Diagnostic Ultrasound Flashcards
What are the 3 main equations for acoustic impedance? (z)
z = P/v (P = local pressure, v = local particle velocity)
z = pc (pressure x velocity)
z = sqrt(pk) (sqrt(pressure x stiffness))
What are the symbols for density, stiffness and acoustic impedance?
p, K, Z
Generally, what is the factor of difference between the strongest and weakest echoes returning?
10 **5
What is the Doppler Equation?
fr = 2 ft v/c cos(theta)D
fr = frequency received
ft = frequency transmitted
v = target velocity
c = speed of sound (1540)
fD = Doppler frequency of Doppler shift
What is the technical name for a Doppler Waveform Display?
Sonogram
What is the mathematical algorithm that converts the time domain of a sonogram to frequency?
Fourier transform
How would you describe slow moving blood flow in a normal vessel?
Parabolic flow
What does the amplitude of the Doppler signal depend on?
The number of RBCs reflecting back the signal
What is the greyscale spectrum divided into?
Frequency bins
What is a disadvantage of CW Doppler?
Multiple vessels within the relatively large sample volume will be detected together and superimposed
What is an advantage of CW Doppler?
It has no velocity limit - can detect very high velocities
What is range gating?
Only detecting signals from a particular depth
What does sample volume depend on?
Pulse length and range gate length
What condition needs to be met to detect the Doppler frequency?
The wave must be sampled at least twice in each period - this is known as the Nyquist Limit
- PRF = 2.fmax
What happens when a wave is sampled at less than twice per time period?
Aliasing
How can the maximum velocity of particles detected without aliasing be increased?
Decrease transmit frequency or depth
What happens to maximum particle velocity detected without aliasing as depth increases?
This decreases
How many cycles do PW Doppler pulses usually consist of?
6 - 7 cycles per pulse
What happens to bandwidth as pulse length increases?
It becomes narrower
What phenomena describes how a shorter pulse length (e.g. to look at narrow region of vessel) causes Doppler frequencies to become less well defined?
ISB - Intrinsic Spectral Broadening
What is ISB?
A single target moving at velocity v will produce a range of Doppler frequencies, not just 1. This is related to the US scanner’s construction and cannot be avoided
What are the 2 explanations of ISB?
1.Transmitted pulses have a finite bandwidth and produce a range of velocities.
The shorter the pulse, the greater the pulse bandwidth, hence the greater the ISB.
- The beam is produced by a transducer of finite width. There will be a range of Doppler angles - producing a range of Doppler frequencies
What factors increase ISB?
- A very short range gate
- A short pulse length
- A very narrow beam
- Larger Doppler angles of Ionisation i.e. closer to 90 degrees
Why do we not use angles of <45 degrees for PW Doppler?
At shallow angles, there is poor penetration of ultrasound into the vessel lumen - can cause errors in velocity measurement
