Ultrasonic maging Ultrasound The propagating media interaction Scattering

  • Slides: 20
Download presentation
Ultrasonic İmaging

Ultrasonic İmaging

Ultrasound – The propagating media interaction • Scattering (Uniform and. . ) • The

Ultrasound – The propagating media interaction • Scattering (Uniform and. . ) • The reflected wave from a boundary deviates, • Reflection • Cannot be interpreted as reflection or refractio • Refraction • The Phenemenon is called as diffraction • Absorbtion • Huygens principle expresses diffraction 2

Huygens Principle • Huygens principle states that every point in the surface can be

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 3

Beam Pattern • Rearrange field intensity at P point using paraxial, fresnel and fraunhofer

Beam Pattern • Rearrange field intensity at P point using paraxial, fresnel and fraunhofer approximatio • 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 4

Near Field Transition • The assumptios are not valid for near field which is

Near Field Transition • The assumptios are not valid for near field which is smaller than D 2/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 poin 5

Pulse-Echo • Some imaging systems rotates the transducer in order to steer its receive/transmit

Pulse-Echo • Some imaging systems rotates the transducer in order to steer its receive/transmit bea • 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 6

C-Mode Display

C-Mode Display

Ultrasonic Imaging System • The major blocks of an imaging system; • Transducer array

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 17

Sampled Transducer (Array) • Single transducer enables fixed focused or not focused operation. That

Sampled Transducer (Array) • Single transducer enables fixed focused or not focused operation. That disables in-ph 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 18

Steering and Focusing • Multiple focal zones are possible using an array. • Multiple

Steering and Focusing • Multiple focal zones are possible using an array. • Multiple transmit focus is not practical; dynamic focusing is employed only in receive mo • 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 sig 19

Beamforming Techniques • Full Phased Array • All array elements simultaneously activated for transmit

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 20