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Ultrasound Principles

Velocity of sound in soft tissues: 1540 m/s, lowest in gases, faster in fluid, faster in ST and fastest in bones. Reflection = backscatter

Harmonic imaging: filtering fundamental echoes

Real time compounding: averaging signals from multiple angles, improving signal-to-noise ratio

Power output: strength of the pulse in dB, has thermal index, mechanical index

Gain: amplification of signal from deeper tissue to compensate for more attenuation when compared with superficial structures; TGC (Time Gain Compensation)

Focal zone: focusing the beam at different depth improves resolution at this level

Doppler effect: sound that reflects off a moving object changes frequency: object moving toward the transducer reflect sound at a higher frequency and vice versa. The difference in transmitted and received frequency is Doppler frequency shift and depends on Doppler angle (axis of probe and axis of moving object)

Resistive index = (peak systolic velocity - peak diastolic velocity) / peak systolic velocity, nl 0.5 - 0.7; RI goes up when resistance to flow goes up (thrombosis)

Duplex Doppler sonography: Gray-scale sono + Pulsed Doppler (waveform)

Shadowing: due to sound absorption (stones, calcs; anechoic, clean shadow) or reflection (gas; dirty shadow); fat produce partial shadowing (AML)

Posterior enhancement/Increased through transmission: less attenuation in fluid so pulse is stronger after passing through

Mirroring: gas is the best acoustic mirror, interface between lung and adjacent ST (liver, liver lesion, diaphragm, Doppler)

Ring Down artifact: gas, series of bright echoes

Aliasing: Wrap around from the positive extreme of the scale to the negative extreme; identifies areas of high flow velocities; Increase Pulse Reptition Frequency to decrease aliasing

Tissue vibration: turbulent blood flow - AV fistulas, stenosis, aneurysms