Elastography in Breast Imaging

Elastography has been one of the most intensely investigated additions to clinical ultrasound examinations for almost two decades. Elastography guidelines for many different anatomical regions have been published by different medical scientific societies [1–5]. In general, two different methods of elastography are available – strain (SE) and shear wave elastography (SWE) [6, 7]. Shear wave elastography has been most extensively researched for liver applications, but within recent years the method has also been evaluated for focal lesions in the breast, the thyroid and other organs. In shear wave elastography, the speed of shear waves, which arise perpendicularly to a push pulse from the transducer, is measured. In strain elastography, mechanical stress is applied to the transducer, and the deformation of tissue is shown on a color map, which may be interpreted visually or semi-quantitatively calculating a ratio. This method is mainly used for the evaluation of focal lesions. One of the first and most widely examined clinical applications for ultrasound elastography was the evaluation of breast tumors. In breast elastography, the evaluation of the strain images may be performed qualitatively or semi-quantitatively. The qualitative method uses the Tsukuba score with 5 categories, where an increasing number indicates an increased risk of malignancy [8]. The semi-quantitative measure is based on calculating a strain ratio, which is the deformation of a lesion relative to the surrounding tissue [9]. Several groups have tried to establish a cut-off for strain ratios to separate benign and malignant lesions, but without reaching consensus. Recent research has indicated that strain elastography may play a role mainly in distinguishing BIRADS 3 and 4 lesions [10, 11]. Early studies of ultrasound elastography focused on the feasibility of the different techniques and the comparison of elastography compared to B-mode ultrasound as a stand-alone examination [8, 12–14]. Most studies found, that elastography could not replace the B-mode examination, although specificity tended to be higher than for B-mode examinations as also demonstrated by a recent article in EJU [15]. Later, the combination of ultrasound elastography with the conventional B-mode examination was the primary focus of several studies, generally showing convincing results [11, 16]. In accordance with this, the latest edition of the BI-RADS classification system incorporated ultrasound elastography, either SE or SWE, as one of the associated features to assess the risk of malignancy [17]. More recently the focus of the diagnostic work-up of breast tumors with elastography has been centered on the evaluation of the difficult intermediate-risk BI-RADS classes, namely BI-RADS 3 and 4a, as was the case with a recent article published here in EJU [10]. In this study of more than 1000 breast tumors the pre-test probability of breast cancer of 1.3 % for BI-RADS 3, was improved to a post-test probability of 29.8 after using strain ratio calculation. Likewise, for BI-RADS 4 the pre-test probability of breast cancer was improved from 8.5 % to a post-test probability of 48.7 % following strain ratio calculation. In another study using both strainand shear-wave imaging, 290 out of 494 BI-RADS 4a tumors were downgraded to BI-RADS 3 with only one malignant lesion incorrectly downgraded [18]. A study using known breast Jonathan Frederik Carlsen Kristoffer Lindskov Hansen Caroline Ewertsen Michael Bachmann Nielsen Editorial