Magnetic resonance imaging is sensitive to motion. Just like with other images, movement may cause blurring and distortion (‘artefacts’). To counteract this, motion correction methods are often used. These include devices that track motion as well as software that can correct some of the artefacts after the images have been collected. We have just published a paper on a potential new way to do this, using a wireless accelerometer (link, open access [1]), so here is a quick blog post about motion and MRI, explaining some of our findings along the way.
The GE Firefly scanner, 3T
One of the reasons for doing this work is that we are using a new scanner for newborn babies. Motion is always an issue in MRI, even for adults, but the scanning of newborns may be particularly vulnerable. It is not always easy to convince a newborn baby to remain still. Newborns may move several centimetres where adults only shift a few millimetres. As the newborn is smaller in size, this movement also has greater impact in that it can completely displace the (much smaller) structure of interest. Newborn babies also show differences in physiology compared to adults, which can affect the scan. For example, they breathe faster and less regular, and the resulting motion is transmitted to the head to a greater degree (due to the smaller distance between head and chest) [2].
Types of motion
Motion comes in many types. There is microscopic motion, related to for example circulation of blood or water diffusion, and there is macroscopic motion, related to whole-body movement and physiological functions, for example breathing movement. It may be periodic (e.g. breathing movement), intermittent (e.g. yawns, hiccups) or continuous (e.g. general unsettledness, scanner vibrations). In research settings, noise and motion may be induced by experimental procedures [3]. Motion causes artefacts such as blurring, signal loss, loss of contrast and even the replication of signal in wrong places (‘ghosting’) – all lowering the quality of the image. An example of a motion artefact can be seen in the image below.
Fast Spin Echo image. Left: no motion artefact; Right: artefact due to in-plane rotational head movement. Image from Paley et al. 2017. Technologies, 5(1); 6. (CC BY 4.0)
In the figure above, there are lines on the right-hand scan (red arrow), which are distortions. These distortions were created because the head rotated slightly whilst it was being scanned. Too much distortion and the image will become less useful for clinical and experimental purposes.
Types of motion correction
There are many types of MRI motion correction. The simplest may often be to prevent and minimise movement using coaching of the patient, sedation and fast and/or motion-resistant imaging protocols. A fast scan with a still individual will usually give very little motion. However, this may not always be possible. Patients do not always lie still, sedation may not always be a good idea, and even our best imaging sequences can be vulnerable to movement to some extent. Large movement is therefore often best tackled through different means: it is detected and corrected for. Correction can be done during the scan (real-time) or after the scan (creation of the image from the raw data and/or post-processing of the image).
There are limits to this type of large movement correction. For example, we can use so-called navigator pulses during the scan to correct for movement in real time, but they tend to make scans take much longer. We can also use tracking devices to correct for motion both during and after a scan, but such devices are limited by the level of motion they can detect and require a fair bit of extra equipment to work inside or interact with the scanner. Finally, we can correct for motion in reconstruction or post-processing, but this too usually takes a lot of time and effort. Which type(s) of correction method is best may differ between different types of scan, patients, and experimental protocols and so on.
In our paper, we used an external motion measuring device – a wireless accelerometer, similar to one that you may buy for fitness purposes – to measure motion of the head. The nice thing about this is that it can give us full real-time 3D motion information about how the head moves. It is not like a visual device, which needs a clear line of sight to be able to ‘observe’ the head at all times. The accelerometer gave us continuous, wireless feedback on the angle of the object being scanned. We could then use this information to adjust the MR data, using a motion correction algorithm. The algorithm, using movement data from the accelerometer, adjusted how the MR signal was recorded at each given time point. We were in short using the signal from the accelerometer to shift k-space.
This meant that shifts in signal due to movement could in theory be recorded and, at least partly, fixed. Conversely, it also meant that motion could be introduced in a motion-free image. To introduce motion, we first made a motion data file with the accelerometer, simply by manually rotating it and recording the angles. We then applied this motion data file to the raw data of a motion-free scan. The motion file was used to shift signal in k-space for each affected phase encode step. Doing this, we could distort the image in the same way that real motion would cause distortions, despite there being no original motion in the MR data. We could ramp this up as we pleased, adding more and more ‘motion’, as shown in the figure below.
MR images incorporating increasing amounts of motion. (a) Original no-motion image, (b-f) motion applied, starting with 2 × 10−2 radians (b) and doubled for each successive image. Image from Paley et al. 2017. Technologies, 5(1); 6. (CC BY 4.0)
In principle, reversal of the motion effects should be possible. The motion in the figure above was introduced using a standard rotation matrix which multiplied the k-space locations by the measured angle, and if we reverse this process (i.e. counter-rotate the k-space data according to the measured angles), removing the noise should be possible. As with most things, it is easier to break than fix, yet we did see a subtle reversal of motion artefacts for a simple side-to-side rotation. This means that a wireless accelerometer may eventually be used to retrospectively correct for motion in neonatal MRI scans. It is also possible that it could be used for guiding real-time correction methods.
References:
1. Paley, M., Reynolds, S., Ismail, N., Herigstad, M., Jarvis, D. & Griffiths, P. Wireless Accelerometer for Neonatal MRI Motion Artifact Correction. Technologies. 2016; 5(1): 6. doi:10.3390/technologies5010006
2. Malamatenioua, C., Malika, S., Counsella, S., Allsopa, J., McGuinnessa, A., Hayata, T., Broadhousea, K., Nunesa, R., Ederiesc, A., Hajnala, J. & Rutherford, M. Motion-compensation techniques in neonatal and fetal MR imaging. Am J Neuroradiol. 2013; 34(6):1124-36.
3. Hayen, A., Herigstad, M., Kelly, M., Okell, T., Murphy, K., Wise, R., & Pattinson, K. The effects of altered intrathoracic pressure on resting cerebral blood flow and its response to visual stimulation. NeuroImage. 2012; 66: 479-488. doi: 10.1016/j.neuroimage.2012.10.049.
The Firefly 3-Tesla neonate MRI scanner in the Jessop Wing. Image from Paley et al. 2017. Technologies, 5(1); 6. (CC BY 4.0)