Intensity

Intensity is a crucial concept in ultrasound physics, particularly for understanding image quality and patient safety.

Here's what you need to know about intensity:

• Definition and Units: Intensity describes the energy density within a sound beam. Its units are watts per square centimeter (W/cm²).
• Calculation: Intensity is calculated as beam power divided by the cross-sectional area.
  • Intensity (W/cm²) = Power (W) / Beam Area (cm²).
• Determinants and Adjustability: Intensity depends on the source specifications and can be adjusted by the operator.
• Behavior in Tissue: As sound propagates through tissue, both intensity and power diminish. This reduction in intensity is part of the overall attenuation of the sound wave.
• Safety and Bioeffects: Intensity is a key metric for evaluating safety and bioeffects. Standard imaging intensities in ultrasound are generally considered safe, with no known bioeffects. However, ultrasound can potentially cause thermal and mechanical effects, and the thermal effect arises from temperature increases due to ultrasound waves being absorbed by biological tissues.
• Relationship with Duty Factor: The duty factor 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 permit 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.
• Relationship to Output Power and Image Brightness: Transducer output (also known as acoustic power, pulser power, or transmitter output) directly influences the brightness level across the entire ultrasound image and is regulated by the excitation voltage from the pulser. Adjusting transducer output upwards can improve the signal-to-noise ratio, enhancing image clarity. However, increasing transducer output also increases patient exposure to ultrasound. The ALARA (As Low As Reasonably Achievable) Principle guides these adjustments, prioritizing minimizing patient exposure by first adjusting receiver gain to optimize image brightness before increasing output power.