Sonography 1 + 2 Flashcards
What is a ultrasound?
Sound waves of frequencies higher than the range of
human hearing and their movement (propagation) in a
medium
What is converted into a grey scale value in US?
The echo amplitude.
What is compression?
increase of density during propagation
What is rarefaction
decrease of density during propagation
Does sound move along the waveform?
no
What is a complete cycle?
compression followed by a rarefaction
How many cycles in pulsed US?
2-5
What frequency does diagnostic US travel at?
-2 MHz to 20 MHz
Is a high frequency attenuated more or less?
more.
Which frequencies have the higher spatial res?
High frequencies
Best frequencies for deep tissues and why?
Low because high frequencies will be attenuated before they can hit the structure.
What is wavelegnth?
the physical length of the cycle
What is period? (t)
Length of time it takes for a cycle to start and finish.
What is the strength of the US beam?
its amplitude.
What is the velocity of propagation?
Speed of which US travels..
CONSTANT FOR A SPECIFIC TISSUE
1540 ms-1
What is the speed of sound in soft tissue?
1540 m/s
Which has a faster speed of sound: air or bone?
bone
What is acoustic impedance?
A measure of the resistance of a medium to the transmission of sound.
The product of velocity (c) of the medium and the density (p) of the medium
The acoustic impedance is a constant for each medium.
measured in rayls
If there is an increased acoustic impedance BETWEEN materials, will there be less or more reflection?
more
What do we spend 99.9% of the time in US doing?
listening for echoes.
can a transceiver emit and receive a pulse SIMULATEANOUSLY?
no
How long does the reflection from a depth of 1cm take in soft tissue?
13micro seconds
What is PRF max?
maximum pulse repition frequency
What is PRF max limited by?
Imaging depth.
at a larger depth what has to happen for the PRF
Less pulses per second due to longer listening times
INCREASED IMAGING DISTANCE = DECREASED PRF.
What is PRS
Pulse repetition period :
The time between the start of one pulse to the start of the
next pulse.
The time required to transmit a pulsed ultrasound wave plus the time devoted to listening for the returning echoes from that wave
If PRS increases what happens to PRF max?
Decreases
What is SPL and what does it determine?
spatial pulse length.
This is like wavelength .. it is a physical length measurement and hence affects Axial resolution
What is PD?
pulse duration
If SPL increases what happens to PD
also increases
If SPL increase what happens to axial resolution?
decreases
What is Duty factor?
Measure of the fraction of time that the transducer is emitting sound (or is active).
What is axial resolution?
Resolution along the axis of the beam.
THe smaller the resoltuon the better…
in US it should be 1 mm or better.
Does a longer or shorter pulse length give a better axial resoltuon?
Shorter is better as its not travelling as far.. so higher frequencies will be attenuated less.
Which beam width increases lateral resolution: wide or narrow?
Narrow.
Generally which is better in US? axial or lateral resolution?
Axial as the US beam is usually shorter than it is wider.
What is attenuation?
Reduction of intensity as sound propagates through a medium
Will an echo reflected at the surface of the body be stronger or weaker than an echo reflecting from an identical structure, deep within the body?
Stronger
What affects attenuation in US?
Tissue density and shape
Distance travelled
Frequency of the beam
What is the approximate rate of attenuation in soft tissue?
1dB/MHz/cm
What is a positive dB value?
When a wave has a higher intensity than the reference
wave. AMPLIFIED THE SIGNAL
What does doubling and halving the intensity do to the dB?
Doubling : +3 dB
Halving : -3dB
Why is dB used?
compresses large attenuation values into a simpler scale to use.
What is the reference wave in US?
The intensity produced by the transeiver.
What dB values do we expect in US?
negative
What does a higher frequency have on attenuation
More attenuation
what are the 4 types of US interactions with a medium?
Reflection
Refraction
Scattering
Absorption
Which US interaction produces heat?
Absorption… can be used in US therapy to help create bloodflow.
Which US interaction produces artefacts?
Refraction
Is US absorption useful for image quality?
No. it doesn’t contribute to the image at all.
but the heat can be used for US therapy.
Why is absorption more common in higher frequencies?
Because higher frequencies are absorbed more.
Which tissues absorb more sound?
denser structures such as bone.
Where does reflection occur?
at large SMOOTH surfaces like diaphragm and organ boundaries.
Why do we aim to get the US beam to hit the structure at 90 degrees?
So it reflects back at that angle.
What is angle of incident equal to?
angle of reflection
Is reflection related to the frequency of the wave or changes in acoustic impedance?
it is NOT related to frequency..
it is an attenuation element and thus related to acoustic impedance.
Why is gel and drinking water used in US?
to decrease the large differences in acoustic impedance.. so some sound can transmit and not hit a wall.
WHy is scatter important in US?
shows the ‘texture’ of a surface and shows internal structures of organs.
WHat is scattering dependent on?
Frequency… a higher frequency will increase scattering.
When is scatter really useful?
Doppler or moving objects.
Why is there more scattering in surface imaging?
BEcause higher frequencies are used during surface imaging
Why does refraction occur?
Only occurs if the velocity of sound is different in the two media and when angle of incidence is not perpendicular.
Tissue one density is differnt to tissue 2.
If the velocity of speed of sound is lower in the second medium… will it bend towards or away from the ‘normal’
towards.
If there is larger differences between two mediums what will happen?
larger angle of refraction
If the refracted beam has bended AWAY from the normal, what has occured?
The velocity of the speed of sound in the second medium is higher so the angle has INCREASED.
Which effect is used to generate US?
CONVERSE piezoelectrict effect.
Electrical energy -> mechanical energy.
WHat is the piezoelectric effect?
When the echo returns and mechanical energy is converted into electric energy.
What is the material in PZT?
lead zirconium titanate
What do materials need to contain for piezoelectric effect to occur?
Dipoles
What temperature must be exceeded to cause dipoles to un-freeze?
the curie temperature.
What does rapidly changing polarity (alternative current) cause to the dipoles?
Constant expansion and contraction causing the CONVERSE PEIZO ELECTRIC EFFECT.
Will a thicker transducer crystal produce higher or lower frequencies?
Low
Are transducers typically multi or single frequency?
single
If the crystal was placed directly onto the skin, about __% of the ultrasound would be reflected.
80%
Other than gel what else does the transducer use to lower differences in acoustic impedence?
the matching layer.
why is the matching layer sometimes called the ‘1/4 wave matching layer?
IT HAS A THICKNESS OF ONE QUARTER OF ITS WAVELENGTH
What layer is used in pulsed US mode?
Dampening layer
WHat is the purpose of the dampening layer?
To stop the transducer making sounds after it is turned off… to keep the pulses at 2-5 cycles.
What does the backing layer improve in US?
AXIAL RES.
Will a continuous transducer produce multiple frequencies?
No, they are single frequencies.
What frequencies does pulsed transducers produce?
a range of frequencies.
which pulse has a greater bandwidth?
shorter pulses
Will a longer SPL have less or more cycles?
More.
Will a longer SPL have a shorter or longer bandwidth?
Shorter/ narrow.
typically a longer bandwidth will increase/ decrease axial resolution?
Increase as it has a shorter SPL
Which array is used more?
phased
Frequency of linear array
5-13 MHz
Frequency of curvilinear array
1-8 MHz
What is the advantage of only activating certain crystals?
can create a narrow beam = increased lateral resolution
Frequency of phased array and what they are used for typically?
2-8 MHz.
good for cardiac images, small area i.e. between ribs.
Frequency of endocavitary array
8-13 MhHz
Why is CMUT considered better than PZT?
better acoustic matching.
Wider bandwidth capabilities (smaller pulse lengths)
Potentially improved axial resolution
Potentially lower costs with easier fabrication,
Why is a focused beam desirable in US?
Produces pulses with a small diameter.
Gives better visibility of detail in the image.
Best detail will be obtained for structures within the focal zone.
Distance between the transducer and the focal zone is the focal depth.
Two zones in the unfocused beam?
Near : Frensel zone
Far : Fraunhofer Zone
What is the near field the same as?
same diameter of the transducer.
What is the NZL and what affects the length of it?
NEAR ZONE LENGTH
Related to diameter of the transducer.
wavelength
what will an increase in frequency do to the NZL?
increase it
What two things decrease the divergence at the far field?
Increasing diameter
and increasing frequency,
What affects the diameter of the US pulse?
amount of crystals activated or number we use
What affects the width of the US beam
This is the length and related to the number of cycles we have
For a single element transducer, side lobes are about __% of the main beam energy
15%
