SSI is a quantitative stiffness imaging technique based on the combination of a remote palpation induced into tissues by the radiation force a focused ultrasonic beam and an ultrafast ultrasound imaging sequence. Such ultrafast frame rates permit to track in real time the displacements induced by the propagation of the resulting shear waves. For each pixel, the shear wave speed can be estimated locally and enables quantitative mapping of the local shear elasticity (characterizing the stiffness in kPa). In this study, we will use a new generation of ultrafast ultrasound scanners (Aixplorer®, Supersonic Imagine, Aix en Provence, France) with a linear L10-2 probe (256 elements, 6 MHz) in neonates born between 25 and 40 weeks' gestation (n=100). Three separate acquisitions will be obtained for each area of interest both on right and left sides and stiffness was measured using a unique ROI of 2.5 cm².