Download In vivo skeletal muscle tension measurement using Magnetic

In vivo skeletal muscle tension measurement using
Magnetic Resonance Elastography (MRE)
TR Jenkyn, PhD1, RL Ehman, MD2, KR Kaufman1, PhD, K-N An1, PhD
1
Biomechanics Laboratory, Div. of Orthopedic Research, Mayo Clinic, Rochester, MN, USA
2
Dept. of Diagnostic Radiology, Mayo Clinic, Rochester, MN, USA
Email: [email protected]
INTRODUCTION
Indeterminacy is the primary obstacle
for inverse kinematic modeling in musculoskeletal biomechanics. Force transducers and
EMG provide clues of muscle load sharing,
but do not completely solve joint
indeterminacy.
Magnetic Resonance Elastography
(MRE) is a new technique for quantifying
tissue stiffness in vivo (Muthupillai, 1995).
Muscle stiffness has been shown to change
with state of contraction (Dresner, 2001).
MRE applies shear waves to muscle and
images the wave propagation through the
tissue. Imaged wavelength changes with
muscle stiffness and is therefore directly
related to muscle tension.
This study assesses MRE
applicability to biomechanics by directly
measuring isometric muscle tension.
METHODS
Subjects lay supine in a GE MRI
scanner (aged 26-32, 3 female, 4 male)
within an ankle loading apparatus. Moment
was applied to the ankles in neutral position
(8Nm, 16Nm plantar-flexing and 30Nm,
60Nm dorsi-flexing) which was isometrically
opposed. A vibrator applied shear waves of
frequency 150Hz (f) and amplitude 30µm.
Tibialis anterior (TA) and triceps surae (TS)
were imaged with a gradient-echo, cyclic
motion sensitizing sequence (TR/TE of
100ms/min full, 256x64 acquisition matrix,
24cm FOV). To determine muscle tension,
muscle tissue was modeled as fibers in a
viscous medium. Tension (T) is therefore a
function of wavelength (λ) as in Equation 1.
T=
1
1
 ζ
λ2
ζ 2ω 2 + ρ 2 A2ω 4 2 cos  arctan
2
 ρAω
4π
2


(
)




Equation 1: The λ− Τ relationship where ζ
is viscosity, ρ is density, A is cross-sectional
area, and ω=2π f (Graff, 1975).
Surface EMG was also collected
from TA and TS (sampled at 1000Hz,
bandpass 30-500Hz, gain 350) while
repeating the experiments outside the MRI.
EMG was integrated and normalized with
maximum voluntary contraction (IEMG).
RESULTS AND DISCUSSION
MRE shear wavelength increased in
TA as it opposed increasing plantar-flexing
moment (Figure 1). Wavelength increased to
a lesser extent with applied dorsi-flexing
moment as TA acted as an antagonist.
TA tension and IEMG both
increased with applied plantar-flexing
moment (Figure 2). TS tension increased
with applied dorsi-flexing moment. This
agreed qualitatively with IEMG activity
(Figure 2). MRE measurement of tension in
multiple muscles simultaneously has been
demonstrated. Since not all the ankle
muscles were imaged, the internal joint
moment due to muscle tensions did not
match the externally applied ankle moment.
Imaging all active muscles about the ankle
and balancing internal muscle moment with
externally applied moment is currently
underway.
MRE is sensitive to the muscle λ− Τ
relation used. Currently, this relation is
derived from wave motion of strings with
tension in a viscous medium. This models the
contracting muscle well but models relaxed
muscle less well. Further derivation of the
λ− Τ relation is on-going to address this.
MRE noninvasively quantifies tension
in multiple muscles simultaneously. MRE
can potentially overcome joint indeterminacy
and yield tremendous insight into
musculoskeletal biomechanics.
Plantar-flexing Load
Dorsi-flexing Load
Relaxed
Figure 1: MRE imaged shear waves in TA
for each applied ankle moment.
TRICEPS SURAE
TRICEPS SURAE
1.000
Tension [N]
1500
IEMG [fraction of MVC]
2000
Dorsi-flex
Plantar-flex
1000
500
0
-16
-8
0
30
0.750
Plantar-flex
0.250
0.000
60
-16
Applied Moment [Nm]
TIBIALIS ANTERIOR
-8
0
30
Applied Moment [Nm]
60
TIBIALIS ANTERIOR
2000
IEMG [fraction of MVC]
1
1500
Tension [N]
Dorsi-flex
0.500
Plantar-flex
Dorsi-flex
1000
500
0
0.75
Plantar-flex
Dorsi-flex
0.5
0.25
0
-16
-8
0
30
Applied Moment [Nm]
60
-16
-8
0
30
Applied Moment [Nm]
60
Figure 2 MRE tensions (left) and IEMG
(right) in TA (top) and TS (bottom) for each
applied moment.
ACKNOWLEDGEMENTS
Funded by NIH-NICHD.
REFERENCES
Dresner MA, et al. (2001) “MRE of Skeletal
Muscle” JMRI 13(2) pp269-76
Graff KF Wave Motion in Elastic Solids.
Ohio State Univ Press:1975
Muthupillai R, et al. (1995) “MRE by direct
visualization of acoustic strain waves”
Science 269 pp1854-7