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
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.

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

16. C-Mode Display

17. 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

18. 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

19. 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

20. 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