## 1 Introduction  
Ultrasound imaging (US) is a safe, portable, low-cost imaging method for assessing soft tissues, including skeletal muscle or viscera, and is widely used by different specialties (e.g., physiotherapists, cardiologists, radiologists, hepatologists or gynecologists) [1]. During the last years, several studies have developed technical reports to assess the validity and/or reliability of different imaging procedures [2] and imaging methods [3, 4].  
Elastography is a US physics-based imaging technology sensitive to tissue stiffness. It has been further developed and refined in recent years to make quantitative assessments of tissue stiffness [5]. Although the first elastography method was \"strain imaging\" (which consists of manual compression on the tissue with an ultrasound transducer), the most recent technology is the \"shear wave imaging\" method, which measures physical tissue displacement generated by shear waves perpendicular to the direction of the force produced with the transducer [6].  
Recent developments have made strain elastography more accurate by providing real-time feedback regarding the optimal pressure needed, with an indication bar scaled from 1 to 6 (where 1 is not appropriate and 6 is the most appropriate) [7]. Thus, strain elastography provides qualitative information based on a color map and semi-quantitative information expressed as the stiffness comparison between two areas within the same image. This strain ratio (SR) is calculated as the mean strain in the reference divided by the mean strain in the target.  
Prior evidence assessing SR accuracy by using both strain and shear wave elastography in calibrated phantoms presented greater accuracy in the shear wave method [8; 9]. One of the reasons explaining these findings is that strain elastography applies an unknown compression to the tissue and the deformation obtained is relative. Although shear wave elastography does not depend on the manual compression of the operator, as in strain elastography, it is more expensive, less accessible and it should be considered that all US methods are susceptible to be biased by shadowing, reverberation, clutter artifacts or operator experience [10].  
Therefore, considering (1) the accessibility advantages of strain elastography and (2) inconsistency in semi-quantitative SR results (evaluated methods, the system used and the position of the reference region of interest) [8], an important preliminary step for semi-quantitative elastography to be used and interpreted correctly in research or clinical practice is establishing validity, reliability and variability of measurements.  
The rationale for conducting this study is based on two reasons. Firstly, the accuracy of SR is directly dependent on the magnitude of the stiffness difference between both targets [11]. Although strain elastography has been widely assessed in phantoms under optimal conditions previously, these reports compared the stiffness difference between targets and backgrounds in big magnitudes ranging from 11-55 kPa [8; 9; 10; 11]. However, a recent study assessing stiffness differences between active and latent myofasical trigger points (MTrP) with control points within the same muscle showed smaller differences than those assessed in vitro (ranging from 0.04-2.76 kPa) [12]. Therefore, further research assessing the accuracy of SR calculation in tissues with similar stiffness is needed. Secondly, the influence of the examiners' experience for assessing SR is not consistent [8; 11; 13].  
The aims of this study were to determine semi-quantitative elastography SR validity, inter-examiner reliability and variability of measurements considering different locations within the images and the operators' experience using a phantom with homogeneous stiffness (since homogeneous tissues are characterized by zero stiffness difference) under optimal conditions.