Figure 1.

(A) Snell’s law is depicted in three dimensions at the interface between medium 1 (top), having propagation velocity c₁, and medium 2 (bottom), having propagation velocity c₂. The white arrow indicates the direction of a wavefront incident on the interface at angle ξ₁ with respect to the normal (dark gray arrow). Upon entering the second medium, the wavefront travels at an angle ξ₂ with respect to the normal. (B) Propagation paths for the proposed two layer model (shown in two dimensions only). Steering vector F is depicted for an outer element (black arrow); red and blue arrows indicate the actual propagation paths used to compute corrected delays for center and outer elements, respectively. F is the steering vector, n is the normal to the bone–tissue interface, a_x is the element coordinate in the x direction, c₁ and c₂ are the propagation velocities in tissue and bone, respectively, z is the focal distance, and t is the thickness of the planar layer (skull or Lucite plate).