US Flashcards
(26 cards)
Frame rate
frame rate to make each image.
shallower images=>higher FR
Additional mode of US would increase or decrease the frame rate
decrease
how can the opearator increase the frame rate
narrowing the FOV `or zooming
what is time gain compensation
in order to make equally echogenic tissues look the same even if they are located in different depths.
Dynamic range compression.
shifting the contrast window to lower end to increase contrast of soft tissue (low signal)
what Doppler shift (change of frequency) is proportional to?
Speed of movement of the object relative to the probe
What would happen if doppler angle is more than 60degress
reduce accuracy of velocity calculations.
Differences between pulse doppler, colour doppler and continuous wave doppler
Pulse doppler:
small sample volume.
flow direction adjusted by angle cursor.
Colour doppler:
Abilities to display the mean doppler doppler shift value.
Not using quantitative measurement.
CWD:
Probe continuously transmitting US beam.
ability of displaying extremely high velocities
Limitation of US
Attenuation => depth of penetration=> spatial resolution and measurement accuracy.
Propagation speed=>Frame rate=>temporal resolution
Depth of penetration would affects the temporal resolution as well.
Artefacts=> contrast resolution/ unable to access to some region.
what affect US attenuation?
Frequency. strongly affect the depth of penetration as well.
Spatial resolution of US
Lateral resolution= beam width=typically a few mmvbn
Axial resolution (depth)= cT/2= less than 1 mm
C=propagation speed, T = pulse duration.
Temporal resolution
FR = 15-50 per second
factors affect temporal resolution.
1. frame averaging
2. compound imaging
contrast reolution
ability to see variation of tissue properties.
best way to improve is to minimise artefacts.
Attenuation artifects
- shadowing (pass through high attenuation)
- enhancement (pass through low attenuation)
- edge shadowing
depth artifacts
1.reverberation
2. ring-down (usually due to gas bubble)
beam artifect
- Beam width artefact
- Sidelobe artefact
- Slice thickness artefact
beam path artefact
mirror artefact
refraction artifact
doppler artifacts: frequency aliasing
Happen when doppler PRD is too low
Doppler shift >PRF/2
Exist if negative flow in spectral display.
US safety
thermal- heating (TI e.g 1 max temp increase is 1)
mechanical - cavitation (possibility of cavitation occur)
Compounding imaging
machine create multiple images. each with the beams steered in different direction.
Advantages: improved tissue boundaries, reduce speckle, reduce artefacts.
Disadvantage: poor temporal resolution, useful artefacts may be reduced.
harmonic imaging
preferable in echocardiography
Enhance contrast agent.
improves grey-scale images
1. better lateral and slice thickness resolution
2. reduced beamwidth, slice thickness and sidelobe artifacts
Disadv
Frame rate half
reduce penetration
us contrast agent
microbubbles
improving cardiac wall outlining, liver function, perfustion
speckle tracking
looks at how the speckle in ROI move from one image to the next
calculate speed and direction of motion
strain imaging
measuring thickness of certain structure. measure systolic and diastolic muscle thickness.
