Ultrasound Flashcards
what are uses of ultrasound in medicine?
Imaging
Doppler (vascular and cardiology)
Therapeutic (lithotripsy)
what are ultrasounds?
Sound energy that has frequencies higher than is detected by the human ear
what are properties of sound waves?
longitudional waves
compression waves
needs a medium to propagate
has a low propagation speed compared to light
what determines the speed of the soundwave?
frequency x wavelength
what frequency are ultrasounds above?
20kHz
what is the average speed of sound tissue?
1540m-1
Varies slightly in different tissues
what are the different interactions ultrasound has with tissue?
Absorption
Reflection
Refraction
Scatter
what is absorption?
Ultrasound energy is converted to heat which reduces amplitude
what is reflection?
Energy reflected from a boundary (acoustic interface) The boundary is between two materials that have different acoustic impedances and some ultrasound will pass through the boundary and some will reflect.
The amount of reflection depends on the differences in acoustic impedence
what is refraction?
some or all energy is diverted from its original path
what is scatter?
energy is dispersed in all directions
what is frequency?
how many cycles occur per second
what is wavelength?
distance travelled by one cycle
what does amplitude indicate?
how strong the ultrasound is represented by the height of the wave
*reduces with absorption
what does absorption vary with?
tissue properties
frequency (the higher the frequency the more absorption)
distance travelled
what determines acoustic impedence?
density x speed of sound
what is the amount of reflection and transmission of an ultrasound depend on?
difference in acoustic impedance
the larger the difference in acoustic impedance the more energy is reflected and less transferred.
what is gel used for ultrasound imaging?
The air acoustic impedance is very different to that of tissue.
Gel has an acoustic impedance that is more similar to that of tissue.
The air-fat interface has such a large difference in acoustic impedance that only 0.1% of energy is transferred and passes through the air layer and the rest reflected
With gel-fat interface there is less of a difference on acoustic impedance values so 99.5% of energy is transferred so this allows you to get an image.
how does reflection effect the image?
the greater the reflection the brighter the image will appear on USS.
what is the result of high frequency?
High resolution
HOWEVER
shorter wavelength
more attenuation
what is attenuation?
loss of power or amplitude of the ultrasound signal as it passes through the tissue
what makes up attenuation?
absorption
scatter
reflection
what are biological effects of ultrasound?
- Mechanical: cavitation and streaming.
2. Thermal
what are the safety indicators of ultrasound?
mechanical index
thermal index
what is mechanical index?
the likelihood of causing cavitation. The highest allowed is 1.9
what is the relationship between frequency and wavelength?
The shorter the wavelength, the higher the frequency
what frequency is best for deep structures?
A lower frequency and it has a longer wavelength and better penetration
what is a sound wave?
a longitudional compression wave so it needs a medium to travel in
what does the amount of refraction depend on?
speed of sound
what is scatter?
where the energy goes in all directions. Scatter from both the surface and inside the tissue.
what is attenuation?
how much power/amplitude is lost as it passes through the tissue.
what makes up the transducer?
piezoelectic element electrodes backing block Matching layer housing
what is the most common material for the piezoelectric element?
zirconate titanate
what is good about continious waves on doppler?
can be used to measure higher velocities
No aliasing artefact
Good resolution
what isn’t good about continious waves on doppler?
no information about depth of vessel
what is good about pulsed wave on doppler?
gives information about depth and can work out velocity
what isn’t good about pulsed wave on doppler?
aliasing when velocity is too high
what is a linear array?
Produces a rectangular field of view with uniform beam density throughout
useful for shallow structures and small parts
what is linear array good for?
shallow structures
what is curvilinear array good for?
deeper structures
what is curvilinear array?
linear elements arranged along a curved transducer allowing a wider field of view
what is a phased array?
A small footprint probe with a wide range of view
what is phased array good for?
because it has a small footprint it’s good for in between ribs etc as the arrays can change the direction of the us
What is m mode?
Motion: uses short pulsed waves
what is TGC Mode?
Time gain compensation
Used for unified brightness so deeper structures aren’t darker.
Compensates for attenuation due to absorption
what is bright on us imaging?
fatty structures as they have high reflectivity
what are advantages of ultrasound?
allows for real time imaging
no radiation
can be used for biopsy
relatively cheap
what are disadvantages of US?
operator dependant
diagnostic errors
what is microcavitation?
a biological mechanical effect of US.
Tiny bubbles that have been produce rapidly collapse and release energy.
when do you get shadowing?
when there is high attenuation
when do you get enhancement?
when there is low attenuation and the tissue behind it appears brighter than expected
what is reverberation?
when sound waves bounce back and fourth between two reflective surfaces before returning to the transducer
what are different forms of artefact?
shadowing
enhancement
repeat reflection
reverberation
