Module 7 : Doppler Principles Flashcards
what is a doppler shift
- a change in pitch results form a relative motion of the source of the sound or the reciever
- when a sound source is moving towards you the wave crests are closer together and pitch is higher or moving away from you the crests are further apart and pitch is lower
what 3 things does doppler give us information on
- presence of flow
- direction of flow
- quality of flow
what are three different types of doppler
- spectral trace
- color image
- audible sound
how is the Doppler effect produced with blood
- sound reflects of moving RBCs which alter the frequency that is returned
- the amount of change is related to the direction and speed of the RBCs
what does the intensity of the signal relate to
- proportional to the number of RBCs and thus indicates the quantity of blood in the sample
what is the doppler shift equation
- doppler shift = received - transmitted
how is the doppler shift determined
- uses the beat frequency
- reflected wave differs from transmitted frequency
what is the more detailed doppler shift equation
^F = 2Fo x v x cos0 / c
does doppler use a higher or lower frequency
- lower frequency
- RBCs very small so already reflect very little of the echoes
what is the most accurate angle of insonation for doppler
- most accurate would be 0º
- we use 60º accurate and reproducible
what is the nyquist limit
- reached when RBC velocity faster than machines ability to sample
- pulses only sent out as fast as they can be returned and are limited by the speed of sound in tissue
- causes aliasing
what is the nyquist limit equation thing
1/2 PRF
what is aliasing
- wrapping of the color or the spectral trace around the baseline
what are the five ways we can correct for aliasing
- move the baseline
- increase the PRF
- increase doppler angle
- lower the operating frequeny
- change to continuous wave
what two methods o correct for aliasing do we use in practice
- move baseline
- increase PRF
where should the sample volume be placed in the vessel
- middle of vessel
- where the highest velocity is
what are the 3 components of the sample volume
- beam width (3D)
- receiver gate length
- length of emitted pulse
what decreases the gate length
- pulse length reduced
- improve spectral display
do you want long or short pulse for doppler
- long pulses to interact with the RBCs longer
instead of range gating what is used with continuous wave doppler
- no image so uses zone of sensitivity
what is the zone of sensitivity
- intersection of transmitted and received beam
what is the spectrum
- array of components of a wave arranged in order of increasing frequency over time
what are the three components of the spectral trace
- time (x)
- frequency or velocity (y)
- power (z)
what is the power (Z axis) of the spectral trace
- represented by brightness of the pixels
- proportionate to the number fo blood cells moving at one velocity at a given time
what is the way in which a system creates a spectral trace
- fast Fourier transform
what is the Y axis of the spectral trace
- represent either frequency of the doppler shift or velocity
- velocity is preferred
what is the flow direction of RBCs relative to
- the transducer
what does antegrate flow mean
- towards the probe expressed as positive number
what is retrograde flow
- flow away from the transducer expressed as a negative number
what are the 3 different categories of pulsatility
- high
- moderate
- low
what are the characteristics of high pulsatility or high resistance flow
- tall narrow sharp systolic peak
- reversed or absent diastolic flow
where is high resistance flow seen in body
- arteries of the extremities
what are the characteristics of low pulsatility or low resistance flow
- broad systolic peaks
- forward flow through diastole
where do we see low resistance flow
- vessels that feed the vital organs
characteristics of moderate pulsatile or moderate resistance flow
- tall narrow sharp systolic peak
- forward flow through diastole
what is the pulsatility index equation
- PI = A (peak velocity) - B (lowest velocity) / mean
what is the resistivity index equation
- RI = A - B / A
what is the systolic diastolic ratio
S/D = A/B
acceleration equation
acceleration = ^v / ^t
what is spectral broadening
- vertical thickening of the spectral trace or envelope
what is false spectral broadening
- artifact that occurs when doppler gain is set too high
