Unit 6A: Attenuation in Sonography

Introduction

Unit 6 is divided into two parts: 6A (Attenuation) and 6B (Echo Creation).
Focuses on what happens to sound as it travels through a medium.
Attenuation refers to sound weakening as it travels, important for understanding limitations in diagnostic imaging.
Physics of ultrasound impacts daily sonographic decision-making.

Seven parameters of a sound wave include amplitude, power, and intensity.
Amplitude: Describes maximum variation of an acoustic variable (density, pressure, movement).
Power: Rate at which work is performed or energy is transmitted.
Intensity: Power distributed over an area.
- Larger area = weaker intensity; smaller area = stronger intensity.
All parameters weaken with propagation.

Decrease in amplitude and intensity as sound travels through a medium.
Depends on initial intensity, wave frequency, and medium.
Examples:
- Whispering vs. shouting: Higher amplitude sound (shout) takes longer to attenuate.
- Sonographer's decisions affected by attenuation (e.g., transducer selection).
Machines compensate for attenuation; sonographers can adjust using machine tools.
- Weak structures cause strong reflectors behind them (e.g., cysts, gallbladder).
- Strong absorbers (e.g., gallstones) lead to shadowing.

Decibels: Measure change in amplitude, power, or intensity. Use a logarithmic scale.
Rules:
- Positive 3 decibels = double intensity.
- Positive 10 decibels = tenfold increase in intensity.
- Negative 3 decibels = half intensity.
- Negative 10 decibels = one-tenth intensity.
Attenuation implies negative decibel changes.
Can calculate decibel changes from intensity factors (e.g., double, half).

Attenuation caused by absorption, scattering, and reflection.
Absorption: Main cause, sound energy converted to heat.
- Concerns about thermal bio-effects (e.g., bone, lung, air are high absorbers).
Scattering: Happens with small interfaces; crucial for imaging organs (e.g., liver, spleen).
- Less attenuation but necessary for tissue visualization.
Reflection: Occurs at large interfaces (organ borders).
- Specular reflection: Mirror-like, strong, smooth surfaces.
- Diffuse reflection: Irregular, sends echoes in multiple directions.

Attenuation coefficients report how much sound attenuates per cm.
Formula for soft tissue: Frequency (MHz) / 2.
Total attenuation = attenuation coefficient x distance traveled.
Higher frequencies and longer distances increase attenuation.
- Example: 12 MHz transducer attenuates much quicker than 2 MHz.

Different tissues attenuate sound differently.
- High attenuators: Air, bone, lung.
- Low attenuators: Water, body fluids (e.g., blood, urine).
Sonographers use knowledge of attenuation for choosing imaging windows and diagnosing.