Reading...
Play button
Play button
Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
5 Cards in this Set
- Front
- Back
|
Define acoustic impedence
|
It is the product of velocity and density.
Velocity of ultrasound depends of the material through which it travels - more compressibility = lower velocity. (eg. air) At any interface the % of reflection is dependant on the degree of acoustic matching. Also depends of angle of incidence - angle of incidence = angle of reflection in a smooth object *specular reflection - in rough objects the reflected beam spreads out over an angle which increases with roughness and reduced wavelangth. Therefor transducer will receive some reflections * diffuse reflection - structures smaller than wavelength will scatter the sound in all directions |
|
|
Explain the concept of resonant frequency
|
If a transducer operates with a frequency that produces a wavelength 2 x the thickness of the transducer sounds waves coming off the back of the transducer will be in phase (constructive interference). To change frequency need to change transducer.
|
|
|
Explain the mechanical coefficient (Q)
|
Q is the ratio of the mean frequenct to bandwidth.
Bandwidth is the width of the frequency spectrum. Damping vibration results in low Q. Low Q = short ring down (heavy damping), short pulses, broad range of frequencies -> good for pulsed USS. high Q good for continuous wave ultrasound |
|
|
Explain near and far zones
|
Sound energy is confined in a beam of diameter (D). In the near field (Frsnel region) the beam is parallel. The length of this region = fD2
At the far field (Fraunhofer region) the interference is lost and the beam diverges. The diverge angle is less with increasing fD. Therefor less divergence with greater frequenct and diameter. Diameter of Fresnel region defines best lateral resolution. Reducing diameter has disadvantage of producing shorter near field and more divergent far field. |
|
|
What is the purpose of beam focusing and how is it done
|
Done to improve lateral resolution by focusing beam to depth of interest.
Done either by using a curved piezoelectric element or electron focusing with an array of seperate elements which energise in sequence to produce a beam that is maximally constructive at a given focal depth. |
