Ultrasonic bone testing apparatus with repeatable positioning and repeatable coupling

An ultrasonic bone testing apparatus has a foot well assembly and a shin guide assembly which are mechanically coupled to secure a foot and lower leg of patient in a comfortable position, during the measurement process. The shin guide assembly includes a molded form lined with contoured foam lining which has a shin restraint section, an instep guide section and a foot restraint section. An adjustable strap attached to the shin restraint section is placed around the calf. An instep support guide having sliding blocks is mounted on the molded form. The sliding blocks are inserted into respective channels of respective bridge brackets of the foot well assembly to attach the shin guide assembly to the foot well assembly. The channels and sliding blocks are lined with respective ratchet teeth to permit the latching of the shin guide assembly to foot well assembly at a selected one of multiple levels, and thereby the foot and lower leg can be secured comfortably. The apparatus further includes a transducer drive mechanism for positioning a pair of transducer assemblies. A controller automatically modifies the positioning of the transducer assemblies until ultrasonic coupling is achieved and a receiving transducer receives a signal of a predetermined quality. The distance between the transducers is continuously measured by a position encoder. The controller calculates a speed of the ultrasonic signals through the foot using the distance between the transducers which is determined by the position encoder. The controller uses temperature readings from a temperature sensor to improve the accuracy of the position encoder measurements and correct for temperature dependent inaccuracy in the ultrasound measurement. A coupling pad and acoustical delay line of the transducer assembly provide a waveguide function to collimate an acoustical beam a sufficient distance along a propagation axis to allow wavefronts to evolve onto a more uniform intensity pattern. The elastomer coupling pad has an angled surface which expels air bubbles from a contact area when pressure is applied to the pad. The coupling pad preferably is made of polyurethane with a durometer of 10 to 15 Shore A. Petroleum jelly is used as a coupling gel to more efficiently couple the ultrasonic energy. The coupling gel may be replaced by self-wetting coupling pads.