Elastography Approaches Based on Ultrasound

MedTech Outlook | Friday, April 22, 2022

Over the last two decades, several elastography methods for measuring the mechanical properties of different organs have been developed and then made available for use in the clinic.

Fremont, CA: Different imaging techniques can offer morphologic and functional data, but they cannot provide quantitative data on soft tissue mechanical properties. Multiple elastography approaches have been developed to measure the mechanical properties of various organs. These many elastography procedures have two things in common: they all involve delivering stress to the tissue and monitoring the tissue's response to that stress. Stress can be applied by external mechanical actuation, external compression, an internally applied acoustic radiation force (ARF), or endogenous motion, such as motion induced by the heart pressure pulse. The mechanical properties of the motion caused by the applied stress can be examined using sensitive high-frame-rate ultrasonic measurements that can detect micron-level displacements.

The ultrasound push beam can be focused on precise regions within the imaging field of vision using ultrasound array transducers. Long tonebursts, on the range of 100–1000 seconds, are utilized to generate a force that will disrupt the tissue to the point where motion can be detected, resulting in hundreds to thousands of ultrasonic cycles. Pulses used for B-mode imaging or Doppler measurements, for example, can be as short as 1–10 cycles or bursts as short as a few microseconds.

The tissue is pushed in the direction of the ultrasonic propagation due to the transfer of momentum from the ultrasound waves to the absorbing tissue. The tissue may continue to move in the direction of the ultrasound after the pushing toneburst stops, but it will gradually begin to go back to its equilibrium position. The motion within the push beam region of excitation (ROE), including the level of peak displacement and other aspects of the time-dependent behavior of the response, can be examined to evaluate the tissue. Multiple ARF pushes can be employed at the same time to examine the response, as in viscoelastic response (VisR) imaging. The tissue's elastic or viscoelastic properties govern these motion characteristics.

When a force is given to an elastic material, it deforms instantaneously. The material will return to its original location after the force has stopped. The response of a viscoelastic material to a force will have some delay or time dependence. Furthermore, once the force is removed, the material will take some time to return to its former location. The mechanisms of a spring mattress and a memory foam mattress are two common examples. When a depressing force is applied, a spring mattress responds swiftly and returns to its original position nearly instantly; however, a memory foam mattress takes time to deform and return to its original shape.