How does ultrasound bidirectional axial transmission reflect geometry of long bones?

Axial transmission (AT) techniques assess bone properties along the long bones axis over a length of a few cm. This technique has been advocated to be appropriate for cortical thickness assessment. Some techniques measure the speed of waves transmitted axially in one direction (monodirectional AT) whereas other techniques average the speed of waves which are transmitted axially in two opposite directions (bidirectional AT). The issues addressed here are the comparison between monodirectional and bidirectional techniques and the test of a simplified model to reflect propagation on realistic geometry. It was found that the model of the plate of constant thickness could represent well propagation on realistic geometry as long as bidirectional velocity was considered.