US 2024/0201303 A1Published application

Method of estimating motion of an object and/or magnetic field offsets during an mri scan

AssigneesEidgenoessische Technische Hochschule (Eth), Universitaet Zuerich

InventorsThomas ULRICH, Klaas PRUESSMANN

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Abstract

In a method of estimating motion of an object and/or magnetic field offsets in a region surrounding the object during a magnetic resonance (MR) imaging scan of the object, superimposed magnetic fields and radiofrequency fields are generated according to an MR sequence for forming images, the MR sequence comprising a train of sequence modules, each sequence module comprising a radiofrequency (RF) excitation segment and an image encoding gradient segment, the MR sequence further comprising a plurality of navigator gradient segments. The navigator signal is acquired along a trajectory in k-space and expressed as a discrete time series comprising a predefined number of complex-valued signal datapoints. The navigator signal acquired in a first sequence module is used to calculate a transformation matrix which relates rotation angles and translational shifts and/or changes of the magnetic field to corresponding changes in navigator signal in a first order approximation. The navigator signal acquired in a subsequent sequence module is used to estimate object motion in terms of a translational displacement and a rotational displacement of the object and/or magnetic field offsets in terms of a scalar field variation and a vectorial field variation between the first sequence module and the subsequent sequence module by solving a corresponding linear least-squares estimation problem.

Claims20

Dependent claims

A method of estimating motion of an object during a magnetic resonance (MR) imaging scan of the object comprising:

generation of a main magnetic field in the object by a main magnet and generation of superimposed magnetic fields and radiofrequency fields according to an MR sequence for forming images,
the MR sequence comprising
a train of sequence modules,
each sequence module comprising a radiofrequency (RF) excitation segment and an image encoding gradient segment,
said MR sequence further comprising
a plurality of navigator gradient segments,
in which method
an object signal is acquired with an RF receive coil or coil array during a measurement segment of each sequence module of the train of sequence modules,
a navigator signal is acquired with said RF receive coil or coil array during each one of said navigator gradient segments of the plurality of navigator gradient segments, wherein
said navigator signal is acquired along a trajectory k(t) in k-space,
said navigator signal, in a given sequence module, is expressed as a discrete time series comprising a predefined number N of complex-valued signal datapoints
s
⁡
(
k
⁡
(
t
i
)
)
,
wherein
i
=
1
⁢
to
⁢
N
,
a transformation matrix M which relates rotation angles and translational shifts to corresponding changes in the navigator signal in a first order approximation is calculated from the navigator signal s(k(ti)) acquired in a first sequence module, optionally including the navigator signal sj(k(ti)) acquired in at least one further sequence module,
the navigator signal {tilde over (s)}(k(ti)) acquired in a subsequent sequence module is used to estimate object motion in terms of a translational displacement Δx and a rotational displacement θ of the object between said first sequence module and said subsequent sequence module by solving the following linear least-squares estimation problem:
min
Δ
⁢
x
1
,
Δ
⁢
x
2
,
Δ
⁢
x
3
,
θ
1
,
θ
2
,
θ
3

(
s
˜
-
s
)
-
M
⁡
(
Δ
⁢
x
1
Δ
⁢
x
2
Δ
⁢
x
3
θ
1
θ
2
θ
3
)

2
2
.

88 more elements

A method of estimating magnetic field offsets in a region surrounding an object during a magnetic resonance (MR) imaging scan of the object, comprising:

generation of a main magnetic field in the object by a main magnet and generation of superimposed magnetic fields and radiofrequency fields according to an MR sequence for forming images,
the MR sequence comprising
a train of sequence modules,
each sequence module comprising a radiofrequency (RF) excitation segment and an image encoding gradient segment,
said MR sequence further comprising
a plurality of navigator gradient segments,
in which method
an object signal is acquired with an RF receive coil or coil array during a measurement segment of each sequence module of the train of sequence modules,
a navigator signal is acquired with said RF receive coil or coil array during each one of said navigator gradient segments of the plurality of navigator gradient segments, wherein
said navigator signal is acquired along a trajectory k(t) in k-space
said navigator signal in a given sequence module is expressed as a discrete time series comprising a predefined number N of complex-valued signal datapoints
s
⁡
(
k
⁡
(
t
i
)
)
,
wherein
i
=
1
⁢
to
⁢
N
,
a transformation matrix Q which relates changes of the magnetic field to corresponding changes in the navigator signal in a first order approximation is calculated from the navigator signal s(k(ti)) acquired in a first sequence module, optionally including the navigator signal sj(k(ti)) acquired in at least one further sequence module,
the navigator signal s(k(ti)) acquired in a subsequent sequence module is used to estimate magnetic field offsets in terms of a zeroth-order field variation ΔB0 and a first-order field variation
(
G
1
G
2
G
3
)
between said first sequence module and said subsequent sequence module by solving the following linear least-squares estimation problem:
min
Δ
⁢
B
0
,
G
1
,
G
2
,
G
3

(
s
˜
-
s
)
-
Q
⁡
(
Δ
⁢
B
0
G
1
G
2
G
3
)

2
2
.

79 more elements

A method of estimating motion of an object and magnetic field offsets in a region surrounding the object during a magnetic resonance (MR) imaging scan of the object comprising:

generating superimposed magnetic fields and radiofrequency fields according to an MR sequence for forming images,
the MR sequence comprising
a train of sequence modules,
each sequence module comprising a radiofrequency (RF) excitation segment and an image encoding gradient segment,
said MR sequence further comprising a plurality of navigator gradient segments,
in which method
an object signal is acquired with an RF receive coil or coil array during a measurement segment of each sequence module,
a navigator signal is acquired with said RF receive coil or coil array during each one of said navigator gradient segments,
wherein
said navigator signal is acquired along a trajectory k(t) in k-space
said navigator signal, in a given sequence module, is expressed as a discrete time series comprising a predefined number N of complex-valued signal datapoints
s
⁡
(
k
⁡
(
t
i
)
)
,
wherein
i
=
1
⁢
to
⁢
N
,
a transformation matrix R which relates rotation angles, translational shifts and changes of the magnetic field to corresponding changes in the navigator signal in a first order approximation is calculated from the navigator signal s(k(ti)) acquired in a first sequence module, optionally including the navigator signal sj(k(ti)) acquired in at least one further sequence module,
the navigator signal {tilde over (s)}(k(ti)) acquired in a subsequent sequence module is used to estimate object motion in terms of a translational displacement Δx and a rotational displacement θ of the object and magnetic field offsets in terms of a scalar field variation ΔB0 and a vectorial field variation
(
G
1
G
2
G
3
)
between said first sequence module and said subsequent sequence module by solving the following linear least-squares estimation problem:
min
Δ
⁢
x
,
θ
,
Δ
⁢
B
0
,
G
1
,
G
2
,
G
3

(
s
˜
-
s
)
-
R
⁡
(
Δ
⁢
x
1
Δ
⁢
x
2
Δ
⁢
x
3
θ
1
θ
2
θ
3
Δ
⁢
B
0
G
1
G
2
G
3
)

2
2
.

103 more elements

Dependencies are inferred from the claim text.

Facts

Application: 18/557,980Filed 30 Apr 2022
Priority: EP 21171727.730 Apr 2021
Published: 20 Jun 2024
Term ends: Not estimatedThe source records don’t support an estimate.

Classification4 codes

G01R 33/56563
Correction of image distortions, e.g. due to magnetic field inhomogeneities: caused by a distortion of the main magnetic field B0, e.g. temporal…
G01R 33/543
Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console: control of the operation of…
G01R 33/56509
Correction of image distortions, e.g. due to magnetic field inhomogeneities: due to motion, displacement or flow, e.g. gradient moment nulling
G01R
Measuring electric variables

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