US 2014/0111203 A1Published application

Reducing artifacts in magnetic resonance images acquired using propeller echo planar imaging

AssigneeThe Board of Trustees of the University of Illinois

InventorXiaohong Joe Zhou

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Abstract

In PROPELLER utilizing EPI k-space sampling, phase errors arising primarily from eddy currents can considerably degrade image quality. The phase errors include spatially constant phase errors, spatially linear phase errors, and oblique phase errors. Methods to measure and correct for these phase errors are disclosed. Two or three reference scans are acquired, each reference scan being mutually orthogonal along the orthogonal physical gradient axes in a MRI system. A spatially constant phase error and a spatially linear phase error are determined from each of the reference scans for each relevant physical gradient axis. These phase errors can be used to predict the constant, linear, and oblique phase errors in each blade of an EPI PROPELLER k-space data set. With the known phase errors for each blade, constant, linear, and/or oblique phase correction is applied prior to or during PROPELLER image reconstruction, producing an image with substantially reduced artifacts.

Claims20

Dependent claims

A method comprising:

acquiring a first reference scan of an object in a magnetic resonance imaging system, wherein acquiring the first reference scan comprises applying a first reference readout gradient along a first axis;
based on the first reference scan, determining a constant phase error cx associated with the first axis and a linear phase error lx along the first axis;
acquiring a second reference scan of the object, wherein acquiring the second reference scan comprises applying a second reference readout gradient along a second axis, the second axis being orthogonal to the first axis;
based on the second reference scan, determining a constant phase error cy associated with the second axis and a linear phase error ly along the second axis;
acquiring a third reference scan of the object, wherein acquiring the third reference scan comprises applying a third reference readout gradient along a third axis, the third axis being orthogonal to the first axis and the second axis;
based on the third reference scan, determining a constant phase error cz associated with the third axis and a linear phase error lz along the third axis;
acquiring a plurality of intersecting blades around a center of k-space, the plurality of intersecting blades being in an imaging plane of k-space, wherein:
each of the plurality of intersecting blades is offset from the first axis by a respective blade angle θ, and
the imaging plane is defined by a rotation matrix A given by:
A
=
[
a
0
a
1
a
2
a
3
a
4
a
5
a
6
a
7
a
8
]
;
for each of the plurality of intersecting blades, determining a constant phase error cθ, a linear phase error lθ, and an oblique phase error oθ, wherein:
the constant phase error cθ is given by:
cθ=(a0 cos θ+a1 sin θ)cx+(a3 cos θ+a4 sin θ)cy+(a6 cos θ+a7 sin θ)cz;
the linear phase error lθ is given by:
lθ=(a0 cos θ+a1 sin θ)2lx+(a3 cos θ+a4 sin θ)2ly+(a6 cos θ+a7 sin θ)2lz;
the oblique phase error oθ is given by:
o
θ
=
F


O


V
pe
F


O


V
ro
·
N
ro
N
pe

{
sin


2

θ

[
(
a
1
2
-
a
0
2
)

l
x
+
(
a
4
2
-
a
3
2
)

l
y
+
(
a
7
2
-
a
6
2
)

l
z
]
-
2

cos


2


θ

[
a
0

a
1

l
x
+
a
3

a
4

l
y
+
a
6

a
7

l
z
]
}
,
FOVro is a field of view of an applied blade readout gradient,
FOVpe is a field of view of an applied phase-encoding gradient,
Npe is a number of lines in a direction of the phase-encoding gradient, and
Nro is a number of samplings in a direction of the applied blade readout gradient along the number Npe of lines;
and
displaying an image, wherein the image is generated based on at least one of the constant phase error cθ, the linear phase error lθ, and the oblique phase error oθ.

155 more elements

A method comprising:

determining a constant phase error cx associated with a first axis and a linear phase error lx along the first axis, based on a first reference scan of an object in a magnetic resonance imaging system, the first reference scan being acquired using a first reference readout gradient along the first axis;
determining a constant phase error cy associated with a second axis and a linear phase error ly along the second axis, based on a second reference scan of the object, the second reference scan being acquired using a second reference readout gradient along the second axis, the second axis being orthogonal to the first axis;
acquiring a plurality of intersecting blades around a center of k-space, the plurality of intersecting blades being in an imaging plane of k-space, wherein each of the plurality of intersecting blades is offset from the first axis by a respective blade angle θ;
for each of the plurality of intersecting blades, determining a constant phase error cθ, a linear phase error lθ, and an oblique phase error oθ, wherein:
the constant phase error co is given by:
cθ=cx cos θ+cy sin θ,
the linear phase error lθ is given by:
lθ=lx cos2θ+ly sin2θ,
the oblique phase error oθ is given by:
o
θ
=
F


O


V
pe
F


O


V
ro
·
N
ro
N
pe

(
l
y
-
l
x
)

sin


2


θ
,
FOVro is a field of view of an applied blade readout gradient,
FOVpe is a field of view of an applied phase-encoding gradient,
Npe is a number of lines in a direction of the applied phase-encoding gradient, and
Nro is a number of samplings in a direction of the applied blade readout gradient along the number Npe of lines;
and
displaying an image, wherein the image is generated based on at least one of the constant phase error cθ, the linear phase error lθ, and the oblique phase error cθ.

53 more elements

A method comprising:

determining a constant phase error cx associated with a first axis and a linear phase error lx along the first axis, based on a first reference scan of an object in a magnetic resonance imaging system, the first reference scan being acquired using a first reference readout gradient along the first axis;
determining a constant phase error cy associated with a second axis and a linear phase error ly along the second axis, based on a second reference scan of the object, the second reference scan being acquired using a second reference readout gradient along the second axis, the second axis being orthogonal to the first axis;
determining a constant phase error c, associated with a third axis and a linear phase error lz along the third axis, based on a third reference scan of the object, the third reference scan being acquired using a third reference readout gradient along the third axis, the third axis being orthogonal to the first axis and the second axis;
receiving a plurality of intersecting blades around a center of k-space, the plurality of intersecting blades being in an imaging plane of k-space, wherein:
each of the plurality of intersecting blades is offset from the first axis by a respective blade angle θ, and
the imaging plane is defined by a rotation matrix A given by:
A
=
[
a
0
a
1
a
2
a
3
a
4
a
5
a
6
a
7
a
8
]
;
for each of the plurality of intersecting blades, determining a constant phase error cθ, a linear phase error lθ, and an oblique phase error oθ, wherein:
the constant phase error cθ is given by:
cθ=(a0 cos θ+a1 sin θ)cx+(a3 cos θ+a4 sin θ)cy+(a6 cos θ+a7 sin θ)cz,
the linear phase error lθ is given by:
lθ=(a0 cos θ+a1 sin θ)2lx+(a3 cos θ+a4 sin θ)2ly+(a6 cos θ+a7 sin θ)2lz,
the oblique phase error oθ is given by:
o
θ
=
F


O


V
pe
F


O


V
ro
·
N
ro
N
pe

{
sin


2

θ

[
(
a
1
2
-
a
0
2
)

l
x
+
(
a
4
2
-
a
3
2
)

l
y
+
(
a
7
2
-
a
6
2
)

l
z
]
-
2

cos


2


θ

[
a
0

a
1

l
x
+
a
3

a
4

l
y
+
a
6

a
7

l
z
]
}
,
FOVro is a field of view of an applied blade readout gradient,
FOVpe is a field of view of an applied phase-encoding gradient,
Npe is a number of lines in a direction of the applied phase-encoding gradient, and
Nro is a number of samplings in a direction of the applied blade readout gradient along the number Npe of lines;
and
displaying an image, wherein the image is generated based on at least one of the constant phase error cθ, the linear phase error lθ, and the oblique phase error oθ.

152 more elements

A method comprising:

acquiring a plurality of reference scans of an object in a magnetic resonance imaging system, wherein acquiring the plurality of reference scans of the object comprises applying a first reference readout gradient along a first axis of a plurality of orthogonal axes, and applying a second reference readout gradient along a second axis of the plurality of orthogonal axes;
based on the plurality of reference scans, determining a plurality of constant phase errors associated with corresponding axes of the plurality of orthogonal axes and determining a plurality of linear phase errors along corresponding axes of the plurality of orthogonal axes;
acquiring a plurality of intersecting blades around a center of k-space, the plurality of intersecting blades being in an imaging plane of k-space, wherein each of the plurality of intersecting blades is offset from the first axis by a respective blade angle θ, and wherein the imaging plane is defined by a rotation matrix A;
determining a constant phase error cθ, a linear phase error lθ, and an oblique phase error oθ for each of the plurality of intersecting blades, wherein:
the constant phase error cθ is a function of at least the rotation matrix A, the blade angle θ, and the plurality of constant phase errors associated with the corresponding axes of the plurality of orthogonal axes,
the linear phase error lθ is a function of at least the rotation matrix A, the blade angle θ, and the plurality of linear phase errors along the corresponding axes of the plurality of orthogonal axes, and
the oblique phase error oθ is a function of at least the rotation matrix A, the blade angle θ, the plurality of linear phase errors along the corresponding axes of the plurality of orthogonal axes, a field of view of an applied blade readout gradient, a field of view of an applied phase-encoding gradient, a number of lines in a direction of the phase-encoding gradient, and a number of samplings in a direction of the applied blade readout gradient along the number of lines;
and
displaying an image, wherein the image is generated based on at least one of the constant phase error cθ, the linear phase error lθ, and the oblique phase error oθ.

6 more elements

Dependencies are inferred from the claim text.

Facts

Application: 14/113,044Filed 23 Apr 2012
Priority: 21 Apr 2011
Published: 24 Apr 2014
Term ends: Not estimatedThe source records don’t support an estimate.

Classification7 codes

G01R 33/565
Image enhancement or correction, e.g. subtraction or averaging techniques , e.g. improvement of signal-to-noise ratio and resolution: correction…
G01R 33/4824
MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented…:…
G01R 33/56518
Correction of image distortions, e.g. due to magnetic field inhomogeneities: due to eddy currents, e.g. caused by switching of the gradient…
G01R 33/56554
Correction of image distortions, e.g. due to magnetic field inhomogeneities: caused by acquiring plural, differently encoded echo signals after…