A polysiloxane (i.e., silicone) (ECOFLEX-0010, Smooth-On, Inc., Easton, Pennsylvania, USA) was selected as the polymer matrix due to its tissue mimicking properties (25 ), durability and reasonable stability over time. It is less vulnerable to change over time as compared to water-based agar gel or gelatin counterparts that are widely used as phantoms in MRE studies (26 (link)). Also, Ecoflex provides a strong and homogenous cohesion to the container walls used as the mechanical actuation source. Ecoflex is formed by mixing parts 1A and 1B in 1:1 by weight or volume and cure at room temperature with negligible shrinkage. Two methods were utilized to minimize air bubble trapping during the mixing process prior to sample curing. In the first approach the Ecoflex mixture was poured with a very thin strip into the container inside a vacuum chamber (5305-1212, Thermo Scientific-Nalgene, Rochester, NY). In the second method, the Ecoflex was poured onto larger flat plates in a vacuum chamber to speed up the escape of air bubbles from the mixture; then, the air bubble-free Ecoflex was poured into the test container slowly before it sets.