**2. Two elastography techniques**

Two different sorts of medical equipment can make use of elastography:


US elastography is of two different types:

Real Time Elastography Strain (RTES) using an external constraint Shear Wave Elastography (SWE) using an internal constraint.

For both techniques, the results of the 'estimation of tissue stiffness' are either qualitative or quantitative.

#### **2.1 RTES: (SE)**

This technique uses an external constraint induced to the tissue which is generated by the physician himself by positioning the probe in contact with the skin. The probe allows him to apply repetitive minimal compression/vibration to the breast tissue, the compression wave being propagated at the speed of 1500 m/s. The software of the echography machine allows the analysis of the relative movements of the different tissues thanks to the stress/strain ratio distribution providing an image of relative tissue stiffness, classified with the Tsukuba coloured criteria of stiffness of the lesion itself and of the surrounding tissues (classification in keeping with the BI-RADS radiological classification) with five scores [3]:

*Breast Elastography DOI: http://dx.doi.org/10.5772/intechopen.102445*

Score 1—No difference between the different tissues Scores 2 and 3: Increasing proportion of the stiffer zone Score 4: typical permanent internal stiffness of the lesion itself Score 5: stiffness extends beyond the margins of the lesion to the background (**Figures 1** and **2**).

The real cut-off point between benign and malignant corresponds to the score 3–4 boundary.

The use of the (ROI) region of interest on the most reproducible frame (selected with cine-loop) can be placed to cover the lesion and its adjacent tissue (as softly as possible, ideally just under the skin) to allow calculation of the fat/lesion ratio.

These data increase the diagnostic confidence in the differentiation between benign and malignant diseases without adding significantly to the length of the examination.

The sensitivity is over 90%, the specificity 88% when adding US elastography in multicentric series of several thousands of breast lesions with a good accuracy and reproducibility [4].

#### **Figure 1.**

*Strain elastography: benign fibro-adenoma: fat to lesion ratio = 1.65.*

**Figure 2.** *Strain elastography: malignant lesion with a fat/lesion ratio = 17.47.*

#### **2.2 SWE**

A third dimension, that is a piece of histo-pathological information obtained through the assessment of time elasticity is achieved by the Shear Wave Elastography technique (SWE), the first two dimensions being morphological grey scale images on the one hand and functional flow imaging of soft tissue on the other.

The waves travel in these tissues at 1–10 m/s with Shear Waves.

Focused ultrasound beams are automatically generated at supersonic speed by the probe and received back for analysis. The analysis of the propagation of ultrasound wave trains in the tissue structures is the trademark of SWE.

A mind-boggling calculation capacity is needed to capture shear waves and measure their propagation speed at the frame rate of up to 20,000 images per second.

Shear waves propagate faster in hard tissue than in soft one. The shear wave speed can be converted in colour (optionally in grey scale) or in kilo Pascal (kPa).

A specific region of interest (ROI) of different stiffer parts of the lesion or of adjacent tissue gives precise information on the variations in stiffness. This measurement in kilo-Pascal is a characteristic of the SWE technique. It allows extremely precise results.

Fat elasticity varies from 3 to 9 kPa; glandular stroma elasticity from 11 to 50 kPa. The average figure for the elasticity of malignant lesions is higher than 100 kPa and can even reach over 150–200 kPa (in 13 published studies).

All the lesions with a figure higher than 50/60 kPa are to be biopsied, thanks to an impressive improvement in the sensitivity and the negative predictive value up to 100% according to EVANS. The specificity as well as the positive predictive value are also significantly improved with a better evaluation of the 3–4 BI-RADS

**Figure 3.** *SWElastography: benign lesion without colour modification +28 kPa.*

#### **Figure 4.** *SWE with a sub-cutaneous malignant invasive carcinoma = 284 kPa.*

cases which can thus be reclassified upward or downward as the case may be, which leads to a considerable reduction in the number of unnecessary biopsies (**Figures 3** and **4**) [5].
