Mechanical properties of tissues such as shear modulus (elasticity) are related to the state of tissue health. Therefore, noninvasive methods for measuring tissue elasticity have important clinical applications. The elasticity of a soft tissue is associated with its shear wave propagation speed. Acoustic radiation force from a focused ultrasound beam can be used to generate shear waves within tissues. The shear wave propagation speed can be measured using pulse-echo ultrasound based on time-of-flight principle. However, through a systematic phantom study, we found that shear wave speed measurement based on such approach is dependent on transducer type, focal depth, push aperture size, and lateral tracking range. The measurement bias can be explained by the geometrics of the focused push beam intensity distribution.

As an alternative to traditional focused method, we proposed shear wave speed measurement using radiation force from an unfocused ultrasound beam. Phantom study results show consistent measurement of shear wave speed can be achieved over a relatively long axial extent. In vivo measurements on the biceps of a healthy volunteer show consistent increase of shear wave speed for the biceps under 0, 1, 2, and 3 kg loading. Advantages and limitations of the proposed method are discussed.

Based on the unfocused method, a special push scheme called comb-push is proposed. Several unfocused push beams are transmitted simultaneously with gaps in between to form a comb shaped radiation force field, generating a shear wave with known wavelength (spatial frequency). Only traditional single A-line motion tracking on the shear wave propagation path is required to measure the shear wave speed. Both phantom and in vivo muscle results show consistent shear wave propagation speed measurement can be achieved using comb-push. Such a method potentially can be readily implemented in current commercial ultrasound imaging systems.