Anisotropic Properties of Breast Tissue Measured by Acoustic Radiation Force Impulse Quantification

Abstract

The goal of our study was to investigate the anisotropy of normal breast glandular and fatty tissue with acoustic radiation force impulse (ARFI) quantification. A total of 137 breasts in 137 women were enrolled. These breasts were divided into the duct-apparent group and the duct-inapparent group, divided into the ligament-apparent group and the ligament-inapparent group. Shear wave velocity (SWV) in the radial (SWV^r) and anti-radial (SWV^a-r) directions was measured. The elastic anisotropy of glandular tissue and fatty tissue was evaluated as the ratio between SWV^r and SWV^a-r. The SWV ratio was 1.30 ± 0.45 for glandular tissue and 1.27 ± 0.53 for fatty tissue in the total group. In glandular tissue, the SWV ratio of the duct-apparent group was higher than that of the duct-inapparent group (p = 0.011). In both glandular and fatty tissue, the SWV ratio was higher in the ligament-apparent group than in the ligament-inapparent group (p < 0.05 for both). SWV^r was higher than SWV^a-r in both glandular tissue and fatty tissue in all groups (p < 0.05 for all) except in breast fatty tissue in the ligament-inapparent group (p = 0.913). It is concluded that both breast glandular tissue and fatty tissue exhibited anisotropy of elastic behavior. To improve the diagnostic power of elastography in breast lesions, the elastic anisotropy of glandular tissue and fatty tissue should be taken into account in calculating strain ratio or elasticity ratio.

Introduction

Ultrasound plays a very important role in breast lesion characterization (Hooley et al. 2013). Breast ultrasound elastography, a new ultrasound imaging technique providing information on stiffness, has become a routine tool in addition to conventional ultrasound in clinical practice (Barr et al., 2015, Cosgrove et al., 2013, Hooley et al., 2013, Li et al., 2013, Sadigh et al., 2012). Strain elastography and shear wave elastography are the two types of elastography currently available. In strain elastography, in addition to qualitative elasticity score, the semiquantitative strain ratio is employed to discriminate benign and malignant breast lesions. The strain ratio is calculated by comparing the strain of the lesion with that of the adjacent normal glandular tissue or fatty tissue (Zhi et al. 2008). Similarly, in some studies using shear wave elastography, glandular tissue or fatty tissue is used as the reference normal tissue to calculate the shear wave velocity (SWV) ratio (Jin et al. 2012) or the elastic modulus ratio (Youk et al. 2013).

The aforementioned studies, however, do not take into account the possibility of anisotropy (i.e., the tissue's material properties are orientation dependent) in breast glandular tissue or fatty tissue. It is commonly known that anisotropy in biological tissue is a rule more than an exception (Gennisson et al., 2003, Levinson, 1987). This anisotropy is a consequence of the orientation of the tissue fibers (Bouchard et al., 2009, Gennisson et al., 2010, Mace et al., 2011). The breast is composed of glandular and fatty tissue fixed by fibrous bands known as Cooper's ligaments. The glandular tissue is drained by ducts that are arranged in a radial fashion (Going, 2011, Love and Barsky, 2004). Similarly, Cooper's ligaments are distributed in a radial orientation in the whole breast (Oliveira et al. 2012). Therefore, we hypothesized that breast glandular and fatty tissue might exhibit an anisotropic elasticity distribution, which might have an effect on the diagnostic performance of elastography using strain ratio or elasticity ratio.

To the best of our knowledge, to date there has been no research exploring the anisotropic property of normal breast tissue using ultrasound shear wave elastography. Few studies have investigated the elastic anisotropy of breast tissue using ultrasound, and all were focused on detecting the anisotropy of only breast lesions, not normal breast tissue (Skerl et al., 2013, Skerl et al., 2016). These studies indicated that malignant breast lesions are more anisotropic than benign lesions. Therefore, anisotropy has the potential to be used to differentiate benign from malignant solid breast masses using shear wave elastography. The aim of this study was to investigate the anisotropy of normal breast glandular and fatty tissue with acoustic radiation force impulse (ARFI) quantification technology.