# Intra-System Reliability Assessment of 2-Dimensional Shear Wave Elastography
## 2 Materials and Methods  
This study utilised a model 049A human tissue mimicking quality assurance elastography phantom produced by Computerized Imaging Reference Systems, Inc., Norfolk, VA, USA (CIRS) made from Zerdine(r), a poly-acrylamide polymer [24] (Figure 2). This phantom has acoustic properties comparable to human tissue, with a reported acoustic velocity of 1540 m/s \(\pm\) 10 m/s and an attenuation coefficient of 0.5 dB-cm\({}^{-1}\).MHz\({}^{-1}\)[25]. Zerdine(r) phantoms from CIRS have been validated and have demonstrated adequate homogeneity and low viscosity levels [9; 18; 26]. Other investigations using tissues mimicking phantoms have shown that ultrasound frequencies used in the MHz range can be translated in vivo [27; 28]. Figure 1: 2D-SWE Ultrasound elastography display of two phantom targets using circular ROI (**a**) Type IV (85 kPa) and (**b**) Type I (10 kPa). Each image display is split into the parametric colour map on the left, (higher SWV coded as red/orange and slower SWV as blue/green) and reliability propagation map on the right (parallel lines indicating increased reliability). The phantom contains a series of eight cylindrical tubes which taper from 16.7 mm to 1.6 mm at set intervals from one end of the phantom to the other. Each tube provides an imaging target containing one of four known elasticity values, Type 1 (10 kPa), Type II (17 kPa) Type III (47 kPa) and Type IV (85 kPa). These targets offer a range of elasticity values dependent on the clinical area of interest. Current machine dependent reference ranges for the liver, report values <7 kPa for normal tissue and >17 kPa for advanced fibrosis [2]. Elastography studies of the breast have demonstrated values between 30 and 50 kPa for normal parenchymal tissue and much higher values in malignant breast lesions [29]. The four targets are located at 3 cm and repeated at 6 cm depth. The background medium surrounding the targets can be sampled and has a known elasticity of 25 kPa. The largest diameter (16.7 mm) was chosen to limit the potential for shear wave refraction at the target boundary [30] and to accommodate the sampling ROI. SWV measurements were performed using a Canon Aplio i-series 600 machine (Canon Medical, Otawara-shi, Japan) with a curved 8C1 probe. The system can make the conversion of SWV to Young's modulus E (kPa) using the equation E = 3\(p\)(SWV)\({}^{2}\). This equation assumes the tissue density is equal to water (\(\rho\) = 1 g/cm\({}^{-3}\)), and that the relationship between Young's modulus and shear modulus (G) is E \(\approx\) 3 G, which occurs for incompressible materials having a Poisson's ratio of 0.5. Soft tissue is often regarded as incompressible [30]. In this study, we chose not to convert the machine measured values of SWV to kPa to focus on the sources of intra-system variability as they applied to SWV. Elastography guidelines recommend reporting values in SWV, as this is the quantity measured by the machine and  
Figure 2: Characteristics of the Model 049A elastography phantom (CIRS, Inc., Norfolk, VA, USA) (**a**) Top view showing the position of the phantom target cylinders (**b**) Dimensions of stepped target cylinders (**c**) Side View—Target types and depth. (**d**) Phantom enclosure. Used with permission from CIRS, Ltd.  
the conversion to kPa relies on a range of assumptions, which can introduce additional errors [21; 31]. Elastography samples were acquired using the system's single-shot method, which activates a 2.5 MHz acoustic radiation force pulse over a single frame. SWV measurements were independently performed by 2 experienced Sonographers (CE and EC), each with over 15 years of ultrasound experience, and 1 less experienced practitioner (AJ) to evaluate the potential of a training effect. To ensure the distribution of the sample means from each operator were approximately normally distributed and to conduct parametric tests, 30 consecutive SWV measurements were obtained from each phantom target and background [32]. SWV measurements were repeated at two available depths 3 cm and 6 cm.