Week 3 - L1 Amplitude, Power and Intensity

Learning Objectives for Week 3

  • Define key concepts: amplitude, intensity, and power.

  • Describe the interrelationship between amplitude, intensity, and power.

  • Illustrate a sound wave, distinguishing frequency from amplitude.

  • Identify effects of increasing amplitude, power, and intensity in diagnostic ultrasound.

  • Discuss the necessity of decibels (dB) for measuring attenuation.

  • Identify variables affecting sound attenuation in the human body.

  • Determine the most significant variable contributing to attenuation.

  • Identify factors contributing to sound absorption.

  • Relate attenuation concepts to diagnostic ultrasound terms: anechoic, posterior shadowing, posterior enhancement, and penetration limits.

Amplification and Measurements

Amplitude, Intensity, and Power

  • Amplitude:

    • Definition: The maximum variation of a variable from its mean value, particularly in sound concerning pressure, density, and particle motion.

    • Units vary based on the acoustic variable:

      • Pressure: Pascals (Pa) or Megapascals (MPa)

      • Density: Kilograms per cubic meter (kg/m³) or grams per cubic centimeter (g/cm³)

      • Particle Motion: Newtons (N) or Volts (V)

    • Represents how “loud” a sound is.

  • Power:

    • Definition: Measured as the amount of work or energy expended per unit time.

    • Units: Watts (W)

    • Observation: Doubling the transmit voltage (amplitude) results in quadrupling the power, indicative of a non-linear relationship (Power $ hicksim$ Amplitude²).

  • Intensity:

    • Definition: The distribution of power over an area.

    • Formula: extIntensity=racextPowerextAreaext{Intensity} = rac{ ext{Power}}{ ext{Area}}

    • Units: Watts per area (W/m²).

    • Example: Intensity increases if transmit voltage is doubled.

    • Relatable to sunlight ignition having low intensity until focused.

Relationship Between Amplitude, Intensity, and Power

  • As amplitude increases:

    • Power: Increases as Power $ hicksim$ Amplitude².

    • Intensity: Inversely affected by area; doubling amplitude quadruples power but needs adjustment by area for intensity.

  • Implications for Clinical Practice: Higher transmit voltages lead to increased acoustic energy and stronger echoes, but caution is advised regarding potential bioeffects.

Acoustic Phenomena

Acoustic Shadowing

  • Observed in conditions like calcified plaque leading to an absence of echoes behind the structure, visualized in medical imaging.

Acoustic Enhancement

  • Occurs in areas like anechoic masses (cysts), resulting in brighter areas on ultrasound beneath the mass due to weakened attenuation of sound waves.

Attenuation

  • Definition: Refers to the reduction in intensity of ultrasound waves as they pass through tissue.

  • Influenced by multiple factors:

    1. Absorption

    2. Scattering

    3. Reflection

  • Significant variable affecting attenuation is the tissue type through which the sound travels.

  • The most significant factor relating to absorption is the frequency of the sound wave used.

  • Terms related to attenuation include:

    • Anechoic: Indicates the absence of echoes.

    • Posterior Shadowing: Indicates structures that inhibit sound transmission.

    • Posterior Enhancement: Indicates areas where sound propagation is increased.