A Conversation with. . . Jacques Souquet, PhD

The diagnostic capabilities of ShearWave Elastography

The name of Jacques Souquet, PhD, is synonymous with ultrasound innovation, having been on the forefront of the technological development for more than 30 years, to the extent that the French newspaper Le Monde has called him the Steve Jobs of medical imaging. In 2005, Souquet
founded SuperSonic Imagine, based in Aix-en-Provence, France, where he and his colleagues have developed methods to harness shear waves to evaluate tissue elasticity, an indicator of pathology, including cancer. SuperSonic Imagine's Aixplorer ultrasound system received FDA approval in 2009.  We talked to Souquet about ShearWave Elastography (a trade name used by SuperSonic Imagine) and its diagnostic capabilities.

Q   rt image: What is ShearWave Elastography in ultrasound and how does it work?

A   Jacques Souquet: ShearWave Elastography is approved outside the U.S. to quantitatively assess the true elasticity of tissue in realtime. Tissue elasticity is related to pathology and is therefore an important diagnostic tool.

ShearWave Elastography leverages the interaction between ultrasound waves and shear waves to determine tissue elasticity. Shear waves are a natural occurrence in the body. A beating heart produces shear waves. Up until now, no ultrasound system has been able to exploit shear waves because of technological constraints.

ShearWave Elastography can be explained in three steps.

• The generation of shear waves: ShearWave Elastography uses the acoustic radiation force induced by ultrasound beams to displace tissue and create shear waves. To do this, pulses are successively focused at different depths in tissue at supersonic speed and are enhanced by forming a mach cone, which increases the shear wave propagation. This is done by a patented technology that does not require a change in workflow or a cool down period for the transducer.
• The capture of shear waves: ShearWave Elastography relies on an ultra-fast software platform that can acquire images up to 20,000 Hz. As shear waves move quickly through the body, an acquisition speed of at least 5,000 Hz is necessary to detect them. To capture in full the shear wave propagation, a flat insonification is used instead of a line-by-line acquisition method found in conventional ultrasound.
• The computation of shear waves: The Young’s Modulus tells us the speed of the shear wave propagation is directly related to tissue elasticity values. Shear wave propagation speed is calculated and true tissue elasticity is displayed in kilopascals on a color-coded elastography map of the region of interest. Each pixel in the ShearWave Elastography image has a kilopascal value, rendering a local reading of tissue elasticity, all in realtime.

Q   image: How is it different from conventional elastography?

A   Souquet: Conventional elastography is called strain or static elastography. Strain elastography uses a uniform mechanical compression at the surface of the body to cause deformation of the tissue. The compression is applied by the user and the ultrasound scanner calculates and displays the induced deformation and not the true elasticity of the tissue being imaged. This method is not quantitative; it is user-dependent and has poor reproducibility rates.

Q   image: What are the current and future applications?

A   Souquet: The current applications available are the breast, thyroid, and abdomen, most particularly liver and kidney. Future applications will include prostate, musculoskeletal,
and cardiovascular.

Q   image: You mentioned that ShearWave Elastography is user skill-independent. Please elaborate on what that means and how it works?

A   Souquet: ShearWave Elastography is user skill-independent because the generation of the shear waves is generated by software. The user does not have to induce pressure on the tissue being scanned and therefore has a lower risk of creating artifacts from compression. Strain elastography methods rely on the user’s skill to correctly apply the needed pressure to have a valid elastography reading. ShearWave Elastography takes out the guesswork, as there is not workflow change from B-Mode imaging to ShearWave Elastography imaging. Simply push a button and ShearWave Elastography is activated.

Q   image: What, if any, are the implications of user skill independence both for patient management and quality?

A   Souquet: Patient management can be improved through ShearWave Elastography. As ShearWave Elastography is user skill-independent, physicians and sonographers can scan the same patient and display the same results, reducing time in patient management.  In addition, as no transducer compression is required to do ShearWave Elastography, artifacts are reduced and image quality is enhanced.

Moreover, patient management and quality can be enhanced through reproducible Shear Wave Elastography results. Quantitative tissue elasticity parameters give an added diagnostic tool to characterization of tissue while also providing accurate lesion monitoring and therapy follow-up. Lesions can be followed over time in order to determine future therapy or to understand the reactivity of a lesion to therapeutic drugs.

–  rt image