Diagnostic studies Flashcards
what are the types of mechanical wave propagation?
- transverse - particles vibrate in direction perpendicular to the wave2. longitudinal - particles vibrate back and forth in the same direction
what are the acoustic variables
change in pressurechange in densitychange in temperaturechange in particle location
bioeffect
- mechanical - related to cavitation if the acoustic energy is too high, gas boubles can form in the medium and implode- stable cavitation - vibration of bubble that doesnt result in violent colaps- transient - violent collapse of bubbles in medium resulting in thermal release2. thermal
wave parameters
frequencypropagation velocitywavelengthamplitude related to power and intensity
what has the lowest speed of propagation
gas
what has the highest speed of propagation
solid material
what is the tissue speed of propagation
1540m/sec
what is frequency
measure how often the event occurs per time (in Hertz)corresponds to the number of compression - rararefractiontypically 2-30mHz
what is period
reciprocal of frequency. Time between event occurring and event being repeated.Typically 0.5 - 0.083 micro seconds
propagation velocity
determined by the stiffness and density of medium higher density = higher stiffness = higher propagation velocity
wavelength
propagation velocity over frequencydistance between compression (peak) and rarefraction (minima) of a waveshorter wavelength = better resolution
what happens when the color gain is too low
color dropout through the vessell
what happens when the color gain is too high
color can bleed over regions of tissue
wall filter
eliminates Doppler signal below a set velocity as a means of reducing color clutter signals produced by relatively slow movement from strong reflectors
what does a low wall filter allow you to do
detect very low velocity flow (color scale has to be set low as well)
what happens with high pulse repetition frequency
high wall filtercant see lower velocitiesonly middle of the vessel fills
reasons to detect higher PSV than reality in spectral doppler
1) overgaining –> leads to blossoming
2) too high transmit power –> leads to blossoming
3) artifacts related to large Doppler angle (>60*)
4) incorrect angle correction
what does spectral doppler measure
velocity directly
characteristics of power doppler
does not display flow direction
gives mean velocity
has the worst temporal resolution
characteristic of color power doppler
much less angle dependant
encodes signal based on strength (power)
more sensitive of low flow velocities
what is required for vascular lab accreditation
- standardized protocols for exmaination performance and interpretation
- protocols in place for urgent reporting of critical findings
- ongoing quality assurance program must provide clinical correlation of lab results with oher confirmatory studies or operative findings
- current examination findings must be composed with previous studies
- lab medical director has to have formal (residency/fellowship) or informal (through CME) trainng in vascular interpretation + RPVI
resistance equasion
8 x length x viscosity/pi x radius to 4th power
Reynauld’s number
determines likelyhood of turbulence
inertial forces / viscous forvces
=diameter x volume x velocity of fluid / viscosity
>2000 results in turbulence
Bernoulli’s equasion
the greatest pressure gradient exists where distal velocity is greatest
lowest pressure exists where the highest velocity exists (just distal to stenosis)
Power Injector Guidelines for aortic arch
- flush catheter
- pressure 900-1200
- rate of rise 0.1
- injection rate 15
- total volume 30
Power Injector Guidelines for carotid artery
- end hole catheter
- pressure 300
- rate of rise 0.7
- injection rate 3-5
- total volume 6-10
Power Injector Guidelines for subclavian and brachiala rtery
- end hole catheter
- pressure 300
- rate of rise 0.7
- injection rate 3-5
- total volume 6-10
Power Injector Guidelines for abdominal aorta
- flush catheter
- pressure 900
- rate of rise 0.1
- rate 15
- total volume 30
Power Injector Guidelines for SMA / Celiac
- end hole catehter
- pressure 200
- rate of rise 1
- rate 3
- total volume 6-20
Power Injector Guidelines for renal artery
- end hole catheter
- pressure 200
- rate of rise 1
- rate 3
- total volume 6
Power Injector Guidelines for pelvic vessels
- flush catheter
- pressure 900
- rate of rise 0.1
- rate 10
- total volume 20
Power Injector Guidelines for infrainguinal segment
- end hole catheter
- pressure 300
- rate of rise 0.7
- rate 3-5
- total volume 6-10
characteristics of balloon expendable stents
- simple precise deployment
- can be dilated beyond reported diameter but with correspondent shortening of the stent
- higher hoop strength
- better for orificial lesions
characteristics of self - expanding stents
- require greater degree of operator stabilization
- cannot be postdilated beyond their normal diameter
- more flexible but less hoop strength
- good for tortuous or mobile vessells
pulsatility index definition
value of resistance of peripheral bed,
with increased peripheral vasodilation, there is an increase in diastolic runoff and decreased pulsatility index
What does increasing sample size do
Increases signal to noise ration (SNR) improving sensitivity to low velocity flow
What does increasing Doppler gain do
Amplifies both signal and noise so SNR is knot improved
What doe e wall filter do
Removes the low frequency Doppler shifted signal (low velocity flow had low copper frequency)
What does low PRF do
Appropriate for detecting and displaying low velocity flow
What is plus repetition frequency
Pulsed wave Doppler instruments determine blood flow velocity by taking intermittent samples of the flow. The rate of sampling is the pulse repetition frequency
What is the Nyquist limit
The upper limit of the Doppler shift frequency that can be detected accurately
Commonly defined as pulse repetition frequency /2
What’s aliasing
Artifact of the Doppler flow detection that occurs when the Doppler frequency shift exceeds the Nyquist limit
When aliasing occurs in a spectral waveform, the velocity information is displayed improperly and the systolic peaks are cut off at the top of the velocity scale and appear below the baseline as flow in the reverse direction
Increasing the PRF will increase the sampling rate and reduce or eliminate the aliasing effect. Also achieved by decreasing Doppler angle or using lower frequency transducer
Doppler effect
Change in frequency of a sound wave produced by a relative motion between a sound source and a listener
When moving towards each other frequency change is positive - increase
When moving away from each other frequency change is negative - decreased
Transmitting frequency
Depth of imaging is inversely proportional to transmitting frequency. Low frequency penetrates more deeply than high
Continuous wave Doppler
Transmitting and receiving transducers operated simultaneously and continuously
Can not identify flow st specific site / depth
Qualitative assessment of flow
Bedside Doppler
Pulsed wave Doppler
Signal transducer alternated between transmitting and receiving
Able to determine flow st specific site / depth
Burst of ultrasound (pulse) is transmitted and after specific time interval receiver is activated
Fast Fourier transform
Generates amplitude vs frequency profile through analysis of the detected signals
Aliasing
Occurs when sampling is not frequent enough to capture actual movement
Occurs when nyquist limit (frequency, 1/2 PRF) is exceeded
Is decreased by increasing pulse repetition frequency
Waveform wraps around and appears as flow in the opposite direction
Normal carotid bulb flow on duplex
Unidirectional flow along the flow divider throughout systole
Reversal of flow at power players region at peak systole
No flow along the outer wall at the end of diastole
