Physics 1 Module 7 & 8 Flashcards
What is the Doppler shift formula?
Doppler shift = 2fo x V x Cos(-) / c
Why use the lowest possible frequency when performing doppler?
Reduce attenuation (esp. absorption)
What happens to velocity as angle of insonation increases?
Increased percent error for velocity
RBC velocities faster than the machine’s ability to sample
Aliasing/nyquist limit exceeded
Nyquist limit =
1/2PRF
What are the 5 ways to correct for aliasing?
- Move the baseline *
- Increase the scale/PRF/velocity range *
- Increase doppler angle
- Lower frequency
- Change to CW
What determines the sample volume?
- Beam width
- Gate length
- Length of the emitted pulse (SPL. We want longer SPL with doppler b/c longer interaction with RBCs and shorter with 2D)
How does a smaller gate length affect the spectral display?
Improves it
What is the minimum number of cycles/pulse required for doppler?
4
Where is the sample volume found for CW?
Zone of sensitivity where the transmitted and received beam intersect
What are the axes on a spectral tracing?
X time
Y velocity
Z power/RBC density
Mathematical technique that separates the individual doppler shifts from the complex beat frequency
Fast Fourier Transform (FFT)
Tall, narrow, sharp systolic peaks with reversed or absent diastolic flow
High pulsatility/resistance
Broad systolic peaks, forward flow through diastole
Low pulsatility/resistance
Tall, narrow, sharp systolic peaks with forward flow through diastole
Moderate resistance/pulsatility
Vertical thickening of the spectral trace or envelope
Spectral broadening
What happens to flow rate if the pressure gradient increases?
It increases
What happens to volume flow rate if the resistance to flow increases?
It decreases
What does Poiseuille’s Law describe?
The relationship between pressure and flow
What happens to flow velocity when there is an increase in tube length or increase in viscosity?
It decreases
What happens to flow when there is a decrease in the diameter?
A large decrease in flow
All RBCs travelling at the same velocity, usually seen at the opening of large vessels (Aorta)
Plug flow
Most common flow pattern in normal arteries, the middle RBCs moving faster than the ones near the wall.
Laminar/parabolic flow
Seen when there is a significant reduction in the vessel diameter (stenosis)
Jet flow
