Ultrasound Flashcards
(49 cards)
PEI - Interface Depth Formula
d = vt/2
d = depth of reflector (m)
v = velocity of sound (1540 m.s)
t = roundtrip time of the pulse/echo
What is the Pulse Repetition Frequency?
Rate at which pulses are emitted from the transducer
Deeper the tissue imaged -> longer the transducer must wait for all echoes to return –> reduced PRF
Different Transducer Types and application
Curvilinear Probe -> Liver and deeper abdominal structures
Linear Array -> Cardiovascular, lens, retina, musculoskeletal (tendons)
Intracavity Probe -> Gynae, Uterus, Ovaries
Phased Array -> Used to squeeze between ribs and fan our prior to imaging heart
Examples of acoustic windows
Referring to a structure or anatomical configuration that allows deeper anatomy to be visualised by ultrasound (causes little attenuation of the beam)
Amniotic fluid (for developing foetus)
Aqueous and vitreous humor (eye structures, e.g., retina)
Full bladder when imaging pelvic organs (uterus, ovaries and prostate gland)
Effect of increasing Gain
Allows processing of more incoming echoes
-> brighter image, increase in noise and artefact
-> less contrast and less fine details
Effect of Decreasing Gain
Processing of less incoming echoes
–> darker image
B-Mode Control - Depth Function
Can be altered to instruct machine to wait longer for echoes to return
Penetration of US on a particular transducer can be altered by manipulating the frequency of the probe
-> reduce frequency —> pulse transmits further
B-Mode Control - Focus Function
Important to manually adjust the depth of the focus –> want target structure within the focal zone of the US beam
What is the focal zone
The narrowest part of the US beam
Provides the greatest lateral resolution during the scan
Effect of focus on the the US beam
Beam width is narrowest at the focal zone
Beam intensity is at its highest at the centre of the beam
B-Mode Control - Zoom function
Allows for the image size to be increased –> can result in improved resolution of structure
Y-Axis Movements (Longitudinal)
Slide
- slide transducer along patient
Rock
- rock transducer back and forth along y-axis
- used when imaging arteries -> improves doppler effect
Z-axis movements
Compression
- Used to push structures in and out (e.g., blood vessels)
- can be used to determine presence of blood clot
Rotation
- Rotate transducer from
What is the use of the QA phantom?
Assists in determining limits of axial and lateral resolution
Provide a definition for resolution and state the differing types
The degree of detail that structures can be seen on images
Spatial, temporal, contrast, colour
What is spatial resolution and the two types
ability to differentiate small structures on a B-mode image
axial and lateral
What is spatial resolution affected by
Beam characteristics
Line density
Resolution on the viewing monitor
What is axial resolution
Closest distance two structures can be along the axis of the beam (up and down) and which can still be seen as different entities
Relationship between axial resolution and spatial pulse length
Axial resolution directly related to spatial pulse length
AR = 1/2SPL
SPL = no. of cycles x wavelength
What is the spatial pulse length affected by
frequency
- higher frequency = shorter wavelength = shorter SPL
transducer design
- better damping = fewer cycles = shorter SPL
What is axial resolution affected by
Output power and gain
Increased output power/gain –> effective pulse length will increase
How can you optimise axial resolution
using highest frequency possible
using low power / gain setting
Is axial resolution affected by distance from transducer
It is not affected
What is lateral resolution
Closest distance two structures can be at 90 degrees across the axis of the beam (left to right across the beam) at the same depth and still be seen as separate entities
Is related to and is approximately equal to the beam width
If entities are the same distance from each other as the beam width, they will appear as a single entity on the image (interfaces are not resolved)
