Elastography

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Elastography
Diagnostics
Manual compression elastography of invazive ductal carcinoma 00132.gif
Manual compression elastography of invasive ductal carcinoma
MeSH D054459

Elastography is a medical imaging modality that maps the elastic properties of soft tissue. The main idea is that whether the tissue is hard or soft will give diagnostic information about the presence or status of disease. For example, cancerous tumours will often be harder than the surrounding tissue, and diseased livers are stiffer than healthy ones.

Elastography is a relatively new technology, and entered the clinic primarily in the last decade. The most prominent techniques use ultrasound or magnetic resonance imaging (MRI) to make both the stiffness map and an anatomical image for comparison.

Applications[edit]

Elastography is used for the investigation of many disease conditions in many organs. It can be used for additional diagnostic information compared to a mere anatomical image, and it can be used to guide biopsies or, increasingly, replace them entirely. Biopsies are invasive and painful, presenting a risk of infection, whereas elastography is completely noninvasive.

Elastography is used to investigate disease in the liver. Liver stiffness is usually indicative of fibrosis or steatosis, which are in turn indicative of numerous disease conditions, including cirrhosis and hepatitis. Elastography is particularly advantageous in this case because when fibrosis is diffuse, a biopsy can easily miss sampling the diseased tissue, which results in a misdiagnosis.

Naturally, elastography sees use for organs and diseases where manual palpation was already widespread. Elastography is used for detection and diagnosis of breast, thyroid and prostate cancers. Certain types of elastography are also suitable for musculoskeletal imaging, and they can determine the mechanical properties and state of muscles and tendons.

Because elastography does not have the same limitations as manual palpation, it is being investigated in some areas for which there is no history of diagnosis with manual palpation. For example, magnetic resonance elastography is capable of assessing the stiffness of the brain, and there is a growing body of scientific literature on elastography in healthy and diseased brains.

Historical background[edit]

Palpation has long been used to detect disease. In a breast self-examination, women look for hard lumps, as cancer is usually stiffer than healthy tissue.

Palpation is the practice of feeling the stiffness of a patient's tissues with the practitioner's hands. Manual palpation dates back at least to 1500 BC, with the Egyptian Ebers Papyrus and Edwin Smith Papyrus both giving instructions on diagnosis with palpation. In Ancient Greece, Hippocrates gave instructions on many forms of diagnosis using palpation, including palpation of the breasts, wounds, bowels, ulcers, uterus, skin and tumours. In the modern Western world, palpation became considered a respectable method of diagnosis in the 1930s.[1] Since then, the practice of palpation has become widespread, and it is considered an effective method of detecting tumours and other pathologies.

Manual palpation, however, suffers from several important limitations: it is limited to tissues accessible to the physician's hand, it is distorted by any intervening tissue, and it is qualitative but not quanitative. Elastography, the measurement of tissue stiffness, seeks to address these challenges.

How it works[edit]

There are a host of different elastographic techniques, running the spectrum from extensive clinical use to early stages of research. Each of these techniques works in a different way. What all methods have in common is that they create a distortion in the tissue,A observe and process the distortion to infer the mechanical properties of the tissue, and then display the results as an image to the operator. Each elastographic method is characterized by the way it does each of these things.

Inducing a distortion[edit]

To image the mechanical properties of tissue, we need to see how it behaves when deformed. There are three main ways of inducing a distortion to observe. These are:

  • Pushing or vibrating the surface of the body (usually the skin) with a mechanical device or the practitioner's arm
  • Using ultrasound to create a 'push' or a high or low frequency mechanical wave inside the tissue
  • Observing distortions created by normal physiological processes, like the pulse or heartbeat (this is called endogenous motion imaging)

Observing the distortion[edit]

The primary way elastographic techniques are categorized is by what imaging modality (type) they use to observe the distortion. At the present time, elastographic techniques using ultrasound and magnetic resonance imaging (MRI) dominate the field.[1] There are a handful of other methods that exist as well, including using light or using mechanical pressure sensors.

The observation of the distortion can take many forms. In terms of the image obtained, it can be 1-D (i.e. a line), 2-D (a plane) or 3-D (a volume), or just a single value, and it can either be a video or a single image.

Processing the distortion to find the stiffness[edit]

A scoring card allowing comparison of the liver's Young's modulus (stiffness) to fibrosis stage with elastography. Quantitative methods have the advantage of allowing comparison with reference values.

Once the distortion has been observed, the stiffness can be found from it. Most elastography techniques find the stiffness of tissue based on one of two main principles:

  • For a given applied force (stress), stiffer tissue deforms (strains) less than does softer tissue.
  • Mechanical waves (specifically shear waves) travel faster through stiffer tissue than through softer tissue.

Some techniques will simply display the distortion or the wave speed to the operator, while others will compute the stiffness (specifically the Young's modulus or similar shear modulus) and display that instead. Some techniques present results quantitatively, while others only present qualitative (relative) results.

Ultrasound elastography[edit]

There are a great many ultrasound elastographic techniques. The most prominent are highlighted below.

Quasistatic Elastography / Strain Imaging[edit]

A strain elastography image (centre) of the prostate gland detects a cancer (dark red area at lower left). The finding is confirmed by histology.

Quasistatic elastography (sometimes called simply 'elastography' for historical reasons) is a pioneering elastography technique. In this technique, an external compression is applied to tissue, and the ultrasound images before and after the compression are compared. The areas of the image that are least deformed are the ones that are the stiffest, while the most deformed areas are the least stiff.[2] Generally, what is displayed to the operator is a an image of the relative distortions (strains), which is often of clinical utility.[1]

From the relative distortion image, however, making a quantitative stiffness map is often desired. To do this requires that assumptions be made about the nature of the soft tissue being imaged and about tissue outside of the image. Additionally, under compression, objects can move into or out of the image or around in the image, causing problems with interpretation. Another limit of this technique is that like manual palpation, it has difficulty with organs or tissues that are not close to the surface or easily compressed.[3]

Transient Elastography[edit]

Main article: FibroScan

Transient elastography gives a quantitative one-dimensional (i.e. a line) image of tissue stiffness. It functions by vibrating the skin with a motor to create a passing distortion in the tissue (a shear wave), and imaging the motion of that distortion as it passes deeper into the body using a 1D ultrasound beam. It then displays a quantitative line of tissue stiffness data (the Young's modulus).[4] This technique is used mainly by the FibroScan system, which is used for liver assessment,[5] for example, to diagnose cirrhosis.

Acoustic Radiation Force Impulse imaging (ARFI) and Shear Wave Elasticity Imaging (SWEI)[edit]

Acoustic Radiation Force Impulse Imaging (ARFI) uses ultrasound to create a qualitative 2-D map of tissue stiffness. It does so by creating a 'push' inside the tissue using the acoustic radiation force from a focused ultrasound beam. The amount the tissue along the axis of the beam is pushed down is reflective of tissue stiffness; softer tissue is more easily pushed than stiffer tissue. By pushing in many different places, a map of the tissue stiffness is built up.

A related method is called Shear Wave Elasticity Imaging (SWEI). Like ARFI, a 'push' is induced deep in the tissue by acoustic radiation force. The disturbance created by this push travels sideways through the tissue as a shear wave. By using ultrasound to see how fast the wave gets to different lateral positions, the stiffness of the intervening tissue is inferred. SWEI therefore shows a quantitative stiffness value at a few locations, while ARFI shows only a qualitative value, but in a full 2-D map. Despite their differences, SWEI is sometimes grouped with ARFI or considered a form of ARFI, and is sometimes called quantitative ARFI.[6]

Methods of elastography that vibrate or push the surface of the tissue have some trouble with imaging deep tissues because the distortion used to find the elasticity is very, very weak by the point when it reaches deep tissues. Methods that use radiation force, however, can create distortions in deep tissue relatively easily, making them generally better at imaging deep tissues.[3]

Supersonic Shear Imaging (SSI)/Shear Wave Elastography (SWE)[edit]

Conventional ultrasonography (lower image) and elastography (supersonic shear imaging; upper image) of papillary thyroid carcinoma, a malignant cancer. The cancer (red) is much stiffer than the healthy tissue. The scale is in kPa of Young's modulus.

Supersonic Shear Imaging gives a quantitative, real-time two-dimensional map of tissue stiffness. It is often called 'Shear Wave Elastography', though it is not the only method to use shear waves. Like ARFI and SWEI, supersonic shear imaging uses acoustic radiation force to induce a 'push' inside the tissue of interest, and like SWEI, the tissue's stiffness is computed from how fast the resulting shear wave travels through the tissue. By using many near-simultaneous pushes, and by using an advanced ultrafast imaging technique to track the wave, supersonic shear imaging can make a two-dimensional quantitative map of the tissue's stiffness (the Young's modulus), and create one every second.[7][8]

It has demonstrated clinical benefit in breast, thyroid, liver, prostate and MSK imaging. Ultrasound Elasticity Imaging is used for breast examination with a number of high-resolution linear transducers.[9] A large multi-center breast imaging study has demonstrated both reproducibility [10] and significant improvement in the classification[11] of breast lesions when shear wave elastography images are added to the interpretation of standard B-mode and Color mode ultrasound images.

Magnetic Resonance Elastography (MRE)[edit]

An anatomical MRI image of a brain (top) and an MRE elastogram of the same brain (bottom). The stiffness is in kPa of shear modulus.

Compared to all the variation in ultrasound elastography techniques, elastography with magnetic resonance imaging (MRI) is relatively uniform. For this reason, the dominant technique is simply called Magnetic Resonance Elastography (MRE).

In MR elastography, a mechanical vibrator is used on the surface of the patient's body; this creates shear waves that travel into the patient's deeper tissues. An imaging acquisition sequence that measures the velocity of the waves is used, and this is used to infer the tissue's stiffness (the shear modulus).[12][13] The result of an MRE scan is a quantitative 3-D map of the tissue stiffness, as well as a normal 3-D MRI image to compare it to.

One strength of MR elastography is the resulting 3D elasticity map, which can cover an entire organ.[14] Because MRI is not limited by air or bone, it can access some tissues ultrasound cannot, notably the brain. It also has the advantage of being more uniform across operators and less dependent on operator skill than most methods of ultrasound elastography.

On the other hand, MR elastography suffers from long acquisition times, in the neighbourhood of 15 minutes per direction. This makes it time-consuming, and also impractical for tissues that move or are close to other tissues that move. MR imaging is additionally more expensive than ultrasound and less convenient for patients and physicians.[1]

Other techniques[edit]

In addition to the dominant ultrasound and magnetic resonance methods there are a handful of others. These include elastography with optical coherence tomography (i.e. light) and tactile imaging.

Tactile Imaging[edit]

Main article: Tactile imaging

Tactile Imaging is a medical imaging modality that translates the sense of touch into a digital image. Tactile Imaging is used for imaging of the prostate,[15] breast,[16] vagina and pelvic floor support structures,[17] and myofascial trigger points in muscle.[18]

Notes[edit]

A.^ In the case of endogenous motion imaging, instead of inducing a disturbance, disturbances naturally created by physiological processes are observed.

References[edit]

  1. ^ a b c d Wells, P. N. T. (June 2011). "Medical ultrasound: imaging of soft tissue strain and elasticity". Journal of the Royal Society, Interface 8 (64): 1521–1549. doi:10.1098/rsif.2011.0054. 
  2. ^ Ophir, J.; Céspides, I.; Ponnekanti, H.; Li, X. (April 1991). "Elastography: A quantitative method for imaging the elasticity of biological tissues". Ultrasonic Imaging 13 (2): 111–134. doi:10.1016/0161-7346(91)90079-W. 
  3. ^ a b Parker, K J; Doyley, M M; Rubens, D J (February 2011). "Imaging the elastic properties of tissue: the 20 year perspective". Physics in Medicine and Biology 56 (2): 513. doi:10.1088/0031-9155/57/16/5359. 
  4. ^ 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 control 49 (4): 436–446. doi:10.1109/58.996561. 
  5. ^ 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". Hepatology 44 (6): 1511–7. doi:10.1002/hep.21420. PMID 17133503. 
  6. ^ Nightingale, Kathy (November 2011). "Acoustic radiation force impulse (ARFI) imaging: a review". Current medical imaging reviews 7 (4): 328–339. doi:10.2174/157340511798038657.Acoustic. 
  7. ^ 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.
  8. ^ 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
  9. ^ Mendelson EB, Chen J, Karstaedt P. Assessing tissue stiffness may boost breast imaging specificity. Diagnostic Imaging. 2009;31(12):15-17.
  10. ^ 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.
  11. ^ 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, Tardivon A, Gay J, Henry JP, Cohen-Bacrie C, and the BE1 Investigators. Radiology 2012;262:435-449
  12. ^ 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].
  13. ^ Manduca A, Oliphant TE, Dresner MA, et al. Magnetic resonance elastography: Non-invasive mapping of tissue elasticity. Med Image Anal 2001; 5: 237-54.
  14. ^ 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): 255–282. 
  15. ^ 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.
  16. ^ Egorov V, Sarvazyan AP. Mechanical Imaging of the Breast. IEEE Transactions on Medical Imaging 2008; 27(9):1275-87.
  17. ^ Egorov V, van Raalte H, Sarvazyan A. Vaginal Tactile Imaging. IEEE Transactions on Biomedical Engineering 2010; 57(7):1736-44.
  18. ^ 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.

See also[edit]