Elastography is a new technique that evaluates tissue stiffness. There are two elastography methods, strain and shear wave elastography. Both techniques are being used to evaluate a wide range of applications in medical imaging. Elastography of breast masses and prostates have been shown to have high accuracy for characterizing masses and can significantly decrease the need for biopsies. Shear wave elastography has been shown to be able to detect and grade liver fibrosis and may decrease the need for liver biopsy. Evaluation of other organs is still preliminary. This article reviews the principles of elastography and its potential clinical applications.

Keywords

Elastography

Ultrasound

Breast mass

Breast cancer

Prostate cancer

Liver fibrosis

Thyroid mass

Key points

Introduction

With the recent Food and Drug Administration approval for quantification using ultrasound elastography, these techniques are rapidly gaining acceptance for many clinical applications. Multiple vendors have some form of elastography available on their systems. In this article, the principles of the 2 major types of ultrasound elastography are briefly reviewed, highlighting their advantages and disadvantages. The clinical applications that are becoming widely accepted as the standard of care are discussed. Emphasis is placed on how to incorporate these techniques into your practice. A brief review of other potential applications that are not yet mature enough for routine clinical use is provided.

Principles

There are 2 types of ultrasound elastography presently available, strain elastography (SE) and shear wave elastography (SWE).1,2 Although these both measure tissue stiffness, there are differences in the techniques and how they are used in clinical practice.

SE is a qualitative (not quantitative) technique. The images obtained demonstrate the relative stiffness of the tissues within the field of view (FOV). However, the absolute stiffness of the tissue is not known; other factors, such as relative stiffness compared with normal tissue or change in size compared with B-mode, must be used to obtain clinically useful information.2,3 SE images are obtained by comparing the frame-to-frame changes of tissues when a vibratory or compression/release force is applied to the tissue. Soft tissues deform more, whereas hard tissues deform less (Fig. 1). The amount of compression/release force to obtain optimal SE images varies by vendor. In some cases, only the patients’ breathing and/or heartbeat are required to generate optimal elastograms, whereas, in others, compression and release using the transducer are required. With practice, the optimal scanning technique can be learned and be reproducible. Some vendors have a bar, or quality measure, on the monitor that provide real-time feedback on the appropriateness of the amount of compression/release being applied.2

The results are provided in an image that can be displayed in gray scale or in a variety of color maps. The data provided are the same regardless of which color map is used, and the map choice is often dictated by the user’s experience. It is important to remember that because SE is a relative measure of stiffness, the FOV should contain a range of tissues with varying stiffness to allow for an appropriate dynamic range in the scale for adequate interpretation.2

SWE is a technique that provides a quantitative measure of stiffness that is expressed in meters per second (the shear wave speed) or in kilopascals (Young’s Modulus).1,2 Most systems allow for either to be displayed, and they are easily converted from one to the other. In SWE, a push pulse, often referred to as acoustic radiation force impulse (ARFI), is used to generate shear waves within the tissues, which is similar to dropping a stone into a pond (the push pulse) and generating waves on the water (shear waves). Note that the shear waves are generated perpendicular to the push pulse. Conventional B-mode imaging is used to monitor the shear waves generated through the tissue and calculate the shear wave speed (Fig. 2).

In this technique, either a single measurement over a small FOV can be obtained (point quantification SWE) or color mapping of a large FOV of individual pixel shear wave speeds is depicted (2-dimensional [2D] SWE). The color map used to display the data is usually red as hard and blue as soft. The color scale can be adjusted to allow better depiction of the range of shear wave speeds within the FOV, and careful attention to the scale is advised if interpretation based on color is used.

A limitation of SWE is that the push pulse is attenuated as it traverses tissue and ultimately reaches a point where it is too weak to generate shear waves. In this case, the area/tissue where adequate shear waves are not obtained is not color coded (black) on 2D SWE or a value of x.xx or 0.00 is obtained with point quantification SWE.1,2 In SWE, the absolute stiffness value can be used for lesion characterization or the ratio of the lesion stiffness to normal tissue stiffness can be used.

Another elastography technique is available whereby ARFI is used to generate tissue deformation and the deformation is used to calculate a strain image. This method is a qualitative strain method and not an SWE method. This technique is called Virtual Touch Imaging (VTI, Siemens Ultrasound, Mountain View, CA).

One factor that is critical in obtaining accurate elastograms using either the SE or SWE techniques is the amount of precompression or preload used when obtaining the elastogram.2 This precompression is the amount of pressure exerted on the tissues when the scan is taken. As tissues are compressed with the transducer (ie, using a heavy hand), the tissues become stiffer. In general, softer tissues are affected more than stiffer tissues; therefore, both the relative values in SE as well as the absolute values of SWE can be affected enough to lead to inaccurate tissue assessment. A method of applying minimal precompression has been described and found to be highly reproducible.4

Breast elastography

Both SE and SWE have been shown to have high sensitivity and specificity for characterization of breast masses as benign or malignant.5–13 Breast elastography has been recommended for breast lesion characterization in the guidelines of both the European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB) and the World Federation for Ultrasound in Medicine and Biology (WFUMB).14,15 Previous work in vitro has demonstrated that breast cancers are significantly stiffer than benign breast lesions with little overlap, suggesting elastography would be an excellent method for breast lesion characterization.16 Careful attention to technique is important for both types of elastography and are descripted in detail...