Ultrasound Flashcards
(10 cards)
How is the ultrasound pulse generated in the piezoelectric crystal
A high frequency AC current will cause a the material to vibrate.
What is one way to change the direction of the beams
by firing the transducers at different times
What are the 4 elements of resolution
Axial resolution - Pulse / wave length
Lateral resolution - beam width / focus
Contrast resolution - transducer design
Temporal resolution - fram rate (PRF & lines per frame)
What are the main features of axial resolution
Resolve objects at different depths
Depends on spatial pulse length, structures < 1/2 pulse length cannot be resolved.
Pulse length depends on wavelength / frequency and the amount of damping.
Higher frequency gives better axial resolution however at a higher depth F, penetration depth is reduced
What are the main features of Lateral Resolution
Resolve objects perpendicular to the beam.
- Best lateral resolution is at the focus
- Structures will only be resolved if the seperation between them is greater than the beam width
- Higher Frequency gives longer near field with less divergent far field
- Focussing is crucial
What is Temporal Resolution
The ability to determine if an object as moved over time
What are the 5 main factors in Attenuation
Absorption, Scattering, Reflection, Refraction, Divergence
How does frequency affect penetration
Higher frequency gives less depth of penetration
How does divergence of the beam come about and what are some of its affects
- Due to diffraction effects.
- Power is spread over a larger area so intensity is reduced.
- Affects both beam and echo
- Particularly affects far field in non focussed beam & areas distal to the focal zone in the focussed beam
What is the main risk associated with ultrasound
Sustained intense ultrasound will cause interaction in tissues. Thermal & Mechanical, as air bubbles expand under the influence of ultrasound.
