What Is Elastography?

Elastography is to convert the acquired elasticity information of biological materials into visible light images used by doctors, so that doctors can judge the mechanical properties of materials of tissues through visible light images, and then judge the possible pathology of corresponding tissues or organs based on the soft and hard conditions of the tissues. Change and its position, shape and size.

Elastography vision

Vision is a physiological function of most living things. The purpose of seeing is to obtain information. The information that biological vision functions can only obtain is optical information. The information provided by optics includes size, color, binocular or multi-eye optical acquisition. After the information is processed by the brain, distance information can also be judged. Tactile sensation is also a physiological function of most living things. It can only be sensed by the sense organs of the living body, but cannot be seen by the visual system. For example, the temperature of the object and the degree of softness and hardness of the substance can only be known by touching. . In principle, our vision system can only feel the visible light band information of electromagnetic waves. The electromagnetic waves in this band cannot penetrate the interior of the object, so it can only provide information on the surface of the object. The question is, how can we see what is inside the object? Can you convert sensory information into visible optical information to facilitate observation and analysis?

Elastography

Imaging is the process and result of the coordinated work of the human visual system. It obtains optical signals through the eyeball and converts them into electrical signals, which are transmitted to the brain through the nervous system through electrochemical processes, and processed by the brain to make judgments and issue corresponding responses. Instructions to direct other systems of the organism to carry out corresponding activities. Imaging technology is a technology integration process in which people use various possible technologies to convert various characteristic information of objects into optical information that can be recognized by the biological vision system in order to make corresponding judgments. In fact, the significance and value of imaging technology is that it can effectively expand our visual range and improve our recognition ability. When we understand the imaging technology, we need to make some basic judgments. First, what is the substance you want to see, what properties do you want to understand, and what can you use as a detection messenger to help you effectively obtain this substance? Attributes, how can the obtained physical attributes be converted into optical signals that your visual system can perceive and easily identify. For example, telescopes and microscopes can effectively enlarge our range of viewing angles and help our auto vision system to obtain clear images. This is a direct aid; X-rays can penetrate objects and are absorbed by matter during penetration. The larger the atomic number Matters (substances with a larger mass of individual atoms) absorb more. If we irradiate an object with uniform intensity X-rays, the intensity of X-rays after passing through the object becomes uneven due to the different absorption of substances in different parts of the object. It is X-ray imaging to convert this uneven absorption situation into a visually distinguishable image. Our X-ray film is the simplest example, but it only gives plane information without depth information, which is what we often say Two-dimensional information. If you want to obtain three-dimensional information, that is, three-dimensional information, we can illuminate the object from different directions, and we can obtain three-dimensional information through calculation. This is the CT scan that our people often say; radio waves in electromagnetic waves It can enter the interior of the object, and when it interacts with the substance, it will react with water molecules in the substance. Resonance absorption, if the intensity of the radio wave when entering is uniform, the uneven distribution of the water component inside the material will cause the intensity of the radio wave passing through the material to be uneven. Through irradiation in different directions, the internal water of the material can be obtained through calculation. The stereoscopic image of the distribution of the components, we convert it into a recognizable image of our visual system, which is the magnetic resonance imaging of matter. In human tissues, the water composition of different tissues or the same tissue after normal and pathological changes is significantly different, and we can use MRI to diagnose the image of the disease. The result can be seen, but the organism itself can be recognized only by seeing, as a medical imaging method to reflect the elastic properties of soft tissues.

Elastography

Elasticity is a mechanical property of a material. What is said everyday is that the material has good elasticity, which means that the material is easy to deform, and the material is not elastic, which means that the material is not easy to deform. Of course, this means that the material acts under the same force. under. In the discipline of mechanics, the elasticity of a material refers to its ability to deform and return to its original shape under the action of external forces. It is generally expressed in terms of modulus. The force on a material can be in different forms. Its elasticity also reflects its Different deformation capabilities, for example, a material is subjected to a force in the same direction as its deformation direction. The deformation that occurs is called stretching (the direction of the force is consistent with the direction of deformation) or compression (the direction of the force is opposite to the direction of deformation). The modulus of this material's deformability is called Young's modulus. The larger the Young's modulus, the less easily the material deforms. The smaller the Young's modulus, the more easily the material deforms. The modulus that describes the shear deformation of the material is called shear. Shear modulus, the modulus that describes the volumetric deformation of a material is called bulk modulus. In short, the larger the material modulus, the less likely it is to deform. In organisms, the soft and hard condition of tissue materials is often related to the health of their tissues. For example, cancerous lumps are usually harder than normal tissues. The question is, can we turn the information about the softness and hardness of the human tissue (that is, its ability to deform) into image information used by doctors to provide doctors with a disease diagnosis technology? The answer is yes, this is elastography.

Elastography has multiple technical implementations, and its application extends from a large number of clinical applications to early research and exploration. Each elastography technique works differently, all of which require the deformation of the tissue. The mechanical properties of the tissue are inferred by observing and processing these deformations, and the results are usually displayed to the operator as images. Elastography methods are divided by how to achieve tissue deformation and obtain elastic properties of the tissue.

Elastography is divided into the following steps:

1 Elastography 1. Induced Deformation

Generally, the mechanical properties of tissue materials are sensed through tactile sensation. The essence of haptic perception is how easy it is to deform the material after it is touched. The material is easy to deform. In other words, if you want to know the soft and hard degree of the material, you must make it sexually change. In order to show the mechanical properties of the internal tissues of the human body, one must deform the tissue to a certain degree. This is the deformation induction. At present, there are three main methods used to induce deformation. They are:

1) Push or vibrate the surface of the body (usually the skin) through external mechanical devices or the subject's own limbs, transmitting external forces into the body, causing deformation of the tissue of interest;

2) Apply external ultrasound to the human body to cause vibration-induced deformation in the tissues of the subject;

3) Observe the deformation of the tissues of interest during normal physiological activities, such as pulse or heartbeat-induced deformation.

2 Elastography 2. Deformation observation

There are many ways to observe deformation. Depending on the image obtained, it may be one-dimensional (one line), two-dimensional (planar) or three-dimensional (volume), or just a single value, or it may be a video or an image. . In most cases, the results presented to the operator are accompanied by a conventional image showing the distribution of different hardnesses in the tissue. The classification of elastography technology is mainly based on the way of observing deformation. At present, elastography technology using ultrasound and magnetic resonance imaging occupies a dominant position, and there are many other elastography methods, including the application of light or mechanical pressure sensors.

3 Elastography 3, processing of deformation and obtaining hardness

Observing the deformation can obtain the hardness of the corresponding material through the mechanical relationship. Most elastography techniques obtain tissue stiffness based on two main mechanical relationships:

1) For a given force, harder tissue deforms less than softer tissue;

2) Mechanical waves travel faster in harder tissues than in softer tissues.

There are three ways to provide doctors with information about the mechanical properties of tissue or organ materials: they are:

1) The direct measurement information providing method, that is, the deformation or ultrasonic velocity obtained by the measurement is simply displayed to the doctor, and the doctor makes his own judgment. This requires the doctor to have a certain clinical biomechanical basis, and he must know in advance what kind of deformation or ultrasonic velocity What kind of organizational status it represents;

2) The method of providing mechanical information, which is to convert the measured deformation or ultrasonic wave speed into the hardness of the tissue material, such as Young's modulus or shear modulus, and display it to the doctor. This also requires that the doctor can accurately correspond to the hardness and tissue cases. relationship;

3) The direct diagnosis image information providing method, that is, the tissue deformation or ultrasonic wave velocity information obtained by the measurement is converted into the hardness information of the tissue by the computer according to the determined diagnosis rule, and then converted into imaging information that can determine the position, shape and size. Let doctors visually understand the extent of tissue lesions and make reasonable judgments based on their experience. This is true elastography.

Application of elastography

Elastography is mainly used in the diagnosis of diseases of soft tissues and organs. Compared with anatomical images, elastography can provide auxiliary diagnostic information for the mechanical status of tissues, can guide biopsy, and can sometimes replace biopsy in combination with other tests. . For example, patients with liver diseases such as liver fibrosis and fatty liver usually have higher hardness than normal liver. Elastography has huge advantages in the diagnosis of liver disease.

Elastography can be used to identify and diagnose breast, thyroid, and prostate cancer. Certain types of elastography are also suitable for skeletal muscle imaging. They can identify the mechanical properties and status of muscles and tendons.

Elastic imaging avoids the limitations of manual palpation, and its application can be extended to areas that cannot be reached by manual palpation clinics. For example, magnetic resonance elastography can assess the stiffness of brain tissue.

There are many methods of elastography, such as ultrasound elastography, quasi-static elastography / strain imaging, and magnetic resonance elastography. The dominant technology is magnetic resonance elastography.

In magnetic resonance elastography, a specific kind of mechanical vibration is placed on the surface of the subject's body, and the generated shear wave is propagated into the patient's deep tissue. The hardness of the tissue is inferred using an image acquisition sequence capable of measuring wave velocity (Shear modulus). The result of the scan is a three-dimensional image of quantitative tissue hardness, as well as a normal three-dimensional nuclear magnetic resonance image compared to it. One of its advantages is that it can give a three-dimensional elastic map covering the entire organ. Since magnetic resonance imaging is not limited to air and bone tissue, it can display tissues that cannot be displayed by ultrasound, especially brain tissue. Has the advantage of operator consistency, compared to most ultrasound elastography methods, it has been less dependent on the operator. However, magnetic resonance elastography requires a longer image acquisition time, about 15 minutes in each direction, which makes it time consuming and has a poor effect on moving tissue or tissue adjacent to the moving tissue. In addition, magnetic resonance imaging is more expensive than ultrasound imaging and is not convenient for patients and physicians.

Elastography References

[1] Wells, PNT (June 2011). "Medical ultrasound: imaging of soft tissue strain and elasticity". Journal of the Royal Society, Interface8 (64): 15211549.doi: 10.1098 / rsif.2011.0054.

[2] Muthupillai R, Lomas DJ, Rossman PJ, et al. Magnetic resonance elastography by direct visualization of propagating acoustic strain waves. Science 1995; 269: 1854-7. [49, 219, 220].

[3] Manduca A, Oliphant TE, Dresner MA, et al. Magnetic resonance elastography: Non-invasive mapping of tissue elasticity. Med Image Anal 2001; 5: 237-54.

[4] Sarvazyan; Hall, TJ; Urban, MW; Fatemi, M; Aglyamov, SR; Garra, BS. "An overview of elastography-an emerging branch of medical imaging". Current medical imaging reviews 7 (4): 255282 .

[5] Ophir, J .; Céspides, I .; Ponnekanti, H .; Li, X. (April 1991). "Elastography: A quantitative method for imaging the elasticity of biological tissues". Ultrasonic Imaging13 (2): 111 134.doi: 10.1016 / 0161-7346 (91) 90079-W.

[6] Parker, KJ; Doyley, MM; Rubens, DJ (February 2011). "Imaging the elastic properties of tissue: the 20 year perspective". Physics in Medicine and Biology56 (2): 513.doi: 10.1088 / 0031- 9155/57/16/5359.

[7] Sandrin, Laurent; Tanter, Mickaël; Gennisson, Jean-Luc; Catheline, Stefan; Fink, Mathias (April 2002). "Shear elasticity probe for soft tissues with 1-D transient elastography.". IEEE transactions on ultrasonics, ferroelectrics, and frequency control49 (4): 436--446.doi: 10.1109 / 58.996561.

[8] Ganne-Carrié N, Ziol M, de Ledinghen V et al. (2006). "Accuracy of liver stiffness measurement for the diagnosis of cirrhosis in patients with chronic liver diseases". Hepatology44 (6): 15117.doi : 10.1002 / hep.21420.PMID17133503.

[9] Nightingale, Kathy (November 2011). "Acoustic radiation force impulse (ARFI) imaging: a review". Current medical imaging reviews 7 (4): 328339.doi: 10.2174 / 157340511798038657.Acoustic.

[10] Supersonic Shear Imaging: A New Technique for Soft Tissue Elasticity Mapping. Bercoff J. et al., IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 51, No. 4, April 2004.

[11] Acoustoelasticity in soft solids: Assessment of the nonlinear shear modulus with the acoustic radiation force, J.-L. Gennisson, a M. Rénier, S. Catheline, C. Barrière, J. Bercoff, M. Tanter, and M Fink, J. Acoust. Soc. Am. 122 [1] 6, December 2007

[12] Mendelson EB, Chen J, Karstaedt P. Assessing tissue stiffness may boost breast imaging specificity.Diagnostic Imaging.

[13] Shear wave elastography for breast masses is highly reproducible. Cosgrove DO, Berg WA, Doré CJ, Skyba DM, Henry JP, Gay J, Cohen-Bacrie C; the BE1 Study Group. Eur Radiol. 2011 Dec 31.

[14] Shear-wave Elastography Improves the Specificity of Breast US: The BE1 Multinational Study of 939 Masses. Berg WA, Cosgrove DO, Doré CJ, Schäfer FKW, Svensson WE, Hooley RJ, Ohlinger R, Mendelson EB, Balu-Maestro C , Locatelli M, Tourasse C, Cavanaugh BC, Juhan V, Stavros AT, Tardlon A, Gay J, Henry JP, Cohen-Bacrie C, and the BE1 Investigators. Radiology 2012; 262: 435-449

[15] Kennedy BF, Kennedy KM, Sampson DD. [1] A review of optical coherence elastography: fundamentals, techniques and prospects. IEEE Journal of Selected Topics in Quantum Electronics 2014; 20 (2): 7101217.

[16] Weiss RE, Egorov V, Ayrapetyan S, Sarvazyan N, Sarvazyan A. Prostate mechanical imaging: a new method for prostate assessment. Urology 2008; 71 (3): 425-429.

[17] Egorov V, Sarvazyan AP. Mechanical Imaging of the Breast. IEEE Transactions on Medical Imaging 2008; 27 (9): 1275-87.

[18] Egorov V, van Raalte H, Sarvazyan A. Vaginal Tactile Imaging. IEEE Transactions on Biomedical Engineering 2010; 57 (7): 1736-44.

[19] Turo D, Otto P, Egorov V, Sarvazyan A, Gerber LH, Sikdar S. Elastography and tactile imaging for mechanical characterization of superficial muscles. J Acoust Soc Am 2012; 132 (3): 1983.