[Mristudio-users] Head Motion issue

[Susumu Mori]

Yes, motion always degrades MR images. It's not just DTI, but DTI is one of
the most susceptible.If data degrade in the time domain, there is only so
much you can do in postprocessing, which is always "after the fact" remedy.

1> Any MR image degrades if the subject moves more than 1 pixel during the
scan (I mean "DURING" the scan, not in between the scan). This is a
macroscopic motion in the order of 1-2mm or more. It is not a DTI specific
issue. During one phase encoding to the next phase encoding, if the subject
moved more than 1 voxel, the fourier transform won't work. When we talk
about motion-correction for DTI, this type of motion correction is not
included. This effect is more severe for 3D T1 scans which usually take 2-5
min for one image, during which the subject could move. For DTI, we
consider this effect is much less because we usually use a single-shot EPI,
which takes only less than a second. To correct this motion, we need a
real-time motion monitoring, which usually involves external motion tracker
or extra imaging schemes, but we assume this problem is negligible for DTI.

2> There are true-3D MRI and multi-slice 3D MRI approaches. T1 is usually
the former and T2 and DTI are usually the latter. The former suffers more
of the #1 problem. The latter has it's own problem; between-slice
misalignment within one 3D volume. As I mentioned in #1, one 2D DWI is
taken within less than a second. However, to obtain a whole 3D volume with
multi-slice acquisitions, I believe it takes 10-15 s. Any macroscopic
motion (>1-2mm) during this time period would cause among-2D-slice
misalignment within a 3D volume. In DTI motion correction, this type of
misalignment is NOT corrected while this problem does happen. To correct
this problem, a real-time motion tracker and real-time FOV alignment DURING
the data acquisition is needed. And even with such a method, it may be
difficult to eliminate the problem completely. There are some technique
already presented but the important point is, this problem cannot be
corrected after the scan, just like the problem #1.

3> Motion correction between 3D volumes. This is a problem common to all
quantitative MRIs that require calculation using multiple 3D MRI data, like
DTI, fMRI, quantitative T2 map, etc. What is specific to this group is the
mis-alignment among 3D data. Here we assume that each 3D data is free from
#1 and #2 problems and therefore each data are not degraded. This is one of
the motion corrections we routinely do for DTI and all other quantitative
MRI. This method is well established and we can expect the errors are
minimized. Of course, this correction can introduce resolution degradation
but we usually ignore it.

4> Image distortion correction. There are B0 susceptibility and
eddy-current distortion, while the former is common to all EPI-based
acquisition (DTI, fMRI, etc) and the latter is specific to DWI. These
corrections are approximation at best but they do substantially reduce the
error. So, it is a good idea to do it. The degradation by eddy current can
especially be bad in old days and the correction schemes helped us a lot.
B0 problem does not degrade DTI data because all DWIs and b0 are deformed
in the same way, but it can cause some interpretation errors of the data.
Morphologic results (like volumes) could be inaccurate without correction,
although, we can't expect perfect correction by postprocessing regardless
of the method we use because sometimes the degradation occurs in the time
domain (can't retrieve in the spatial domain anymore).

5> motion-caused phase error and intensity drop. This is DWI specific.
Microscopic motion (like brain pulsation) less than one pixel (<1-2mm)
causes signal drop IF it happens during a pair of diffusion-weighting
gradients. Because our brains are constantly moving, this is an significant
issue. The higher the b-value, more problem we have. Thus, HARDI-type scans
with higher b-values suffer more. Once this happens, the only thing we can
do is to discard the image or affected pixels. Many DTI postprocessing
software has pixel-rejection programs. This is again, an after-the-fact
remedy, not a fundamental solution. Even if it works perfectly, we suffer
from loss of data points. For example, if we acquire 30 DWIs and 3 of them
are badly affected and removed, we loose SNR. For high-b-value scans with
poor SNR, I'm not sure how you can reliably detect affected voxels or
images. I believe there are some papers discussing about it.

So, overall, you can see that the motion problems for DTI (and MRI) are not
solved but many software tries to reduce it.

As some of the issues are inherent to MRI, which is one of the slowest
imaging techniques, we can't dismiss MRI entirely due to its sensitivity to
motions. We always need to be careful when we interpret the data,
especially when you suspect that the patient group may have more
motion-related problems. Otherwise, we would end up in using MRI and
sophisticated analysis only to say, "patients moved more."



On Sun, Mar 26, 2017 at 2:05 PM, Shaimaa Abdelsattar <shaimaa96 at hotmail.com>
wrote:

> Hello,
> There are papers in the literature discussing the effect of head motion on
> DTI analysis as it can induce spurious differences among groups. I would
> like to ask if this motion is still a concern even after eddy current
> correction and registration into b0 image using AIR tool in DTI studio for
> tractography.
> Thank you in advance
> Dr.Shaimaa Mohammad, MD
>
> Sent from my iPad
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