Unit 6A: Attenuation

Introduction

• Key Focus: Understanding attenuation in sound waves as they travel through a medium.
• Purpose: Helps sonographers understand physical limitations in creating diagnostic images.
• Units 6A & 6B:
  • 6A: Focuses on attenuation (weakening of sound).
  • 6B: Focuses on echo creation.

Importance for Sonographers

• Physics of ultrasound impacts daily decisions.
• Key in transducer selection and image optimization.

Section 6A.1: Strength Parameters

• Strength Parameters: Amplitude, Power, Intensity.
  • Amplitude: Maximum variation of an acoustic variable.
  • Power: Rate of energy transmission.
  • Intensity: Power distributed over an area.
• Key Point: All parameters weaken with propagation.

Section 6A.2: Attenuation

• Definition: Decrease in amplitude and intensity of a sound beam through a medium.
• Factors Affecting Attenuation:
  • Initial intensity
  • Frequency of the wave
  • Medium it travels through
• Key Considerations:
  • Limits the depth of image creation.
  • Machines compensate for weak echoes.

Section 6A.3: Decibels

• Decibels: Measure change in amplitude, power, intensity.
  • Logarithmic scale (base 10).
• Rules:
  • Increase of 3 dB = Intensity doubled.
  • Increase of 10 dB = Intensity 10 times stronger.
  • Decrease of 3 dB = Intensity halved.
  • Decrease of 10 dB = Intensity 1/10th as strong.

Section 6A.4: Causes of Attenuation

• Three Physical Phenomena:
  • Absorption
  • Scattering
  • Reflection
• Factors of Attenuation:
  • Frequency
  • Propagation distance
• Absorption: Main cause, energy converted to heat.
• Scattering: Allows organ imaging, causes slight attenuation.
• Reflection: Defines organ borders, also causes attenuation.

Specific Concepts

• Higher Frequencies:
  • Attenuate more quickly.
  • Cause more scattering.
  • Are absorbed more.
• Longer Distances: More attenuation.
• Transducer Frequency: Impacts depth and clarity of images.

Section 6A.5: Reporting Total Attenuation

• Reported in Decibels: Negative change.
• Attenuation Coefficient: Decrease in dB per cm of travel.
• Formula:
  • Attenuation Coefficient = Frequency / 2
  • Total Attenuation = Attenuation Coefficient × Distance
• Half Value Layer Thickness: Depth where intensity is half the original.
  • Calculated as 3 dB / Attenuation Coefficient

Section 6A.6: Attenuation in Other Tissues

• Air, Bone, Lung: High attenuators.
• Muscle: Depends on fiber direction.
• Water, Blood, Urine, Bile: Low attenuators.
• Fat: Less than soft tissue.

Practical Applications

• Choice of Imaging Window: Critical in avoiding high attenuators.
• Using Fluid-Filled Structures: As windows to reduce attenuation.

Conclusion

• Materials Provided: Workbook with activities and study questions.
• Focus: Understanding attenuation aids in making informed imaging decisions.