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 Huygens principle expresses diffraction

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 Here A(xs,ys) defines the source distribution along the surface S, λ represents the wavelength and k is called as wave number, it is given by 2π/λ.

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 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

C-Mode Display

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) 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 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 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