Ultrasound Physics and Instrumentation Flashcards

The Doppler equation used to calculate flow velocity is: v = (f_d × c) / (2 × f_0 × cos θ). What does 'θ' represent?

Answer: The angle between the ultrasound beam and the direction of blood flow

θ is the Doppler angle—the angle between the insonating beam and the axis of blood flow; as θ approaches 90°, cos θ approaches 0 and velocity calculation becomes unreliable.

Aliasing in color flow Doppler imaging appears as:

Answer: An abrupt color reversal in the center of a high-velocity jet

When peak Doppler shift exceeds the Nyquist limit, aliasing causes the color to wrap around and display the opposite color (e.g., red becomes blue) at the site of high velocity.

Which technique is most effective for eliminating aliasing in pulsed-wave Doppler without switching to continuous-wave mode?

Answer: Increasing the pulse repetition frequency (PRF)

Increasing PRF raises the Nyquist limit (PRF/2), allowing higher Doppler shift frequencies to be sampled without aliasing.

Power Doppler differs from conventional color flow Doppler primarily in that it displays:

Answer: The amplitude (power) of the Doppler signal rather than frequency shift

Power Doppler maps the intensity (energy) of the returning Doppler signal, making it more sensitive to slow flow and small vessels but providing no directional information.

The range resolution (axial resolution) of a pulsed ultrasound system is primarily determined by:

Answer: Spatial pulse length

Axial resolution ≈ spatial pulse length / 2; shorter pulses (higher frequency, fewer cycles) improve the ability to distinguish two closely spaced reflectors along the beam axis.

A duplex ultrasound system combines which two imaging modalities?

Answer: B-mode (gray-scale) imaging and pulsed-wave Doppler

Duplex ultrasound integrates real-time B-mode imaging for anatomical visualization with pulsed-wave Doppler spectral analysis for hemodynamic assessment.

Time gain compensation (TGC) in ultrasound imaging is used to:

Answer: Compensate for the increasing attenuation of sound with depth

TGC applies progressively increasing amplification to returning echoes from greater depths to counteract attenuation, producing a uniform brightness display regardless of depth.