Ultrasound elastography (UE) depicts the stiffness of tissues [1,2,3,4,5]. Just like manual palpation, elastography can distinguish a hard focal lesion from a soft lesion. Compared with manual palpation UE has the advantage of evaluating deeper lying lesions, and furthermore UE is semi-quantifiable. Several methods for semi-quantification exist today. One method uses a visual-scale score to evaluate the focal lesions; another uses strain-ratios and compares the strain of a region of interest in a focal lesion with the strain in the surrounding tissue. Elastography is available in most high-end ultrasound systems. This article presents an overview of strain elastography (SE) and its applications in breast cancer diagnostics.

UE measures the strain of tissues [1,2,3,4,5]. The strain of a tissue is defined by the change in length during compression divided by the length before compression. The relationship between the compression, or the stress, and the strain is calculated in the Young’s modulus, E = stress/strain, which estimates the stiffness of a certain tissue [4]. Most elastography techniques measure the strain and not the Young’s modulus, and therefore a direct quantification is not possible.

Strain elastography is available in most high-end ultrasound systems. SE measures axial displacement of tissue caused by mechanical stress in real-time. The stress is either applied externally with the transducer by the operator or by physiological shifts inside the patient. Transducer stress is applied by continuously compressing and decompressing the skin of the patient, a few millimeters of axial transducer movement being sufficient.

The elastogram is derived from data of the change of radio frequency signals before and after compression. The elastogram is displayed in a split-screen mode with the conventional B-mode image and the elastogram on the monitor (Figure 1). The elastogram may be displayed as a color-overlay on the B-mode picture. Most SE-systems display tissue stiffness in a continuum of colors from red to green to blue, designating soft (high strain), intermediate (equal strain) and hard (no strain) tissue. However, there is no color standard as yet, and some SE-systems have an inverse color scale of the others. As mentioned, SE measures the strain of tissues and not the Young’s modulus; thus, per definition the resulting elastogram is not quantifiable. There are several ways of providing a semi-quantifiable elastographic measure applicable in a clinical setting. These are especially useful when evaluating whether focal lesions are malignant or not.

Semi-quantifying elasticity was first described and used in the elastographic evaluation of breast tumors by Itoh et al. [6]. The visual scoring system compares the strain, i.e., the elastographic color of the lesion, with the strain of the surrounding tissue and can be applied in other anatomic areas as well [7,8,9,10,11].

The elastograms are divided into certain categories correlating to the likelihood of malignancy and range from benign (I), probably benign (II), uncertain (III), probably malignant (IV) to malignant (V) [6]. Another way of semi-quantifying the stiffness of a tissue is to use strain-ratios (SR) [12,13]. A SR-measurement compares the strain in two manually selected regions of interest (ROIs) on the elastograms. One ROI is placed in the focal lesion, and the reference ROI is placed in the surrounding normal tissue, preferably in the same depth as the lesion. The SR is automatically calculated by the elastography software and yields the fraction of the average strain in the reference area divided by the average strain in the lesion. The higher the SR, the higher the likelihood of malignancy. Cut-off values of the SR provide the highest diagnostic accuracy [13].

Elastographic examinations are highly operator dependent. Several studies have evaluated the interobserver variability of SE both in a phantom [14] and in vivo [7,8,9,10,11], but the results are not unanimous, since interobserver agreement ranges from high to low.

SE has been applied in the ultrasound examinations of suspected breast cancer. The 10 largest published studies on visual scored elastography and breast cancer are listed in Table 1. These 10 studies were found on a search in Pubmed on the 20 August 2012. The search query contained the words “elasticity imaging techniques” (Mesh terms) or “ultrasound elastography”, and the words “Breast Neoplasms” (Mesh terms) or “Breast cancer” and yielded 200 abstracts. The abstracts were read by one author (JFC) and 33 articles met the inclusion criteria, i.e., reporting the diagnostic values of five-point scale visual-scored SE in the evaluation of breast lesions. Only studies on lesions without prior histological diagnosis were included. Of the 33 articles, 10 articles with the largest number of evaluated lesions were included in Table 1. The algorithm for the inclusion of these articles is illustrated in Figure 2.

The inclusion criteria in the 10 selected studies differed somewhat, but all studies used the same five-point scoring system and a cut-off value between 3 and 4 for assessing the likelihood of malignancy, categories 1–3: benign and categories 4–5: malignant. The inclusion criteria for the lesions varied slightly, but all final diagnoses were made by pathological assessment of biopsies. Some studies only assessed small breast lesions (<2 cm [16,17] or <l cm [16]) as these are difficult to evaluate on B-mode imaging.

SE is widely available and easy to use in a clinical setting. The fact that SE is real-time and can be done bedside along with the B-mode examination makes the use of SE feasible in a lot of different anatomic areas. In breast cancer, SE has shown great potential and a good diagnostic performance in several studies. While sensitivity decreased in all eight studies comparing B-mode US with SE included in this review, specificity and accuracy increased in seven of eight studies.

When comparing B-mode US with combined B-mode and SE, data are more controversial. The specificity and overall accuracy of the sonographic evaluation of breast lesions increased when combining B-mode- and SE-imaging in four of five studies presented in this article.

Although SE has poor sensitivity, SE in combination with B-mode imaging shows better diagnostic performance.

We suggest that elastography should be used as an adjunct to the clinical B-mode examination of suspected breast cancer. However, the algorithm used to combine the two diagnostic methods differed between the studies presented here. The best algorithm for the combination of B-mode and SE-imaging still needs to be established in future clinical studies.