Motion Compensation Strategies
David Atkinson D.Atkinson@ucl.ac.uk
Imaging Strategies Course, ISMRM 2011 Montreal
Introduction
Motion during MR imaging remains a significant problem. This article outlines the some of
the acquisition strategies available for motion compensation. The article and the accompanying
lecture are intended as an educational overview and not a comprehensive review. More details
of many of the pulse sequences are described in the book by Bernstein et al [1]. Further details
about motion correction in image reconstruction are available in the Friday Sunrise Educational
Course - Image Reconstruction.
The Effect of Motion
A conventional MR image is the Fourier Transform (FT) of the acquired k-space data. The
acquisition of k-space is often slow compared to physiological motion due to timing constraints
imposed by nuclear time constants (T1 and T2), safe gradient switching speeds and power deposition. The summation operations in the discrete FT mean that each image pixel is composed
of a weighted sum of every point in k-space. Thus any alteration to k-space at any time during
the acquisition can potentially affect every pixel in the image - typical motion artefacts being
image blurring or ghosting [2]. Motion can alter k-space in three ways; movement during the
application of a gradient can cause phase errors, motion between gradients can result in phase
and amplitude inconsistencies in k-space, and the movement of tissue can affect steady state conditions or result in tissue moving out of plane. Positional information is encoded by gradients
into the phase of k-space, thus unintended alterations to the phase (or magnitude) of k-space get
translated into positional errors by the FT operation, typically leading to blurring and ghosting.
Acquisition Strategies to Reduce Motion Effects
The rest of this article concentrates on technical issues but good patient preparation can reduce
motion associated with anxiety and claustrophobia. In babies tricks such as ‘feed-wrap-scan’
can allow a baby to be scanned asleep. In other children, sedation or anesthetics may be used
where the cost and risks are outweighed by the expected diagnostic value of the scan. If scan
time is short enough, breath-holding can be used to reduce respiratory artefacts.
Many acquisition strategies have been proposed to reduce motion effects. The problem is fundamentally challenging because ideally we need to know the motion of every pixel at every time
point and change the running scan and/or reconstruction. However, even though such complete
information is hard to obtain, motion compensation strategies can be effective. The techniques
can be broadly divided into sequences with reduced motion sensitivity, and, measurements that
enable corrections.
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Sequences With Reduced Motion Sensitivity
Averaging
Many sequences acquire multiple signal averages during free breathing. Although multiple averaging does not correct for motion, it weights coherent signal (from stationary tissue or that
with a consistent average position) greater than incoherent motion, often resulting in improved
quality scans.
Reducing Signal from Moving Tissue
Spatial saturation bands can be positioned to prevent moving tissue from providing a signal, for
example upstream of inflowing blood or over abdominal regions to reduce respiratory artefacts.
Suppression of either fat or CSF can also be used, especially with imaging sequences where fat
or CSF would appear bright and lead to prominent artefacts.
Phase Encode Re-Ordering
Motion affected k-space can be viewed as the ideal, motion-free, k-space multiplied by some
motion-dependent modulation function. In the image domain, this k-space multiplication is
equivalent to convolution of the ideal image with the Fourier Transform of the modulation
function. If the convolved function has multiple peaks, then replications (ghosts) will be seen
in the image. To reduce this ghosting, the order of k-space acquisition can be changed so that
the modulation function (and thus its FT) does not oscillate. Typically these schemes re-order
during respiration and include ROPE [3], COPE [4] and HOPE [5]. Re-ordering does not correct
for motion, but it can have a positive impact on image appearance.
Gating and Triggering
Prospectively triggering a sequence using an ECG or pulse oximeter is commonly used to capture
k-space at consistent times in the cardiac cycle over multiple heart beats. Retrospective gating
records k-space and the cardiac signal continuously and then assembles k-space into the relevant
cardiac phases after the acquisition. These techniques work well, especially if the heart rate is
regular. Respiratory triggering and gating can also be used with the respiratory phase inferred
from pneumatic bellows or the position of the diaphragm as determined from navigators (see
later section on navigation).
Gradient Moment Nulling
A gradient means that the magnetic field strength varies as a function of position. When the
overall area (zeroth moment) of a series of gradients is zero, stationary spins experience no net
phase changes. However, spins that move in the gradient direction experience different fields and
will not always be re-phased at the time when the net area is zero. This phase accrual is used
to provide diffusion contrast in DWI and velocity measurements in phase contrast sequences. In
general, the phase accrued by spins moving with a constant velocity is determined by the first
order moment of the gradients (the integral over time of the product of the gradient and time).
When the first order moment is non-zero, unwanted motion leads to phases that can cause signal
loss and image ghosting. The technique of gradient moment nulling aims to provide a set of
gradients with a low first order moment at relevant times. For slice selection, the relevant time
is the end of the slice select gradients whilst for the readout and phase-encode, the relevant
time is at the echo peak (which occurs after the phase encode gradients and during the readout
gradients). More references, details about the design, and compensating for acceleration in
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addition to velocity are available in Bernstein et al [1]. The subject is still a topic of on going
research, e.g. [6], and is also related to isotropic diffusion weighting e.g. [7] in moving structures.
Short TE and Non-Cartesian Sequences
For sequences such as spiral and radial where the acquisition starts near the centre of k-space,
there is relatively little gradient-on time before passing through the centre of k-space (where the
echo signal is largest) and thus the first-order moment at this point in time is low, and hence
the sensitivity to motion can be low. Often the motion artefacts associated with non-Cartesian
sequences are more benign than for regular Cartesian sampling (they can be less pronounced
and also may appear outside the region of interest) [8].
Motion Measurements: Tracking or Retrospective Compensation
Techniques such as navigators can be used to measure at least some components of motion which
can then be used either to alter a running scan (prospective correction), or to guide reacquisition
of parts of the data, or to guide a retrospective correction. Prospective correction requires rapid
data processing and feedback, but has the advantage of potentially avoiding missing data and
spin history problems. Retrospective correction is the only technique that can in principle correct
for non-affine components of motion.
Navigation
Navigation refers to inferring some components of motion from MR data, usually some additional
measurements within a sequence, rather than an external measure. One of the simplest forms of
navigation is to acquire extra non phase-encoded lines of k-space. Fourier Transformed, these give
projections through the object from which some components of motion can be inferred. This was
one of the early techniques [9] and recent applications of this central profile approach include its
use in balanced cardiac sequences [10]. When the acquisition is from multiple coils with different
local sensitivities, motion about both respiratory and cardiac cycles can be obtained from these
projections, e.g. [11].
Commonly the term ‘navigators’ refers to the signal from a cylindrical or ‘pencil beam’
shaped excitation through the diaphragm. The liver/lung border provides good contrast and
it is relatively easy to track the diaphragm here. From this position information, the scan
can be gated to acquire only during a user-determined window of diaphragm positions and the
slice/volume position may also be updated (also see [12] and Prospective Correction below).
The pencil beam profile is achieved through the use of a spiral excitation k-space. The previous
techniques are often known as 1D navigators or navigator echoes.
More complex rotational motion can be measured using orbital [13], spherical [14] and rapid
cloverleaf [15] paths through the acquisition k-space, as well as floating navigators with a small
phase encoding [16, 17].
As a means of trying to detect specifically cardiac motion, navigators from the fat signal
near the coronaries have been developed [18]. It is also possible to capture the FID following an
RF pulse and obtain motion information [19, 20, 21, 22] or make use of the butterfly or bow tie
pattern when k-space is being rewound or prepared [23, 24].
The central region of k-space can be scanned rapidly and provides a low resolution image
from which motion can be determined. Sequences such as radial and spiral can be inherently selfnavigated having a high sampling density near k-space centre (as well as some natural robustness
to motion) and can be used to infer motion, e.g. in whole heart coronary imaging using a 3D radial
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acquisition [25]. The PROPELLER [26] method obtains low resolution images from rotating
blades of k-space that are used for motion compensation.
In multi-shot diffusion weighted imaging, 2D navigators have been used to measure the phase
changes caused by motion during diffusion sensitization [27] for retrospective correction (e.g. [28])
or as a guide to re-acquisition of the worst affected shots [29].
External Sensors
External sensors can provide additional information with no compromise to the scan sequence
or imaging. Optically tracked markers or active markers cannot directly measure the motion
of internal tissues but have been the subject of recent demonstrations for neurological imaging
[30, 31, 32]. Recently an ultrasound probe has been used to monitor internal motion for cardiac
imaging [33].
Prospective Correction
The position of an imaged slice or volume can be adjusted during acquisition by altering the
gradients and/or RF pulses with the intention of tracking a region of interest. Successful tracking
can reduce k-space inconsistencies due to both position changes and problems associated with inconsistent spin histories. A 1D navigator can be used for slice tracking [34], but for more complex
motion compensation, a more complete description of motion is required. This can come from
rapid low resolution images with motion determined by image registration [35, 36, 37], or using
orthogonal spiral navigators [38, 39]. Note that using prospective alterations to gradients, it is
possible to compensate for 3D affine motion. For non-rigid motion components that are more
complex than affine, currently the only general method for compensation is to use retrospective
techniques in image reconstruction (see Sunrise Educational Course on Motion Correction in Image Reconstruction). Although prospective correction aims to effectively fix the imaging volume
of interest, this can lead to a relative motion of coil sensitivities, distortions associated with B0
variations [40] and other tissue moving non-rigidly. Many prospective techniques also include
some postprocessing to reduce these effects and overcome any latency issues. Spectroscopy as
well as imaging can also benefit from these methods e.g. [41].
Outlook
It seems likely that in the near future, most diagnostic quality scans will not be faster than
physiological motion. This means there will continue to be a role for developments in motion
correction. In the future we might expect to see scan time being fully exploited for motion
measures and a more coherent integration of prospective scanner control, motion measures,
learned models of motion and reconstructions that include not just inter-shot motion but effects
such as motion during a shot, T2 signal decay and the relative motion of field inhomogeneities
and coil sensitivities. The area of parallel RF transmit offers the possibility of new ways to
monitor and perhaps compensate prospectively for motion during acquisition.
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