Ultrasound Flashcards
what is the velocity of the wave
Velocity of the wave = wave frequency x wavelength
what type of wave is ultrasound?
ultrasound is a longitudinal wave
ultrasound requires a medium. T/F?
True
what are the two components of the wave as it prorogates through tissue?
the wave undergoes a series of compressions and rarefractions.
is ultrasound higher or lower frequency than what the human ear can detect
it is higher than what we can hear (x1000)
what is the difference between doppler and normal ultrasound?
Doppler has much higher pressure than normal ultrasound
what is young’s modulus?
young’s modulus is a measure of the elasticity of a medium
what is young’s modulus equation?
c = ⎷E/⍴
⍴ = density of medium
E = youngs modulus
what needs to be matched for passing of ultrasound from one medium to another
you need to match the acoustic impedance
what is acoustic impedance
a measure of how “easy” it is for sound to pass through a medium
what is high power ultrasound used for?
lithotripsy
is the intensity of the ultrasound beam uniform?
no
write down equation used to compare intensities of the beam
dB = 10log10 I2 / I1
what unit is the loss of power of the US beam
1Db/cm/MHz
what is the effect when acoustic impedance difference is very big?
there is a lot of reflection when the acoustic impedance difference is large
what are the different fates of US beam
refraction, reflection, scatter
what is the angle of incidence?
the angle of incidence is the angle of reflection
what happens to the beam as it passes through one medium to another
the beam bends
what is diffraction?
the bending of the US beam into the shadow of a strong absorber
where does diffraction commonly occur?
absorber edge, ie: gallstone
how are images constructed?
images are constructed by computing the time is takes for the beam to travel from the transducer and return from a reflecting surface. this is our depth data
what dictates the brightness of the display
the magnitude of the echo
what is a transducer?
a device that converts one form of energy into another form
what happens to the piezoelectric crystal when an electric current is applied?
the crystal alters shape, this is because the molecular dipoles change direction
name components of the transducer and their basic function
acoustic insulation
acoustic lens
backing block - absorbs US going backwards, attenuates stray beam
matching layer - allows 100% transmission of beam into tissues, minimising reflection due to traversing mediums (acoustic impedance)
piezoelectric crystals - converts electrical to mechanical energy and forces the ceramic plate to move
what is the function of the crystal in receive mode?
the small voltage generated by deformation of the crystal is detected and amplified, crystal compresses and expands, wave sent into patient and backwards into the probe, bounces back to the crystal and registered as an impulse
what does the matching layer improve?
improves sound transmission by compensating for large acoustic impedances
what does the US gel do?
it eliminates the air layer
what is continuous wave be used for
measuring femur length in utero
are higher or lower frequency waves absorbed more?
Higher frequency waves are absorbed more
when SPL changes, what is also effected?
the spatial resolution
what is meant by the term dispersion
the filtering out of the higher frequencies
when SPL is longer, what can happen to two adjacent objects
when the SPL is longer, two adjacent objects can be seen as overlapping objects
what is the pulse repetition frequency?
the number of pulses per second
image depth
the time between two pulses must be greater than the time for the “return trip” which is equivalent to twice the image depth
what determines the imaging depth
the PRF
what is the bandwidth
the number of frequencies within a pulse
what effect does increasing spatial pulse length have on bandwidth
increasing SPL will decrease the bandwidth
short pulses have greater or lesser mixtures of frequencies?
short pulses have a greater mixture of frequencies
SPL in imaging vs doppler
SPL shorter in regular US, longer in doppler
what is specular reflection
occurs when the boundaries of two structures are smooth, generates echoes that define organ boundaries like liver.
what happens when sound waves encounter structures much smaller than their frequencies
they are scattered in all directions, happens with RBCs for example
what do scattered waves produce on the image
a speckle appearance, does not correspond to anatomical detail
what is the mirror artefact?
when reflecting surface meets the beam at a large angle of incidence, such as the diaphragm, part of the beam not reflected directly back to the transducer, produces a secondary reflection
effect of higher frequency on near & far fields
higher frequency, longer near field
what is lateral resolution?
spatial resolution in a plane perpendicular to the beam
another term for the near field
fresnel zone
another term for far field
Fraunhoffer zone
in which field is lateral res preserved
lateral resolution is preserved in the near field/fresnel zone
what happens to the beam after the fresnel zone
the beam diverges
what optimises lateral resolution at depth
a wide transducer of high frequency as divergence decreases with wider transducer or increasing frequency
what does the acoustic lens do
narrow the beam to improve lateral resolution
