Among other things, there is disclosed structure and methods for registering images obtained through internal (e.g. intravascular) ultrasound devices. Embodiments of a device with a rotating ultrasound beam is provided, with a wall of the device being anisotropic in ultrasound passage. As examples, a cable opaque to ultrasound is attached along the wall of the device, so that the ultrasound beam at the location of the cable is blocked, reflected or scattered. As another example, a thin film of metallic material is placed on or in the wall to allow a portion of the beam to be blocked or attenuated. The imaging system recognizes the changes to the signals made by the anisotropic wall, and registers successive images according to those changes.