- greatest contributor to attenuation in tissues
- Potential source of bio-effects (discussed later)
acoustic speckle - noise resulting from the constructive and destructive interference of small
sound wavelets
-grainy appearance
Amplitude - -For a wave or vibration, the maximum displacement o...
- Potential source of bio-effects (discussed later)
acoustic speckle - noise resulting from the constructive and destructive interference of small
sound wavelets
-grainy appearance
Amplitude - -For a wave or vibration, the maximum displacement on either side of the equilibrium
(baseline) position.
- positive amplitudes (above baseline) and negative amplitudes (below baseline)
-units that are appropriate to the acoustic variable considered
EX: pressure amplitude = pascals (Pa)
Attenuation (a) - -is the weakening of sound while it propagates
- limits the imaging depth (high frequencies are attenuated faster therefor are shallower than lower
frequencies)
-its weakening effect must be compensated by the transducer
- a = 1/2(f x l) Units = decibels (dB)
, -3 mechanisms of attenuation: absorption, reflection and scattering
-DIRECTLY RELATED TO DISTANCE
-DIRECTLY RELATED TO FREQUENCY
Absorption - - conversion of sound energy to heat
Attenuation Coefficient - - the number of decibels of attenuation that occurs when sound travels
one centimeter
- Unit: dB/cm
- 0.5 dB of attenuation per centimeter for each megahertz of frequency (in soft tissues)
- the farther the sound travels, the more attenuation
- equal to 1/2 of the operating frequency
- f/2
constructive interference of scatter - - The interference that occurs when two waves combine to
make a wave with a larger amplitude
- In phase
- reinforce each other
Contrast Agents - -Micro-bubbles of gas stabilized by a protein, lipid or polymer shell create
stronger harmonics than tissue therefore detecting these harmonic frequencies increases the contrast
between the micro bubbles and the surrounding tissues.
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