15. The velocity of blood in an artery can be determined using the Doppler effect with
ultrasound. Blood in an artery is moving directly towards a piezoelectric transducer,
which emits ultrasound at a frequency of 7.5MHz. The shifted frequency is:
a. Either high or low depending on blood velocity
b. Either high or low depending on blood pressure
c. Higher than 7.5MHz
d. Lower than 7.5MHz
Introduction to Doppler Effect Ultrasound Blood Velocity
Doppler effect ultrasound blood velocity measurement revolutionizes non-invasive vascular assessment.
A piezoelectric transducer emits 7.5 MHz ultrasound; blood in arteries moving directly towards it produces
a reflected frequency higher than 7.5 MHz due to wavefront compression.
This principle enables precise artery flow detection in clinical settings such as echocardiography.
How Frequency Shift Works in Blood Flow
The Doppler equation:
fd = (2v f0 cosθ) / c
where fd is the frequency shift,
v is blood velocity, f0 is the transmitted frequency (7.5 MHz),
θ is the angle between flow and beam (0° for direct approach),
and c is the speed of sound in tissue.
A positive fd occurs when blood approaches the probe, making the received frequency f0 + fd
greater than 7.5 MHz. The magnitude of the shift scales with velocity, but flow direction determines whether it increases or decreases.
Tools like spectral Doppler visualize this as elevated peaks in the waveform display.
- The shifted frequency is higher (option c).
- It is not variable by velocity magnitude alone (option a), not influenced by pressure (option b), nor lower (option d).
- Used clinically for carotid artery stenosis detection via characteristic “whining” Doppler audio shifts.
Clinical Notes
- Typical frequencies: 2–10 MHz balance penetration and spatial resolution.
- Advantages: real-time, radiation-free perfusion imaging and dynamic vascular assessment.
