System and method for z-shim compensated echo-planar magnetic resonance imaging

Patent metadata

Field	Value
Publication number	US-2016209487-A1
Application number	US-201414915556-A
Country	US
Kind code	A1
Filing date	Aug 28, 2014
Priority date	Aug 28, 2013
Publication date	Jul 21, 2016
Grant date	—

Abstract

A system and method for imaging a subject includes a first imaging pulse sequence having gradient blips along an x-direction and a y-direction to acquire calibration image data from multiple slices. The imaging pulse sequence also includes a plurality of Z-shimming gradient blips coincident in time with the gradient blips along the x- and y-directions and varied within each slice. A plurality of calibration images are reconstructed from the calibration image data and a comparison image is formed by selecting an image from the calibration images corresponding to at least one of the varied Z-shimming gradient blips for each slice to determine a desired value of the Z-shimming gradient blips. The desired values are used to perform a second pulse sequence to acquire clinical image data from the subject. The second pulse sequence is used to acquire clinical images having been compensated for magnetic susceptibility variations within the subject.

First claim

1 . A method for compensating for magnetic susceptibility variations within a subject to be imaged using a magnetic resonance imaging (MRI) system, the method including steps comprising:
a) performing, with the MRI system, a first pulse sequence to select desired shimming parameters, the pulse sequence comprising:
i) an imaging pulse sequence including gradient blips along two orthogonal gradient directions to acquire calibration image data from a plurality of slices across the subject;
ii) a plurality of shimming gradient blips applied along a third gradient direction orthogonal to the two orthogonal gradient directions, wherein the shimming gradient blips are coincident in time with the gradient blips along two orthogonal gradient directions and are varied within each of the plurality of slices;
b) reconstructing a plurality of calibration images from the calibration image data; c) forming a comparison image by selecting an image from plurality of calibration images corresponding to at least one of the varied shimming gradient blips for each slice and combing the selected images into the comparison image; d) comparing the plurality of calibration images to the comparison image to determine a value of the shimming gradient blips to be included in the desired shimming parameters; e) performing, with the MRI system, a second pulse sequence using the value of the shimming gradient blips included in the desired shimming parameters to acquire clinical image data from the subject; f) reconstructing the clinical image data to form a first set of clinical images of the subject; and g) selectively combining the clinical images to form a plurality of clinical composite images of the subject.

2 . The method of claim 1 further comprising h) applying a spatial filter to control noise at high spatial frequencies.

3 . The method of claim 2 wherein the spatial filter includes using a Hanning window to control noise at the high spatial frequencies.

4 . The method of claim 2 wherein step h) includes applying an inverse-reconstruction to transfer the clinical composite images of the subject to k-space.

5 . The method of claim 4 wherein step h) includes applying the spatial filter to control noise at high spatial frequencies in k-space to generate filtered image data and reconstructing the filtered image data to generate a final set of the plurality of clinical composite images of the subject.

6 . The method of claim 1 wherein step g) includes combining every two consecutive images within the first set of clinical images to form the plurality of clinical composite images of the subject.

7 . The method of claim 1 wherein step g) includes performing a maximum intensity projection (MIP) to combine every two consecutive images within the first set of clinical images to form the plurality of clinical composite images of the subject.

8 . The method of claim 1 further comprising correcting for slice-time differences in real space.

9 . The method of claim 8 wherein correcting for slice-time differences in real space includes applying a temporal linear interpolation based on a voxel-wise triplet of previous, current, and subsequent time points.

10 . The method of claim 1 further comprising combining the calibration images to form the plurality of clinical composite images of the subject.

11 . The method of claim 10 wherein step c) includes selecting from the plurality of clinical composite images of the subject to form the comparison image.

12 . The method of claim 1 wherein step d) includes determining the value of the shimming gradient blips to control slice intensity inhomogeneities.

13 . The method of claim 1 wherein the plurality of clinical composite images of the subject form functional magnetic resonance images (fMRI) of the subject.

14 . A magnetic resonance imaging (MRI) system, comprising:
a magnet system configured to generate a polarizing magnetic field about at least a portion of a subject arranged in the MRI system; a magnetic gradient system including a plurality of magnetic gradient coils configured to apply at least one magnetic gradient field to the polarizing magnetic field; a radio frequency (RF) system configured to apply an RF field to the subject and to receive magnetic resonance signals therefrom; a computer system programmed to:
a) control the gradient system and the RF system to perform a first pulse sequence to select desired Z-shimming parameters, the pulse sequence comprising:
i) an imaging pulse sequence including gradient blips along an x-direction and a y-direction to acquire calibration image data from a plurality of slices across the subject;
ii) a plurality of Z-shimming gradient blips along Z-direction, wherein the Z-shimming gradient blips are coincident in time with the gradient blips along the x-direction and the y-direction and are varied within each of the plurality of slices;

b) reconstruct a plurality of calibration images form the calibration image data;
c) select a comparison image;
d) compare the plurality of calibration images to the comparison image to determine a value of the Z-shimming gradient blips that provided calibration images approximate to the comparison image;
e) control the gradient system and the RF system to perform a second pulse sequence using the value of the Z-shimming gradient blips included in the desired Z-shimming parameters to acquire clinical image data from the subject;
reconstruct the clinical image data to form a first set of clinical images of the subject; and
g) selectively combine the clinical images to form a plurality of clinical composite images of the subject by combining consecutive images acquired with a given Z-shimming gradient doublet indicated by the desired Z-shimming parameters.

15 . The system of claim 14 wherein the computer system is further programmed to utilize a maximum intensity projection algorithm to selectively combining the clinical images to form the plurality of clinical composite images.

16 . The system of claim 14 wherein the computer system is further programmed to transform the plurality of clinical composite images to k-space as a plurality of clinical composite image data sets, filter the plurality of clinical composite image data sets to remove noise in high spatial frequencies, and reconstruct a final plurality of clinical composite images from the filtered plurality of clinical composite image data sets.

17 . The system of claim 14 wherein the computer system is further programmed to track a stimulus of the subject during the second pulse sequence and correlating the stimulus with the plurality of clinical composite images of the subject.

18 . The system of claim 14 wherein the computer system is further programmed to correct for slice-time differences in real space by applying a temporal linear interpolation based on a voxel-wise triplet of previous, current, and subsequent time points.

19 . The system of claim 14 wherein the computer system is further programmed to determine the value of the Z-shimming gradient blips to control slice intensity inhomogeneities in step d).

20 . The system of claim 14 wherein the first pulse sequence and the second pulse sequence include a gradient echo, echo planar imaging (GE-EPI) pulse sequence.

Assignees

• Silbersweig David
• Pan Hong
• Brigham & Womens Hospital

Inventors

• Stern Emily
• Silbersweig David
• Pan Hong

Classifications

• G01R33/4806

  Functional imaging of brain activation · CPC title

• G01R33/5616

  using gradient refocusing, e.g. EPI · CPC title

• G01R33/56536 Primary

  due to magnetic susceptibility variations · CPC title

• G01R33/4835

  of multiple slices · CPC title