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.
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.
- 1 Applications
- 2 Historical background
- 3 How it works
- 4 Ultrasound elastography
- 5 Magnetic Resonance Elastography (MRE)
- 6 Other techniques
- 7 Notes
- 8 References
- 9 See also
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.
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. 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
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,† observe and process the distortion to infer the mechanical properties of the tissue, and then display the results to the operator, usually as an image. Each elastographic method is characterized by the way it does each of these things.
Inducing a distortion
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
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. 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. In most cases, the result is displayed to the operator along with a conventional image of the tissue, which shows where in the tissue the different stiffness values occur.
Processing the distortion to find the stiffness
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.
There are a great many ultrasound elastographic techniques. The most prominent are highlighted below.
Quasistatic Elastography / Strain Imaging
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. Generally, what is displayed to the operator is an image of the relative distortions (strains), which is often of clinical utility.
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.
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). This technique is used mainly by the FibroScan system, which is used for liver assessment, for example, to diagnose cirrhosis.
Acoustic Radiation Force Impulse imaging (ARFI) and Shear Wave Elasticity Imaging (SWEI)
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.
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 (namely ARFI, SWEI, SSI and SWE), however, can create distortions in deep tissue relatively easily, making them generally better at imaging deep tissues.
Supersonic Shear Imaging (SSI)/Shear Wave Elastography (SWE)
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.
It has demonstrated clinical benefit in breast, thyroid, liver, prostate and musculoskeletal imaging. Ultrasound Elasticity Imaging is used for breast examination with a number of high-resolution linear transducers. A large multi-center breast imaging study has demonstrated both reproducibility  and significant improvement in the classification 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)
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). 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. 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.
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 is a medical imaging modality that translates the sense of touch into a digital image. Tactile Imaging is used for imaging of the prostate, breast, vagina and pelvic floor support structures, and myofascial trigger points in muscle.
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- †^ In the case of endogenous motion imaging, instead of inducing a disturbance, disturbances naturally created by physiological processes are observed.
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