Ultrasound Flashcards
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)
Beam focussing effect on lateral resolution
Beam focussing reduces beam width –> improves lateral resolution
What is contrast resolution
ability to differentiate tissues of different echogenicity
no echoes (anechoic)
low (hypoechoic)
high (hyperechoic)
What can affect the contrast resolution
Increase in background noise and backscatter interference in the image –> decrease in contrast resolution
Thin slices –> improve contrast resolution (minimises chance of tissue surrounding a lesion reducing contrast separation between the two)
Why does backscatter interference occur
Echoes returning to transducer will interact with each other –> causes complex low intensity interference –> causes haze over the image
What is temporal resolution
ability to resolve rapidly moving structures
What is temporal resolution dependent on
high frame rate –> increased temporal resolution
What is colour resolution
used to describe the spatial resolution of the colour display when defining moving substances (usually blood)
Difference between red shift and blue shift
Red shift
- longer wavelength and lower frequency
Blue shift
- shorter wavelength and higher frequency
What is the doppler effect
Assumed change in frequency that occurs due to relative motion between:
o Wave source
o Receiver
o Reflector of the wave
When US beam is directed at target, what frequencies are detected by the wave source
transmit frequency
received frequency
What is the doppler shift
Difference between received and transmitted frequencies due to motion of blood flow relative to the beam
o Change in f = Fr – Ft
What is the doppler shift dependent on
Dependent on
o Transducer frequency
o Velocity of blood flow
o Intercept angle
What is a colour doppler
Doppler overlay on the background b-mode picture
Doppler shift is coded to an area of interest -> a hue is allocated to represent to when a doppler is moving away or towards a transducer
Colour represents the average doppler shift.
What is the colour doppler dependent on
blood velocity
doppler angle
What are the 3 variables measured in a colour doppler
mean doppler shift
variance
doppler signal power
what is the mean doppler shift in a colour doppler
Proportional to mean velocity
What is the variance in a colour doppler
- Measure of spectral broadening
- Particularly important in detecting stenosis -> if present, area has large variance.
- often green in image
What is a pulsed weight doppler
- Relies on the same ‘pulse echo principle’ as grey scale imaging.
- Provides detailed information about a single sample volume.
How is ultrasound used in RT
Clarity
Intrafraction motion estimation in breast and prostate
Ovary localisation, gynae application and placement of brachytherapy
X axis movements
Sweep and fan
B mode controls
Frequency, focal zone, gain controls, depth
Frequency control
Displayed frequency is the median operating frequency that the system is set at
Lower frequency increases penetration into soft tissue but reduces resolution
Effect of focus on the beam
Any given echo will give rise to a stronger echo when it lies within the focal zone because the beam is at its highest intensity at the centre of the beam
The beam width is also narrowest at focus and lateral resolution is improved
TGC or DGC
Time gain compensation or depth gain compensation
Gain applied according to the depth or attenuation of the images as ultrasound travels through it
Provide a definition for acoustic window
Referring to a structure or anatomical configuration that allows deeper anatomy to be visualised by ultrasound
