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Ultrasonic İmaging. Ultrasound – The propagating media interaction. Scattering (Uniform and ..) Reflection Refraction Absorbtion. The reflected wave from a boundary deviates, Cannot be interpreted as reflection or refraction The Phenemenon is called as diffraction

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ultrasound the propagating media interaction
Ultrasound – The propagating media interaction
  • Scattering (Uniform and ..)
  • Reflection
  • Refraction
  • Absorbtion
  • The reflected wave from a boundary deviates,
  • Cannot be interpreted as reflection or refraction
  • The Phenemenon is called as diffraction
  • Huygens principle expresses diffraction
huygens principle
Huygens Principle
  • Huygens principle states that every point in the surface can be modeled as a source
  • emitting ultrasonic waves
  • The effects of all individual point sources should be accumulated in order to determine
  • The field intensity on a particular point, mathematically
beam pattern
Beam Pattern
  • Rearrange field intensity at P point using paraxial, fresnel and fraunhofer approximations,
  • Result is important because it states that the far-field intensity is the fourier transform
  • of aperture function; kx/z and ky/z are spatial frequencies
  • U(P) shows far-field beam pattern of A(x,y) and it defines the beam quality
  • Wider apertures results narrower beams, thus aperture size affects beam width

near field transition
Near Field Transition
  • The assumptios are not valid for near field which is smaller than D2/λ distant
  • The flat aperture may be assumed that it is focused to infinity; emitted waves have
  • the same phase at infinity
  • If the aperture is shaped to focus a certain point, the assumptions are valid at that point
pulse echo
  • Some imaging systems rotates the transducer in order to steer its receive/transmit beam
  • Transducer transmits US signal to the each angle in imaging area and receives the
  • reflected signal, The TX/RX operation is known as pulse echo
  • Echo carries impedance information of corresponding steering angle
ultrasonic imaging system
Ultrasonic Imaging System
  • The major blocks of an imaging system;
  • Transducer array receives or transmits the US signal
  • Transmit beamformer focuses the array to half-depth of the imaging area
  • Receive beamformer dynamicaly focus the array to different depths
  • Signal processor adopts the data for standart video monitors
sampled transducer array
Sampled Transducer (Array)
  • Single transducer enables fixed focused or not focused operation. That disables in-phase
  • sum of signals out of focal point.
  • Instead of single mechanical focus transducer can be sampled in order to form a
  • transducer array, which enables multiple focus by applying proper delays
steering and focusing
Steering and Focusing
  • Multiple focal zones are possible using an array.
  • Multiple transmit focus is not practical; dynamic focusing is employed only in receive mode
  • The beamforming can mathetmatically be expressed as follows,
  • , , where s(.) is input signal,
  • τ is beamforming delays, c is velocity of US, F is focal distance and b(t) is beamformed signal
beamforming techniques
Beamforming Techniques
  • Full Phased Array
    • All array elements simultaneously activated for transmit and receive
    • Requires complex front-end electronics
    • Improved SNR, Proportional with N√N
  • Classical Synthetic Aperture
    • The same element is activated for transmit and receive
    • Simple front end
    • Poor SNR, Proporional with
  • Synthetic Phased Array
    • All array element pairs individually activated using multiple pulse-echo
    • Average SNR