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Ultrasonic İmaging

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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Ultrasonic İmaging

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  1. Ultrasonic İmaging

  2. 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

  3. 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

  4. 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  

  5. 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

  6. 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

  7. C-Mode Display

  8. 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

  9. 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

  10. 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

  11. 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

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