Onur Afacan1, Burak Erem1, Diona P. Roby1, Noam Roth2, Amir Roth2, Sanjay P. Prabhu1, and Simon K. Warfield1
1Radiology, Boston Childrens Hospital and Harvard Medical School, Boston, MA, United States, 2Robin Medical Inc., Baltimore, MD, United States
Synopsis
In
this work we report results from a large pediatric study that shows
the effect of motion. Motion patterns were measured on 82 children,
mean age 13.4 years, in a T1 weighted brain MRI. An expert
radiologist graded the images using a 4-point scale ranging from
clinically non-diagnostic to no motion artifacts. We used these
grades to correlate motion parameters such as maximum motion, mean
displacement from a reference point and motion free time. The results
will help the motion correction community in better understanding
motion patterns in pediatric populations and how it effects image
quality.Purpose
Motion
is the most prevalent of MRI artifacts especially in pediatric
populations. Most young children are either sedated or anesthetized
in order to have a successful imaging session, which is undesirable
due to their potentially adverse health effects. Although a number of
works have tried to eliminate or reduce motion artifacts through
retrospective [1] or prospective correction [2], to our knowledge
there is no study on pediatric populations to show typical motion
patterns and their effect on image quality.
Methods
We
performed a motion measurement study on 82 subjects at our children’s
hospital. The study was performed on a 3T Siemens TRIO scanner
(Erlangen, Germany). Subjects were selected from patients who require
a brain screen study and were cleared to be scanned without sedation
or anesthesia. Mean age of the group was 13.4 years, with a minimum
of 5 years and a maximum of 24 years. Five scans were removed from
the study due to technical problems with the motion measurement
system, i.e. the system reported errors or we encountered
difficulties with sensor placement.
Motion
was measured using an electromagnetic tracker [3] developed by Robin
Medical Inc. (Baltimore, USA). The tracker had two sensors that were
attached to the foreheads of subjects and whose locations and
orientations were reported in real-time. We used a modified
T1-weighted 3D GRE sequence as our test sequence. The product
sequence was modified to include short and low amplitude bipolar
gradients in the x-y-z axes after each RF pulse. The sensors used
these gradients to measure the changes in the magnetic field and
estimate motion parameters. Sequence parameters were TE=2.93ms,
TR=15ms, FA=20 degrees, with a 1mm isotropic resolution.
The
data from the sensors were analyzed retrospectively. We used three
different criteria to characterize motion: maximum displacement from
the reference position, mean distance traveled from the reference
position, and motion-free time. The reference position was the
position at the time that the center of k-space was acquired.
Motion-free time was defined as the percent of the scan duration for
which the position was less than 0.1mm from the reference position.
The scans were then graded by an expert radiologist according to the
severity of their motion artifacts using a Likert scale of 1 to 4; 1)
images contain severe motion artifacts that can not be used for
diagnosis; 2) images contain motion artifacts, but can be used for
gross diagnostic implications; 3) images contain some motion
artifacts and can be used for diagnostic purposes; 4) images do not
contain visible motion artifacts. We have used these grades to
classify what kind of motion patterns might result in non-diagnostic
images.
Results
Figure 1 shows
examples of images graded by the expert radiologist in terms of
motion artifacts (from grade 1 (left) to grade 4 (right)). Figure 2
shows motion patterns corresponding to each of those cases, with the
y-axis showing the average sensor displacement from the reference
position for the whole scan (~6 minutes). Figure 3 summarizes the
relationship between the 3 motion criteria and the motion artifact
grades. The average motion-free time for grades 1 to 4 were 0.35,
0.72, 0.91 and 0.98 respectively, while the mean distances traveled
from the reference point for the same grades were 1.76mm, 0.84mm,
0.5mm and 0.29mm.
Conclusion
In
this work we reported results from a large pediatric study that shows
the effect of motion. Our results show that both motion-free time and
average displacement from the center of k-space had high correlation
with image quality, whereas maximum displacement was not a good
predictor. Among the 77 subjects where motion was measured
successfully, 17 of them had average motion larger than 0.5mm, and
this resulted in non-diagnostic images in 11 of them (14.2 percent).
Similarly, 14 subjects had less than 90 percent motion-free time,
which also led to non-diagnostic images. It should be noted that
these study subjects were selected to undergo MRI without sedation
because they were cooperative, and the broader pediatric population
(especially younger children) is generally sedated because even
greater motion is expected from them during scans.
Acknowledgements
This
work was funded in part by NIMH project 5R42MH086984 and NIBIB
project 1R01EB019483.References
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44.3 (2000): 457-465.
[3] Gholipour,
Ali, et al. "Motion-robust MRI through real-time motion tracking
and retrospective super-resolution volume reconstruction."
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