Overview of Medical Ultrasound Elastography

Conventional ultrasound imaging technology uses the difference in acoustic impedance of body tissues for imaging. The impedance echo signal is difficult to distinguish between different tissues with the same acoustic impedance, such as breast hard tumors and healthy cysts, which are both darker areas and difficult to distinguish.B-MODEThey are both darker areas, making them difficult to distinguish.^[1]Ultrasound elastography (Ultrasound Elastography) is a new application of ultrasound technology. It mainly uses the correlation between the elastic modulus of tissues and the biological characteristics of lesions for imaging. It was first proposed byOphiret al. in1991year^[2]Similar to the method of diagnosing diseases by tapping and touching the softness and hardness of tissues, the elastic modulus of tissue can be expressed by the formulaE=T/S whereEis the elastic modulus,Trepresents stress,Srepresents strain. Ultrasound elastography applies an external or internal(including the body's own respiration and heartbeat)static/dynamic pressure to the tissue. Using ultrasound imaging methods, combined with digital signal processing or digital image processing technology, the corresponding changes in parameters such as tissue displacement, strain, and velocity are estimated, and the distribution of mechanical properties such as the elastic modulus inside the tissue is obtained indirectly or directly.

According to the different ways of applying mechanical stimulation, elastography can be divided into two categories: static/quasi-static imaging and dynamic imaging^[3]. Static imaging applies a constant/low-frequency stress to the tissue, measures the displacement before and after tissue deformation, obtains the strain,Sand then estimates the elastic modulusE. Because of the constant/low-frequency stressTis unknown, static elastography only reflects the contrast of elastic modulus between different tissues and cannot quantitatively calculate the elastic modulus of tissues; dynamic elastography mainly uses tissue shear waves (Shear Wave) to detect the elastic modulus, and most can perform quantitative detection of the elastic modulus. Longitudinal waves(Compressional Wave)`nbsp_tagThe speed of sound in tissue is around1500 m/s`nbsp_tagwhile the speed of shear waves is closely related to the shear modulus of the tissueμgenerally around1-50 m/s. There is a relationship between the speed of sound and shear modulus,while the density of tissue is generally around1000 kg/m³and does not fluctuate much. Therefore, the shear modulus can be indirectly measured through the shear wave velocity.μBecause biological tissues are incompressible, there is an approximate relationship between Young's modulus and shear modulus of3timesE=3μTherefore, the relationship between Young's modulus and shear wave velocity of biological tissues is. By measuring the shear wave velocity, the distribution map of Young's modulus can be obtained, and the elastic spectrum of the tissue can be obtained. Data show that the shear modulus of tissueμhas a larger variation range than Young's modulus,Ecan obtain better image contrast, so shear modulus can also be usedμfor direct imaging^[4]。

OphirThe idea of static elastography proposed early by the group has been adopted by many medical ultrasound companies. For example,Hitachiuses hand pressure to apply stress“Real-time Tissue Elastography”elastography technology, the corresponding model isEUB-8500；Philips`nbsp_taguses respiration and heartbeat to apply stress technology, the corresponding model isiU22etc. The imaging effect of the static elastography method is greatly affected by factors such as the personal experience of the probe operator and the magnitude of the applied pressure, and the repeatability is poor. However, due to the simple imaging algorithm and low hardware requirements, it is widely used in imaging of organs such as breasts and thyroid.

With the progress of ultrasound technology, in the proposal of ultrasound elastography25In recent years, especially in the last decade or so, new ultrasound elasticity methods have emerged continuously and have gradually been applied in commercial ultrasound imaging systems.1998Year,James Greenleaf The group proposed vibration ultrasound imaging technology, simultaneously applyingω₀andω₀+ωtwo beams of focused ultrasound pulse signals. The focal point will vibrate at a frequency ofωAnalyzing the vibration data can obtain the vibration displacement and elastic modulus information of the focal point. However, when performing a full-image scan, the ultrasound pulse energy attenuates severely and is time-consuming. This technology has not yet been put into practical application.2001Year, AmericanKathy Nightgale The group proposed acoustic radiation force impulse imaging technology (ARFI)They use the sound pressure generated by the focused sound beam to push the tissue to produce a slight displacement, then switch the transducer to the imaging mode to receive the ultrasound speckle signal generated by the tissue vibration, and obtain the displacement and elastic parameters of the tissue through signal analysis.Nightgale Later combinedARFItechnology with shear wave technology to formARFI-SWSThis imaging method wasSiemens Acuson S2000system.Philips`nbsp_tagEPIQ7 andEPIQ5system usesElastPQ The shear wave imaging scheme also uses a similar technical scheme^[5]。

The first1Ddynamic elastography probe was developed byLangevinlaboratoryCathelinein1995year. It consists of a conventional ultrasound probe and a mechanical pulse driver that generates tissue shear waves. Using ultrafast imaging (Ultrafast Imaging) method, at1000frames/sspeed to record ultrasound speckle information and measure tissue displacement. This technology was2001 year byEchosenscompany launched a commercial productFibroScanBased on1Ddynamic elastography probe,Langevinlaboratory1997year launched2Ddynamic elastography probe. The probe consists of a linear array probe and a driver that excites shear waves,2003 In vivo experiments were conducted in the year. Although the results were encouraging, due to its large size and heavy weight, practical application is still difficult.

To overcome2Dthe problems of elastography probes,2004yearBercoffet al. combined the two basic technologies of ultrasound elastography: supersonic shear wave technology and ultrafast imaging technology, and proposed supersonic shear wave elastography technologySSI（Supersonic Shear Imaging), allowing the sound beam focal point to move at a speed greater than the shear wave velocity, and then using ultrafast imaging technology at5000-30000frames/sspeed to collect information such as displacement and velocity of the tissue at the Mach cone boundary. This method can obtain excellent signal-to-noise ratio and imaging speed.2008In the year, Curie Laboratory conducted in vivo experiments on this technology,2012yearSupersonic Imaginecompany applied this technology to itsAixplorersystem, showing very good reproducibility in breast detection^[6]。

Ultrasound elastography is currently mainly reported in a few organs such as breast, thyroid, prostate, and liver. The elastic modulus data of tissues and organs are still lacking, and the data discrepancies are large in different literatures. Looking forward to the future development of medical ultrasound elastography,SSItechnology, shear modulus imaging,2Delastography undoubtedly represent the future development direction of elastography technology. However, with the improvement of elastography image quality and the quantitative detection accuracy of elastic modulus, conducting elastography research on more organs, establishing a clinical database of elastography, and forming industry standards as soon as possible will be important research content and development directions for ultrasound elastography in the future.

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