstructing magnetic resonance images from the first magnetic resonance data and the second magnetic resonance data, wherein the magnetic resonance images depict the first imaging volume and the second imaging volume; and
(c) displaying the reconstructed magnetic resonance images to a user.
2. The method of claim 1 , wherein the first RF excitation pulse is applied before the second RF excitation pulse and the second RF refocusing pulse is applied before the first RF refocusing pulse, such that the first echo time is different than the second echo time.
3. The method of claim 1 , wherein the first RF excitation pulse is applied before the second RF excitation pulse and the first RF refocusing pulse is applied before the second RF refocusing pulse, such that the first echo time and the second echo time are equal.
4. The method of claim 1 , wherein the pulse sequence further comprises:
at least a third RF excitation pulse that excites spins in a third imaging volume;
at least a third RF refocusing pulse that that refocuses spins in the third imaging volume at a third echo time relative to the third RF excitation pulse; and
a third readout during which third magnetic resonance data are acquired from the third imaging volume.
5. The method of claim 4 , wherein the first, second, and third RF excitation pulses are applied in a different temporal order than the first, second, and third RF refocusing pulses such that the first, second, and third echo times are different from each other.
6. The method of claim 1 , wherein the first RF excitation pulse is a single band RF excitation pulse and the first imaging volume comprises a first slice.
7. The method of claim 6 , wherein the second RF excitation pulse is a single band RF excitation pulse and the second imaging volume comprises a second slice.
8. The method of claim 1 , wherein the first RF excitation pulse is a multiband RF excitation pulse and the first imaging volume comprises a first plurality of simultaneously excited slices.
9. The method of claim 8 , wherein the second RF excitation pulse is a multiband RF excitation pulse and the second imaging volume comprises a second plurality of simultaneously excited slices.
10. The method of claim 1 , wherein the first readout and the second readout comprise echo planar imaging (EPI) readouts.
11. The method of claim 1 , wherein the first imaging volume comprises a first slice and the second imaging volume comprises a second slice.
12. The method of claim 1 , wherein the first RF refocusing pulse causes spins in the first imaging volume to refocus into a first echo signal and the second RF refocusing pulse causes spins in the second imaging volume to refocus into a second echo signal that is non-overlapping in time with the first echo signal.
13. The method of claim 1 , wherein the first readout is configured to generate multiple spin echoes at different first echo times within the first imaging volume, and the second readout is configured to generate multiple spin echoes at different second echo times within the second imaging volume.
14. The method of claim 1 , wherein the pulse sequence further comprises an inversion recovery (IR) RF pulse applied before the first and second RF excitation pulses.
15. The method of claim 1 , wherein the pulse sequence further comprises at least one diffusion-encoding gradient that applies diffusion encoding to spins in each of the first and second imaging volumes, such that the first and second magnetic resonance data are first and second diffusion-weighted magnetic resonance data.
16. The method of claim 15 , further comprising generating T2 relaxation maps from the first and second diffusion-weighted magnetic resonance data and generating diffusion parameter maps from the magnetic resonance images.
17. The method of claim 16 , further comprising generating multidimensional correlation data by computing correlations between the T2 relaxation maps and the diffusion parameter maps.
18. The method of claim 17 , wherein the correlations are computed based in part on a joint probability distribution of T2 relaxation parameters and diffusion parameters.
19. The method of claim 1 , wherein the pulse sequence further comprises echo-shifting gradients that alternately dephase and rephase echoes in order to separate sampled k-space data for each of the first and second imaging volumes.
20. The method of claim 19 , wherein at least one of a magnitude, direction, and duration of the echo-shifting gradients are adaptively optimized to reduce additional diffusion encoding caused by the echo-shifting gradients.