Ultrasound elastography: the new frontier in direct measurement of muscle stiffness

Abstract

The use of brightness-mode ultrasound and Doppler ultrasound in physical medicine and rehabilitation has increased dramatically. The continuing evolution of ultrasound technology has also produced ultrasound elastography, a cutting-edge technology that can directly measure the mechanical properties of tissue, including muscle stiffness. Its real-time and direct measurements of muscle stiffness can aid the diagnosis and rehabilitation of acute musculoskeletal injuries and chronic myofascial pain. It can also help monitor outcomes of interventions affecting muscle in neuromuscular and musculoskeletal diseases, and it can better inform the functional prognosis. This technology has implications for even broader use of ultrasound in physical medicine and rehabilitation practice, but more knowledge about its uses and limitations is essential to its appropriate clinical implementation. In this review, we describe different ultrasound elastography techniques for studying muscle stiffness, including strain elastography, acoustic radiation force impulse imaging, and shear-wave elastography. We discuss the basic principles of these techniques, including the strengths and limitations of their measurement capabilities. We review the current muscle research, discuss physiatric clinical applications of these techniques, and note directions for future research.

Keywords: Diagnostic imaging; Elasticity; Hardness; Muscles; Rehabilitation; Ultrasonography.

Figure 1

Images of the sternocleidomastoid (SCM) muscle. A and B, B-mode ultrasound images. C and D, Ultrasound elastograms. Normal muscle (arrowheads in part A) is mostly soft. Mass in an affected SCM (arrowheads in part B) shows increased hardness from normal. C indicates carotid artery; purple, soft tissue; red, hard tissue. (Adapted from Kwon and Park [15]. Used with permission.)

Figure 2

Ultrasound elastogram (left) and B-mode scan (right) of the medial gastrocnemius. Left image shows the soft and hard references used to calculate a semiquantitative strain ratio. (Adapted from Chino et al [35]. Used under the terms of the Creative Commons Attribution License.)

Figure 3

Images produced from custom transient elastography with continuous shear waves using a linear array (B-mode) transducer and a mechanical handheld vibrator. B-mode (gray scale) and phase plot images of shear-wave vibrations 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 represent areas where phase lag was measured. Note the limited gray-scale image quality. (Adapted from Ballyns et al [13]. Used with permission.)

Figure 4

Representative shear-wave elastograms at 20° (A), 0° plantar flexion (B), and 10° dorsiflexion (C) in a pediatric patient’s left lateral gastrocnemius muscle. The blue-shaded box over the lateral gastrocnemius muscle represents the area of measurement. Purple indicates softer tissue (lower Young modulus) and green indicates stiffer tissue (higher Young modulus). Numbers indicate the lateral gastrocnemius (no. 1), the soleus (no. 2), and the flexor hallucis longus (no. 3).