generally cardiac ultrasound uses frequencies of 2.5-5 MHz.
transducer acts as:
source of ultrasound - majority of energy emitted is absorbed or scattered. However some sound energy is reflected at interfaces between tissues of different acoustic impedence (eg muscle and blood).
receiver of ultrasound - recieves reflected sound. The transducer acts as source and receiver by emitting short bursts of ultrasound and then 'listening' for returning echoes.
determination of depth of structures:
sound travels at a relatively constant speed (1540m/s). The depth of a structure which has reflected a particular echo can thus be determined by the delay between emission of the signal and the recording of the echo on the transducer. The position of the spot on the display screen is determined by the delay, and the brightness of the spot is determined by the magnitude of the returning signal.
resolution of images:
the higher the frequency the better the axial resolution (ie discrimination between structures along the direction of the beams of ultrasound). However higher frequencies are attenuated more quickly and have less penetration. Thus the image generated for a particular study uses the highest frequency that still allows visualization of the deep structures being studied.
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