US2012271583A1PendingUtilityA1
System and method for retrospective correction of high order eddy-current-induced distortion in diffusion-weighted echo planar imaging
Est. expiryApr 19, 2031(~4.8 yrs left)· nominal 20-yr term from priority
G01R 33/5616G01R 33/56518G01R 33/56341
49
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Claims
Abstract
A computer is programmed to acquire calibration data from a calibration scan, the calibration data configured to characterize high order eddy current (HOEC) generated magnetic field error of an imaging system. The computer is also programmed to process the calibration data to generate a plurality of basis coefficients and a plurality of time constants and to calculate a plurality of basis correction coefficients based on the plurality of basis coefficients, the plurality of time constants, and gradient waveforms in a given pulse sequence. The computer is further programmed to execute a diffusion-weighted imaging scan that comprises application of a DW-EPI pulse sequence to acquire MR data from an imaging subject and reconstruction of an image based on the acquired MR data. The computer is also programmed to apply HOEC-generated magnetic field error correction during image reconstruction configured to reduce HOEC-induced distortion in the reconstructed image.
Claims
exact text as granted — not AI-modified1 . An MRI apparatus comprising:
a magnetic resonance imaging (MRI) system having a plurality of gradient coils positioned about a bore of a magnet, and an RF transceiver system and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images; and a computer programmed to:
acquire calibration data from a calibration scan, the calibration data configured to characterize high order eddy current generated magnetic field error of an imaging system;
process the calibration data to generate a plurality of basis coefficients and a plurality of time constants;
calculate a plurality of basis correction coefficients based on the plurality of basis coefficients, the plurality of time constants, and gradient waveforms in a given pulse sequence;
execute a diffusion-weighted imaging scan comprising:
applying a DW-EPI pulse sequence to acquire MR data from an imaging subject; and
reconstructing an image based on the acquired MR data; and
apply high order eddy-current-generated magnetic field error correction during image reconstruction configured to reduce high order eddy-current-induced distortion in the reconstructed image.
2 . The MRI apparatus of claim 1 wherein the computer is further programmed to calculate a high order eddy-current-related field map.
3 . The MRI apparatus of claim 2 wherein the computer in being programmed to calculate the high order eddy-current-related field map, is programmed to calculate the high order eddy-current-related field map based on the equation:
f
(
u
,
v
)
=
∑
n
c
n
(
t
0
)
B
n
(
u
,
v
,
w
0
)
,
where c n (t 0 ) are basis correction coefficients of the plurality of basis correction coefficients for a basis function at a time point t 0 at which the high order eddy current field is approximated, and B n (u, v, w 0 ) are polynomial bases.
4 . The MRI apparatus of claim 2 wherein the computer is further programmed to apply one of an intensity correction and a geometry correction to data based on the high order eddy-current-related field map.
5 . The MRI apparatus of claim 4 wherein the computer in being programmed to apply the one of the intensity correction and the geometry correction, is programmed to apply the one of the intensity correction and the geometry correction based on the equation:
I
corrected
(
u
,
v
)
=
(
1
+
∂
h
(
u
,
v
)
∂
v
)
·
I
distorted
(
u
,
v
+
h
(
u
,
v
)
)
,
where h(u, v) is a pixel shift map.
6 . The MRI apparatus of claim 4 wherein the computer is further programmed to calculate a pixel shift map, h(u, v), based on the high order eddy-current-related field map.
7 . The MRI apparatus of claim 1 wherein the computer is programmed to apply the high order eddy-current-generated magnetic field error correction for an arbitrary imaging plane.
8 . The MRI apparatus of claim 1 wherein the computer, in being programmed to process the calibration data, is programmed to:
apply a 3D phase unwrapping to a phase angle of the calibration data;
scale the unwrapped calibration data to generate a magnetic field data set;
spatially fit the magnetic field data set to a harmonic basis to generate basis coefficients; and
temporally fit the basis coefficients along a time axis using one of a single-exponential model and a multi-exponential model.
9 . The MRI apparatus of claim 1 wherein the computer, in being programmed to process the calibration data, is programmed to:
take a time derivative on a phase angle of the calibration data to obtain a magnetic field offset at a coil location;
spatially fit each time point of the magnetic field offset to a harmonic basis to generate basis coefficients; and
temporally fit the basis coefficients along a time axis using one of a single-exponential model and a multi-exponential model.
10 . The MRI apparatus of claim 1 wherein the computer, in being programmed to calculate the plurality of basis correction coefficients, is programmed to calculate the plurality of basis correction coefficients based on the equation:
d
n
(
t
)
=
∑
m
=
X
,
Y
,
Z
G
m
β
mn
α
mn
τ
mn
-
t
/
τ
mn
,
where G m is the X, Y, or Z component of the diffusion gradient amplitude, β mn is a pulse sequence type and sequence timing related constant, α mn are basis coefficients, and τ mn are time constants.
11 . The MRI apparatus of claim 1 wherein the computer is further programmed to display the reconstructed image to a user.
12 . A method for correcting high order eddy-current-induced distortions in diffusion-weighted echo planar imaging (DW-EPI) comprising:
acquiring calibration data from a calibration scan, the calibration data configured to characterize high order eddy currents of an imaging system; processing the calibration data to generate a plurality of basis coefficients and a plurality of time constants; calculating a plurality of basis correction coefficients based on the plurality of basis coefficients and based on the plurality of time constants; and applying a DW-EPI pulse sequence to acquire MR data from an imaging subject; reconstructing an image based on the acquired MR data; and wherein the reconstructing the image comprises applying high order eddy-current-generated magnetic field error correction configured to reduce high order eddy-current-induced distortion in the image.
13 . The method of claim 12 further comprising calculating a high order eddy-current-related field map.
14 . The method of claim 13 further comprising applying one of an intensity correction and a geometry correction to data based on the high order eddy-current-related field map.
15 . The method of claim 14 further comprising generating the data based on the high order eddy-current-related field map by calculating a pixel shift map, h(u, v), based on the high order eddy-current-related field map.
16 . A non-transitory computer readable medium having stored thereon a computer program comprising a set of instructions, which, when executed by a computer, causes the computer to:
acquire calibration data from a calibration scan configured to characterize high order eddy current generated magnetic field error of an imaging system; process the calibration data; generate a plurality of basis coefficients and a plurality of time constants based on the processed calibration data; calculate a plurality of basis correction coefficients based on the plurality of basis coefficients, the plurality of time constants, and gradient waveforms in a DW-EPI pulse sequence; apply the DW-EPI pulse sequence to acquire MR data from an imaging subject; reconstruct an image based on the acquired MR data; and apply high order eddy-current-generated magnetic field error correction during reconstruction of the image configured to reduce high order eddy current induced distortion in the reconstructed image.
17 . The computer readable medium of claim 16 wherein the set of instructions further causes the computer to:
calculate a high order eddy-current-related field map;
calculate a pixel shift map, h(u, v), based on the high order eddy-current-related field map; and
apply an intensity and geometry correction to the pixel shift map.
18 . The computer readable medium of claim 17 wherein the set of instructions that causes the computer to calculate the high order eddy-current-related field map causes the computer to calculate the high order eddy-current-related field map based on the equation:
f
(
u
,
v
)
=
∑
n
c
n
(
t
0
)
B
n
(
u
,
v
,
w
0
)
,
where c n (t 0 ) are basis correction coefficients of the plurality of basis correction coefficients for a basis function at a time point t 0 at which the high order eddy current field is approximated, and B n (u, v, w 0 ) are polynomial bases.
19 . The method of claim 18 wherein the set of instructions that causes the computer to calculate a pixel shift map causes the computer to calculate the pixel shift map based on the equation:
h
(
u
,
v
)
=
f
(
u
,
v
)
G
PE
,
where G PE =1/(γTL) with γ being the gyromagnetic ratio of the nucleus of interest, T being the EPI echo spacing, and L being the field of view in the phase encoding axis.
20 . The method of claim 19 wherein the set of instructions that causes the computer to apply the intensity and geometry correction causes the computer to apply the intensity and geometry correction based on the equation:
I
corrected
(
u
,
v
)
=
(
1
+
∂
h
(
u
,
v
)
∂
v
)
·
I
distorted
(
u
,
v
+
h
(
u
,
v
)
)
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