Reliability and validity of quantifying absolute muscle hardness using ultrasound elastography

Abstract

Muscle hardness is a mechanical property that represents transverse muscle stiffness. A quantitative method that uses ultrasound elastography for quantifying absolute human muscle hardness has been previously devised; however, its reliability and validity have not been completely verified. This study aimed to verify the reliability and validity of this quantitative method. The Young's moduli of seven tissue-mimicking materials (in vitro; Young's modulus range, 20-80 kPa; increments of 10 kPa) and the human medial gastrocnemius muscle (in vivo) were quantified using ultrasound elastography. On the basis of the strain/Young's modulus ratio of two reference materials, one hard and one soft (Young's moduli of 7 and 30 kPa, respectively), the Young's moduli of the tissue-mimicking materials and medial gastrocnemius muscle were calculated. The intra- and inter-investigator reliability of the method was confirmed on the basis of acceptably low coefficient of variations (≤6.9%) and substantially high intraclass correlation coefficients (≥0.77) obtained from all measurements. The correlation coefficient between the Young's moduli of the tissue-mimicking materials obtained using a mechanical method and ultrasound elastography was 0.996, which was equivalent to values previously obtained using magnetic resonance elastography. The Young's moduli of the medial gastrocnemius muscle obtained using ultrasound elastography were within the range of values previously obtained using magnetic resonance elastography. The reliability and validity of the quantitative method for measuring absolute muscle hardness using ultrasound elastography were thus verified.

Figure 1. Typical ultrasound elastogram of the tissue-mimicking material.

The image on the left is a color-coded elastogram superimposed on a conventional B-mode ultrasound image, whereas that on the right side is the B-mode ultrasound image. On the elastogram, a color-coded scale from red (soft) to blue (hard) and a numeric scale of 1–7 indicating the velocity of the compression–relaxation cycles are presented. Regions of interest within the soft mimicking reference (7 kPa), hard mimicking reference (30 kPa), and tissue-mimicking materials (50 kPa) are indicated by the yellow rectangles.

Figure 2. Typical ultrasound elastogram of the in vivo human medial gastrocnemius muscle (MG).

The yellow rectangles are the regions of interest for the soft reference (7 kPa), hard reference (30 kPa), and MG.

Figure 3. Comparison of the Young’s moduli of seven types of tissue-mimicking materials obtained using the mechanical displacement–load compression method (E _Comp) and ultrasound elastography (E _US).

The two independent methods of calculating Young’s modulus were significantly correlated (r = 0.996, P<0.001).

Figure 4. The Young’s modulus of the medial gastrocnemius muscle (MG).

The Young’s modulus of MG was measured at 20° dorsiflexion, anatomical position, and 30° plantar flexion. The anatomical position of the ankle is considered as 0° (i.e., 90° between the tibia and the sole). Dorsiflexion is considered negative while plantar flexion is considered positive. Young’s modulus significantly decreased with plantar flexion (*P<0.05).