Piezoelectric crystals are made use of that is safe from radiation also:

B i.e. US Type of USG USG frequency - Trans abdominal USG for 3-5 MHzQ obstetric purpose 5- 7.5 MHzQ - Trans vaginal USG for obstetric purpose - USG for breast 15 MHz - Endoscopic USG for gutwall 7.5 - 20 MHz - To image vessel wall 20 MHz - cathater