Ultrasonic İmaging

Ultrasound – The propagating media interaction

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• 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

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• 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

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• 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

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• 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

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• 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

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• 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)

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• 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

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• 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

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• 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