3D ultrasound-based dynamic and transient elastography: first in vitro results

3D magnetic resonance (MR) elastography is a well-established technique based on monochromatic mechanical excitations to study soft tissue mechanical properties. MR imaging observations are used to calculate tissue viscoelastic properties and has been validated in vivo, but the technique is limited by the acquisition time (about ten minutes). We study the feasibility of performing 3D dynamic elastography using an ultrasound based imaging system. The ultrasound approach provides a low cost system and reduces the acquisition time by a factor of 300 (to about 2 seconds). Two ultrasonic arrays are moved perpendicularly using a stepper-motor-control positioning system to acquire ultrasonic images in a given volume of interest. Mechanical excitation is induced by a 20 mm square plate linked to an external vibrator. Displacements induced by the vibrator are calculated from ultrasound images using a 2-dimensional estimator (Tanter, M. et al., 2002). Each ultrasonic array is able to calculate in-plane (axial and lateral) displacements. Lateral displacements are the same from both arrays and are averaged. The configuration leads to the estimation of the three displacement components in a full 3D area. Imaging sequences for 3D displacements have been realized. Experiments were conducted in a phantom mimicking heterogeneous tissue with two harder inclusions. 3D shear elasticity, viscosity and anisotropy maps of the studied phantoms are presented and show the feasibility of building a full ultrasound based system for 3D dynamic elastography providing a complete description of tissue mechanical properties in a few seconds.