Ultrasound Flashcards
how is ultrasound used in RT
- as a diagnostic tool
- image guidance
- interfraction motion estimation in prostate and breast
- gynaecological application, placement of brachytherapy
- localisation tool (eg. ovaries)
initial diagnosis of what tumours are ultrasound used
liver
pancreas
kidney
breast
prostate
electromagnetic waves
- travel at the speed of light
- no medium is required for propagation
- distinguished by energy, frequency and wavelength
- eg. light and x-rays are electromagnetic waves
mechanical waves
- defined as the propagation of energy through a medium by cyclic pressure variations
- need deformable elastic medium for propagation (such as air, water, soft tissue)
- ultrasound propagates by mechanical waves
two types of mechanical waves
- transverse
- the particle motion in the medium is 90 degrees to the direction of the wave
- longitudinal waves
- the particle motion in the medium is in the same plane as the direction of the wave
pulse echo sequence
- once a pulse has been sent into tissue, the transducer is set to receive mode
- the returned echo is converted into an electrical voltage
- the time from pulse transmission to echo receive can be accurately measured, ant this is used to calculate the interface depth
interface depth formula
d=vt/2
d is the depth of reflector (in m)
v is the velocity of sound (1540 m.s.)
t is the roundtrip time of the pulse
different transducer types
- curvilinear
- very good for abdominal structures + obsetrics
- linear array
- used in vascular (arteries and veins), lens and retina, musculoskeletal
- intracavity
- gynae, uterus and ovaries
- phased array
- cardiac
what are the different directions the transducer can move in
in 3 planes - X, Y and Z plane
X plane = sweep and fan
Y plane = slide and rock
Z plane = compression and rotation
what are the 4 different types of resolution
spatial
contrast
temporal
colour
what is resolution
the degree of detail that structures can be seen on images
spatial resolution
the ability to differentiate small structures on a B mode image
two types of spatial resolution
axial and lateral
what affects spatial resolution
beam characteristics
line density (which affects lateral resolution
resolution of the viewing monitor
axial resolution
the closest distance between two structures can be along the beam axis and is differentiated as two entities
lateral resolution
the closet distance between two structures can be at 90 degrees across the axis of the beam and can be seen as different entities
focal zone + its affect on the beam
considered the beams ‘waist’
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 the focus, therefore lateral resolution is improved
contrast resolution + depends on
the ability to differentiate tissues of different echogenicity
depends on
- the amount of background noise and backscatter interference in the image
- slice thickness
- inherent characteristics of the electronics of the machine and transducer construction
what are the terms used to describe echogenicity
- anechoic (echolucent or sonolucent) = absence of echoes
- hypoechoic (echopenic) = low level echo
- hyperechoic (echogenic) = high echo
what are the terms used to describe echotexture
- fine or coarse
- homogenous = very even pattern
- heterogenous = mixed pattern
temporal resolution
ability to resolve rapidly moving structures - dependent on the time frame
high frame rates are required for increased temporal resolution
colour resolution
a term used to describe the spatial resolution of the Doppler colour display when defining moving substances (usually blood)
how well are colour displayed to demonstrate moving substances (eg blood)
Doppler effect
- assumed change in frequency that occurs due to relative motion between
- wave source
- receiver
- reflector of the wave
doppler shift
difference between received and transmitted frequencies due to motion of blood flow relative to beam
change in F = frequency received - frequency transmitted
received frequency dependent on
transducer frequency
speed of sound
velocity of blood flow
intercept angle
overall gain
the amount of gain/brightness applied to the returning echoes in an image
increasing gain too much will add noise, and decreasing too far can potentially lose information
time gain compensation (TGC) or depth gain compensation (DGC)
is the gain applied according to the depth or attenuation of the images as ultrasound travels through it
basic B-mode controls
frequency
focal zone
gain controls
depth
grayscale QA phantom
used to test a range of imaging parameters on an ultrasound system, including the accuracy of measurements and the limits of system resolution
pros of ultrasound in RT
non ionising
cheaper, fast
mobile
more appealing for claustrophobic patients
can be used for pregnant women
cons of ultrasound in RT
highly operator dependent.
possible allergy to the gel
not user friendly, need experience
difficult to visualise bone
low resolution compared to MRI or CT
air or gas blocks visualisation
what are the basic B-mode controls
gain and TCG
frequency
focal zone
depth
spatial pulse length
the length of time that an ultrasound pulse occupies in space or the length of the pulse we send out
axial resolution is directly related to SPL
spatial pulse length is affected by
the frequency
- higher frequency = shorter wavelength = shorter SPL
the transducer type
- better damping = fewer cycles = shorter SPL
SPL formula + how it relates to axial resolution
SPL = number of cycles x wavelength
AR = 1/2 SPL
