USS Flashcards
How would the USS waves are produced?
USS waves are produced by a transducer that converts electrical signal into an uss beam.
It consists of a peizoceramic disc or rectangular plate
what is the plate made of in the transducers?
it is made of compressed microcrystalline lead zirconate titanate -PZT- or PVDF
Hows the two flat faces are made electerically conducting?
with a very thin coating of silver.
what would happen when a DC voltage is applied to the flat face of the disc?
it expands and if the voltage is reversed it contracts
The movement of the face is proportional to the voltage.
T/F?
The voltage produced is inversely proportional to the pressure.
False
The voltage produced is proportional to the pressure.
When the disc is compressed equal and opposite charges and corresponding voltage appear on the two flat faces.
T/F
The transducers can act as both transmitter and a reciever.
True
what is a Curie temperature?
when heated above a certain temp, transducers lose their peizoelectric properties
eg 350C for PZT
what type of voltage is used in pulsed mode?
DC
when the Tx is in contact with a patient and DC is applied, it instantly expands and compresses a layer of the material in contact with it, this also affects the adjacent material therefore a layer or wave of compression travels with a velocity through the material followed by wave of decompression or rarefaction
what type of voltage is used in continuous wave mode?
AC
When AC is applied, the crystal face pulses forward and backwards like a piston producing successive compression and rarefaction
Each compression wave has moved forward by a distance called the wavelength by the time the next one is produced.
the graph of pressure against time is what type of graph?
It is a sine wave.
T/F?
The USS waves can travel in vacuume.
False
The sound waves unlike, x rays and light, require a medium to travel through
T/F?
The USS waves are a transverse wave
false
sound wave is a longitudinal wave
T/F?
X and gamma rays and sound waves can be reflected, refracted and focused
false
unlike X and gamma rays, teh sound waves can be reflected, refracted and focused
T/F?
The greater the density the lower the velocity
True
T/F?
The greater the compressibility the greater the velocity
False
The greater the compressibility, the lower the velocity
T/F?
velocity is independent of temperture
False
Velocity depends on temperature.
How long does it take for sound waves to travel 1cm in average soft tissue?
7 micros
Air has ……..density but is much more…… tahn water or tissue, hence the ….velocity.
Air has ..much lower..density but is much more..compressible.. than water or tissue, hence the ..low..velocity.
T/F?
wavelength is proportional to velocity.
True
what is the unit of measuring intensity of USS?
Intensity of USS measured in watts per square mm- W/mm2-. this is proportional to the square of the wave amplitude and is under the operator’s control.
what is constructive interference?
This is when two waves are in phase/in step, their amplitudes add up.
What is destructive interference?
This is when two waves cancel each other out.
what would happen if two waves are only partially out of phase?
They result in reduced intensity.
T/F?
the transducer vibrates to produce a wavelength in the transducer equal to half the thickness (t) of the PZT disc.
False
the transducer vibrates to produce a wavelength in the transducer equal to TWICE the thickness (t) of the PZT disc.
Define resonant frequency:
The frequency at which the transducer is the most efficient as a transmitter of sound is also the frequency at which it is most sensitive as a receiver of sound. It is called its neutral or resonant frequency.
T/F?
The thicker the transducer, the lower the natural frequency and the longer the wavelength.
True
what is the relationship between transducer thickness and the wavelength.
The thicker the transducer–> the lower the natural frequency and–> the longer the wavelength.
what is the natural period of a transducer?
T=1/f
what does the resonant frequency of a transducer depend on?
it depends on
i) its dimension, particularly its thickness
ii) its materials.
To change the frequency , the Tx has to be changed
A 3.5 MHz transducer has a disc about ……..mm thick.
A 3.5 MHz transducer has a disc about ..0.5..mm thick.
T/F?
The amplitude of pressure wave decays linearly.
False
They decay exponentially with time.
what is damping?
What is ringing?
The amplitude of pressure waves decay exponentially with time. This is called damping.
If the vibration continues for an appreciable time, it is called ringing.
What is a low mechanical coefficient/Q value?
If the damping is heavy , it has a short time constant or ring down time.
This is said to have a low mechanical coefficient or Q value.
T/F?
A transducer that is lightly damped has a lower Q.
False
A transducer that is lightly damped has a higher Q.
T/F?
A transducer that is lightly damped has a higher Q, producing longer pulse and higher output of sound.
True
A transducer that is lightly damped has a higher Q, producing longer pulse and higher output of sound.
T/F?
USS travels at a constant speed in tissue.
True
what does the acoustic insulator do in USS probe design?
It stops the transducer vibrating in hand.
What is the function of backing material in USS probe design?
It stops vibrations reverberating back into the PZT material.
It also determines the length of the USS pulse by determining how much it is dampened-Q value
What is the function of piezoelectric material in USS probe design?
this is the material that forms the USS and receives echoes.
it is 1/2 wavelength thick and is usually composed of 256 crystals.
what is the size of PZT material in USS probe design?
its 1/2 wavelength thick and it is usually composed of 256 crystals.
what is the size matching layer in USS probe design?
it is always 1/4 wavelength thick to educe wavelength impedance difference.
Why is matching layer 1/4 wavelength thick?
to reduce wavelength impedance difference.
In continuous mode the transducer emits sound if …….frequency
single
in pulsed mode it emits……….
continuous spectrum of sine waves having a range of frequencies
what is the bandwidth?
The bandwidth is the full width at half maximum intensity- FWHM- of the frequency spectrum.
a short pulse has a wider bandwidth of freq.
What is the mechanical coefficient-Q?
This is the ratio of mean frequency to bandwidth.
The greater the Q, the narrower the bandwidth both as transmitter and receiver.
what is the effect of Q on bandwidth?
The greater the Q, the narrower the bandwidth both as transmitter and receiver.
What sort of Q transducer is suitable for continuous wave imaging?
High Q produces a pure note and responds only to that note, which is good for continuous wave uss.
What sort of Q transducer is suitable for pulsed wave imaging?
One with a low Q has a short ring down time, produces short pulses and responds to a range of frequencies.
What direction would sound waves travel in if the transducer had a diameter equal to one wavelength or less?
Sound would spread out equally in all directions.
T/F?
USS is a higher end of electromagnetic radiation spectrum.
False
USS is not an electromagnetic radiation.
But it does undergo reflection and refraction at the interface.
what is near field or the Fresnel region?
This is the near parallel part of the beam.
It extends a distance D2/4 wavelength
ie the length of the near zone is proportional to fD2
power of 2
What is the far zone? or the Fraunhofer region?
This is when the interference effect is lost and the beam diverges.
The angle of divergence is larger for …..wavelength/D
The angle of divergence is larger for ..Larger..wavelength/D
ie the far zone divergence is larger for smaller fD
T/F?
The side lobes may cause image artefact.
True.
using a transducer of higher frequency ……….the length of thenear zone and ……the divergence of far zone.( for the same diameter of the disc.
The beams become more directional.
using a transducer of higher frequency ..increases..the length of the near zone and ..decreases..the divergence of far zone.( for the same diameter of the disc.
using a transducer of larger diameter ……… the length of the near zone and …… the divergence of the far zone.
using a transducer of larger diameter ..increases.. the length of the near zone and ..decreases.. the divergence of the far zone.
how do you measure the near field length?
D2*/4wavelength
What are the ways to focus the beam? (3)
- using a curved PZT element
- using a plastic acoustic lens.
- electronic focusing
T/F?
The greater the curvature, the longer the focal length.
False
The greater the curvature, the SHORTER the focal length.
Where would you place the plastic acoustic lens?
This is cemented to the transducer face.
what could you use instead of plastic acoustic lens?
a curved mirror is used instead.
The shorter the focal length , the ……and ……..the focal region or depth of focus over which the beam is reasonably narrow.
The shorter the focal length , the ..narrower.. and ..shorter..the focal region or depth of focus over which the beam is reasonably narrow.
where would you use electronic focusing/ annular array?
This is used in mechanical sector scanners.
T/F?
The focallength cannot be changed without changing the transducer.
false
The focal length can be changed without changing the Tx: The greater the time delay bwn energising successive annular ring , the shorter the focal length.
The focal length can be changed without changing the Tx: The greater the time delay bwn energising successive annular ring , the …………. the focal length.
The greater the time delay bwn energising successive annular ring , the ..shorter.. the focal length.
what is a acoustic impedence-Z-? ( how do we measure it?)
This is the product of density (p) of the material and the velocity (v)
Z = v x p
what does the proportion of energy that is reflected and transmitted depend on?
It depends on the acoustic impedance of the material.
It depends on the density and elasticity of the material.
T/F?
The energy reflected and transmitted in uss depends on frequency.
False
The energy reflected and transmitted in uss is INDEPENDENT of frequency.
How do we measure the fraction of sound that reflected (R) at two interfaces?
R= (Z1-Z2)2/(Z1+Z2)2
the fraction of sound that is reflected at the interface bwn two materials also depend on the angle of incidence.
The greater the difference in Z, the ………. the fraction R reflected.
The greater the difference in Z, the ..greater.. the fraction R reflected.
The less the difference in Z the ……..the fraction transmitted.
The less the difference in Z the ..greater..the fraction transmitted.
what happens when Z1=Z2 ?
There will be 100% transmission and no reflection. eg the transducer and the backing block.
at interface bwn bone and tissue the fraction reflected is about……..
30%
about 70% is transmitted.
what would happen at interface with gas/air and why?
There will be total reflection. As Z is negligible.
gas filled organs cast a shadow and structures underneath cannot be imaged.
Give examples.
the bowel wall can be imaged but not the lumen.
USS can be used to check for air in vessels eg liver.
T/F?
It is possible to get sound from the Tx to the patient and vice versa if there is air trapped bwn the Tx and the skin.
false
It is impossible. It is all reflected back. for this reason the transducer is pressed against the patient and a coupling gel is used. Bubbles of air must be avoided.
because of the mismatch of Z bwn the Tx and tissue only……… of the sound would be transmitted in either direction bwn the Tx and the pt.
because of the mismatch of Z bwn the Tx and tissue only..20%.. of the sound would be transmitted.
How would you overcome the mismatch of Z bwn the Tx and the patient?
by attaching a matching plate to the front face of the trnsducer.
what is the size of the matching plate?
it is a quarter of a wavelength thick and made of a plastic compound that has an intermediate Z.
T/F?
small fractions of USS are reflected at interface bwn different soft tissue- eg 1% at fat/kidney interface.
True
T/F?
Reflections
False
T/F?
The angle of reflection = the angle of incidence
True
What is Snell’s law?
This is when the ratio of sines of the incidence and refraction angle is equal to the ratio of sound velocities in the two material.
Sin1 : sin 2 = V1 : V2
What is the effect of wavelength and roughness of the surface on the spread?
The shorter the wavelength , the rougher the surface, the wider the spread
T/F?
In imaging the transducer will receive some reflections even if the beam does not strike an interface exactly at right angles.
True
T/F?
When sound encounters a structure that is much smaller - eg red blood corpuscle or tissue parenchyma- than the wavelength it is scattered more or less equally in all directions.
True
T/F?
Sound is attenuated exponentially with the depth of travel.
True
What is attenuation measured in ? (unit)
Decibels dB- this allows a wide range of power or intensity ratios to be expressed by using a log scale.
10 x log (power or intensity ratio ) = no of decibels.
T/F?
The higher the frequency the lower the attenuation
False
The higher the frequency the greater the attenuation.
Why is sound attenuated exponentially?
Energy is absorbed and converted into heat by frictional and viscous forces in the material
energy leaves the forward travelling beam due to scattering and to partial reflection by the multitude of interfaces that the beam encounters on route.
The decibel valuse are additive.
What does the +ve and -ve value mean?
+ve values are amplification and -ve values are attenuation.
Define half value layer-HVL?
The thickness of tissue that reduces the sound intensity to half its original value is called HVL.
The dB loss in tissue per cm is proportional to …..
frequency.
ie at 3.5 MHz the loss is about 3.5 dB/cm
There is ………..absorption or scatter in water, so a full baldder can help in USS penetration.
little
in bone the attenuation is ……
much greater
35 dB/cm at 2.5 MHz
Air attenuates………
heavily
attenuation in lung 40 dB/cm at 1 MHz
T/F?
The higher the frequency the higher the penetration.
False
The higher the frequency the less the effective penetration of the beam.
What is the formula for penetration?
penetration (cm) = 40/ frequency (MHz)
what are the uses of A-mode imaging?
Examining the eye
identification of cysts in the breast
showing midline displacement in the brain.
What does A-mode imaging show?
It shows the position of tissue interfaces only.
It is the basic principles of USS imaging.
How does the A mode scanning work?
- probe is held stationary against the patient
- it is pulsed and the light spot starts to move from the left hand edge of the display screen.
- The light spot moves at a constant speed tracing out a horizontal line across the screen.
what is the velocity of the pulse of USS in A-mode scanning?
it averages 1.5 mm/micros
T/F?
In A-mode scanning the signal voltage produced after amplification which produced a blip, the height is proportional to the echo stength.
True
the pulse repition frequency is ………KHz in A mode.
1
what does TGC do to attenuation and echoes?
attenuation is compensated and the echoes equalised electronically
T/F?
Only boundaries perpendicular to scan lines will be imaged.
True
How does the returning echo in B mode display as?
The returning echo pulses are displayed as small bright dots
what does the bright small dots correspond to?
they correspond to the amplitude of the signal for each of the interfaces encountered.
The transducer which is a circular disc and may be an annular array moves within a …….. pressed against the body.
fluid filled plastic dome
What is the advantage of oscillation in sector scanning?
the wobble, or sector angle and the field size can be changed.
what are the two different types of scanners?
- mechanical/sector scanning
2. electronic scanning
what happens in electronic scanning?(in stepped linear array)
an elongated transducer is divided into a large number of separate narrow strips.
These would have short near field and widely diverging far field.
The PZT elements are energised in overlapping groups in succession so that a well defined uss beam comes in effect from a small transducer and scans a rectangular area in the body with 120 scan lines.
What are the two types of electronic scanning?
stepped linear array.
steered or phased array.
Ho could you change the focal length P in stepped linear array scanning?
By changing the timing of the applied pulses.
what is steered or phased array scanning?
similar but shorter Tx contains fewer elements
If they are energised simultaneously they act as a single transducer and the beam travels forward.
If they are energised separately in rapid sequence the pulses reinforce only in one direction.
What would happen if they are energised simultaneously or separately in steered phased array?
If they are energised simultaneously they act as a single transducer and the beam travels forward.
If they are energised separately in rapid sequence the pulses reinforce only in one direction. They interfere destructively in all others and the beam swings to the right. If they are energised in reverse order they swing to the left.
T/F?
With both types of array the focal length cannot be changed electronically by the operator.
False.
It can be changed electronically by the operator.
what is the relationship between the time delay and the focal length?
The greater the time delay between energising the successive pairs of element, the shorter the focallength.
What is the plane in which the beam is electronically focus called? what is the direction of this plane relative to the length of the array?
Unlike the circular annular array the beam is focused electronically in one plane only- the azimuthal plane, parallel to the length of the array.
what defines the slice thickness?
focusing in the perpendicular elevation plane is done by shaping each transducer into a curve or using an acoustic lens. This effectively defines the slice thickness.
what type of transducer would you use for electronic focusing in the elevation mode? and why?
1.5 D transducer.
inter row spacing = 10wavelength and
inter element spacing = wavelength/2
These produce better resolution of small lesions with greater uniformity at depth.
Focusing of the beam improves the image in the focal region but makes it worse beyond there. how could you reduce this effect?
This can be mitigated by multiple zone focusing.
The Tx is gated so that it receives echoes from the corresponding focal zone only. Others areblocked.
how do you measure PRF?
PRF = frame rate x lines per frame.
what is teh effect of depth on PRF?
The depth of view is increased by reducing PRF.
depth of view = 0.5x sound velocity / PRF
When would you use a linear scan?
this requires a larger area of patient contact so it gives a better quality image and maintains a wide field of view near the skin.
It is used in imaging the whole abdomen, liver, superficial vessels and thyroid and in obs and gynae.
When would you use a sector scan?
This is easier to manipulate , requires a smaller acoustic window , has a narrower field near the skin but a wider field at depth.
It is used to image heart through intercostal space or subcostally and the infant brain through the fontanelle. it is also used in intracavitary probes.
T/F?
A linear array can be made in a curved format to produce sector type imaging
True
beam divergence can limit the useful depth and the line density with depth.
what would you do to avoid the obscuring effect of bone or gas?
Two types of scanner may be used endoscopiccaly:
a linear array or a single high frequency transducer rotating through 360.
Ex: heart through oesophagus, prostate through rectum, fetus through vagina.
what is a transvascular transducers? what do they do?
they use very small crystal arrays on the end of a catheter and miniaturised electronics.
They operate at 10-20 MHz and are used in assessing cardiac vessels and stents.
how would contrast improve the image quality?
They do this by increasing the reflection from the tissue containing the agent.
what sort of contrasts are used in USS?
They should have low toxicity and they sould be readily eliminated by the body.
They are based on microbubbles (less than 4 microm) or nanoparticles
How would the microbubbles destroyed?
they are destroyed by USS of high intensity.
After the uss examination normal static diffusion leads to total destruction within few hours.
when would you use air filled microsphere encapsulated in a thin shell of Albumin?
These are injected iv.
they increase back scatter from ventricular border to improve visualisation and flow evaluation.
They adhere to thrombi making the DVT diagnosis easier.
when would you se low solubility gas encapsulated in a lipid shell?
They are used for all vascular application. especially in small vessels in pancreas, kidneys and liver, peripheral vascular disease and tumour vasculature.
What sort of contrast would you use in vascular application?
Low solubility gas encapsulated in a lipid shell
what sort fo contrast would you use in DVT scanning?
air filled microsphere encapsulated in a thin shell of albumin.
what sort of contrast would you use for metastases?
perfluorocarbon nanoparticles.
They stay in blood for many hours post iv and are slowly taken up by liver and spleen
What contrast would you use for immunologically targeted ?
Gold bound colloidal microtubes.
Echo enhancement is possible when antibodies are conjugated with microtubes.
T/F?
The targeted contrast can only be used for diagnosis.
False
They could eventually be used for drug and gene therapy.
what is the first harmonic?
This is the fundamental frequency.
The subsequent harmonics are integral multiplies of the first harmonic.
if the USS freq probe operates at 2 MHz what would be the 2nd, 3rd and 4th harmonics be?
They would be 4,6, and 8 MHz.
what is the disadvantage of pulse inversion?
It preserves axial resolution but takes longer than standard so artefact can occur.
what is involved in pulse inversion in USS?
every other pulse has its polarity reversed and the echoes received from each pair of transmitted pulses are summed, all the odd harmonics disappear from the signal, including the initially transmitted frequency.
at the same time the amplitude of each harmonic is double which enhances SNR for the 2nd harmonic
what are the advantages of tissue harmonic?
- reverberation artefacts are reduced-
- distortion and scattering from fatty tissues are reduced
- low contrast lesions and liquid filled cavities are better visualised as there is reduced acoustic noise.
reverberation artefacts are reduced in harmonics. why?
this is because low amplitude echoes do NOT produce harmonics.
distortion and scattering from fatty tissues are reduced in harmonics. Why?
as these are at the fundamental frequency and so suppressed , hence contrast resolution is improved.
low contrast lesions and liquid filled cavities are better visualised. Why?
As there is reduced acoustic noise.
in harmonic band filtering, the transmitted pulse should not contain high frequency. Why?
because this could corrupt the received signal.
To ensure this the pulse is stretched to produce a narrow transmission band.
This degrades the axial resolution, in part this is compensated by the improved detection of 2nd harmonic..
what type of harmonic preserves axial resolution?
Pulse inversion.
when would harmonics start being produced?
they are being produced when the transmitted beam has passed a few cm through the pt.
They exist but are not produced in the returning echo beam - except those harmonics produced by micro-bubble contrast agents.
how would you remove the fundamental freq in harmonic band filtering?
By using a filter.
The transmitted pulse should contain none of the higher frequency as this could corrupt the received signal.
what is a 4D imaging?
this is when the acquisition and display of volume images is in real time. This is where the time varying spatial relationships of structures are displayed.
T/F?
The matrix size is smaller than the number of scan lines.
False.
The matrix size is bigger than the no of scan lines so that the pixel will not be noticeable. The scan lines can be obvious.
what is a digital scan converter?
after passage through a digital to analogue converter the signal is used to modulate the brightness of pixels on the monitor screen. because the memory locations are written in a different order from which they are read, the system is called a digital scan converter.
T/F?
The smallest signal that can be detected is just greater than the noise.
True
what is the DR in USS
This is the ratio of max intensity of the signal to the minimum that can be detected.
what is the typical DR in USS?
How about after applying TGC?
It is usually 70-80 dB
After TGC the DR is typically 40-50 dB
what does the monitor can display a brightness gain?
25dB
T/F?
The DR of recording film is as good as the monitor
false
The DR of recording film is NOT as good as the monitor
and that of the polaroid film is even worse.
what could be used to increase the change in signal level at an interface?
edge enhancement
what would you use to eliminate low amplitude noise and scatter?
reject control
what is signal filtering?
reject control is used to eliminate low amplitude noise and scatter
name some image processing techniques:
- digital filtering
- temporal averaging
- contrast enhancement.
define axial/depth resolution:
This is the ability to separate two interfaces A and B along the same scan line.
what is the axial resolution in terms of the pulse length?
The axial resolution is about half the pulse length.
The higher the frequency the shorter the pulse.
T/F?
The higher the frequency the shorter the pulse, the better the axial resolution.
True
How could you make the axial resolution worse?
By omitting the backing block and therefore increasing Q
define lateral resolution:
this is the ability to separate two structures side by side at the same depth.
what does the lateral resolution depend pn?
It depends on the beam width being narrower than the gap.
How could you improve the resolution in near field?
by using a smaller transducer and by focusing.
what is the formula for beam width?
Beam width = focal length x wavelength/ diameter.
T/F?
In a focused transducer the beam is narrowest and the resolution best in the focal region.
True
T/F?
The shorter the focal length, the narrower and shorter the focal region.
True
what is the effect of high frequency on resolution and penetration?
It improves the resolution but reduces penetration.
what is the lateral resolution in terms of transducer diameter?
1/3
what is the effect of high frequency probe on image resolution and penetration?
It reduces penetration and improves resolution.
what are the following probes used in?
3.5-5 MHz
5-10 MHz
10-15MHZ
3.5-5 MHz- general purpose abdominal scanning including heart, liver and uterus
5-10 MHz- thyroid, carotid, breast, testis, superficial tissues and for infants.
10-15MHZ- eye which is small and acoustically transparent
what is a speckle?
This is the interference bwn the waves scattered from many small structures, too small and too close to be resolved.
This produces a textured appearance within tissues.
what is reverberation artefact?
Multiple reflections to and fro bwn transducer face and a relatively strongly reflecting interface near the surface produces a series of delayed echoes equally spaced in time that falsely appear to be distant structures.
Name the artefact:
Multiple reflections to and fro bwn transducer face and a relatively strongly reflecting interface near the surface produces a series of delayed echoes equally spaced in time that falsely appear to be distant structures.
Reverberation.
Name the artefcat:
the diaphragm acting as a mirror, or structures in the liver can appear to lie in the lung. Pacemaker wires can also act as strong reflectors.
Double reflection.
what is acoustic shadowing?
The strongly attenuating or reflecting structures eg bowel gas, lung, bone and gallstones and kidney stones- reduce the intensity of echoes from the region behind them and cast shadow.
Give examples of acoustic shadowing:
bowel gas, lung, bone, gallstones and kidney stones
what is acoustic enhancement?
Fluid filled structures being weakly attenuating increase the intensity of echoes from the region behind them, producing a -ve shadow
T/F?
Both acoustic enhancement and shadow are made better with TGC.
False
They are made worse by TGC.
what is refraction?
refraction of beam falling obliquely on the two surfaces displaces the beam and the images of structures beyond. It distorts the image.
what is the other name for ring down artefact?
comet tail artefact.
what is ring down artefact?
Give example
when a small gas bubble resonates it emits uss continuously resulting in a track throughout the scan.
can be caused by air in the stomach.
T/F?
The heart valves and the wall move too quickly to be followed with a normal real time scanner.
True
How is the M mode scanning performed in case of heart valves?
Movement of the heart valves along the line of sight may be displayed by stepping the vertical line of dots slowly and steadily in a horizontal direction across the screen.
quantitative analysis is possible.
T/F?
The change of frequency is proportional to the velocity of the interface.
True
The higher the frequency or the faster the interface moves, the greater the doppler frequency shift.
T/F?
motion at right angle to the transducer shows Doppler effect.
False
Motion at right angle to the transducer shows NO Doppler effect.
when would the max doppler shift is obtained?
when the angle is 0
whereas in imaging the strongest echoes occur when angle is 90.
what is actually measured by the doppler shift of uss?
the blood flow velocity
to make the resonant freq precise and max sound output, a ………….is necessary and no backing block is used..
to make the resonant freq precise and max sound output, a ..high Q..is necessary and no backing block is used.
The higher the pitch the……..the velocity.
The higher the pitch the..greater..the velocity.
T/F?
With continuous wave it is not possible to locate the moving reflector or to distinguish between the flow in two overlapping vessels at different depth.
True
what is duplex scanning?
this is doppler measurements combined with real time B mode imaging.
the scanning head combines a single pulsed doppler transducer offset to a mechanical or electronic sector scanner
how often the B mode scan is updated in duplex scanning?
once a second. Most of the time is spent in Doppler mode.
the doppler transducer might operate at a lower frequency which allows ……..flow to be measred.
the doppler transducer might operate at a lower frequency which allows ..faster..flow to be measred.
what is the cursor is set to do?
It is set to identify the sampling volume.
what is the use of measuring the diameter of the lumen of the vessel?
it can be estimated to allow volume flow rates to be calculated.
what is the depth and width of sampling tissue in pulsed doppler?
The depth of tissue depends on the time and its thickness on the time for which it is open.
The width of sampling volume - pear shaped- is the width of doppler beam.
what PRF is chosen for superficial and deeper vessels?
for superficial vessels a high PRF is chosen and a lower one for deeper vessel.
what is the effect of range setting on PRF and TGC?
As the range setting is increased the PRF is reduced and the TGC automatically increased.
what is a sonogram?
This is a graph of doppler frequency against time and it displays the variations of blood flow velocity and direction during the heart cycle.
T/F?
in a sonogram each pixel coresponds to a different freq or flow velocity.
True
what could you measure from a sonogram?
parameter such as peak velocity, mean velocity, and variance of velocity can be evaluated.
T/F?
when blood flows through a constriction the flow velocity and corresponding pressure increases
true
Bernoulli formula
what is Bernoulli formula?
when blood flows through a constriction the flow velocity and corresponding pressure increases.
P= 4v2*
This is based on Bernoulli equation = blood is incompressible and that energy is conserved but ignores any effect of viscosity and turbulance.
What is Bernoulli equation?
This is based on Bernoulli equation = blood is incompressible and that energy is conserved but ignores any effect of viscosity and turbulance.
define aliasing in USS;
with pulsed doppler it is not possible to measure very high flow velocities with accuracy. If the flow is too fast it will be shown in the wrong direction and its velocity is underestimated.
how would aliasing be shown in sonogram?
as ‘wrap around’ top and bottom in the sonogram.
what is aliasing consequence of?
its a consequence of sampling requirement asso with Shannon and Nyquist- that the waveforms being measured must be at least sampled twice in each period.
T/F?
aliasing occurs with continuous wave doppler.
False
Aliasing does NOT occur with continuous wave doppler.
what is the fastest flow that could be measured with accuracy?
the fastest flow that can be measured with accuracy is the velocity that produces a doppler shift freq equal to half the PRF being used.
A greater flow velocity than this produces aliasing.
T/F?
it is easy to measure fast flow in deep vessels.
False
It is difficult.
The deeper the gate has to be set the smaller the PRF that could be used and so the smaller, the fastest flow that can be measured without aliasing.
what determines the PRF needed?
the depth of the sampling volume determines the PRF needed.
The PRF determines the max velocity that can be measured without aliasing.
what are the ways of reducing aliasing?
- by reducing Doppler effect either by:
1. using a probe of lower frequency
2. increasing the angle.
but both increase the error in the measured flow. - by increasing PRF
T/F?
doubling the PRF, doubles the max measurable blood flow velocity.
true
T/F?
when the PRF becomes very high the pulse merge and we would have continuous wave doppler with no aliasing with no range data.
True
what shows the direction and velocity of movements or flow?
a colour mapped doppler.
T/F?
colour scheme depends on themanufacturer.
true
what is the difference bwn colour doppler and normal B scan mode?
The doppler pulse is longer in colour doppler.
how many frames are scanned a sec in colour doppler?
typically 25 frames are scanned a sec over 60 degree sector.
compare the sonogram in a gated doppler with colour doppler;
the sonogram in a gated doppler is produced from eg 100 consecutive pulses( infinite no in continuous doppler) in colour scanning there is only 4-12 pulses along each scan line.
they only need to estimate the mean velocity in each sample volume.
to produce the sonogram of any sample, vol or to measure peak velocity, a single sample volume is selected on the ……. and the instrument switched to ………
to produce the sonogram of any sample, vol or to measure peak velocity, a single sample volume is selected on the ..colour scan.. and the instrument switched to ..spectral doppler mode..
the performance of colour scan doppler is limited by…..
small time available to collect the datafrom each beam position.
comment on:
- frame rate
- penetration depth
- Field width
- line density
- number of pulses in a train.
Frame rate- should be fast enough to follow changes of flow velocity
penetration depth- it is inversely proportional to PRF
Field width or sector size- 30-90 degree
line density- ie scan lines per frame should be high enough for good spatial resolution.
number of pulses in a train- should be high enough to give accurate velocity information.
T/F?
if requiring a high frame rate for children, then the sector size and depth range should be set to minimum.
True
T/F?
aliasing is a feature of colour scans.
true
what is power doppler?
Power doppler map the amplitude of the doppler signal without any indication of the velocity.
what does the amplitude depend on in power doppler?
it depends on the number of red blood cells within the volume and on attenuation through the tissue.
all movements regardless of phase will contribute to the amplitude.
so that the power doppler emphasises the quantity of blood flow.
T/F?
The power doppler is subject to aliasing.
false
As this is non directional, it is not subject to aliasing.
T/F?
weaker signals can be imaged on power doppler.
True
it has increased sensitivities for imaging smaller vessel.
how would you test for resolution
it is tested by imaging a test rig composed of parallel wires mounted on a frame and immersed in a perspex bath containing a fluid in which sound travels at 1540 m/s
how would you test for sensitivity, DR and accuracy of A scan calliber?
using a perspex block machined with a number of equally spaced vertical rods, of different diameter and of decreasing depth.
each 7mm of perspex is equivalent to 4mm of tissue ( sound travels faster in perspex)
how would you test for greyscale performance and doppler function?u
usually based on phantom with tissue equivalent properties based on gelatine loaded with different reflecting material often called tissue mimicking phantom.
how would you measure the power out put of transducer
by weighing the sound pressure with a force balance or by measuring the heating effect using a calorimeter.
what is used to measure the heating effect?
calorimeter.
where is the intensity of the uss beam at its greatest?
usually in the focl area- 0. 1mW/mm2*
the time averaged intensity should no where exceed…….. mW/cm2*
The total sound energy (intensity x dwell time) should nowhere exceed…….. J/cm2*
the time averaged intensity should no where exceed..100.. mW/cm2*
The total sound energy (intensity x dwell time) should nowhere exceed..50.. J/cm2*
what are the risks associated from USS;
- local heating- due to frictional, viscous and molecular relaxation process- leading to chemical damage but mitigated by blood flow
- acoustic streaming of cellular content in direction of the beam affecting cell membrane permeability.
- cavitation-
- mechanical damage to cell membrane caused by violent acceleration of particle.
how would uss lead to cavitation?
the high peak pressure changes causing micro bubbles in a liquid or near liquid medium to expand. if they collapse all of a sudden there will be an enormous increase in temp–>chemical damage to cellular constituent- more likely with pulsed beam
define thermal index:
this is the ratio of the power emitted to that required to increase the temp by 1C and gives an indication of the temp rise in tissue.
define mechanical index:
this is a measure the max amplitude of the pressure pulse and is defined as the peak rarefaction pressure divided by the square root of USS frequency
what should happen to exposure time when TI is >1
The exposure time should be reduced as TI increases above 1
particular care is needed in scanning when patient is feverish and when using………..mode.
particular care is needed in scanning when patient is feverish and when using..pulsed doppler system..mode.
what are the sources of noise in USS?
most noise is electronic from the very small current measured. Additional noise comes from reverberation in patient or in the transducer probe.
