Sono 3 + 4 Flashcards
Transducer focussing can be either:
Fixed or adjustable
Disadvantage of fixed transducer?
not able to produce same image detail at all depths of the body
A transducer can be
designed to produce a
focused ultrasound beam by
using:
An acoustic lens in front of the element A concaved piezoelectric element An acoustic mirror behind the crystal
Which transducers are able to adjust focus to a specific depth for each transmitted pulse?
Adjustable array transducers
How can focusing be achieved in adjustable transducers?
Electronic
Uses a series of time delays in triggering each crystal
Dynamic Aperture
Uses changes to the number of crystal elements activated at a single time
In other words there are changes to the diameter of the
crystal
In electronic focussing , which crystals are activated first?
the outer most.
inner most (central crystal) is last to send out a pulse
If there is a greater time delay in electronic focusing.. will the focal depth be longer or shorter?
Shorter meaning more focussing has occurred.
If more elements are activated the larger the diameter of the transducer…. if a transducer has a larger diameter will the focal depth be larger or smaller?
Larger.. the focussing is done further in depth due to larger divergencies..
How many times wider does the transducer width or aperture have to be compared to the wavelength to produce a parallel-sided US beam?
ATLEAST 10 times.
This means that the curved individual sound waves will form together to make a straight-line. (uniform beam)
What is a stepped array?
When adjacent elements are activated on after another to create a large number of spaced lines… kind of like a mexican wave,.
What is a steered array?
Creates a ‘wave’ by triggering the outermost elements first before finally activating the centre one.
essentially sweeps the beam.
What is another name for the steered array?
phased array
If the angle is increased in phased array sweeping what has occurred?
There has been a larger time delay.. (most likely at the outermost crystals).
What does the combination of images from differnt directions produce?
a compound image
what is a compound image best used for?
Looking at curved organs or lesions boundaries.
Also used for moving objects in doppler
What is A mode and B mode imaging?
A mode: CW (continuous wave)
B mode: PW (pulsed wave) doppler
What does the A stand for in A mode?
Amplitude
What information does the A scan show?
Depths of the echo-producing structures and amplitude of the echo.
When is A mode now typically used?
For imaging of the eye.
other than that it is rarely used.
What does the B in B mode stand for?
brightness.
How does B mode differ from A mode?
It gives brightness to the image … so we can correlate the brightness into a grey scale when matched to specific Echo signal amplitudes.
What determines the PRF? (pulse repetition frequency)
The depth at which we are imaging
What is PRF?
Pulse repetition frequency
How many pulses are sent out per second
What is the purpose of the master timer?
To set the PRF and SIMULTANEOUSLY sending timing pulses to the pulse generator , Time gain compensator and display.
How many Pulses per second does the pulse generator produce?
apporx 1000 PPS
What is TGC and what is its purpose?
Time gain compensator
Ensures that echoes from similar structures at different depths have the same amplitude in the signal
What converts the electrical signal to a signal for the video feed?
demodulator
What does the amplitude of the signal depend on?
Reflection coefficient
Angle of incidence
Attenuation
TGC
What does amplitude give in the image?
Brightness scale .. its the ‘strength’ of the echo.
What is doppler US based on?
The apparent shift (change) of frequency
What frequencies (range) are used in doppler?
what ranges are used for shallow vessels?
What ranges for deeper vessels?
2-10 MHz
(5-7MHz) higher frequencies for shallow vessels
2 MHz . lower frequencies for deeper vessels.
What is the doppler shift?
difference in the sent and received frequency
What is the range of frequencies in the doppler shift? and what does that mean?
200 - 15, 000 Hz.
THis is is AUDIBLE range so we can hear it.
What would a higher pitch in doppler shift mean?
increased velocity, so structure is moving faster.
What is the doppler effect?
Apparent change in frequency of sound if there is relative motion between the source of sound and the receiver.
If a source is moving towards the receiver will frequency increase or decrease?
increase
… APPEARS TO INCREASE.. technically still the same frequency.
what is a disadvantage of using dynamic focusing over electronic focusing?
Lose of lateral resolution
If there is 0 time delay on a steered array what will occur?
straight beam produced
what is exponential attenuation?
loss of sound as it moves in a given direction .. often associated with A mode
Why is doppler imaging only a APPARENT change?
Because the change isnt ‘real’ the speed of sound is still at a constant of 1540 m/s
The greater the doppler angle the greater the doppler error , how can we reduce this?
keep the angle between 30-60 degrees.
Which region can doppler detect blood flow?
and what makes this region occur?
Sampling region.
Overlap of transmitted and received beams
Limitations of CW doppler?
units dont indicate direction of blood flow
cannot seperate signals from multiple blood vessels that fall into the same overlap region… Super impsotion
We cannot use the TGC because depth info is lacking (theyre always mvoing)
What is a high pass filter?
Removes low frequencies
Such as strong low frequency shifts (below 200 Hz) due to movement of vessel walls (wall thump).
If high pass filter is set too high (it blocks too much low frequencies ) what may occur?
Slow moving blood vessels may get their echo filtered out.
a longer SPL reduces axial resolution yet PW doppler uses a longer SPL than B mode , why?
to allow better measuring accuracy of the doppler shift.
how does PW doppler achieve depth selection
by using an electroninc time gate window
why is the beat pattern not as well defined in CW doppler?
cause we sample the signal at discrete intervals.
which doppler has a more defined beat frequency?
PW doppler.
Does aliasing occur in CW doppler
no
When will aliasing occur?
When the the PRF is less than twice the doppler signal frequency
WHat is aliasing?
Detection of lower (than actual) frequency sound.
3 Ways to increase correct measurement of max blood velocity?
Increase PRF - limits sampling depth
Decrease transducer frequency - reduces axial resolution
Increase doppler angle
These will also remove aliasing
Tissues damage in unfocussed US?
Below 100mW/cm squared
Tissue damage in focussed US?
Below 1W/cm squared
What is TP?
Temporal peak (TP) – maximum intensity within a pulse
What is PA?
Pulse average (PA) –intensity averaged over the duration of a single pulse
what is TA?
Temporal average (TA) –intensity averaged over pulse
repetition period and includes the “dead” time between the
pulses.
What is SP?
Spatial peak (SP) – maximum intensity within the beam
profile and usually occurs at the centre of the beam profile
What is SA?
Spatial average (SA) – intensity averaged over the crosssectional area of the beam. A cut off point of 0.25 times the spatial peak intensity has been established to limit the area over which the intensity is averaged.
Values of ISPTA are usually below ___________
for imaging.
100 mW/cm2
Values of ISPTA can exceed _________ for certain
Doppler applications.
1,000 mW/cm2
What are some mechanical effects of US?
Damage to bodies with lots of gas e.g. lung and intestine
Some areas with US CMedia
What temp and time can cause tissue damage?
8-10 degrees C for 1-2 mins
in doppler mode what does risk of US heating injuries depend on?
Temperature elevation and dwell time
for every 1 degree celcius increase, how much should the dwell time be decreased by to when attempting to decrease temperature dose to a featus?
a factor of 4
What is the maximum temp allowance above 37 degrees?
1.5 degrees.
how can US damage fluids such as blood?
microstreaming
how is cavitation classified?
stable or transient
what is cavitation
rupturing of gas or air filled cavities
can stable cavitation cause microstreaming?
yes
When does stable cavitiation occur in mammals and at what range?
continuous wave ISPTP of 10 mW/cm squared
What is transient cavitation?
a explosive cavitiation
causes SEVERE microstreaming and rupture to tissues….
and occurs ISPTP at 2000 W/cm2
Should transient cavitiation occur during imaging??
no. we shouldnt be using intensities that hight
at what point in the thermal and mechanical index is the risk v reward discussion held?
thermal : 1.0
Mechanical: 0.5
TI meaning?
thermal index
what does a TI value of 2 signify?
a possible 2 degree C increase
what tissue can cause high amounts of refraction?
fat
WHat is the sign of reverberation in an image?
echoes are equally spaced
What is comet form?
a high form of reverberation
high amp echoes raper and reduce with brightness in depth
what is shadowing?
loss of echoes behind a structure
bone = clean shadow
gas = dirty shaddow
What is enhancement artefact?
converse of shadowing
a form of impedence .. usefull to see cysts…
lesions appearing black with no enhancment = solid
lesions appearing black with enhancement = cyst.
High velocity layer
faster objects appear nearer
slower object appear further away.
(than the 1540 m/s)
What is a common example of a high velocity material?
bone
Where does mirroring often occur?
diaphragm
