U.S. patent application number 11/469617 was filed with the patent office on 2008-03-06 for motion corrected magnetic resonance imaging.
This patent application is currently assigned to THE BOARD OF TRUSTEES OF THE LELAND STANFORD JUNIOR UNIVERSITY. Invention is credited to Murat AKSOY, Roland BAMMER, Chunlei LIU, Rexford NEWBOULD.
Application Number | 20080054899 11/469617 |
Document ID | / |
Family ID | 39150573 |
Filed Date | 2008-03-06 |
United States Patent
Application |
20080054899 |
Kind Code |
A1 |
AKSOY; Murat ; et
al. |
March 6, 2008 |
MOTION CORRECTED MAGNETIC RESONANCE IMAGING
Abstract
A method of correcting for motion in magnetic resonance images
of an object detected by a plurality of signal receiver coils
comprising the steps of acquiring a plurality of image signals with
the plurality of receiver coils, determining motion between
sequential image signals relative to a reference, applying rotation
and translation to image signals to align image signals with the
reference, determining altered coil sensitivities due to object
movement during image signal acquisition, and employing parallel
imaging reconstruction of the rotated and translated image signals
using the altered coil sensitivities in order to compensate for
undersampling in k-space.
Inventors: |
AKSOY; Murat; (Stanford,
CA) ; LIU; Chunlei; (Fremont, CA) ; NEWBOULD;
Rexford; (Menlo Park, CA) ; BAMMER; Roland;
(Palo Alto, CA) |
Correspondence
Address: |
BEYER WEAVER LLP
P.O. BOX 70250
OAKLAND
CA
94612-0250
US
|
Assignee: |
THE BOARD OF TRUSTEES OF THE LELAND
STANFORD JUNIOR UNIVERSITY
Palo Alto
CA
|
Family ID: |
39150573 |
Appl. No.: |
11/469617 |
Filed: |
September 1, 2006 |
Current U.S.
Class: |
324/307 |
Current CPC
Class: |
G01R 33/56509 20130101;
G01R 33/5611 20130101; G01R 33/5676 20130101 |
Class at
Publication: |
324/307 |
International
Class: |
G01V 3/00 20060101
G01V003/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0001] The U.S. government has rights in the claimed invention
pursuant to NIH Grant Nos. NIH-1R01EB002771, NIH-1R01NS35959,
NIH-NCRR-P41-RR09784, 1R01NS047607 and 5R01NS34866 to Stanford
University.
Claims
1. A method of correcting for motion in magnetic resonance images
of an object detected by a plurality of signal receiver coils
comprising the steps of: a) acquiring a plurality of image signals
with the plurality of receiver coils, b) determining motion between
sequential image signals relative to a reference, c) applying
rotation and translation to image signals to align image signals
with the reference, d) determining altered coil sensitivities due
to object movement during image signal acquisition, and e)
employing parallel imaging reconstruction of the rotated and
translated image signals using the altered coil sensitivities.
2. The method of claim 1 wherein in step c) the rotation and
translation to image signals is performed iteratively in image
space and in k-space.
3. The method of claim 2 wherein in step a) image signals are
acquired using a spiral in/spiral out pulse sequence.
4. The method of claim 3 wherein in step a) the spiral in sequence
provides a navigator signal for a k-space acquired signal in the
spiral out sequence.
5. The method of claim 4 wherein step e) employs an iterative image
reconstruction algorithm using parallel processing.
6. The method of claim 5 wherein the image reconstruction algorithm
is iterative SENSE algorithm.
Description
BACKGROUND OF THE INVENTION
[0002] This invention relates generally to magnetic resonance
imaging (MRI), and more particularly the invention relates to the
correction of magnetic resonance imaging for motion of the imaged
body.
[0003] Involuntary patient motion is still one of the greatest
challenges in MRI. Due to the sequential nature of the MR
acquisition process, a significant amount of time can elapse
between different samples in k-space, leaving many MRI sequences
vulnerable to patient motion. The resulting increase in
misregistration can severely impair the diagnostic quality of an MR
examination. The medical condition of a patient, such as tremor,
pain, or mental status, often prevents even willing patients from
holding still. Past attempts at addressing this problem through
sequence approaches, such as snapshot imaging or gated acquisition,
have had limited success. Therefore, efficient motion correction
strategies are timely and have the potential for great impact as
MRI becomes more common in geriatric and pediatric patient
populations.
SUMMARY OF THE INVENTION
[0004] The invention is a novel apparatus and method that can
acquire motion-corrected 2D and 3D Magnetic Resonance Images (MRI).
This invention contributes to improved scan robustness in restless
patients or during long MRI scans where motion cannot be avoided
(e.g. volumetric 3D MRI & PC-MRAs, fMRI, DTI). Specifically,
the method performs a retrospective motion correction by
capitalizing on new motion detection techniques and augmented
parallel imaging methodology.
[0005] Prior art has described the effect of motion in k-space, but
not in the context of multi-coil data and its effect on the final
image. Moreover, parallel imaging has been used to correct for
corrupted k-space samples but thus far it was not realized how 1)
missing data can be synthesized by iterative reconstructions, and
how 2) coil sensitivity changes have to be considered in the
reconstruction. Prospective motion correction has been introduced
in prior art but based on MRI measurements; no motion capture
devices were used. The invention allows one to significantly
diminish k-space fragmentation by means of parallel imaging
reconstruction and, hence, to reduce motion artifacts in the final
image. A comprehensive formalism is provided that characterizes
k-space information relative to receiver coil sensitivities in the
presence of motion.
[0006] Specifically, this invention can be separated into three
parts: First, the determination of motion between subsequent
k-space profiles/interleaves relative to a reference point, for
example, using navigator images. Second, the application of counter
rotation/translation to align each profile/interleave with the
reference point. Third, the performance of an augmented parallel
imaging reconstruction to avoid local undersampling and variable
sampling density as well as to address coil sensitivity alterations
induced by motion within the coil sensitivity field.
[0007] 1.) Prior art in retrospective motion correction often
suffers from partially undersampled k-space after being corrected
for motion. Thus, the resulting images are often frustrated by
ghosting artifacts and signal loss. 2.) Prospective motion
correction based on MRI navigator images/volumes suffer from time
delays since motion parameters need to be extracted from at least
two images/volumes and can only be applied to the third acquisition
after considerable time lag and after completion of k-space
acquisition of the entire slice/volume to maintain consistency. The
present method has very little time lag and can be applied almost
in real time. 3.) The residual k-space inconsistencies due to
residual motion and data acquisition under variable coil
sensitivity (due to motion) can be corrected for by the
aforementioned parallel imaging approach.
[0008] For the first part of the invention, motion can be detected
by intrinsic navigator images (e.g. low resolution spiral).
Specifically, the degree of misalignment relative to a reference
point (e.g. reference image) can be detected by co-registering each
navigator image to the reference image. Alternatively, motion can
be detected via external fiducials (mounted on the subject) and
motion capture devices, for example, optical systems.
[0009] For the second point, translational and rotational motion is
reflected in k-space by linear phase rolls and rotations,
respectively. Hence, corresponding interleaves need to be
counter-rotated and phased appropriately. This can be done best
during gridding by rotating the input gridding trajectory and phase
modulating the acquired k-space data.
[0010] For the third part, an image reconstruction algorithm is
used that considers the altered coil sensitivity to which an object
is exposed to if it moves during MRI image acquisition. Note that
the effect of altered coil sensitivity needs to be considered
regardless of using either prospective or retrospective motion
correction techniques or whether or not parallel imaging based scan
acceleration is used. For the case of retrospective motion
correction, the k-space might appear fragmented and partially
under- or oversampled. Here, both augmented parallel imaging
reconstruction (e.g. iterative SENSE reconstruction) and sampling
density correction algorithms will be applied to diminish these
artifacts.
[0011] The invention and object and features thereof will be more
readily apparent from the following detailed description and
appended claims when taken with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 illustrates a spiral in and spiral out pulse sequence
showing one interleaf of image acquisition and a low resolution
navigator image for the interleaf.
[0013] FIG. 2 illustrates interleaved spirals (a) and interleaved
EPI (b) showing rotation effects.
[0014] FIG. 3 is a flow diagram of the algorithm for producing
motion corrected images in accordance with an embodiment of the
invention.
[0015] FIG. 4 illustrates spin echo interleaved spiral pulse
sequence and k-space trajectories used to obtain navigator
images.
[0016] FIG. 5 illustrates assessment of the effect of object
rotation during an interleaved spiral data acquisition (32
interleaves) simulated in a quality phantom.
[0017] FIG. 6 illustrates assessment of the effect of object
rotation during an interleaved EPI acquisition (32interleaves)
simulated in a quality phantom.
[0018] FIG. 7 illustrates in vivo experiment conducted with a fully
sampled low-resolution (32.times.32) single-shot spiral-in
navigator preceding each conventional spiral interleave.
[0019] FIG. 8 illustrates another in vivo experiment conducted with
a fully sampled low-resolution (32.times.32) single-shot spiral-in
navigator preceding each conventional spiral interleave.
[0020] FIG. 9 illustrates ten out of thirty-two navigator images
obtained from consecutive acquisitions (32 interleaves).
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0021] As noted above, the application of conventional motion
correction strategies often produces undersampled k-space data. As
a result, significant residual artifacts can persist. The present
invention introduces a variant of parallel imaging that reduces the
net k-space undersampling and improves overall image quality. This
correction scheme employs an iterative image reconstruction to
synthesize missing data in k-space and exploits the complementary
image encoding capacity provided by parallel imaging.
[0022] In this implementation, motion is determined from low
resolution navigator images that are co-registered by an automatic
registration routine. Navigator data can be derived from
self-navigating k-space trajectories or in combination with other
navigation schemes that estimate patient motion.
[0023] In applying motion correction in accordance with the
invention, several assumptions are made including only translation
and rotation of rigid body motion is present and negligible motions
occurs during data acquisition.
[0024] FIG. 1 illustrates a spiral in/spiral out pulse sequence
which can be used to determine the extent of motion from an
appropriate set of navigator images. A modified SENSE
reconstruction procedure uses the navigator data to find motion
parameters and eliminates the effects of undersampling k-space.
[0025] Due to the gradient system limitations, a spiral trajectory
mostly starts off in slew rate limited region and switches to
amplitude limited region after a certain time which is determined
by the scan parameters. In the case of the spiral in & out
trajectory used for this study, a spiral in trajectory is used to
get a fully sampled low resolution image for each interleave, and
the spiral out part constitutes one of the interleaves of the final
high resolution image (FIG. 1). One advantage of this pulse
sequence is that the spiral in portion makes use of the dead time
after the 180.degree. degree pulse up to the echo time TE and this
introduces no penalty for scan time in case of T2 weighting. The
matrix size of the low resolution navigator data can be adjusted
interactively before scan by the operator.
[0026] The way in which motion affects the final image depends upon
the magnitude and direction of the motion as well as its temporal
relation to the acquisition of k-space. Overall, the net effect of
patient motion during MR imaging is k-space data inconsistency,
which produces artifacts in the final image. If one can identify
corrupted k-space data and has knowledge of the degree of motion,
it is possible to correct, or at least to reduce, image artifacts
by i) rejecting corrupted k-space data; ii) reacquiring corrupted
k-space data; iii) applying retrospective correction schemes that
undo the effects of motion; or iv) determining motion on-the-fly
and prospectively adjusting the acquisition for the new location of
interest while scanning.
[0027] The simplest form of motion is rigid-body translation
without elastic deformation. In k-space, translational motion is
manifested by a linear phase shift along the direction of motion.
Rotational motion produces a slightly more complicated artifact
pattern. For a given object, a rotation in image space produces the
same rotation in k-space. Therefore, during sequential scanning,
the expected k-space information at a particular position will have
been rotated away and will be partially missing or replaced by data
that have already been acquired. Consequently, these object
rotations can be the source of significant image artifacts.
[0028] In order to correct for rotational motion artifacts, one
needs to synchronize the trajectory and the acquired k-space data
with the rotated object. Specifically, the misaligned k-space
lines, or interleaves, can be counter-rotated relative to a
reference point. However, the rotation procedure usually puts the
corrected k-space data at locations that no longer coincide with an
equidistant Cartesian grid, resulting in local under-sampling and
ill-conditioning of the reconstruction problem (FIG. 2). The
effects of rotation are corrected by counter-rotating the
individual interleaves. The effect of this counter-rotation on the
k-space trajectories is shown for both interleaved spirals (a) and
interleaved EPI (b). It is clearly apparent that the
counter-rotation causes a variable sampling density in various
areas of k-space. Some regions become oversampled compared to the
Nyquist sampling rate whereas other regions become only sparsely
sampled. For the EPI, it is also worthwhile noticing that the
sampled trajectory data are no longer on an equidistant grid.
Therefore, interpolation methods for image reconstruction that
address the ill-conditioning have been proposed. A novel motion
correction approach is presented that avoids some of the
aforementioned problems and improves the conditioning of the
reconstruction by first putting the motion problem into a matrix
formalism and then using a parallel-imaging-based synthesis of the
missing k-space information.
[0029] Parallel imaging was introduced several years ago as a
complement to conventional gradient encoding. Since then, it has
been employed in a broad spectrum of applications ranging from
rapid imaging to the reduction of image artifacts. The
inhomogeneous RF profiles of individual receiver coils can be used
to synthesize missing information in k-space or to remove aliasing.
The method proposed here retrospectively corrects rigid body
rotation and translation as well as compensates for the resulting
variable density sampling pattern. Specifically, the method
utilizes an augmented iterative parallel imaging reconstruction
approach to correct for motion regardless of the type of trajectory
being used. The proposed correction method was tested with a new
variant of spiral navigators that can be incorporated in various
pulse sequences.
[0030] The invention retrospectively corrects rigid body rotation
and translation and compensates for the resulting variable density
sampling pattern. Specifically, the method utilizes an augmented
iterative parallel imaging reconstruction approach to correct for
motion regardless of the type of trajectory being used. This is
illustrated in the flow diagram of FIG. 3 of the motion correction
algorithm.
[0031] In the flow diagram, the low resolution navigator images
from the pulse sequence are obtained for each k-space interleaf.
Registration of the navigators using Simplex Algorithm determines
motion parameters. Optimization routines other than Simplex can be
used as well.
[0032] Translational motion is corrected by applying a linear phase
to k-space data, and rotational motion is corrected by
counter-rotating k-space trajectories.
[0033] Complex coil sensitivities are determined, and
counter-rotation and counter-translation of coil sensitivities are
made according to the motion parameters.
[0034] The corrected k-space interleaves are then processed in
parallel using a generalized SENSE (GSENSE) reconstruction. The
final image results from the parallel imaging reconstruction for
arbitrary k-space data that considers coil sensitivity variations
during data acquired along with motion corrected k-space image
data.
[0035] Consider now the theory for augmented GSENSE reconstruction
for motion correction. For a better understanding of how motion
affects k-space and, thus, the reconstructed image, we will first
describe the formation of an MRI data sample at a particular point
in time, t.sub.i(i=1 . . . .kappa.), using discrete sampling in the
time and spatial domain, r.sub.u (u=1 . . . N.sup.2):
m ( t i ) = u = 1 N .times. N .rho. ( r u ) exp ( - j k ( t i ) r u
) . [ 1 ] ##EQU00001##
Therefore, the entire acquisition of .kappa. k-space points can be
stored in a measurement vector, m=[m(t.sub.1) m(t.sub.2) . . .
m(t.sub..kappa.)].sup.T, so that the signal formation in matrix
form can be expressed as:
( m ( t 1 ) m ( t 2 ) m ( t .kappa. ) ) = ( - j k 1 r 1 - j k 1 r 2
- j k 1 r N 2 - j k 2 r 1 - j k 2 r 2 - j k 2 r N 2 - j k .kappa. r
1 - j k .kappa. r 2 - j k .kappa. r N 2 ) ( .rho. ( r 1 ) .rho. ( r
2 ) .rho. ( r N 2 ) ) = m = Ev , [ 2 ] ##EQU00002##
where, v=[.rho.(r.sub.1) .rho.(r.sub.2) . . .
.rho.(r.sub.N.sub.2)].sup.T is the spatial signal distribution in
vector form; k.sub.i=[k.sub.x(t.sub.i) k.sub.y(t.sub.i)
k.sub.z(t.sub.i)].sup.T|.sub.i=1 . . . .kappa. is an arbitrary
k-space sampling trajectory; and r.sub.u=[r.sub.x,u r.sub.y,u
r.sub.z,u].sup.T|.sub.u=1 . . . N.times.N is the position vector.
The solution vector v for the image signal distribution can then be
obtained by computing the Moore-Penrose inverse of E,
E.sup.+m=v=(E.sup.HE).sup.-1E.sup.Hm. [3]
The encoding matrix E can be replaced by the matrix product GF so
that Eq 2 becomes
[0036] m=GFv. [4]
Here, instead of populating the matrix E with N.sup.2.times..kappa.
complex exponentials and performing a direct computation of Eq. 2,
typically an FFT F (C.sup.N.sup.2.sup..times.N.sup.2) is performed
followed by a resampling procedure of the Cartesian k-space data
along the arbitrary k-space waveform using the matrix G (i.e., the
inverse gridding matrix). For optimal sinc interpolation, one
element in the inverse gridding matrix becomes G.sub.t,i=sinc
({tilde over (k)}.sub.t-k.sub.i), with {tilde over (k)}.sub.t and
k.sub.i representing the k-space grid locations; these positions,
k.sub.i, are defined by the arbitrary sampling trajectory. To boost
computation efficiency, more compact resampling kernels (eg.
Kaiser-Bessel window) are normally used instead of sinc
interpolation. The formalism described in Eq. 4 is also know as the
inverse gridding operation. From a Cartesian image, inverse
gridding is able to generate k-space sample points along an
arbitrary trajectory defined by k.sub.i; therefore, it represents
the inverse operation to conventional gridding reconstruction.
[0037] Most frequently, MR data that are sampled along
non-Cartesian k-space trajectories are reconstructed using gridding
reconstruction; however, Eq. 3 can also be used to form an image
from these arbitrarily sampled k-space data. Because of
computational complexity, the direct inversion of Eq. 3 to find v
is prohibitively inefficient; therefore, a least squares solution
for v,
v = arg min v ' ( E H E ) v ' - E H m 2 , [ 5 ] ##EQU00003##
is normally found in an iterative fashion by means of the conjugate
gradient method. This approach has recently gained popularity since
it provides the foundation for iterative SENSE reconstruction as
well as for non-linear phase correction in DWI.
[0038] In the next step, we demonstrate how motion impacts the
acquired k-space data. Specifically, we assume that motion occurs
between interleaves but not during interleaves. This assumption is
legitimate for all MR applications for which the positional changes
during one interleave are negligible. With FF.sup.H=I, where I is
the identity matrix, let .OMEGA. be a warping matrix that operates
on an unperturbed image v.sub.0 that yields the image v and let
.LAMBDA. be the transformation rule in k-space that corresponds to
.OMEGA. in the image domain so that
v=.OMEGA.v.sub.0=F.sup.II.LAMBDA.Fv.sub.0. [6]
Consequently, in the presence of motion, the signal equation for
the acquired MR data changes to
[0039] m=Ev=GFv=GF.OMEGA.v.sub.0=GFF.sup.H.LAMBDA.F
v.sub.0=G.LAMBDA.F v.sub.0=E'v.sub.0. [7]
Equation 7 clearly demonstrates that the solution of Eq. 4 yields a
solution image v that is corrupted by motion. In order to obtain
the true image v.sub.0, the spatial warping .OMEGA. needs to be
included in the encoding matrix E. An augmented version of Eq. 3
that considers spatial transformation is therefore
(F.sup.II.LAMBDA..sup.IIG.sup.II)D
m=(F.sup.II.LAMBDA..sup.IIG.sup.II)D(G.LAMBDA.F)v.sub.0, [8]
in which the term in the parenthesis on the left-hand side of Eq. 8
reflects the gridding of the acquired data, correction for motion,
and inverse FFT. Equation 8 can be solved for v.sub.0 following the
same scheme as outlined in Eqs. 3 and 5. Here, D
(D.sub.(.gamma.,.kappa.),(.gamma.y,.kappa.)=k_density(k.sub.i).sup.-1)
has been introduced in Eq. 8 as a sampling density correction
matrix exactly like in Pruessmann's original approach. Also, the
rotation of k-space data will affect the sampling density in
k-space. Therefore, in this invention the true sampling density was
determined by a Voronoi tessellation approach after motion
correction was applied to the trajectories. Note, however, the
accurate knowledge of the sampling density distribution is not so
critical for this estimation problem since the iterative conjugate
gradient approach works even without prior knowledge of the
sampling density information. Nevertheless, the exclusion of the
density matrix causes much slower convergence rates because the
density correction acts as a pre-conditioner. Additional
preconditioning can be achieved by adding intensity correction.
[0040] Despite density correction, severe motion can lead to
inconsistencies of k-space data, that is, some areas in k-space are
less densely sampled than required by the Nyquist sampling
criterion. In other words, depending on the severity of motion, the
linear dependency in Eq. 8 increases and, therefore, the
conditioning of the system matrix E' decreases, yielding
considerable reconstruction artifacts. We hypothesize that parallel
imaging helps to ameliorate the conditioning of this matrix system
by adding additional encoding information using spatially varying
coil sensitivities. Following Pruessmann's seminal work, the
k-space data for nc coils can be written as:
m'=[m.sub.1m.sub.2 . . . m=.sub.nc].sup.TE'v, [9]
where E'.sub.(i,.gamma.)u=c.sup..gamma.
(r.sub.u)exp(-jk.sub.ir.sub.u), c.sup..gamma. (r.sub.u) is the
complex coil sensitivity of the .gamma..sup.th coil at position
r.sub.u, and m.sub.1 . . . m.sub.nc are the k-space sampling data
vectors for each of the nc coils. For each coil, one can combine E
with c.sup..gamma. on a per pixel basis using
E-diag(c.sup..gamma.), and v can be estimated based on the
aforementioned iterative conjugate gradient method. For reasons of
increased processing speed and without loss of generality, E is
again replaced by GF. Instead of the conventional gridding and
inverse gridding, we used in our implementation the convolution
approach introduced by Wajer et al. Alternatively, the FFTW
(Fastest Fourier Transform in the West) algorithm could be used for
non-power-of-two matrices and non-integer over-sampling factors to
speed up the gridding and inverse gridding steps during the
conjugate gradient iteration.
[0041] Assuming that there is an inverse warping operation (i.e.,
there exists an matrix .OMEGA.inv so that .OMEGA. .OMEGA.inv
S.sub.0=S.sub.0), our motion correction approach in Eq. 7 can be
expanded to incorporated the coil sensitivity information by
rewritingjthe signal equaiton of the .gamma..sup.th coil as
m.sub..gamma.=G F diag (c.sup..gamma.) v=G F diag (c.sup..gamma.)
.OMEGA. v.sub.0=G F .OMEGA. diag (.OMEGA..sub.invc.sup..gamma.)
v.sub.0 m.sub..gamma.=G .LAMBDA.F diag
(.OMEGA..sub.invc.sup..gamma.) v.sub.0={tilde over
(E)}.sup..gamma.v.sub.0, [10]
so that when all coils are combined, Eq. 10 can be expanded to
m'=diag ({tilde over (E)}.sup..gamma.) v.sub.0. [11]
In this context, it is important to understand that in the case of
patient motion the conventional frame of reference defined by the
imaging gradients and the patient frame of reference are not
necessarily aligned. Here, the matrices G and F and the vector
c.sup..gamma. are all defined in the gradient frame of reference,
whereas the final image v.sub.0 is given in the patient frame of
reference. Specifically, the matrix .OMEGA. transforms voxels from
the patient frame into the gradient frame of reference. Notice that
from the perspective of the patient frame of reference for each
shot, the coil sensitivities do not remain static but, instead,
"move around the object". To transfer the "coil motion" into the
patient frame, the warping .OMEGA..sub.inv c.sup..gamma. is used.
Because we are assuming that the coil elements are static (we are
using a rigid 8-channel head array coil) for the rest of this
project, it is unnecessary to show that any true coil motion in the
gradient frame of reference (theoretically, even without subject
motion) can be modeled by including the motion pattern into the
warping matrix .OMEGA..sub.inv. For example, coil motion relative
to the patient might occur in cardiac or abdominal imaging during
respiratory excursions. However, for self-calibrated data, such as
variable-density spirals (FIG. 4a) or a preceding spiral-IN
navigator (FIG. 4b), from which the coil sensitivity information
can be extracted for each shot, the spatial warping of coil
sensitivities is not required since these sensitivity maps are
inherently registered to the trajectories to form the desired
image. Here are spin echo interleaved spiral pulse sequence and
k-space trajectories used to obtain navigator images. A
variable-density spiral design (a-b) can be designed to cover
enough information around the center of k-space, so that a low
resolution navigator image can be produced. Alternatively, a short
(.about.3-5 ms) single shot spiral-IN trajectory can be used during
the initial spin echo formation followed by a conventional
interleaved spiral trajectory to form the desired image.
[0042] In this invention, we assumed rigid body motion only, which
can be modeled by translations and rotations. Specifically,
rotations can be applied most efficiently during gridding by
applying the appropriate rotation matrix to the initially
prescribed k-space trajectory vector, k'=R.phi.,.sub..phi.,.xi.,n
k, for each profile/interleave, n, where
R .phi. , .PHI. , .xi. , n = ( 1 0 0 0 cos .phi. n sin .phi. n 0 -
sin .phi. n cos .phi. n ) ( cos .PHI. n 0 - sin .PHI. n 0 1 0 - sin
.PHI. n 0 cos .PHI. n ) ( cos .xi. n sin .xi. n 0 - sin .xi. n cos
.xi. n 0 0 0 1 ) [ 12 ] ##EQU00004##
is a 3D rotation around the x, y, and z axes using the angles
.phi., .phi., and .xi., respectively. The direct rotation of the
trajectory avoids interpolation errors that would otherwise occur
using image (in our case k-space) rotation functions. Also,
subvoxel translational motion, .delta.x=(.delta.x .delta.y .delta.z
).sup.T, can be achieved most efficiently by adding corresponding
phase ramps in k-space during gridding by phasing the sampled
k-space data:
m Trans ( i ) = m ( i ) j 2 .pi. ( k x ( i ) k x , max - k x , min
.delta. x FOV x + k y ( i ) k y , max - k y , min .delta. y FOV y +
k z ( i ) k z , max - k z , min .delta. z FOV z ) . [ 13 ]
##EQU00005##
Theoretically, the order in which rotational and translational
motion are applied is irrelevant as long as it follows the order in
which these parameters are acquired during the registration process
(see next section). For implementation reasons in this work, the
rotation was applied first and then followed by the
translation.
Determination of Translational and Rotational Motion Components
[0043] The extent of motion can be determined from an appropriate
set of navigator images. These navigator images can either be
generated by self-navigating trajectories (e.g. variable-density
spiral waveforms and PROPELLER blade) (FIG. 4a-b) or from a
separate acquisition (FIG. 4c-d). Here are Spin echo interleaved
spiral pulse sequence and k-space trajectories used to obtain
navigator images. A variable-density spiral design (a-b) can be
designed to cover enough information around the center of k-space,
so that a low resolution navigator image can be produced.
Alternatively, a short (.about.3-5 ms) single shot spiral-IN
trajectory can be used during the initial spin echo formation
followed by a conventional interleaved spiral trajectory to form
the desired image. After obtaining a template by averaging all
navigator images, all other images will be co-registered to this
reference image by adjusting the amount of rotation and translation
to provide the highest degree of similarity between template image
and the image to be registered. This process is iterated and for
each iteration, a new template is constructed from the
counter-rotated and counter-translated navigator images. In this
invention, a multi-grid registration approach is used to find the
global optimum of the cost function by automatically traversing
parameter space (rotation, translation) using the Nelder-Mead
algorithm (a.k.a. simplex algorithm). During each iteration step,
the source image underwent a rotation and translation.
Specifically, the Pearson correlation coefficient
R = i = 1 N .times. N A i B i - 1 N 2 i = 1 N .times. N A i i = 1 N
.times. N B i ( i = 1 N .times. N A i 2 - 1 N 2 ( i = 1 N .times. N
A i ) 2 ) ( i = 1 N .times. N B i 2 - 1 N 2 ( i = 1 N .times. N B i
) 2 ) [ 14 ] ##EQU00006##
is computed from the resulting magnitude image, A, and the target
magnitude image, B, over all pixels, i=1 . . . N.times.N, to serve
as a similarity measure. Here, the multi-grid approach reduces the
likelihood of the optimizer being trapped in a local optimum. A
nice feature of many registration approaches, including the one
used in this invention, is the ability to co-register images in the
subpixel range. Thus, the detection of motion much smaller than the
size of the navigator voxels is possible.
Computer Simulation
[0044] All simulations and post-processing steps were performed on
a Laptop PC (IBM T43p, Pentium M, 2.13 GHz, 1 GB RAM) running IDL
6.3 (RSI, Boulder, Colo.). The synthetic scan data for an
interleaved spiral and EPI acquisition (32 interleaves) were
generated by using inverse gridding operations on MR images of a
quality phantom (FOV=22 cm). First, to simulate rigid body motion,
the same number of synthetic images as interleaves (i.e., n=32)
were generated, differing only in random object rotation (range
.+-.30.degree.) and translation (range .+-.15 mm). These were
generated by using IDL's built-in image rotation function rot and a
Fourier-domain based continuous image shift operation. Here, the
IDL rot function uses a bicubic convolution method to resample the
rotated voxel on the Cartesian grid. Second, each of these n images
was then multiplied by eight simulated receiver coil sensitivities
that were assumed to be located around the circumference of the
object and did not move. The coil sensitivity information was
generated using the Bio-Savart law. These 256 images were then
zero-padded by a factor of two and underwent 2D-FFT. Thereafter,
artificial k-space data were generated by inverse gridding
operations to create interleaved spiral and EPI k-space trajectory
data. On a per coil basis, one interleaf was taken from each of the
n individually distorted images to create a single,
motion-corrupted k-space data set that was used for testing the
proposed correction approach.
[0045] To assess the degree of effective image artifacts in an
uncorrected scan, the aforementioned motion-corrupted data set was
gridded to Cartesian k-space, using the k-space trajectories as
they were initially prescribed. Thereafter, the correction for
rotation and translation was applied to the simulated,
motion-corrupted k-space data and reconstructed using either
conventional gridding reconstruction or our new parallel imaging
based approach. In order to avoid any bias from inaccuracies of the
navigator image co-registration, the rotation and translation
parameters that were used to generate the corrupted data were
inputted in our motion correction scheme. As outlined in the
previous section, inconsistencies in k-space resulting from the
rotation and translation correction were removed by means of
augmented iterative SENSE reconstruction. The normalized absolute
squared difference between the original quality phantom image and
either the conventionally gridded, motion-corrupted image or the
SENSE reconstructed image was computed and served as a surrogate
measure to determine the efficacy of the accomplished artifact
correction. In addition, other commonly used metrics, such as
normalized mutual information and normalized entropy, were used to
assess the agreement between images. In contrast to absolute
squared differences or normalized entropy, which decreases with
increasing co-registration between images, both image correlation
and normalized mutual information increases with increasing
co-registration between images.
[0046] The registration precision is very important for the
efficacy of our correction method. It determines how much the
rotation and translation parameters that have been measured will
fluctuate and, thus, by how much positional "jitter" the k-space
trajectories (and sensitivity maps) are burdened. The latter causes
ghosting artifacts and adds unwanted blurring to the final image.
Registration accuracy, on the other hand, leads to an overall
mismatch between the registered, motion-corrected image and the
true position, but will not cause any of the aforementioned image
artifacts caused by the jitter. To estimate the dependence of
registration accuracy and precision as a function of navigator
resolution (16.sup.2, 24.sup.2, 32.sup.2, 48.sup.2, and 64.sup.2),
spiral-IN navigator images (for details see next section) were
rotated as well as translated along the x and y dimensions in small
increments. A total number of 200 equidistantly spaced increments
were picked spanning a range of .+-.15.degree. rotation and .+-.10
mm for translation over a FOV of 240 mm. The original navigator
image, free of any rotation or translation, served as a reference
image. Rice-Nakagami-distributed noise was added to all images, so
that the navigator SNR was 5, 10, or 15. Thereafter, each
translated and rotated image was co-registered independently to the
reference image using the registration method outlined in the
Theory section. The observed motion parameters were compared to the
true motion applied by linear regression. Here, the correlation
coefficient and the standard deviation of the observed rotation and
translation from the regression line served as surrogates for
accuracy and precision of our registration method.
MR Experiments
[0047] Axial 2D spin echo scans in two volunteers were acquired on
a 1.5 T whole-body system (Signa CVi, GE, Milwaukee, Wis.) with a
high performance gradient set (maximum gradient strength of 50
mT/m, maximum slew rate of 150 mT/m/s), using a new, self-navigated
spiral imaging sequence. MR signal reception was accomplished using
a rigid 8-channel head array coil (MRI Devices, Milwaukee, Wis.),
whereas signal excitation was performed with the integrated body
coil. All human studies performed were approved by the review board
of our institution. Informed written consent was obtained from each
participating subject after the nature of the study was fully
explained and understood.
[0048] The scan parameters for the spin echo spiral scans were as
follows: TR/TE=4,000 ms/56 ms or 85 ms, slice thickness/gap=5/0 mm,
12 slices, FOV=24 cm, matrix=256, interleaves=32, NEX=1, and a
receiver bandwidth of .+-.125 kHz. The self-navigated spiral
sequence uses a low-resolution, fully sampled spiral-IN readout (24
cm FOV) prior to the formation of the spin echo (FIG. 3c-d) to
generate a navigator image. This was followed by a standard
interleaved spiral-OUT acquisition for high resolution imaging.
Using the same acquisition parameter, navigated gradient echo
spiral images were also acquired with the same sequence by turning
of the refocusing RF pulse and using a TR/TE of 4,000 ms/12 ms and
90 ms. All spiral trajectories were initially slew-rate limited and
then gradient limited. The spiral-IN navigator was a 3.5 ms
single-shot spiral that covered an inner k-space radius of 32
points sampled above the Nyquist rate. The first interleaf of the
acquisition had a 1 ms longer echo time than the remaining
acquisitions. Combining this first navigator image with all
subsequent matching navigator images allowed the generation of a
field map to demodulate data during the gridding process.
Synchronization delays between gradient waveform output and data
acquisition that could lead to rotations between the spiral-IN and
spiral-OUT image were measured and adjusted if necessary at the
beginning of each study.
[0049] During the experiment, the volunteers were either asked to
not move at all or to rotate their heads at two levels of motion: a
rotation of .+-.10.degree., which was deemed mild motion, or a
rotation of .+-.20.degree., which was deemed moderate motion. The
relatively large motion was chosen to reflect the range of motion
we see in uncooperative patients at our institution, such as stroke
patients or unsedated pediatric patients. Thereafter, further
experiments with smaller motion .+-.5.degree. and .+-.5 mm were
also conducted.
[0050] After raw data acquisition, the navigator images were first
reconstructed and corrected for receiver coil inhomogeneities using
the coil sensitivities of the individual receiver coil elements
(using the sum-of-squares image as a reference). The coil
sensitivity information was directly retrieved from the navigator
images and was then filtered and extrapolated as described
elsewhere. Thereafter, the amount of motion relative to the
time-average of all navigator images was determined by
co-registration as outlined in the Theory section. An average of
all navigator images was used because it provides better SNR and
greater registration robustness than a single navigator image; the
use of an average template has already demonstrated great utility
for the correction of eddy current distortions in DTI. After all
images were registered to the average image, a new average image
was computed and the registration of the original data to the new
template was repeated. Here, the convergence of the algorithm was
obvious by the diminishing blur seen in the average image.
Typically, no more than three iterations are required until the
blurring is no longer noticeable. Ultimately, the acquired k-space
data were corrected for translational and rotational motion and
underwent the SENSE reconstruction as outlined previously.
[0051] Computer Simulation--FIG. 5 shows the effects of rotational
and translational motion during an 32-interleave spiral
acquisition. Here, assessment of the effect of object rotation
during an interleaved spiral data acquisition (32 interleaves)
simulated in a quality phantom. Eight receiver coils are
distributed equally around the circumference of the phantom. For
each interleave, a random object rotation within the range of
.+-.30.degree. and a random object translation within the range of
.+-.15 mm was introduced with the following results: a) gridding
reconstruction of a quality phantom without rotation; b) gridded
k-space data; and c) spiral sampling trajectory. Note that for
better visualization of the sampling trajectory only the region
.+-.kmax/4 is plotted. If the acquired data are gridded according
to the prescribed trajectory (c and f), inconsistencies in k-space
(e) cause severe artifacts in the reconstructed image (d). Some of
these distortions can be reduced (g) if one uses the object
rotation to counter-rotate the k-space acquisition trajectory for
that particular interleaf (i) and uses these corrected orientations
for gridding (h). Correction for altered sensitivity and an
iterative SENSE reconstruction (10 iterations) can remove most of
the residual k-space sampling errors (k) and provides an image
almost free of artifacts (j). If the data are gridded along the
desired spiral waveform without correction prior to gridding,
significant artifacts become apparent in the image (FIG. 5d). After
correcting for rotational and translational motion, the image
quality could be improved (FIG. 5g), but considerable variations in
k-space sampling density and regional under-sampling (FIGS. 5h and
i) still led to noticeable artifacts. After 10 iterations using the
augmented SENSE algorithm, the artifacts were barely noticeable in
the final reconstruction (FIG. 5j). The efficacy of this method for
removing k-space inconsistencies can also be seen in FIG. 5 of the
final k-space, which has a more homogeneous appearance overall.
FIG. 6 provides similar evidence for improved image quality after
the application of the correction scheme in a simulated interleaved
EPI acquisition. However, it is apparent from both the k-space data
and the final SENSE reconstruction that interleaved spirals (FIG.
5) have slightly less artifacts than interleaved EPI scans (FIG.
6). This originates from the sparser coverage of the outer portion
of interleaved EPI k-space after rotation correction and the
inherent rotational symmetry of spiral trajectories. The degree of
mismatch between the original image and the motion corrupted data
with and without correction was quantified by three different
similarity measures and is summarized in Table 1. The similarity
measures reported in Table 1 confirm the visual impression of
significant artifact reduction after the application of the
parallel imaging-based motion correction algorithm; in the presence
of motion, the image obtained from conventional gridding
reconstruction has the least similarity with the reference image.
Gridding with motion correction and, even more so, iterative SENSE
with motion correction increases the similarity between the image
obtained under motion and the reference image.
[0052] Table 2 summarizes the results of our experiment assessing
the influence of navigator resolution on the accuracy and precision
of our registration procedure. Even with the smallest navigator
resolution, the simulated motions were captured very well, which is
also reflected by the high correlation coefficient, R. The
fluctuation of the positional changes that were determined by the
registration procedure and that are reflected by the standard
deviation, .sigma., of the differences between the detected
rotations and translations and the corresponding regression line,
was clearly in the submillimeter range. With increasing navigator
resolution and increasing SNR, this fluctuation diminished
further.
MRI Studies
[0053] FIG. 7 shows the new navigated interleaved spin echo
spiral-OUT images obtained in a volunteer who was asked either to
lie still or perform random mild or moderate head movements during
the acquisition of navigated interleaved spiral scans. Here, in
vivo experiment conducted with a fully sampled low-resolution
(32.times.32) single-shot spiral-IN navigator preceding each
conventional spiral interleave. a)-c) are motion corrected spin
echo T2w interleaved spiral-OUT scans with no motion (a); mild
motion (b); and moderate motion (c). d)-f) are the resulting images
reconstructed from the same data (i.e., no motion (a); mild motion
(b); and moderate motion (c)) without any motion correction
applied. Without correction, even mild motion (e) causes
significant artifacts that are mostly apparent in the frontal
brain. This is because the origin of the rotation axis is located
occipitally. (f) Increased image artifacts can be seen for moderate
motion. Without motion, there are no apparent differences between
the reconstruction performed with (a) and without (d) motion
correction. However, obvious artifact reduction can be achieved
with the parallel imaging based correction scheme in cases of both
mild (b) vs. (e) and moderate (c) vs. (f) motion. With moderate
motion (c) some residual artifacts are apparent due to the severity
of motion. The images in FIG. 7 demonstrate significant motion
artifacts when no motion correction was applied. In particular, the
image artifacts become increasingly worse and non-diagnostic with
increasing levels of motion. In comparison, the correction
algorithm was clearly able to reduce the artifact level and
diminish the motion-induced blurring. As required, the application
of navigator registration and subsequent motion correction to data
sets without apparent motion did not lead to any visual changes in
image quality or to significant quantitative differences that could
be captured by either one of the three similarity measures. For the
case of small motion, the quantification of image mismatch between
the static and the moving brain with and without correction is
summarized in Table 3.
[0054] FIG. 8 shows further examples of navigated gradient
echo-based interleaved spiral-OUT images from another volunteer.
Here, In vivo experiment conducted with a fully sampled
low-resolution (32.times.32) single-shot spiral-IN navigator
preceding each interleave of a gradient echo-based interleaved
spiral-OUT scan with a) TE=12 ms and b) TE=90 ms. Top left: the
reference sum-of-squares interleaved spiral image is acquired with
no subject motion. Top right: Significant artifacts are apparent if
the volunteer performs moderate head motion during data acquisition
and if no motion correction is applied. Bottom left: correction of
k-space data for translational and rotation motion results in
sharper object contours, but the image quality is still corrupted
by residual ghosting from local undersampling and sampling density
variations. Bottom right: improved image quality after the
application of the augmented SENSE reconstruction, especially for
the TE=90 ms case (FIG. 8b) the SENSE-reconstructed image shows
less artifacts than the conventionally reconstructed (i.e. gridding
reconstruction) image with supposedly no motion. Due to the
intentional head movements of the volunteer, considerable k-space
sampling density variations occurred, which led to significant
artifacts in the conventional image reconstruction. These artifacts
are predominantly visible around the frontal aspect of the brain
because lying in the coil positions the axis of rotation at the
back of the head. Equal windowing was chosen for all images to
better reveal reduction of ghost artifacts. The amount of head
rotation and translation was estimated from the navigator images.
Correcting the k-space data only for rotational and translational
effects clearly reduced image blurring, but the resulting k-space
fragmentation led to considerable aliasing artifacts. With the
application of the SENSE-based correction scheme, these aliasing
artifacts were suppressed quite well. In FIG. 7b, the motion
corrected image using the parallel imaging method appears to be of
better quality than the image with supposedly no motion. Despite
being asked to remain still, the volunteer moved slightly during
the course of the data acquisition. Therefore, this image is a good
example of how our method performs with a small amount of movement,
although we did not test this range of motion systematically.
[0055] Finally, FIG. 9 demonstrates an example of the efficacy of
the registration algorithm that provided us with the amount of
rotation and translation that had to be applied to each individual
interleaf. Here, Ten out of thirty-two navigator images obtained
from consecutive acquisitions (32 interleaves). Top row: navigator
images before co-registration. Second row: aligned navigator images
after registration of each image to an average image computed from
all 32 navigators. Third row: absolute squared difference images
between the individual navigator images before registration and the
reference image. Spatial misregistration is clearly apparent from
the hyper-intense rim at the edge of the brain. The intensity
scaling is increased 10 times compared to the two rows to better
highlight differences. Bottom row: absolute squared difference
images between the individual navigator images after registration
and the reference image clearly demonstrate a better alignment. The
scaling is identical with the images in the 3rd row. Any spatial
misregistration is typically reflected in the difference images
(i.e., navigator images or reference image) by a hyper-intense rim
around the edge of the brain or other structures, with rapid
contrast gradients. As expected, after the application of the
registration algorithm, this rim was not apparent in the difference
images. Occasionally, we found that certain image frames
demonstrated global hyper-intensities despite successful
co-registration. This is most likely due to through-plane motion.
Here, spins that are either partially saturated or fully relaxed
enter the slice and change the overall signal intensity within the
section. These spin history effects can also lead to baseline
intensity changes of a particular interleave and are, therefore,
potentially responsible for residual artifacts. This is a known
problem for 2D retrospective motion correction schemes and is
independent from the method proposed herein. In the current
implementation, no measures were taken to account for this
confounder; however, in the case of redundant sampling, it has been
suggested that data sets with significant spin history problems
should be rejected.
[0056] Patient motion can significantly reduce the diagnostic
quality of MR studies. Often the condition of the patient precludes
them from being cooperative or from remaining still for the
duration of the exam. This is especially true for longer
examination periods in which the likelihood for patients to become
uncomfortable and begin to move increases as the scan progresses.
This, in turn, can require repeated exams and lead to overly
prolonged studies. In the pediatric setting, often sedation or
anesthesia is needed to avoid motion artifacts, leading to
increased logistic problems and higher exam costs. It is therefore
of great relevance to reduce the degree of motion artifacts in MRI
scans. In the past several years, several attempts have been made
to minimize artifacts from patient motion. These correction
approaches have been carried out either prospectively or
retrospectively.
[0057] In this invention, a variant of parallel imaging
reconstruction for arbitrary k-space acquisitions has been proposed
that allows for efficient retrospective in-plane motion correction.
An excellent review of the potential of parallel imaging to correct
for motion has been presented recently by Larkman et al. The work
here has some resemblance to the work of Atkinson et al, Batchelor
et al, and Pipe et al. The novelty of this work is i) a framework
for motion correction that can be applied to arbitrarily sampled
k-space data; ii) the capacity to overcome local k-space
under-sampling and different sampling densities by improving the
conditioning of the inverse problem by means of additional spatial
encoding information from coil sensitivities; iii) a rapid
translation correction by the proper phasing of each k-space sample
point and rotation correction by applying a rotation matrix to each
k-space vector k(ti) that is part of the gridding step; and iv) the
extraction of motion information from either a navigator scheme
built into the trajectory itself or an additional navigator image.
This new method relies heavily on the pioneering work of Pruessmann
et al and Kannengiesser et al regarding the fast reconstruction of
under-sampled, non-Cartesian k-space data. Potential future
developments include finding alternative ways to reconstruct the
data by synthesizing missing k-space parts using information from
adjacent points in k-space, such as GRAPPA-based methods.
[0058] A comparative evaluation of image quality for
motion-corrupted data prior to and after correction for rigid body
motion was performed both in vivo and using computer simulations.
The rigid body motion correction proposed in this effort (i.e.,
correcting k-space data for rotation and translation) with
conventional gridding reconstruction was able to reduce artifacts
to some extent, although considerable residual artifacts were still
apparent due primarily to local violation of the Nyquist sampling
criterion in k-space. By virtue of an augmented iterative parallel
imaging reconstruction, the level of residual artifacts was
significantly reduced. The amount of motion simulated in this
embodiment was fairly large (.+-.30.degree. rotation and .+-.15 mm
translation), but was selected to cover the typical amounts of
motion encountered in clinical studies.
[0059] Exact knowledge of the rotation and translation components
is crucial for any motion correction scheme to be able to fully
correct image artifacts. Like many other correction schemes, the
efficacy of our method strongly depends on the ability of the
navigator image and the co-registration technique to pick up on
small amounts of motion in vivo. However, for the navigated
interleaved spiral trajectories that we have tested in vivo, the
small residual motion errors did not cause any significant residual
artifacts. If the residual motion error increases, mismatch will
first be noticeable as a slight blurring in the corrected image
and/or by "swirl artifacts." Conversely, the great advantage of low
resolution navigator images is that the co-registration is much
more robust, (i.e., the parameter search does not get so easily
trapped into the local optima of the cost function, and the smaller
image size allows very rapid registration). In assessing the
influence of navigator resolution to the registration quality, it
was found that the fluctuations resulting from the registration
were in the submillimeter range. A potential difficulty for the
registration step could be the variable coil sensitivity modulation
that affects the image intensity. These additional signal intensity
modulations might reduce the effectiveness of traditional
similarity measures, and this effect needs to be investigated
further. To minimize this confounder, the navigators were first
intensity corrected. In the future, similarity measures that are
known to be more insensitive to differences in image contrast, such
as (normalized) mutual information, might be more suitable than the
Pearson correlation coefficient. However, in this embodiment, no
influence from the choice of similarity measure on the registration
could be noticed, regardless of whether Pearson correlation or
normalized mutual-information was used.
[0060] The motion correction scheme presented here can also be used
in conjunction with parallel imaging for scan acceleration. There
is essentially no difference in under-sampled k-space resulting
from conventional parallel imaging or from local sampling density
variations caused by reorganizing k-space as a result of motion
correction. The degree to which aliasing can be avoided is
determined by the amount of net regional under-sampling in k-space.
In other words, with increasing data reduction as a result of
parallel imaging, the method's ability to correct for motion
diminishes, whereas if one wants to put more emphasis on reducing
severe motion, the parallel imaging reduction factor has to be
derated accordingly. The ability of the reconstruction ultimately
depends on the number of coils, the geometry factor, and the
general conditioning of the design matrix.
[0061] One of the shortcomings of the method proposed here is its
inability to correct for through-plane motion. We have found that
spin history effects can reduce the quality of the reconstruction,
although the final outcome is still significantly better than
without any correction. Ultimately, spin history effects will
always challenge 2D acquisition techniques that are exposed to
potential movement with higher degrees of motional freedom.
Nevertheless, in this work the goal was to investigate the
possibility of correcting motion and spatio-temporal coil
sensitivity variations using an augmented generalized parallel
imaging reconstruction method. This method, in its general form, is
expressed as a three-dimensional problem with six degrees of
freedom. The application of this technique to 3D acquisitions
should also improve the quality of 3D scans, but further work on
retrieving 3D motion information in a rapid fashion is warranted.
It is important to note, that even with prospective motion
correction and regardless of whether Cartesian or arbitrary k-space
acquisitions are used, the modifications to the original
sensitivity information (i.e., translation and rotation) are
required since the patient is exposed to different coil
sensitivities during the course of motion. Therefore, conventional
Cartesian parallel imaging algorithms or standard iterative SENSE
algorithms do not work properly even when the scan plane can follow
the movement of the object.
[0062] Although the navigator image technique was initially
demonstrated in this invention by using a spin echo approach, we
also applied the navigator scheme to regular gradient echo scans.
The limitation is that the minimum echo time for the pulse sequence
is delayed by a few milliseconds to accommodate the short spiral-in
navigator image. If needed and if T2*-decay permits, the navigator
signal could also be acquired at the end of the spiral-OUT train
(i.e., after the interleaf is acquired that contributes to the
high-resolution image). For spin echoes the spiral-IN navigator is
particularly useful because it takes advantage of the high SNR
available as the signal increases prior to the spin echo formation.
As shown in FIGS. 4a and b, a variable-density spiral-out scan
could be used as well. The image quality of variable-density spiral
scans is affected somewhat more by off-resonance effects and signal
decay than conventional, constant-pitch spirals, and it depends on
the pitch-factor and overall readout length. Nevertheless, this
trajectory has great potential because of its inherent navigation
capabilities and is a viable alternative that has been used
successfully for self-navigated diffusion-tensor MRI. Due to the
spiral readout the overall image quality of our sequence is
slightly impaired by the T2* decay during the readout, but could be
easily improved by increasing the number of interleaves. The
spiral-IN navigator and the parallel imaging based correction
scheme can be also combined with conventional FSE readouts.
[0063] In summary, a new motion correction scheme and a pulse
sequence that takes advantage of the navigator image technique. Our
motion correction scheme capitalizes on the ability of parallel
imaging methods to correct for missing data in k-space. The amount
of motion-induced image displacement and rotation is determined by
the means of registration algorithms applied to navigator images
and can be included in the iterative parallel imaging
reconstruction. An appropriate navigator image can be produced with
either a variable-density spiral waveform or an additional,
low-resolution, single-shot spiral scan prior to regular image
formation.
[0064] While the invention has been described with reference to
specific embodiments, the description is illustrative of the
invention and is not to be construed as limiting the invention.
Various applications and modifications may occur to those skilled
in the art without departing from the spirit and scope of the
invention as defined by the appended claims.
TABLE-US-00001 TABLE 1 Amount of mismatch between reference image
and distorted (simulated) image assessed by similarity measures.
Normalized squared difference normalized entropy normalized mutual
information trajectory GSENSE trajectory GSENSE trajectory GSENSE
uncorrected corrected corrected uncorrected corrected corrected
uncorrected corrected corrected EPI rotation 98.74 380.68 9.35
0.3485 0.3793 0.2955 1.1287 1.1359 1.1853 translation 138.19 498.20
5.79 0.3939 0.4247 0.2799 1.1181 1.1392 1.2198 combined 226.57
384.12 8.85 0.4062 0.4118 0.3082 1.1079 1.1358 1.1933 spiral
rotation 119.84 52.01 11.13 0.3546 0.3409 0.2990 1.1318 1.1512
1.1784 translation 157.00 19.42 5.90 0.3889 0.3087 0.2802 1.1222
1.1714 1.2227 combined 232.42 71.20 15.50 0.4000 0.3793 0.3317
1.1097 1.1405 1.1779
TABLE-US-00002 TABLE 2 Correlation between Simulated and Estimated
Rotations and Translations. navres = 16 navres = 24 navres = 32
navres = 48 navres = 64 x SNR = 5 R 0.999433 0.999595 0.999795
0.999905 0.999927 .sigma..sup.1 0.177930 0.154053 0.112422
0.0781978 0.0692372 SNR = 10 R 0.999692 0.999822 0.999896 0.999952
0.999970 .sigma..sup.1 0.130823 0.102574 0.0799846 0.0552091
0.0445193 SNR = 15 R 0.999782 0.999841 0.999925 0.999961 0.999976
.sigma..sup.1 0.110199 0.0968776 0.0680726 0.0498330 0.0397912 y
SNR = 5 R 0.999546 0.999758 0.999830 0.999923 0.999959
.sigma..sup.1 0.162598 0.121137 0.102916 0.0704131 0.0519895 SNR =
10 R 0.999747 0.999881 0.999931 0.999966 0.999977 .sigma..sup.1
0.122041 0.0847798 0.0654645 0.0465636 0.0388907 SNR = 15 R
0.999802 0.999910 0.999948 0.999976 0.999982 .sigma..sup.1 0.107670
0.0738078 0.0566544 0.0392259 0.0341824 .phi. SNR = 5 R 0.998987
0.999643 0.999842 0.999938 0.999954 .sigma..sup.2 0.344842 0.220522
0.150436 0.0958053 0.0822952 SNR = 10 R 0.999294 0.999770 0.999917
0.999966 0.999979 .sigma..sup.2 0.289213 0.176899 0.109040
0.0711539 0.0554753 SNR = 15 R 0.999349 0.999807 0.999925 0.999968
0.999984 .sigma..sup.2 0.277557 0.162262 0.103673 0.0684389
0.0485496 .sup.1standard deviation of differences (measured -
regression) of translation in mm .sup.2standard deviation of
differences (measured - regression) of rotation in degrees
TABLE-US-00003 TABLE 3 Amount of mismatch between reference image
and distorted image assessed by similarity measures. normalized
squared difference normalized entropy normalized mutual information
trajectory GSENSE trajectory GSENSE trajectory GSENSE Motion
uncorrected corrected corrected uncorrected corrected corrected
uncorrected corrected corrected Mild 3.30 3.48 2.55 0.4085 0.4075
0.3979 1.1354 1.1147 1.1397 Moderate 5.33 3.53 2.36 0.4162 0.4087
0.3972 1.1252 1.1179 1.1414
* * * * *