Regarding safety in ultrasound, there are several key points to understand, primarily concerning potential bioeffects and operational principles designed to minimize patient risk:

1. Bioeffects of Ultrasound Ultrasound can potentially cause effects on biological tissues. These are generally categorized into thermal and mechanical mechanisms.

• Thermal Effect: This arises from temperature increases in tissues due to the absorption of ultrasound waves. Localized hot spots can also be caused by Rayleigh scattering, which might lead to thermal injury.
• Mechanical Effects (Cavitation): These occur when ultrasound excites microbubbles within tissues.
  • Stable Cavitation: Microbubbles expand and contract.
  • Transient Cavitation: Microbubbles burst, which could potentially result in tissue damage. It is important to know that standard imaging intensities in ultrasound are generally considered safe, with no known bioeffects under typical conditions.

2. Safety Limits and Regulatory Agencies Understanding the safety limits of ultrasound exposure is crucial. Regulatory agencies play a role in setting guidelines for the safe use of ultrasound equipment.

3. The ALARA Principle The "As Low As Reasonably Achievable" (ALARA) Principle is a guiding philosophy in ultrasound imaging. This principle emphasizes the importance of minimizing patient exposure. When adjusting imaging parameters to optimize image brightness, the ALARA principle dictates that sonographers should prioritize adjusting receiver gain before increasing output power.

4. Transducer Output and Patient Exposure The transducer output, also referred to as acoustic power, pulser power, or transmitter output, directly influences the brightness level across the entire ultrasound image. It is regulated by the excitation voltage from the pulser. While increasing transducer output can improve the signal-to-noise ratio and enhance image clarity, it also increases patient exposure to ultrasound. Therefore, careful consideration is needed when adjusting this parameter.

5. Intensity and Duty Factor

• Intensity: Intensity describes the energy density within a sound beam. It is a key metric for evaluating safety and bioeffects. Both intensity and power diminish as they propagate through tissue.
• Duty Factor: This represents the proportion of time a pulse is actively propagating and is important for quantifying intensities. Clinical duty factors are typically low (0.001-0.01), as listening intervals dominate to allow for echo reception. Low duty cycles combined with short pulse lengths ensure that only one pulse traverses the imaging plane at a time, localizing energy deposition spatially and temporally, which helps improve safety. Understanding duty factor principles aids in evaluating potential bioeffects.