Ultrasound elastography is the use of ultrasound to produce measurements or images of tissue stiffness. Several different approaches are used, but all rely on creating some form of displacement of the tissue and measuring the response. Ultrasound elastography can be implemented in a number of ways; from simple add-ons for conventional ultrasound scanners, to dedicated ultrasound elastography units. Different techniques are used to generate the results, depending on the particular approach and clinical application.
Ultrasound elastography is a new ultrasonic imaging technique introduced to produce images of the Young’s modulus distribution of compliant tissue. This Young’s modulus distribution is derived from the ultrasonically estimated longitudinal internal strains induced by an external compression of the tissue. The displayed two-dimensional images are called elastograms.
Tissue strain analytics could represent the most important development in ultrasound technology since the advent of Doppler imaging. Tissue Strain applications provide a new dimension of diagnostic information through either qualitative assessment or quantitative measurement of the mechanical stiffness of tissue. Tissue strain analytics compliments conventional ultrasound exams by providing an extra dimension of information – mechanical stiffness. This additional information can be combined with anatomical (B-mode) as well as vascular (Doppler) information such that the actual diagnosis is contained by the intersection of three dimensions of information.
In order to produce ultrasound elastography images or measurements, the tissue to be assessed are stressed by the application of a force. This force may be applied by the operator, an external mechanical device, physiological motion, or by the ultrasound probe itself in the form of acoustic waves. The response of underlying tissue structures to such stress varies according to tissue stiffness. Perhaps the simplest form of ultrasound elastography is to acquire data before a stress is applied. By comparing these two sets of data, it is possible to derive a ‘stiffness’ measure. A stiff object will tend to move as a whole, whereas soft tissue compresses more unevenly, with tissue closer to the applied force compressing more than those further away.
Ultrasound elastography has been most frequently used in the assessment of liver fibrosis in a non-imaging, quantitative capacity, but in recent years the technique has been investigated in a broad array of clinical applications. Of particular note is the potential of ultrasound elastography in the assessment of liver fibrosis and in breast imaging. These subjects have been the subject of a significant body of research and were the primary focus of this review. Other clinical applications for which the ultrasound elastography technique has been investigated include: endoscopic, vascular and prostate imaging, which were briefly reviewed for this evaluation, as well as thyroid, skin and brain tumors, which were outside the scope of this review.
Clinical research has shown very promising results in differentiation between benign and malignant tissue in thyroid gland, breast, and prostate and to assess liver fibrosis.
Also, endoscopic ultrasound elastography is a new application in the field of the endosonography and seems to be able to differentiate benign from the malignant nodes and pancreatic lesions with a high-sensitivity specificity and accuracy. Ultrasound elastography is superior compared to conventional B-mode imaging and the interobserver reproducibility is satisfying.
Intravascular ultrasound elastography is a promising tool for studying atherosclerotic plaque composition and assessing plaque vulnerability. Current ultrasound elastography techniques can measure the 1D or 2D strain of the vessel wall using various motion tracking algorithms. Since biological soft tissue tends to deform non-uniformly in 3D, measurement of the complete 3D strain tensor is desirable for more rigorous analysis of arterial wall mechanics. Atherosclerotic plaque rupture is the major cause of acute coronary syndromes. Currently, there are no reliable diagnostic tools to predict plaque rupture. Knowledge of plaque mechanical properties based on local artery wall strain measurements would be useful for characterizing its composition and predicting its vulnerability.
Currently, a quiet revolution is occurring in diagnostic ultrasound, the results of which are benefiting all ultrasound practitioners. Smaller, portable machines are becoming available, image quality is dramatically improved, real-time imaging is now possible, and solutions are being found for those patients that were previously difficult to image.