An ultrasonic diagnostic imaging system is described in which a multi-element transducer array transmits a focused beam of energy from the transducer elements. As the depth of focus is varied between near and far field focusing under operator control, the nominal frequency of the transmitted ultrasound...http://www.google.com.hk/patents/US4534221?utm_source=gb-gplus-share專利 US4534221 - Ultrasonic diagnostic imaging systems for varying depths of field
