The velocity of sound in the tissues is assumed to be constant, and equal to 1540 cm/s. For typical clinical frequencies of 2 to 5 MHz, the wavelengths are on the order of 0.3 to 0.8 mm. Given the velocity of the wave, pulsed ultrasound methods calculate the distance traveled by the sound wave by measuring the time from the start of the pulse wave to the time taken to reflect from an object and return to the transducer.
ACOUSTIC IMPEDANCE The signal returning to the ultrasound transducer depends on the reflected ultrasound wave. The reflected ultrasound wave is generated at the acoustic interfaces between tissues of different density or through which sounds travel at different velocities. At these tissue interfaces, sound waves are either reflected back to the transducer (specular interface) or scattered into many different directions. The endocardial border represents a specular interface; sound waves are maximally reflected back from this interface when they are incident upon the interface at a 90° angle. As the sound waves become more parallel to the endocardium, the reflected waves are weakest.
Incident ultrasound waves that are scattered as they reflect from red blood cells may be used to assess the Doppler phenomenon, thus measuring the velocity of the red blood cells. The Doppler phenomenon refers to increased frequency of the ultrasound wave when the motion of the red blood cells is toward the transducer, and decreased frequency when the motion is away from the transducer. The Doppler shift, D f, is related to the velocity of the red blood cells, n, by the equation
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