A device for determining the internal structure of a bone along a path directed into the bone (or other tissue) is disclosed. The device comprises a nozzle fluidically connected to a liquid reservoir for providing a liquid jet directed at the bone in the direction of the path; an ultrasonic transducer for generating ultrasonic waves through the liquid jet and for detecting echoes of the ultrasonic waves caused by changes in the acoustical impedance in the bone characterizing changes in the structure of the bone along the path; and an analyzer for interpreting the echoes into meaningful information relating to the location of the structural changes along the path.