One way to generate shear waves is through mechanical vibration: you can literally shake the tissue to make the shear waves. This is actually done as part of the proof-of-concept in a number of papers on shear wave elastography. However, it clearly isn’t practical for clinical use: you’re not going to cut open a patient and shake their liver to measure its stiffness!

Despite its clinical limitations, mechanical vibration offers some distinct advantages for research applications. External shakers can provide precise control over both frequency and amplitude, allowing researchers to study tissue properties across well-defined parameter ranges. This makes mechanical excitation particularly valuable for validating elastography algorithms, characterizing tissue phantoms, and conducting fundamental studies of tissue biomechanics where controlled experimental conditions are essential.

A more practical method for clinical use is to use the acoustic radiation force to cause shear waves. The basic idea is to use a focused ultrasound beam to cause local vibrations at a point in the tissue. This creates shear waves that propagate away from the focal point, and can be observed.

In general, ultrasound shear wave elastography uses time-of-flight methods, which assume a constant shear wave speed, to measure the shear wave velocity. The shear wave speed is then related to tissue stiffness or viscoelastic properties.

The acoustic radiation force approach offers several advantages over mechanical vibration:

• Non-invasive: No physical contact with the patient beyond normal ultrasound probe placement
• Localized excitation: Shear waves can be generated at specific depths and locations
• Real-time capability: Integration with conventional ultrasound imaging systems
• Quantitative measurements: Precise control over excitation parameters