Apparatus for real-time phase correction for diffusion-weighted magnetic resonance imaging using adaptive RF pulses
Phase error in MR imaging is corrected in real time by providing adaptive RF pulses and corresponding adaptive magnetic field gradients to mitigate the effect of phase error in the imaging subject. A real time phase error map is obtained, and then adaptive RF pulses and corresponding field gradients are applied that remove the problematic effects of the phase error. Depending on details of the MR imaging mode being employed, there are several ways this removal can be done. Phase error can be cancelled by providing RF pulses that make the phase in the imaging subject uniform. Another approach is to make the adaptive RF pulses insensitive to the phase errors that are present.
1. A method of performing diffusion weighted magnetic resonance imaging (MRI), the method comprising:
a) operating an MRI system in order to provide a navigator image of an imaging subject, wherein the MRI system comprises:
an MRI system processor,
a main magnet configured to provide a main magnetic field,
one or more gradient magnets configured to provide controllable magnetic field gradients, and
one or more radio-frequency (RF) sources of RF emission configured to provide controllable RF pulses;
b) computing, in real time with the MRI system processor, a phase error map from the provided navigator image;
c) determining, in real time with the MRI system processor, an adaptive RF pulse from the computed phase error map that compensates for phase errors of the computed phase error map;
d) determining, in real time with the MRI system processor, one or more adaptive magnetic field gradients corresponding to the computed phase error map and the determined adaptive RF pulse;
e) providing the determined adaptive RF pulse to the imaging subject with the one or more sources of RF emission and simultaneously providing the determined adaptive magnetic field gradients to the imaging subject with the one or more gradient magnets; and
f) performing diffusion-weighted MRI of the imaging subject by performing steps a) through e) above in succession one or more times while recording, displaying and/or storing the diffusion weighted magnetic resonance imaging scan results.
2. The diffusion weighted MRI method of claim 1 , wherein the adaptive RF pulse compensates for the phase errors present within the computed phase error map by making a post-pulse phase distribution within the imaging subject substantially uniform.
3. The diffusion weighted MRI method of claim 1 , wherein the adaptive RF pulse compensates for the phase errors present within the computed phase error map by making an RF tip axis map substantially equal to the computed phase error map.
4. The diffusion weighted MRI method of claim 3 , wherein one or more of the adaptive RF pulses comprise phase insensitive echo-reset pulses configured for a phase insensitive diffusion encoding preparation.
5. The diffusion weighted MRI method of claim 3 , wherein one or more of the adaptive RF pulses comprise refocusing pulses configured for fast spin echo imaging.
6. The diffusion weighted MRI method of claim 3 , wherein one or more of the adaptive RF pulses comprise excitation pulses configured for diffusion-weighted steady state free precession imaging.
7. The diffusion weighted MRI method of claim 1 , further comprising:
determining with the MRI system processor one or more adaptive magnetic field gradients that compensate for a linear error in the computed phase error map; and
providing the one or more adaptive magnetic field gradients to the imaging subject with the gradient magnets during the diffusion weighted MRI scan.
8. The diffusion weighted MRI method of claim 1 , wherein the one or more RF sources comprise two or more RF sources configured to operate in parallel with one another.
9. The diffusion weighted MRI method of claim 1 :
wherein the phase errors present within the computed phase error map include motion phase errors due to motion of the imaging subject;
wherein the motion phase errors include both predictable motion phase errors and random motion phase errors;
further comprising providing an estimate of the predictable motion phase errors with the MRI system processor;
wherein compensation of the random motion phase errors is provided by the adaptive phases of RF pulses and the adaptive magnetic field gradients; and
wherein compensation of the predictable motion phase errors is provided by pre-computed RF pulses and pre-computed magnetic field gradients that are determined from the estimate of the predictable motion phase errors by the MRI system processor.