5. Doppler Technology Flashcards
Doppler Effect
- if RBCs are stationary, the signal is reflected at the same frequency as the transmitted signal
- when blood flows towards the transducer, the returning signal is compressed by flow of blood producing a shorter wavelength and higher frequency
- when blood flows away from the transducer, the returning signal is stretched and wavelength is larger and frequency is lower
How is the velocity of blood flow determined when using Doppler?
- examination of the frequency change between ultrasound emitted and ultrasound reflected from red blood cells
Modulation
- term for the alterations in the frequency of ultrasound signals that produces the Doppler effect
How is the Doppler signal affected by hematocrit?
Higher hematocrit produces more particles available to reflect back the ultrasound, producing a stronger return signal
- maximized at hct 30, above which destructive interference degrades the signal
- modern echos function well at a wide range of hematocrit
Doppler Equation
Frequency change = v x cos angle x 2f/c
v = blood velocity angle = angle of incidence of beam with blood flow f = transmitted frequency c = speed of sound in blood (1540 m/s)
** only blood velocity and angle are non-constants **
Beam angle and Doppler
Doppler equation relies on cos angle
Cos 1 = 1 ; cos 90 = 0
** at angle of incidence greater than 30 the value of cos rapidly decreases to zero and makes it difficult for Doppler to appreciate movement of blood **
** to simplify mathematics and strengthen the Doppler signal, most machines default to cos 0 (=1) and thus ignore this part of the equation **
Isolating the Doppler frequency shift
Steps:
- Quadrature demodulation
* distinguishes red cell modulated echoes from non-frequency-shifted returning echoes
* compares returning echoes to reference signals that are in phase and 90 degrees out of pahse - Fast Fourier transformation
* isolates specific Doppler frequencies out of the Doppler frequents shifts - Doppler frequency converted to velocity and displayed
Ways to display Doppler data
- Audible
2. Spectral
Audible broadcast of Doppler data
- frequency shifts of blood flow in the heart and great vessels is within the audible range
- texture, pitch, loudness and frequency are used by echo provider to optimize signal
- ideal location = highest pitch and loudest signal
Spectral display of Doppler data
= presenting Doppler data as time-velocity plot
- the spectrum of velocities detected by Fourier transformation are displayed
- frequencies with greater amplitude (loudness) are brighter
- excellent temporal resolution
- basis for quantitative hemodynamic calculations
VTI
Velocity time integral
= area under the velocity-time plot for a single cardiac cycle
Markers of a high quality Doppler signal (spectral display)
- clean envelope
- sharply demarcated borders
- bright pixels
- clear peaks
Pulsed wave Doppler
- uses single crystal as emitter and receiver of ultrasound waves
- emits a short burst and then switches to receive mode
- time delay for signal to leave probe and return depends solely on distance of point from probe
- speed of ultrasound beam constant @ 1540
- delay = (2 x distance)/speed
- transducer interprets echoes only after this time delay has elapsed
- pulse length determines length of the sample volume
- transducer size, signal frequency and beam focus determine sample width and height
Sample volume (PW)
- refers to volume of blood being analyzed at a specific location when using pulsed wave Doppler
Time gating
- selecting signals at a specific depth or location for interpretation based on calculated time delay
Aka pulsed wave Doppler
Time delay equation (pulsed wave Doppler)
Time delay = (2 x distance)/speed
Speed = 1540 constant
Pulse length
= wavelength x # cycles contained in each sound pulse
Pulse wave Doppler trade off
- good range resolution (small sample length) vs accurate determination of velocity
- contrary to 2d echo which prefers short pulse lengths to maximize axial resolution, pulse wave Doppler prefers longer pulse length to maximize the amount of Doppler signal received back at transducer for analysis
Pulse repetition frequency (PRF)
- in pulsed wave Doppler, refers to rate at which the device repeatedly generated sound bursts (ie the time between sending one signal and then sending the next)
- directly related to the depth
- the longer the distance to the sample volume, the lower the PRF
- maximal frequency at which pulse wave Doppler information can be refreshed = 1/2 x PRF
Limitations of pulsed wave Doppler
- pulsed wave Doppler samples intermittently
- system can only resolve frequency that are less than half the PRF (pulse repetition frequent)
- frequencies above this are ambiguous and produce aliasing
1/2 PRF = nyquist limit
Nyquist Limit
= 1/2 pulse repetition frequency
- that is, the highest frequency of change that the pulsed wave Doppler can resolve
- higher velocities are ambiguous and produce aliasing
- changes based on distance from transducer
Maximizing Pulsed wave Doppler
- Choose image that limits distance from transducer to area of interest
- Select low transmitted frequency
* returning Doppler signal with shift = original frequency plus shift. If original frequency is lower, it gives you more room before meeting frequency ceiling (nyquist limit)
* lower frequencies are less attenuated and provide stronger returning signal - Set baseline to greatest range of velocities in the direction of interest
High-frequency pulsed Doppler
- emits second or third pulse signal before original has returned
- increased pulse repetition frequency, thereby raising nyquist limit and allowing for calculation of higher velocities
- operator cannot be sure that the reflected echoes have come from the intended target (vs somewhere more proximal)
- sacrifices spatial resolution for higher velocities/frequencies
Continuous Wave Doppler
- composed of two crystals: one continuously emitting, one receiving
- nyquist limit not applicable
- very blood velocities can be measured
- measures velocities along its entire path
- limited to detecting highest velocity along path, but unable to provide where that velocity is occurring
Color Flow Mapping
- overlays flow velocities and 2D imaging
Important difference from PW and CW
1. Color flow performs multiple pulsed wave samples along the depth of each scan line and obtains multiple scan lines as it sweeps through a sector
- Flow direction and velocity are color coded and overlaid on gray 2D image
- Aliasing will occur because color mapping is based on pulsed wave
* aliasing occurs at velocities lower than with regular PW since part of the signal is used for image generation, and this lowers PRF - Cannot measure blood flow velocities nor track alterations in the cardiac cycle with the precision of PW or CW
* color mapping identifies abnormal flow that is then assessed by conventional Doppler
3.
