ARDMS SPI Ultrasound Bioeffects and Safety 2 — Questions and Answers
Question 1: Which index displayed on modern ultrasound systems specifically relates to the risk of acoustic cavitation?
- Thermal Index (TI)
- Mechanical Index (MI) (Correct answer)
- Dynamic Range Index
- Pulsatility Index
Correct answer: Mechanical Index (MI)
The Mechanical Index (MI) quantifies the likelihood of cavitation by comparing peak negative pressure to the square root of frequency.
Question 2: Stable cavitation differs from inertial cavitation in that stable cavitation involves:
- Violent bubble collapse that produces free radicals
- Bubbles oscillating in a sustained, non-destructive manner (Correct answer)
- Immediate tissue destruction at the focal zone
- Complete vaporization of the liquid medium
Correct answer: Bubbles oscillating in a sustained, non-destructive manner
Stable (non-inertial) cavitation describes gas bubbles that oscillate repetitively without collapsing, whereas inertial cavitation involves violent implosion.
Question 3: The ALARA principle in diagnostic ultrasound means the operator should:
- Maximize output power to shorten scan time
- Use the lowest output that still yields diagnostic information (Correct answer)
- Avoid all pulsed Doppler modes in obstetrics
- Keep MI below 0.1 at all times
Correct answer: Use the lowest output that still yields diagnostic information
ALARA (As Low As Reasonably Achievable) requires minimizing acoustic exposure while still obtaining the necessary diagnostic image quality.
Question 4: Which ultrasound mode typically produces the highest time-averaged acoustic intensity and greatest potential for thermal bioeffects?
- B-mode imaging
- M-mode
- Pulsed-wave Doppler (Correct answer)
- Harmonic imaging
Correct answer: Pulsed-wave Doppler
Pulsed-wave Doppler has a high duty factor and focuses the beam on a small sample volume, resulting in the highest time-averaged intensity and greatest heating potential.
Question 5: A Thermal Index value of 2.0 indicates that the ultrasound system is capable of raising tissue temperature by approximately:
- 0.2°C
- 0.5°C
- 2.0°C (Correct answer)
- 20°C
Correct answer: 2.0°C
The TI is defined as the ratio of acoustic power to the power needed to raise tissue temperature by 1°C, so TI = 2.0 suggests a potential 2°C rise.
Question 6: During a first-trimester obstetric exam, which combination of indices represents the greatest concern for bioeffects?
- Low MI, low TI
- High MI, low TI
- Low MI, high TI
- High MI, high TI (Correct answer)
Correct answer: High MI, high TI
Both a high MI (cavitation risk) and high TI (thermal risk) together represent maximal bioeffect potential and should be minimized during embryonic development.
Question 7: Which biological tissue is considered MOST sensitive to the thermal effects of diagnostic ultrasound?
- Liver parenchyma
- Skeletal muscle
- Developing embryonic/fetal tissue (Correct answer)
- Adult bone cortex
Correct answer: Developing embryonic/fetal tissue
Developing embryonic and fetal tissues are most sensitive to thermal bioeffects because rapidly dividing cells have lower heat tolerance and reduced repair capacity.
