We wish to thank Drs. Zhang and Rubin for their comments and the editors of Academic Radiology for giving us the opportunity to clarify the issues raised. We agree that further research should be undertaken considering both physical/technical and medical questions. With our clinical feasibility study (“Anisotropy of Solid Breast Lesions in 2D Shear Wave Elastography Is an Indicator of Malignancy”), we hope to inspire other researchers to contribute to the question of anisotropy in solid breast masses on shear wave elastography (SWE) imaging.
Our clinical study aimed to improve the differentiation of benign and malignant solid breast masses using two-dimensional SWE (SuperSonic Imagine, Aix-en-Provence, France), with a shear wave frequency range of 70–800 Hz in homogeneous media. We investigated the influence of the imaging plane, that is, the position of the ultrasound (US) probe during the clinical scanning procedure. We obtained four images of each lesion, two each in two orthogonal planes. We compared the values derived from one imaging plane with the values derived from the orthogonal imaging plane to assess the directional dependence of the SWE measurements, that is, the anisotropy . For this purpose, we defined the anisotropic difference (AD) as the difference of the measurements and the anisotropic factor (AF) as the squared difference to assess the degree of anisotropy observed. We introduced these parameters as their assessment is feasible in a clinical study. Although we understand Drs. Zhang’s and Rubin’s concern for our parameter definition evaluating raw values only, practically it would not have been possible to measure intact breasts in all three dimensions during our clinical study.
It would be of interest in the future to determine the causes for the higher AF we observed in malignant and invasive breast lesions. We agree with Drs. Zhang and Rubin that our observation might be caused by the heterogeneity present in malignant tissues.
We agree that inclusion of an illustration of the imaging planes would have been beneficial. Unfortunately, an inclusion of figures is not allowed in a letter to the editor. We defined the radial and anti-radial imaging plane relative to the orientation of the lesion to the nipple. The radial plane connects the center of the lesion with the nipple. This plane is very easily defined. The sagittal and axial planes, on the other hand, are defined relative to the human body and are thus in random orientation to the breast anatomy. The position of the US probe on the skin of the patient does not affect the depth of the lesion, and the heterogeneity from the overlying tissue layers is minimal. Thus, the SWE measurements should not be impeded by rotating the US probe over the breast mass.
Our study included lesions of various sizes with a mean diameter of 15 mm. We evaluated the SWE measurements performed in the clinical standard presetting for breast imaging. Hence, all measurements were derived as Young’s modulus E in kPa instead of the shear wave speed m/s. Although it is possible to change the display unit at the device into shear wave speed, we have not changed the clinical presetting, thus ensuring comparability to other clinical studies. As equation 1 is applied internally by the US device, Drs. Zhang’s and Rubin’s concerns do not negate the clinical utility of our results. However, we agree with the authors that this should be considered by future studies.
All measurements were derived using a region of interest (ROI) size of 2 mm. We are unaware of any recommendation to use larger ROI sizes in the order of several centimeters to obtain measurements with supersonic SWE. Indeed, the recommendation by Berg et al. was the use of an ROI of 2 mm , and our previous study showed that using an even smaller ROI of 1 mm gives the best diagnostic performance if Emean is evaluated . Both AD and AF were ascertained using the Emean values in our study.
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References
1. Wikipedia contributors : Anisotropy. Wikipedia, 2017; 13:06UTC; Available at: https://en.wikipedia.org/w/index.php?title=Anisotropy&oldid=763124843 Accessed February 16, 2017
2. Berg W.A., Cosgrove D.O., Doré C.J., et. al.: Shear-wave elastography improves the specificity of breast US: the BE1 multinational study of 939 masses. Radiology 2012; 262: pp. 435-449.
3. Skerl K., Vinnicombe S., Giannotti E., et. al.: Influence of region of interest size and ultrasound lesion size on the performance of 2D shear wave elastography (SWE) in solid breast masses. Clin Radiol 2015; 70: pp. 1421-1427.