Looking into teeth with ultrasound.
Ultrasound is readily conducted across a boundary when the specific acoustic impedances of the two media are about equal. The specific acoustic impedance of dental enamel is about that of aluminum. A longitudinal sonic pulse, less than 250 nanoseconds in duration, conducted to the tooth through an aluminum rod, has positively detected the enamel-dentin junction as well as the dentin-pulp interface.
D000535:Aluminum; D000818:Animals; D002417:Cattle; D003743:Dental Enamel / Q000033:anatomy & histology*; D003782:Dental Pulp / Q000033:anatomy & histology; D003804:Dentin / Q000033:anatomy & histology*; D007180:Incisor / Q000033:anatomy & histology; D014159:Transducers; D014465:Ultrasonics
D000535:Aluminum