Ultrasound Physics and Instrumentation Flashcards

The piezoelectric effect in ultrasound transducers refers to the ability of certain crystals to:

Answer: Convert electrical energy into mechanical (sound) energy and vice versa

The piezoelectric effect describes how piezoelectric crystals (e.g., PZT) convert electrical pulses into mechanical vibrations (ultrasound) and convert returning echoes back into electrical signals.

Which relationship best describes how ultrasound frequency affects axial resolution?

Answer: Higher frequency produces shorter wavelengths and better axial resolution

Higher frequency transducers produce shorter wavelengths, which improves axial resolution (ability to distinguish two structures along the beam axis), though at the cost of tissue penetration.

The Nyquist limit in pulsed-wave Doppler is defined as:

Answer: Half the pulse repetition frequency (PRF)

The Nyquist limit is PRF/2; when the Doppler shift frequency exceeds this value, aliasing occurs and velocities are incorrectly displayed on the opposite side of the baseline.

Acoustic impedance (Z) of a tissue is determined by:

Answer: The product of tissue density and the speed of sound in that tissue

Acoustic impedance Z = ρ × c, where ρ is tissue density and c is the propagation speed of sound; mismatches at tissue interfaces determine the magnitude of reflection.

Ultrasound attenuation in soft tissue is approximately:

Answer: 0.5 dB/cm/MHz

Soft tissue attenuates ultrasound at roughly 0.5 dB/cm/MHz, meaning a 5 MHz beam loses about 2.5 dB of intensity per centimeter traveled.

The assumed propagation speed of sound used by diagnostic ultrasound equipment in soft tissue is:

Answer: 1,540 m/s

Ultrasound systems use the fixed assumption of 1,540 m/s in soft tissue to calculate depth from round-trip travel time; deviations cause range errors.

Which of the following transducer types produces a sector-shaped image that is useful for accessing small acoustic windows?

Answer: Phased array

Phased array transducers electronically steer a narrow footprint beam to create a wide sector image, making them ideal for small acoustic windows such as between ribs.