Flashcards in RPVI-Physics Deck (107)
Things we need to know about sounds.
sound doesn't travel instantaneously (predictable velocities)
sound travels in a straight line
sounds create pressure energy
sounds create echoes
Does sound travel faster in water or air? low vs high pitch.
low pitch (higher travel less distance)
Is high frequency sounds better at locating smaller or larger objects?
small (like a bat)
What is stiffness and what does is mean for sound velocity?
Stiffness refers to the stiffness of bonds between particles and sounds travels faster the stiffer bonds are
What does density refer to and what does that mean for sound velocity?
how close particles are together and low density media has fast ultrasound velocities
which is slowest to fastest
gases, liquids/soft tissue, solids
air, fat, water, soft tissue, liver, kidney, blood, muscle, bone
fat is not stiff but so low density it travels faster
what is the measure of sound velocity
M/sec or mm/usec
What is the equation to determine depth?
depth=1/2 velocity mm/us X round trip time ms
What is the velocity of sound?
What is frequency?
one cycle of compression and rarefaction of a sound wave.
frequency is measured by hertz
what is one Hertz?
one cycle per second
what range is audible sound in hertz?
what do we use for vascular applications?
What is the equation for wavelength?
V = lamda x Hz
distance of one cycle
wavelength is lamda
wavelength is distance
how many micro sec in a sec?
what gives better resolution higher or lower frequencies?
high frequencies but the don't travel as far into the tissue
what is amplitude measure by?
it is a pressure
what happens to sound intensity as it propagates?
it loses energy
what is this loss called?
What is one decibel?
i1/i2 is the intensity ratio the bigger the ratio the greater the decibel
if there is 100000 (5 zeros) then the log of this is 5.
What things is attenuation a sum of?
divergence of the wave front
What is the half power distance of sound?
the distance sound can travel before only half of the sound remains
Does air have a short or long half power distance?
What is the average attenuation for soft tissue?
what is reflection depended on ?
where is reflection occur?
are all the sounds waves reflected?
why is reflection important?
at soft tissue interfaces (acoustic interface)
no some continues on
if forms US images
what are smooth or specular reflectosreflectors?
needs a 90 angle to be seen
imtima, walls of cyst, venous valves
why does acoustic interface cause reflection?
because has different transmission of velocity and different density.
Z= density X velocity = acoustic impedence value (rayles)
how do you calculate impedance?
R=(z2-z1)/(z2+z1) x 100 = % sound reflected
(ie X% of sound is being reflected)
if we send the US there and back the % happens twice as it travels back
If the speed of sound becomes faster at the interface does is move towards or away from the perpendicular line of the interface?
What is snells law?
V1 x sin(O1) = V2 x sin (O2)
O = theta
theta is the angle from the perpendicular line of the interface
what happens to heat and sound travel?
sounds is converted to heat
more energy is lost at higher frequencies
conversion to heat is less with stiffer media
What is sound scattering?
redirection of sound in several directions so only a small fraction of sound energy returns
What is it caused by?
interaction with a small reflector (rbc) (rayleigh scatter) or a rough interface
rayleigh scatter makes US possible
what is the number of events per unit time?
frequency in time
What is relationship between wavelength and frequency?
frequency and penetration?
frequency and heat production
What is a transducer?
a machine that converts one kind of energy into another energy.
what is the most common vascular transducer? what about for abdo?
what are other special transducers?
curved-linear for abdo (convex)
trans-cranial sector transducer
small curved-linear--transvag can be used for deep vessel in grion, under a rib.
What modes are available for linear transducers?
what does the image look like for normal mode on a linear transducer? for wide?
trapazoid to see wider images (sound is angled at edges0
What are curvilinear transducers used for?
what does the image look like for curvilinear?
edges are curved
what probe can be used for large patients with folds?
what are sector transducers used for?
they have small footprint
what does the image look like for sector transducer
triangular with tip cut off
wide mode linear looks similar but top is much wider
what is the piezoelectric element in a transducer?
converts sounds to electricity and electricity to sound
what does the matching layer of a transducer do?
create acoustic interface that won't reflect all of our sounds back into the transducer
What is the damping layer
its under the matching layer
material that prevents the piezoelectric element from ringing more then 1-2 wavelengths before it stops sound production
what is the order of these elements of a transducer
what is the pulse echo principle?
Us sent into patient with a brief pulse
travel through patient and is reflected, refracted, absorbed and scattered
some sound returns to transducers and creates an image
what is b-mode
What are prerequisites for B-mode imaging
direction of pulse
multiple lines of info obtained and integrated quickly to form image
What does the brightness of the image mean?
it represents the strength of the echo.
what is depth on an US machine?
the maximum time that the unit 'listens'
longer means deeper
What does overall gain do?
it amplifies all of the echoes.
its magnifying the electrical impulses before it gets to the monitor
What is TGC?
is amplification of distant echoes to correct for attenuation. so farther images are amplified but not closer ones.
what does write magnification do
select small portion of the screen and scan only that portion of the screen. get more precise image and larger image
how is focus best used?
keep the focus at or just below area of focus
what is spatial resolution?
what are the different types of spatial resolution?
ability to discerned on tiny structure from another at closer and closer distances apart.
axial along US beam
lateral across transducer
elevational thickness of US slice (can contrib to error)
What is temporal resolution?
how events are happening over time
i.e. blood flow events
What is image uniformity
machines ability to place echoes at their correct depth.
how many frames per sec do you need for adequate temporal resolution?
16 frames per sec gives smooth image
what factors change the frame rate?
lines per frame
# of focal zones
field of view
# of functions running
will all slow down the frame rate
what is the pulse repetition frequency PRF?
number of echoes produces per second
1 pulse per millisec = PRF of 1000
what depth of field of views correspond to what PRF?
75 cm = 1000
25 cm = 3000
12.5 cm = 6000
7.5 cm = 10 000
what is the usual line density?
why is this important?
how can we overcome that disadvantages?
2-6 pulses per line
every pulse produces a line that must travel from the surface of the imaged area to the deepest point and return. the more lines the longer it takes to display image. you trade of TEMPORAL resolution for spatial resolution
lower # of lines, decrease focal zones, write zoom
What other function affect frame rate?
more lines are needed to get the information
What won't affect frame rate
how should the notch of the transducer be placed?
towards the head for sagittal or longitudinal scans
or towards the right for transverse scanning
Which side does the notch appear on the image in sagittal?
the left (called superior cranial)
right called inferior or caudal
In tranverse image where does the notch display and what side of the patient does that represent?
what do we call the side we are touching the patients skin?
what does echogenic?
subtance has echoes in it
more echoes or brighter then surrounding material
tissue is surrounds by tissue that is brighter then it
there are no echoes in a substance
two areas have same level of brightness
what are specular reflector
smooth reflector require 90 angle to be seen
like gortex graft, intima
rough reflector does not need 90 angle
What US assumption can lead to error?
beam is infinitely thin
each echo comes from shortest pathway
loss of energy is linear and predicatable
What is an artifact?
image that doesn't represent in position or intensity any real structure
What is a mirror artifact?
it creates a pseudo mass.
echo hits vein then behind it the artery the artery reflects an echo and it hits the vein again which sends it back to the artery and then to transducer. a mirror image then appears to be deeper then the artery?
how do we eliminate the mirror image?
can the angle of the transducer
what different ways can artifacts occur?
comet tail artifact
How are side lobes produced
produced by all multi crystal transducers
interference from multiple piezo
what is reverberation?
echoe propagates back and forth multiple times between an object
what is comet-tail artifact?
similar to reverberation
series of closely spaced small lines
decrease by using compound imaging
seen best in hypo echoic areas also calcifications
what is an acoustic shadow?
structure adsorbs, refracts or reflects sound at higher level (>0.5) then there will be an area of decreased brightness behind it
what is acoustic enhancement?
structure adsorbs, refracts or reflects sound at lower level then there will be an area of increased brightness behind it
what mode of imaging compounds and reduces these artifacts?
compound (colour also goes on an angle)
What is twinkling artifact?
calcium can also cause colour
happened because of vibrations
get the patient to fremitus and then you can see that it is artifact
What is the equation for doppler shift?
delta F= (2 f/c) v cos 0
delta F= change in freq (doppler shift)
f = frequency of transducer (average if multiple)
v= velocity of blood flow
0=theta angle of isolation
direction of blood flow and direction of sound waves
what happens when theta = 90?
cos 0 = 0
don't ever really get 90 because of the echoes form a cone
How do we solve for velocity?
v=1/2F deltac /f cos0
what happens to doppler shift when blood velocity increases? when doppler angle approaches 90? increase initial frequency?
decrease towards zero
If you don't adjust angle in pulsed doppler what happens?
what angle should you keep the transducer and why?
can cause error
error increases with increasing angle
still get error 18% at 60 degrees
what is continuous wave doppler?
2 piezo simultaneous transmitt and received
no depth position, minimal global info
aliasing cannot occur
what is pulsed doppler?
timed or pulsed so we can
control the depth
minimal global info
can have aliasing
what is aliasing?
erroneous representation of flow
occurs when doppler shift > 1/2 PRF
how do you optimize pulse doppler?
optimize gray scale
select area of interest with appropriate window with reasonable depth
set scale, baseline and doppler gain
set filter to eliminate noise
what decreases aliasing?
shallower vessel (because info coming back faster)
size of field of view (longer time to collect data if large FOV)
decrease image processing (colour on)
how do we avoid aliasing?
adjust baseline (lower it and get wider range of frequency)
increase doppler angle (as approaches 90 doppler shift approaches zero so decrease accuracy but decreases aliasing)
decrease frequency (higher freq equals higher doppler shift)
turn off colour or power doppler
what can we measure in pulsed doppler?
if you are measuring in cm/s then you need an angle
resistive index RI (S-D)/S
systolic, diastolic, mean
what is spectral analysis?
what is the envelope
what is the spectral window
thickness/thin of line on pulsed doppler
the most outside edge
area of black under the envelope
what is spectral broadening?
when the spectral window is thick
large sample volume (gate is more then 1/3 of the artery)