Adam M.J. van Niekerk1, Paul Wighton2,3, Ali Alhamud1, Matthew D. Tisdall2,3, Andre J.W. van der Kouwe2,3, and Ernesta M. Meintjes1
1Human Biology, MRC/UCT Medical Imaging Research Unit, University of Cape Town, Cape Town, South Africa, 2Athinoula A. Martinos Center, Massachusetts General Hospital, Boston, MA, United States, 3Radiology, Harvard Medical School, Boston, MA, United States
Synopsis
In this study we propose a
novel approach to motion correction in MRI that separates the challenges of
tracking orientation and translation. We developed an external hardware device
capable of high frequency orientation estimates independent of the pulse
sequence. The device takes vector observations of gravity and the MRI scanner’s
static magnetic field (B0) and is therefore free from many constraints of some
existing external motion tracking techniques. Most notably, no scanner specific
calibration is required and the device can be miniaturised. Translation
estimates are achieved through the use of 3 high-speed orthogonal navigators.
Line by line rigid body motion correction is implemented in a spoiled gradient
echo pulse sequence. Taget Audience
This study is relevant to
clinicians and researchers who scan subjects prone to move during image
acquisition or use motion sensitive contrasts. Researchers using markers or
optical motion tracking could benefit from the novel approach used for
orientation estimates.
Purpose
We aim to make in-bore
motion tracking more accessible through the use of miniature, low-cost sensors
found in a large variety of consumer electronics. Most current external motion
tracking techniques rely on a set of translation measurements to estimate
orientation
1,2, or a retrograte marker which spatially encodes motion
parameters on the marker itself
3. This fundamentally limits the
minimum size of the apparatus the patient must wear as the reliability of
orientation estimates falls off as the marker/s are brought closer together
1,2
or reduced in size
3. We identified the need for orientation estimates
independent of the imaging pulse sequence that can be estimated at a temporal
resolution unattainable by navigator-based methods. Many external motion
correction techniques are also limited by complex calibration procedures or an
increased level of subject interaction
4, with line-of-site
1,3
or limited measurement range restricting marker placement
2,3. We address
some of these issues using an approach with zero scanner-specific calibration.
Method
An MRI compatible sensor array was developed comprising
a 3-axis magnetometer, 3-axis accelerometer and 3-axis angular rate gyro
(Figure 1). The sensors allow for high temporal resolution (up to 1 kHz data rate)
measurements of the scanner’s static magnetic field vector and the earth’s
gravitational field vector. Orientation estimates are obtained by comparing vector
observations from the accelerometer and magnetometer in the subject frame with measurements
of the
a priori reference vectors (gravity
[-y] and B0 [+z]) in the scanner frame. To reduce the effects of linear
accelerations on the gravity vector estimate, a modified version of the non-linear
complementary filter proposed by Mahony
5 et al. (originally designed
for unmanned aircraft guidance/control) was introduced, which combines angular
rate measurements with the vector-based orientation. All sensor signals were
digitised onboard the device where a 32-bit micro-controller implemented the complementary
filter with single precision floating point arithmetic. A two step, one-time,
calibration of the device was performed to correct misalignment of the 3-axis
sensors caused by soldering imperfections and variations in the gains of the
sensor axes. Data were transmitted to a laptop over a serial optical link (300
Hz). The laptop acts as a TCIP server on the local measurement network for the
scanner to query for the latest orientation estimate. By combining the
orientation estimates with translation estimates obtained using three
orthogonal high speed linear navigators
6, full rigid body motion
correction is achieved. Translation estimates were obtained in the corrected
imaging frame by computing the frequency shift of the raw MR signal relative to
a reference acquisition. A multi-channel fitting algorithm improved the
stability of the translation estimates (Figure 2). Translation and orientation updates
were applied for each line of k-space in a standard 3D spoiled gradient echo
pulse sequence (TE 7 ms, TR 12 ms, 256x256x64) with a non-selective RF pulse
(Figure 3). Four brain acquisitions were performed in three subjects on a 3 T
Siemens Skyra: (1) no motion, correction active, (2)
occasional pose changes, correction active, (3) occasional pose changes, no correction,
(4) continuous motion, correction active.
Motion correction was applied in real-time and corrected images were generated
immediately on the scanner. All scans were conducted in accordance with
protocols approved by the Faculty of Health Sciences Human Research Ethics
Committee of the University of Cape Town.
Results
The magnetometer
measurements of the fields within the MRI scanner during image acquisition
allow for an exceptionally high signal to noise vector reference. This and that gravity and the static magnetic field are almost perfectly orthogonal
result in more precise orientation estimates than what are conventionally possible
with similar sensor arrangements. Figures 4 and 5 show the motion parameters
measured during each of the corrected scans and representative images for all 4
acquisitions in one subject.
Discussion
High temporal resolution
orientation estimates enable robust motion correction free from scanner
specific calibration despite noise in the translation estimates. Tiny changes
in orientation could result in substantial errors on the periphery of the image
due to a ‘lever arm’ effect. These artefacts are difficult to correct
restrospectively highlighting the importance of reliable orientation tracking. Changes
in posture of 0.1 degrees over one TR (12ms) were often observed showing the
importance of high temporal resolution correction. State parameters such as
angular rate and acceleration could be combined with navigator measurements in
future implementations to improve translation estimate accuracy in high
frequency applications.
Acknowledgements
The South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation of South Africa, Medical Research Council of South Africa, NIH grants R21AA017410, R01HD071664.
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