Apparatus for Real-Time Phase Correction for Diffusion-Weighted Magnetic Resonance Imaging Using Adaptive RF Pulses

• US 20130229177A1
• Filed: 08/30/2012
• Published: 09/05/2013
• Est. Priority Date: 08/30/2011
• Status: Active Grant

• Current Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford Management Co.)
• Original Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford Management Co.)
• Inventors: Bammer, Roland, Van, Anh Tu, O'Halloran, Rafael
• Granted Patent: US 9,329,252 B2
• Time in Patent Office: Days
• US Class Current: 324/309
• CPC Class Codes: G01R 33/56 Image enhancement or correction, e.g. subtraction or averaging techniques , e.g. improvement of signal-to-noise ratio and resolution
• CPC Class Codes: G01R 33/5614 using a fully balanced steady-state free precession [bSSFP] pulse sequence, e.g. trueFISP
• CPC Class Codes: G01R 33/56341 Diffusion imaging
• CPC Class Codes: G01R 33/56509 due to motion, displacement or flow, e.g. gradient moment nulling G01R33/567 takes precedence
• CPC Class Codes: G01R 33/5676 Gating or triggering based on an MR signal, e.g. involving one or more navigator echoes for motion monitoring and correction

Abstract

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.

Claims

1. A method for performing magnetic resonance imaging (MRI), the method comprising:

a) operating an MRI system to provide a navigator image of an imaging subject, wherein the MRI system comprises 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, a phase error map from the navigator image;

c) determining, in real time, an adaptive RF pulse from the phase error map that compensates for phase errors of the phase error map;

d) determining, in real time, one or more adaptive magnetic field gradients corresponding to the phase error map and the adaptive RF pulse;

e) providing the adaptive RF pulse to the imaging subject with the one or more sources of RF emission and simultaneously providing the 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-e above in succession one or more times.

2. The method of claim 1, wherein the adaptive RF pulse compensates for the phase errors by making a post-pulse phase distribution in the imaging subject substantially uniform.

3. The method of claim 1, wherein the adaptive RF pulse compensates for the phase errors by making an RF tip axis map substantially equal to the phase error map.

4. The method of claim 3, wherein the adaptive RF pulses comprise phase insensitive echo-reset pulses for phase insensitive diffusion encoding preparation.

5. The method of claim 3, wherein the adaptive RF pulses comprise refocusing pulses for fast spin echo imaging.

6. The method of claim 3, wherein the adaptive RF pulses comprise excitation pulses for diffusion-weighted steady state free precession imaging.

7. The method of claim 1, further comprising:

determining one or more magnetic field gradients that compensate for linear error in the phase error map; and

providing the magnetic field gradients to the imaging subject with the gradient magnets during the diffusion weighted MRI.

8. The method of claim 1, wherein the one or more RF sources comprise two or more RF sources configured to operate in parallel.

9. The method of claim 1:

wherein the phase errors include motion phase errors due to motion of the imaging subject;

wherein the motion phase errors include predictable errors and random errors;

wherein compensation of the random errors is provided by the adaptive phases of RF pulses and the adaptive magnetic field gradients; and

wherein compensation of the predictable errors is provided by pre-computed RF pulses and pre-computed magnetic field gradients that are determined from an estimate of the predictable errors.