Methods and apparatus for phase correction in magnetic resonance imaging

Patent metadata

Field	Value
Publication number	US-9519040-B2
Application number	US-201514677156-A
Country	US
Kind code	B2
Filing date	Apr 2, 2015
Priority date	Apr 4, 2014
Publication date	Dec 13, 2016
Grant date	Dec 13, 2016

Abstract

Methods, apparatuses, systems, and software for extended phase correction in phase sensitive magnetic resonance imaging utilizing an optimized region-growing based phase correction algorithm. Phase correction is formulated as selecting a vector for each pixel of an image from two input candidate vectors so that the orientation of the output vector is spatially smooth. In certain embodiments, the optimized region growing algorithm uses automated quality guidance for determining the sequence of region growing and jointly considers the two input candidate vectors during region growing. Further, the algorithm tracks the quality and the mode at each step of the processing. Spatially isolated tissue regions are automatically segmented and processed with different threads of region growing and the correct vector is reliably identified as the output vector for each thread of region growing. Final phase correction was performed by pixel level optimization.

First claim

The invention claimed is:

1. A computerized method for generating a phase corrected magnetic resonance image comprising:
(a) acquiring a first magnetic resonance image containing error and intrinsic phase information;
(b) calculating two or more candidate input vector images from the acquired image; and
(c) performing an optimized region growing based phase correction algorithm to determine an output vector image that is spatially smooth in orientation and is substantially represented at each pixel by one of the calculated vector images at the same pixel, wherein phase correction is performed by:
selecting an initial seed pixel for a new thread of region growing and placing the initial seed pixel onto a highest priority pixel stack;
selecting a pixel that is placed on a pixel stack with a highest priority order from a list of pixel stacks that hold unprocessed pixels;
determining an updated vector pair for the selected pixel by jointly considering its input vector pair (A and B) with a reference vector pair (A r and B r );
placing each of neighboring pixels of the selected pixel onto one of pixel stacks according to a quality metric by jointly comparing an input vector pair of a neighbor pixel with a reference vector pair;
recording and monitoring the priority and the mode of region growing for the selected pixel;
using the recorded mode of region growing to determine which vector pairs need to be considered and which vector of the updated vector pair is a correct vector; and
performing a final pixel-level optimization in which a vector O of a pixel is assigned as its original input vector A or B depending on if A or B has a smaller angular difference with the locally average O of the pixel over a selected boxcar.

2. The computerized method of claim 1 wherein performing an optimized region growing based phase correction algorithm comprises:
calculating a first candidate input vector map A and a second candidate input vector map B from the first magnetic resonance image;
generating an updated vector pair (A and B) for a given pixel jointly by comparing the angular differences of A and B, B and Ã, {tilde over (B)} and A, of a given pixel with a reference vector pair (A r and B r ) for the same pixel;
deciding the order by which a pixel is visited by the region growing using a quality metric based on the angular differences of A and B, B and Ã, {tilde over (B)} and A, of the pixel with a reference vector pair (A r and B r ) for the same pixel;
calculating the reference vector pair (A r and B r ) for a given pixel as the updated vector pair A and B averaged over all the pixels that have been previously processed by the region growing and are located within a boxcar neighborhood of the given pixel;
determining the mode of region growing by recording and monitoring the number of pixels selecting each of the three possible vector pairs (A and B, {tilde over (B)} and A, B and Ã) as their updated vector pair A and B, wherein à is the mirror vector of A with respect to B and {tilde over (B)} is the mirror vector of B with respect to A;
using the mode of region growing to select the correct vector O from the updated vector pair A and B; and
detecting spatially isolated tissue regions and starting new threads of region growing by recording and monitoring the quality of region growing.

3. The method of claim 1 further comprising:
(e) generating the phase corrected magnetic resonance image or images from the acquired magnetic resonance image or images using a final output vector O to remove the error phase; and
(f) displaying or archiving the phase corrected magnetic resonance image or images.

4. The method of claim 1 further comprising:
(g) selecting a seed pixel or pixels and assigning A and B of the seed pixel or pixels as the updated vector pair A and B for the seed pixel or pixels;
(h) selecting a second seed pixel from a pixel stack and determining its updated vector pair A and B jointly as its A and B, or B and Ã, or {tilde over (B)} and A, wherein à is the mirror vector of A with respect to B, and {tilde over (B)} is the minor vector of B with respect to A, depending on which of the three vector pairs has the smallest angular difference with a reference vector pair for the updated vector pair A and B of the second seed pixel;
(i) determining for the second seed pixel a local quality metric for each of nearest neighbor pixels of the second seed pixel for which the updated vector pair A and B have not been determined and assigning a priority to each of the nearest neighbor pixels using the local quality metric in order to determine the sequence by which each of the nearest neighbor pixels is to be selected as a further seed pixel; and
(j) repeating the steps of (h) and (i) to complete optimized region growing with respect to further seed pixels and to construct the updated vector image pair A and B for all pixels that are processed by the optimized region growing, wherein:
an initial seed pixel or pixels are placed onto a high priority pixel stack or stacks among a series of pixel stacks that are initially empty and which facilitate a sequencing of the optimized region growing;
a pixel is selected as a seed pixel if it has not been processed previously as a seed pixel and it is on a pixel stack that has a highest priority among pixel stacks that contain at least one pixel that has not been processed as a seed pixel; and
the local quality metric of a pixel is calculated as the smallest of the angular differences of A and B, B and Ã, {tilde over (B)} and A, of the pixel with an estimated reference vector pair A r and B r for the same pixel.

5. The method of claim 4 , wherein the values of the vector A and vector B for an initial seed pixel are assigned as the updated vector pair A and B for the initial seed pixel, and an optimized region growing is performed to jointly construct an updated vector pairs A and B for all the pixels processed by the region growing, wherein a final correct vector O is selected by recording and monitoring the mode of region growing of the processed pixels.

6. The method of claim 4 wherein acquiring the magnetic resonance image comprises acquiring two-point Dixon water and fat images, wherein a first image S 1 is acquired at a first echo time TE 1 and a second image S 2 is acquired at a second echo time TE 2 .

7. The method of claim 6 , wherein the images S 1 and S 2 are expressed according to the following equations:
S 1 =( W+δ 1 Fe iθ 1 ) P 1
S 2 =( W+δ 2 Fe iθ 2 ) P 1 P

where W and F are amplitudes for water and fat respectively, P 1 is a phase factor of image S 1 , P is an additional phase factor of image S 2 relative to image S 1 and is determined by a background or error phase, and the method further comprises determining an amplitude attenuation factor (δ 1 , δ 2 ) and phase (θ 1 , θ 2 ) as a function of two echo times (TE 1 , TE 2 ) using a known or pre-calibrated fat signal model.

8. The method of claim 6 , wherein the images S 1 and S 2 are used to generate two vector images A and B as expressed according to the following equations:
A=S* 1 S 2 [Q A +δ 1 (1− Q A ) e iθ 1 ][Q A +δ 2 (1− Q A ) e −iθ 2 ]

B=S* 1 S 2 [Q B +δ 1 (1− Q B ) e iθ 1 ][Q B +δ 2 (1− Q B ) e −iθ 2 ]

where Q A and Q B are the two mathematically possible solutions of the following quadratic equation of Q, which is defined as

Q
=

W

W
+
F

Assignees

• Univ Texas

Inventors

• Ma Jingfei
• Hazle John
• Son Jong Bum

Classifications

• G01R33/5611

  Parallel magnetic resonance imaging, e.g. sensitivity encoding [SENSE], simultaneous acquisition of spatial harmonics [SMASH], unaliasing by Fourier encoding of the overlaps using the temporal dimension [UNFOLD], k-t-broad-use linear acquisition speed-up technique [k-t-BLAST], k-t-SENSE (structural details of arrays of sub-coils G01R33/3415) · CPC title

• G01R33/5602

  by filtering or weighting based on different relaxation times within the sample, e.g. T1 weighting using an inversion pulse · CPC title

• G01R33/4828

  Resolving the MR signals of different chemical species, e.g. water-fat imaging · CPC title

• G01R33/56527

  due to chemical shift effects · CPC title

• G01R33/565 Primary

  Correction of image distortions, e.g. due to magnetic field inhomogeneities · CPC title