Ultrasound Flashcards
How many wave cycles are usually transmitted from an ultrasound probe each time?
3-5
What is the rough speed of US
1500m/s
What are the advantages/ disadvantages of higher frequency ultrasounds?
more details,
a lot quicker attenuation, so can’t see deep
Unit of measurement for ultrasound attenuation?
dB/cm/MHz
What causes a reflection of US?
an interface between two different tissues
What are some highly reflective substances in humans?
gas, bone, plastic
What’s the difference between reflection and scattering in Ultrasound?
Scattering happens with very small objects only, unlike reflection the energy goes in all directions
What is PRF in ultrasound?
Pulse repetition frequency
How often the probe can send out pulses of ultrasound
What is speckle in US?
The echoes from individual scattering objects. Causes the granular appearance of USS.
What is aperture in US production?
the area of the probe face used to create the US
What factors can narrow an US beam?
higher frequency US
larger aperture
How does electronic US focusing work?
US waves from the sides start before waves in the centre, they all add together and should, therefore, create a narrowest beam around the focus area.
What is dynamic receive focusing in US?
the US receiver constantly adjusting the focus because it knows how deep the ultrasounds are coming from based on speed and time
What are sidelobes in US?
beam intensity peaks to either side of the main beam.
Due to constructive interference of the US waves from different transducer parts.
Is axial or longitudinal resolution better in US?
axial (much better, 3x often)
Why is longitudinal resolution limited in US?
smearing due to the beam not being perfectly narrow
What is dynamic range compression?
Compresses the range of ultrasound echoes received into a range that can be displayed on the screen and appreciated by humans
Why is dynamic range compression needed?
As there is a huge range of signal that can return to the US probe, and we can’t appreciate it all visually.
The more intense stuff is pretty useless.
Why is quantitive information regarding highly echogenic tissues not diagnostically useful on US?
As the signal strength is related to the angle the object makes with the probe, as these objects are speculated (mirror-like)
What are the disadvantages of HIGH dynamic range ultrasound
- Little differentiation of tissue type
- artefact echoes more visible
What are the disadvantages of LOW dynamic range ultrasound
- Areas where the signal drops out
- Coarser texture
What does a scan converter do in US?
- Takes echo information in real-time and puts it in the geometrically correct area of memory,
- fill in any blank pixels with guesses
Examples of PRE-processing in US:
○ Zoom
○ Depth
○ Persistence
○ Compound imaging
Examples of POST-processing in US:
- Post processing zoom (not as good)
- Colour mapping (B-mode)
- measurements
What are the assumptions that occur in US calculations?
- Propagation speed = 1540m/S
- Narrow beam
- US travels in straight line, directly from and back to the transducer
- Uniform attenuation
What common tissue has notably different US travel speed?
Does it travel faster or slower?
Fat,
it’s slower (1450m/s vs 1540m/s normally)
Why do depth errors occur in US?
As ultrasound beams travel at different speeds through different tissues, but the machine isn’t able to adjust for this, at assumes they always travel at the same speed (1540m/s)
What the doppler effect in US?
When US round-path distance of the ultrasound changes with time
Due to moving scatterers such as blood vessels
Ways to display doppler effect on US
○ Pulsed doppler
○ Colour doppler
○ Cw doppler
What is cavitation in US?
Small as bubbles oscillate in ultrasound field, in some conditions they can grow then collapse creating 1000 degree temperature areas
