Ultrasound Physics and Instrumentation Flashcards

Which artifact appears as a bright echogenic line with posterior shadowing caused by total reflection at a gas interface?

Answer: Dirty shadowing

Dirty shadowing (reverberation shadowing) occurs at gas interfaces where multiple reflections create a heterogeneous shadow with internal echoes.

The time-gain compensation (TGC) control is used primarily to:

Answer: Compensate for attenuation of sound with depth

TGC amplifies returning echoes from deeper structures to compensate for the progressive attenuation of ultrasound as it travels through tissue.

In pulsed-wave Doppler, aliasing occurs when the Doppler shift frequency exceeds:

Answer: Half the pulse repetition frequency (Nyquist limit)

Aliasing occurs when the Doppler shift exceeds the Nyquist limit (PRF/2), causing the waveform to wrap around and appear on the opposite side of the baseline.

Which transducer design uses multiple elements fired in sequence to create a rectangular field of view?

Answer: Linear array

Linear array transducers fire elements sequentially across the footprint to produce a rectangular image with consistent width at all depths.

The acoustic impedance of a medium is determined by:

Answer: Density and propagation speed

Acoustic impedance (Z) equals the product of the medium's density (ρ) and the speed of sound (c) in that medium: Z = ρc.

Color flow mapping assigns color based on:

Answer: Direction and mean velocity of blood flow relative to the transducer

Color Doppler maps the mean frequency shift (proportional to velocity) of moving blood onto a color scale, with direction indicated by hue (typically red toward, blue away).

Lateral resolution in ultrasound is best improved by:

Answer: Focusing the beam at the depth of interest

Lateral resolution equals beam width, which is minimized at the focal zone; therefore focusing the beam at the target depth optimizes lateral resolution.