Ultrasound Ultrasound (US) represents a mechanical pressure disturbance propagating as wave through materials dense enough to transmit the fast oscillations imparted on molecules. 2-12 MHz in medical practice.
Sound is a traveling variation of acustic variables.Acustic variables: particle motion, density and pressure.
Description of US waves Amplitude - decibels (dB) Frequency (f) - Cycles/sec – Hz Wavelength (λ) - mm Velocity of propagation (c)- depends on each carrying medium - approximately 1540 m/s in blood and soft tissue
Amplitude of the Sound Amplitude: the difference between the reference value (1x10-12 W/m2 ) and the maximum value of an acoustic variable (dB). Sound scale: 0 dB hearing treashold 130 dB painful
Amplitude Decibels (dB) are logarithmic units based on a ratio of the measured value V of acoustic pressure to a reference value R ((1*10-12 W/m2) : dB = 20 log (V/R) For example: ratio of 2 20 x log 2 = 20 x 0.3 = 6 dB Ratio of 1000 = 60 dB
Propagation Velocity, Frequency and Wavelength c=fxλ c= m/s, mm/µs λ = mm f = cycles/s (Hz), cycles/µs (MHz)
Propagation velocity of US indifferent tissues (m/s or mm/µs ) m/s mm/µs Air 330 0.33 Lung 500 0.5 Fat 1450 1.45 Soft tissue 1540 1.54 Bone 4000 4C (human soft tissue) = 1540 m/s or 1.54 mm/µs