Lecture Notes: Unit 6A - Attenuation

Introduction

• Presenter: Em from SoundWO Nerds
• Unit Division: Unit 6 split into 6A (Attenuation) and 6B (Echo Creation)
• Focus of 6A: Understanding attenuation to comprehend limitations in diagnostic imaging.
• Focus of 6B: Creation of echoes and their role in diagnostic imaging.
• Importance: Knowledge of ultrasound physics impacts everyday decisions in sonography, such as transducer selection and image optimization.

Section 6A.1: Strength Parameters

• Sound Wave Parameters: Include amplitude, power, and intensity.
  • Amplitude: Describes maximum variation of acoustic variables like density, pressure, and movement.
  • Power: Rate of energy transmission, related to voltage in ultrasound machines.
  • Intensity: Distribution of power over an area, affected by the size of the area.
• Key Point: Amplitude, power, and intensity weaken with propagation.

Section 6A.2: Attenuation

• Definition: Decrease in amplitude and intensity as sound travels through a medium.
• Factors Affecting Attenuation: Initial intensity, wave frequency, and medium.
• Key Concepts:
  • Higher frequencies attenuate quicker.
  • Attenuation limits imaging depth.
  • Machines compensate for attenuation to process and display echoes.
  • Sonographers can adjust machine settings to compensate for attenuation artifacts.

Section 6A.3: Decibels

• Decibels: Measure of change in amplitude, power, or intensity, using a logarithmic scale.
  • Positive Decibels: Indicate amplification.
  • Negative Decibels: Indicate attenuation.
  • Rules:
    • +3 dB: Intensity doubles.
    • +10 dB: Intensity is 10 times stronger.
    • -3 dB: Intensity halves.
    • -10 dB: Intensity is one-tenth.
• Application: Understanding decibel changes helps in processing ultrasound signal strength.

Section 6A.4: Causes of Attenuation

• Primary Causes: Absorption, scattering, reflection.
• Related Factors: Frequency and propagation distance.
• Detailed Causes:
  • Absorption: Main cause, converts sound energy into heat.
  • Scattering: Allows visualization of tissues; less attenuation.
  • Reflection: Allows visualization of organ borders.
  • High Attenuation Structures: Air, bone, and lung are strong absorbers.

Section 6A.5: Reporting Total Attenuation

• Attenuation Coefficient: Decrease in dB per cm traveled.
• Formulas:
  • Attenuation Coefficient (dB/cm) = Frequency (MHz) / 2
  • Total Attenuation = Attenuation Coefficient x Distance
• Half Value Layer Thickness: Depth where intensity is half the original; inversely related to frequency and medium attenuation rate.

Section 6A.6: Attenuation in Other Tissues

• High Attenuation Tissues: Air, bone, lung, muscle (depending on orientation).
• Low Attenuation Tissues: Water, blood, urine, bile, fat.
• Application: Understanding tissue attenuation helps in imaging strategy.

Conclusion

• Materials: Workbook with activities and study questions related to attenuation.
• Next Steps: Proceed to Unit 6B to learn about echo creation.