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Ultrasound Wave Propagation

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This presentation gives the technology behind the Ultrasound wave propagation.

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									Mechanisms of ultrasound wave
        propagation
    Nature of Wave Propagation
• The acoustic wave launched by the transducer
  is propagated in the tissue

  – as a longitudinal pressure wave that causes local
    variations of

     • pressure density and velocity of the medium.
                  Assumptions

• The tissue is assumed

  – to act like an isotropic non absorbing
    homogeneous fluid

     • in which shear forces are negligible
Wave Equation
Final Equation
              Signal Processing
• The interrogating pulse has a
  – Carrier frequency in the range of 2 to 7 MHz,
    duration of 2 to 3 cycles, and
     • With Gaussian envelope


• The pulse repetition rate lies
     • in the range of 1 to 3 KHz
• Considering that the velocity of acoustic
  waves in tissues is approximately that in
  water (viz. 1540 m/s), the wavelengths lie in
  the range of 0.20 to 0.75mm.

• The attenuation depends on the tissue and
  frequency; however, as a rule of thumb it can
  be taken to be 2 db/(depth in cm, the
  frequency in MHz)
• Compensation of attenuation is a
  prerequisite for a uniformly bright image.
  This is achieved by Time gain control. (TGC)
• Modern ultrasound imaging systems use
  digital electronics and signal processing.

  – The sampling rate for the AD converters and delay
    lines must be chosen at a value at least twice the
    highest bandwidth of the incoming signals
   Block Diagram of
B Mode Imaging System
• The transmitting and receiving piezoelements are
  activated in groups.

• By doing this, the beam is focused region-wise in
  several regions.

• The implementation of this technique of
  “dynamic focusing”
     • can be performed elegantly by employing digital signal
       processing.
• The received signals in the individual groups
  are
  – digitized by means of the AD converters,
     • delayed appropriately and finally summed up


• The sum is then demodulated using a digital
  envelope detector, thus
  – achieving a high SNR even for the low-amplitude
    signals
• The signal is processed to compose the B-
  mode image, which consists of lines and pixels
  as in TV frames

• Finally, the signal samples are stored in a 2-D
  array

• The gray level of a pixel is coded with 6 to 8
  bits

								
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