Office-based elastographic technique for quantifying mechanical properties of skeletal muscle

Abstract

Objectives: Our objectives were to develop a new, efficient, and easy-to-administer approach to ultrasound elastography and assess its ability to provide quantitative characterization of viscoelastic properties of skeletal muscle in an outpatient clinical environment. We sought to show its validity and clinical utility in assessing myofascial trigger points, which are associated with myofascial pain syndrome.

Methods: Ultrasound imaging was performed while the muscle was externally vibrated at frequencies in the range of 60 to 200 Hz using a handheld vibrator. The spatial gradient of the vibration phase yielded the shear wave speed, which is related to the viscoelastic properties of tissue. The method was validated using a calibrated experimental phantom, the biceps brachii muscle in healthy volunteers (n = 6), and the upper trapezius muscle in symptomatic patients with axial neck pain (n = 13) and asymptomatic (pain-free) control participants (n = 9).

Results: Using the experimental phantom, our method was able to quantitatively measure the shear moduli with error rates of less than 20%. The mean shear modulus ± SD in the normal biceps brachii measured 12.5 ± 3.4 kPa, within the range of published values using more sophisticated methods. Shear wave speeds in active myofascial trigger points and the surrounding muscle tissue were significantly higher than those in normal tissue at high frequency excitations (>100 Hz; P < .05).

Conclusions: Off-the-shelf office-based equipment can be used to quantitatively characterize skeletal muscle viscoelastic properties with estimates comparable to those using more sophisticated methods. Our preliminary results using this method indicate that patients with spontaneous neck pain and symptomatic myofascial trigger points have increased tissue heterogeneity at the trigger point site and the surrounding muscle tissue.

Figures

Figure 1

A, Vibrating upper trapezius with the office-based custom vibrator. B, Disassembled view of the plastic housing and direct current motor with an offset weight attached to its rotor axis to generate vibrations. The white screw knob fixes the motor in the plastic housing.

Figure 2

B-mode (gray-scale) and phase plot images of shear wave vibrations at 160 Hz for the phantom background material (A and B), type I sphere with surrounding background material (C and D), and type IV sphere with background material (E and F). Numbered lines denote areas where phase lag was measured. Circled areas denote type I and IV sphere locations.

Figure 3

B-mode (gray-scale) and phase plot images of shear wave vibrations at 160 Hz for the biceps brachii (A and B), normal upper trapezius (C and D), and upper trapezius with an active myofascial trigger point (MTrP; E and F). Numbered lines denote areas where phase lag was measured. Circled areas denote the active trigger point.

Figure 4

Top, Shear wave speeds (points) and model fits (lines) for phantom gel studies. Bottom, Average estimated shear modulus for the background (BG) material, type I sphere, type IV sphere, and surrounding background material for both spheres.

Figure 5

Phase shear wave speeds for the biceps brachii. Blue points indicate data collected during this investigation, and the Voigt model fitting (blue line) shows dispersion of the shear wave (5.11 kPa and 6.9 Pa/s). Gray, orange, and red lines denote reported literature range values for the biceps shear speed when the shear wave propagates along the fibers. Green lines denote reported literature range values for the biceps shear speed when the shear wave propagates orthogonal to the fibers.

Figure 6

A and B, Myofascial trigger points observed as hypoechoic regions on a B-mode image (A) and as regions of a color deficit on a color variance image (B) in a patient with acute neck pain and a palpable active myofascial trigger point. The red arrows highlight two hypo -echoic areas in the B-mode image, which are vibrating with a lower amplitude than the surrounding tissue, as shown in the color variance image, leading to the color deficit. C and D, B-mode image (C; low resolution, with reduced line density, 532 frames per second) and shear wave speed at 179 Hz overlaid on the B-mode image (D). There is an increase in the local shear wave speed in the center of the image corresponding to the palpable active trigger point (white arrow). The B-mode image shows the corresponding hypoechoic region (white arrow). The upper trapezius muscle is the region shown by the double-sided arrow. The imaging depth is 25 mm in all images.

Figure 7

Shear wave speed for the upper trapezius. Red points indicate normal tissue; blue points, active myofascial trigger point (MTrP); and green points, surrounding muscle tissue (SMT) around the trigger point. Lines are Voigt model fits showing dispersion of the shear wave within the muscle tissue. The green model fit partially overlaps the blue. Shear modulus and viscosity values used for fitting the data were as follows: active trigger point, 3.5 kPa and 10.8 Pa/s; normal, 3.38 kPa and 6.09 Pa/s; and surrounding muscle tissue, 2.14 kPa and 10.1 Pa/s.