Overview
This lecture covers the concept of attenuation in ultrasound physics, explaining how and why sound weakens as it travels through different tissues and how this impacts sonographic imaging.
Sound Wave Strength Parameters
- Amplitude describes the maximum variation of an acoustic variable (density, pressure, or movement).
- Power is the rate at which work is done or energy is transmitted by the wave.
- Intensity is the power distributed over a specific area.
- Amplitude, power, and intensity are set by the machine and weaken as sound propagates.
Attenuation: Definition and Importance
- Attenuation is the decrease in amplitude and intensity as sound travels through a medium.
- The rate of attenuation depends on initial sound strength, frequency, and the medium.
- Attenuation limits imaging depth and impacts echo quality.
- Sonographers must compensate for attenuation using machine settings or equipment choice.
Decibel Scale and Rules
- Attenuation and amplification are measured in decibels (dB)—a logarithmic, relative scale.
- +3 dB = intensity doubles; +10 dB = intensity increases 10x.
- -3 dB = intensity halves; -10 dB = intensity decreases to 1/10.
- Decibel changes multiply, not add (e.g., +6 dB = 2x2 = 4x stronger).
- Decibel comparisons help determine changes in wave strength.
Causes of Attenuation
- Three main causes: absorption (conversion to heat), scattering (energy redirected in many directions), and reflection (energy sent back to transducer).
- Absorption is the primary cause of attenuation, especially with bone, lung, or air.
- Scattering helps visualize soft tissue but slightly attenuates the beam.
- Reflection allows for border visualization (specular—mirror-like; diffuse—irregular surfaces).
Frequency, Distance, and Attenuation
- Higher frequency = higher attenuation (less penetration, more detail).
- Attenuation increases with longer distances traveled.
- Low-frequency transducers penetrate deeper but with less resolution.
- Choosing transducer frequency involves balancing detail and penetration.
Calculating Attenuation
- Attenuation Coefficient (soft tissue): dB/cm = frequency (MHz) / 2.
- Total Attenuation = Attenuation Coefficient × distance (cm).
- Half Value Layer Thickness: depth where intensity is halved (−3 dB change); 3 dB/Attenuation Coefficient or 6/frequency (cm).
Attenuation in Different Tissues
- Air, bone, and lung attenuate sound rapidly.
- Muscle attenuates more than soft tissue, depending on fiber orientation.
- Water and most fluids (blood, urine, bile) cause minimal attenuation.
- Fat attenuates less than soft tissue, more than fluids.
Key Terms & Definitions
- Attenuation — Weakening of sound intensity and amplitude as it travels.
- Decibel (dB) — Logarithmic unit to express relative changes in intensity, power, or amplitude.
- Attenuation Coefficient — dB loss per cm; depends on frequency and medium.
- Half Value Layer Thickness — Distance where intensity drops to half (−3 dB).
- Absorption — Conversion of sound energy into heat.
- Scattering — Redirection of sound in multiple directions due to small reflectors.
- Reflection — Echo production at tissue interfaces, important for imaging.
- Specular Reflection — Strong, mirror-like reflection from smooth surfaces.
- Diffuse Reflection — Weaker, multi-directional reflection from rough surfaces.
Action Items / Next Steps
- Complete the attenuation workbook activities and open-ended study questions.
- Practice using decibel rules and attenuation formulas.
- Review chart completion exercises for attenuation coefficient, total attenuation, and half value layer thickness.