<prompt> Ultrasonography transducer containSodium flurideLead ZirconateCaesium fluorideNone  <response>: Some naturally occurring materials possess piezoelectric propeies e.g., quaz But most crystals used in medical ultra sound are man-made. This group of aificial piezoelectric materials are known as ferroelectrics. Barium titanate was the first of the ceramic ferroelectrics to be discovered. This has been largely replaced by lead zirconate titanate, commonly known as PZT. Now lead zirconate titanate is the most commonly used Several types of PZT are available, with slight variations in chemical additions and thermal treatment producing different propeies. Ref : Christensens Physics of Diagnostic Radiology, 4E,Pg no.329 