Melissa Tamara Hooijmans1,2, Crystal L. Coolbaugh1, Hannah L. Kilpatrick1, Mark K George1, and Bruce M Damon1,3
1Vanderbilt University Institute of Imaging Science, Nashville, TN, United States, 2Department of Biomedical Engineering & Physics, Amsterdam University Medical Centers, Amsterdam, Netherlands, 3Department of Radiology and Radiological Sciences, Biomedical Engineering, and Molecular Physiology and Biophysics, Vanderbilt University Medical Center, Nashville, TN, United States
Synopsis
In
this study we explore the feasibility of using displacement fields to quantify
whole muscle 3D strain patterns during submaximal voluntary contractions of the
dorsiflexor muscles. Our results showed a consistent pattern, of large negative (shortening)
and a large positive (lengthening) principal strains. Both the magnitude and
pattern of strain agree with other studies performed during submaximal
contractions which indicates the feasibility of this approach to quantify 3D
strain.
Introduction
Neuromuscular
diseases are characterized by a wide variety of pathophysiological changes in
skeletal muscle, such as fat infiltration, inflammation and fibrosis as well as
reduced muscle strength and function1,2. Our understanding of how these
pathophysiological changes directly impair muscle function is incomplete. Spin Tagging
and Phase Contrast (PC) imaging are most frequently used to quantify mechanical
characteristics of skeletal muscle3,4. However, these
techniques are usually performed in a single slice fashion and/or need
repetition of movements to encode in multiple directions. As a result these
techniques generally reflect only localized information of strain patterns
during low force contractions. Consequently, there is need for fast and 3D
mapping of strain during contractions with higher force levels. In this study
we explore the feasibility of using displacement fields, estimated from 3D high
resolution anatomical scans during rest and submaximal isometric contractions,
to quantify whole muscle 3D strain patterns in the Tibialis Anterior (TA)
muscle.Methods
Seven
healthy subjects (5 men; Age:25.1± 2.7yrs. Range: 23-31yrs.) participated in
this study. All subjects underwent a familiarization session and a test
session. During the familiarization session subjects performed three or more
Maximum Voluntary Contractions (MVC) of the dorsiflexor muscles to determine
their maximum force using a homebuilt isometric exercise device(ref). Then the
subject practiced submaximal dorsiflexion contractions at 20% and 40% MVC.
Isometric force was measured using a load cell and recorded with a custom
LabVIEW program (National Instruments, Austin TX). On the testing day, subjects
were placed in supine position with their right foot strapped in the exercise
device in +10° plantarflexion position. MR Imaging was performed with a 3T MR
system (Philips, Best, The Netherlands) using a 16-channel surface coil and
8-channel table top coils. The scan protocol consisted of three 3D high resolution
mDixon scans (3D;
6 echoes; TR/TE/ΔTE 210/1.01/0.96ms; FOV 192x192x308; slice thickness 3.5mm, 36
seconds),
performed with the leg at rest, during 20% and 40% MVC isometric dorsiflexion contractions.
After the scan at
rest, the volunteers were instructed to match the target force, using real time
visual force feedback and hold that position for the duration of the scan. Post-processing
MR
images were analysed using MATLAB (The Mathworks, Inc., Natick, MA). Registration of the water-only image obtained at rest to the images
obtained during contraction was used to estimate the displacement field between
MVC20% and MVC40% and the relaxed condition using 3D Demons5 (Figure
1). The 3D strain tensor was calculated based on the displacement field;
diagonalized, principal strains were
computed as magnitude-sorted eigenvalues. Principal
and linear strain values were computed for the deep and superficial compartment
of the TA muscle, using manually drawn Regions-of-Interest. Additionally, these
segmentations were divided into distal, middle and proximal segments, based on
equal number of slices per segment. The force data acquired during each
contraction were filtered with a 4th-order, low-pass butterworth
filter (cut-off frequency = 5.5 Hz) to remove the interfering signal from the
scanner, after which the relative contraction intensity was calculated for the
full duration of the contraction. Differences in strain between MVC conditions were
assessed with a Mann-Whitney-U-test. Differences in strain between the
different muscle segments (distal, middle, proximal) were assessed with a
Friedman-test. Results
The
mean MVC force was 265.2±130N. The relative contraction intensity during 40%
MVC condition was 35.7±0.6% and during the 20% MVC condition was 19.3±2.6%. Principal strain maps and filtered force data are shown in Figure 2. Linear and principal strain values for deep and superficial compartment during both MVC conditions
are shown in Figure 3. For the principal strains we found a large-magnitude
negative value (En), a large-magnitude positive value (Ep)
and an almost zero value in both compartments during both MVC conditions. Principal and linear strains did not differ between the two MVC
conditions. Higher Ep were found in the superficial compartment
compared to the deep compartment of the TA muscle during both MVC conditions (20%
Ep: p<0.001 40%; Ep: p<0.0001)(Figure 4). Comparison
between the muscle segments showed higher Ep in the proximal segment
of both compartments and higher En in the distal segment of the deep
compartment during the MVC40% condition(Figure 5). Discussion
In
this study we explored the feasibility of quantifying 3D strain values using
displacement fields. Our results showed a consistent pattern, of large negative
(shortening) and a large positive (lengthening)
principal strains. Both the magnitude and pattern of strain pattern agree with
other studies performed during submaximal contractions6,7. The
variability in strain between muscle compartments and between muscle segments
suggests complex and heterogenous strain patterns which agree with previous
work in
healthy skeletal muscle during in-vivo voluntary submaximal contractions7-10. This
heterogeneity in strain within the muscle can be caused by architectural
differences between compartments and segments, differences in motor-unit
activation and neural control11-12. Interestingly, we did not find differences
in strain between the MVC conditions, which could be due the large variability
between subjects or due to only small deformations differences beween the 20% and 40%MVC. Conclusion
We show the feasibility of quantifying 3D whole muscle
strain calculated from displacement fields during submaximal voluntary isometric
contractions. Acknowledgements
No acknowledgement found.References
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