Displacement estimation of arterial wall from multiple directions by utilizing diverging transmit beam for synthetic aperture ultrasound imaging

Measurement of elasticity of the arterial wall is useful for diagnosis of atherosclerosis. We developed an axial displacement estimator for measurement of regional motion and deformation of the arterial wall in the arterial longitudinal section. However, this method cannot be applied to the arterial cross section because the direction of motion due to an increment of blood pressure coincides with the axial direction only at the ultrasonic beam which passes through the central axis of the artery. In the present study, a method using spherically diverging beams from virtual point sources behind an array was examined for synthetic aperture imaging and displacement vector estimation. Diverging beams were produced by sequentially activating array elements from center to edges of an aperture (composed of 96 elements). For each transmission, 61 receiving beams (RBs) were created at angular intervals of 1 degree. The transmit aperture was stepped in the lateral direction at a pitch of 0.2 mm to perform 97 transmissions sequentially. For synthetic aperture imaging, all receiving beams were used (overlapped beams were compounded), and receiving beams from -25 to 25 degrees at intervals of 5 degrees were used to estimate the displacement at a point of interest from multiple directions. In a B-mode image of a urethane phantom obtained by synthetic aperture imaging using diverging beams, echoes from interfaces are imaged in a wider region compared to conventional linear scanning. Also, displacement vectors in the cross section of the phantom could be estimated by the proposed method. Synthetic aperture imaging and estimation of the displacement from multiple directions was possible using diverging transmit beam from virtual point sources.