Medical diagnostic imaging with ultrasound—sonography—is accomplished with a pulse–echo technique similar to radar and sonar. An array transducer emits a short pulse of ultrasound with a frequency between 2 and 15 MHz. As that pulse travels through the anatomical structures of the human body, a stream of echoes produced by impedance changes is generated and returned to the transducer. Asonographic instrument processes the returning echo stream and digitally stores it as a gray-scale scan line. Subsequent pulses directed to different portions of the anatomy are imaged, and in time a two-or three-dimensional image is written into memory. Between 92 and 256 scan lines make up a 2D frame and many 2D frames can be stacked to form a 3D picture.

Sonographic instruments display gray-scale anatomical images, and frame rates are fast enough that movement can be followed in real time. In some cases, Doppler-shifted echoes caused by tissue or blood-flow...

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