Module 3-2
Shear Wave Generation in the Myocardium
Two mechanisms can generate shear waves in myocardial tissue: an externally applied acoustic push, and spontaneous mechanical events that occur naturally within the cardiac cycle.
Acoustic Radiation Force
When an ultrasound wave travels through tissue, part of its energy is transferred into momentum, resulting in a net force in the direction of wave propagation inside the tissue. That force is called acoustic radiation force (ARF), and its magnitude depends on the local acoustic intensity and the tissue's absorption properties.
To generate shear waves, a focused push pulse is delivered to the region of interest. This is a high-intensity ultrasound burst lasting several hundred microseconds, designed to concentrate ARF at the beam focal point. The result is a brief, localized axial displacement of tissue at the focal zone, on the order of micrometers.
Click anywhere on the tissue to fire another push pulse.
ARF acts downward along the z-axis, into the tissue; the shear wave shown here propagates along the x-axis. Note: Timing and amplitude are exaggerated for clarity, not to scale.
Because the displacement is spatially confined, it creates a shear stress gradient at its edges. Shear waves are then launched from these boundaries and travel laterally through the surrounding tissue, perpendicular to the beam axis, in the low-kilohertz frequency range.
A key practical advantage of the ARF approach is timing control. Push pulse delivery can be synchronized to any selected phase of the cardiac cycle, allowing myocardial stiffness to be assessed at whatever moment is clinically relevant.
The limitation is spatial coverage. Due to the physical properties of shear waves and myocardial wall geometry, propagation remains interpretable only within a few centimeters around the push site. ARF-based SWE is therefore a segmental technique.
Natural Shear Waves
Shear waves can also arise without an external push. Mechanical events during valve closure generate transients at the valve annulus that propagate through the adjacent myocardium as shear waves. Both mitral valve closure (occurring during isovolumetric contraction) and aortic valve closure (occurring during isovolumetric relaxation) produce waves detectable by ultrafast imaging.
Compared to ARF-induced waves, natural shear waves offer several practical advantages. They recur with every heartbeat, requiring no dedicated push pulse or additional energy deposition in the tissue. Their amplitude is substantially larger, making them more readily detectable. And because they propagate broadly through the ventricular wall, they allow in principle the assessment of multiple myocardial segments within a single acquisition.
The trade-offs concern timing and analysis. Wave timing is fixed to specific cardiac events and cannot be chosen. Their long wavelengths also make propagation characteristics more sensitive to myocardial wall geometry and fiber orientation. This complicates the direct interpretation of wave velocity as a myocardial stiffness measure. The relative merits of both approaches are examined in Module 3-4.
Click the left half to replay.
Click the right half to replay.
Each wave originates at the moment of valve closure and propagates through the adjacent myocardium. Note: Timing and amplitude are exaggerated for clarity, not to scale.
Summary
Two mechanisms produce shear waves in myocardial tissue.
| ARF-based SWE | Natural shear waves | |
|---|---|---|
| Generation | A focused push pulse creates a localized axial displacement at the beam focal point; shear waves launch from its edges and propagate laterally in the low-kilohertz range. | Arise spontaneously during mitral and aortic valve closure and propagate through the adjacent ventricular wall. |
| Advantages | Push pulse timing is selectable across the cardiac cycle. | Larger amplitude, recur with every heartbeat, and enable broader segmental coverage without an external push. |
| Limitations | Assessment confined to a few centimeters around the push site (a segmental technique). | Timing fixed to specific cardiac events; long wavelengths complicate the direct relationship between velocity and stiffness. |