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Ultrasound Attenuation Principles

Sep 10, 2025

Overview

This lecture covers attenuation in ultrasound physics—how sound weakens as it travels through tissues, what causes this weakening, how to measure and compensate for it, and the impact of different incidence angles on reflection and attenuation. Understanding attenuation is essential for sonographers to optimize image quality and recognize the physical limits of ultrasound imaging.

Strength Parameters of Sound Waves

  • Amplitude: The maximum variation of an acoustic variable (density, pressure, or movement). High amplitude means a stronger wave and greater fluctuations.
  • Power: The rate at which energy is transmitted, determined by the machine’s voltage.
  • Intensity: Power distributed over an area; smaller areas mean higher intensity.
  • All three parameters (amplitude, power, intensity) are set by the machine and can be adjusted by the sonographer, but all decrease as sound propagates through tissue.

Attenuation Fundamentals

  • Attenuation is the reduction in amplitude and intensity of a sound beam as it travels through a medium.
  • The rate of attenuation depends on:
    • Initial intensity of the sound beam
    • Frequency of the wave (higher frequencies attenuate faster)
    • The type of medium (tissue, bone, air, etc.)
  • Attenuation limits how deep an image can be created; higher frequencies provide more detail but less penetration.
  • Machines compensate for attenuation, but sonographers must choose the right transducer and settings for each exam.
  • Attenuation is a key factor in determining the maximum imaging depth and the quality of echoes received.

Decibels and Sound Measurement

  • Decibels (dB) measure relative changes in amplitude, power, or intensity using a logarithmic (base 10) scale.
  • Key rules:
    • +3 dB = double the intensity
    • +10 dB = 10 times the intensity
    • -3 dB = half the intensity
    • -10 dB = one-tenth the intensity
  • Decibel changes are multiplicative, not additive. For example, a +6 dB change means 2 × 2 = 4 times stronger; a -9 dB change means ½ × ½ × ½ = 1/8 as strong.
  • Decibels compare an initial and a final value, expressing the change in intensity, power, or amplitude.

Causes and Types of Attenuation

Attenuation occurs due to three main physical phenomena, each with distinct characteristics and examples:

  • Absorption

    • The main cause of attenuation.
    • Sound energy is converted into heat within the tissue.
    • Highly absorbing structures: bone, lung, and air.
    • Example: Imaging over a rib results in a strong reflection at the rib’s surface, but a dark shadow behind it because the rib absorbs most of the sound energy.

  • Scattering

    • Occurs when sound interacts with small structures (less than one wavelength in size) within tissue, causing the sound to scatter in many directions.
    • Allows visualization of organ parenchyma (e.g., liver, spleen, muscle).
    • Example: Scattering in the liver tissue enables the sonographer to see the texture of the liver.

  • Reflection

    • Occurs at large interfaces (greater than one wavelength), sending sound back toward the transducer.
    • Enables visualization of organ borders and interfaces.
    • Two types:
      • Specular Reflection: From smooth surfaces (e.g., diaphragm, vessel walls); strong, mirror-like echoes.
      • Diffuse Reflection: From rough or irregular surfaces; echoes are sent back in multiple directions, usually weaker.
    • Example: The bright line seen at the diaphragm-liver interface is due to specular reflection.

  • Other Factors Affecting Attenuation

    • Frequency: Higher frequency waves attenuate more quickly.
    • Propagation Distance: The farther the sound travels, the more it attenuates.

Incidence Angles and Their Effects

  • The angle at which the ultrasound beam strikes a boundary (incidence angle) affects how much sound is reflected, transmitted, or refracted.
  • Perpendicular (90°) Incidence:
    • When the sound beam hits a boundary at 90°, specular reflection is maximized, and the echo returns directly to the transducer, producing a strong signal.
    • This is ideal for imaging smooth, large interfaces (e.g., diaphragm, vessel walls).
  • Oblique Incidence (not 90°):
    • When the sound beam strikes at an angle other than 90°, the reflected sound is sent off at an equal but opposite angle, often missing the transducer.
    • This can result in weaker or absent echoes, making some structures harder to visualize.
    • Most tissue interfaces in the body are not perfectly perpendicular, so many reflections are lost or weakened due to oblique incidence.
  • Diffuse Reflection and Scattering:
    • At rough or irregular surfaces, the angle of incidence is less important because sound is reflected in many directions, increasing the chance that some echoes return to the transducer.
    • Scattering is especially important for visualizing tissue parenchyma, as it provides echoes from many angles.
  • Clinical Implications:
    • Sonographers often adjust the transducer angle to optimize the incidence angle and maximize the strength of returning echoes.
    • Understanding incidence angles helps explain why some structures appear brighter or darker and why certain artifacts (like shadowing or enhancement) occur.

Attenuation Calculations

  • Attenuation Coefficient (dB/cm): In soft tissue, calculated as frequency (MHz) ÷ 2.
  • Total Attenuation (dB): Attenuation coefficient × distance traveled (cm).
  • Half-value Layer Thickness: The depth at which intensity is reduced by half (–3 dB); in soft tissue, equals 6 ÷ frequency (MHz).
  • These calculations help predict how much the sound beam will weaken as it travels through tissue and guide the selection of transducer frequency for different imaging depths.

Attenuation in Different Tissues

  • Air: Extremely high attenuator; even a thin layer of air can block ultrasound, which is why gel is used.
  • Bone and Lung: Both absorb and scatter sound heavily, causing strong attenuation and shadowing behind these structures.
  • Muscle: Attenuates more when the sound beam is perpendicular to the muscle fibers; less when parallel.
  • Water and Fluids (blood, urine, bile): Very low attenuation; these act as acoustic windows, allowing better imaging of deeper structures.
  • Fat: Attenuates less than soft tissue but more than fluids; can affect image quality, especially in obese patients.
  • Clinical Note: Fluid-filled structures (like the bladder) are often used as windows to image deeper organs because they cause minimal attenuation.

Key Terms & Definitions

  • Attenuation: Loss of sound wave strength as it travels through a medium.
  • Decibel (dB): Logarithmic unit for measuring changes in intensity, power, or amplitude.
  • Attenuation Coefficient: The amount (in dB/cm) a sound wave attenuates per centimeter in a medium.
  • Half-value Layer Thickness: The distance at which the beam’s intensity is reduced by half (–3 dB).
  • Incidence Angle: The angle at which the ultrasound beam strikes a boundary, affecting reflection and transmission.

Examples of Attenuation Types

  • Absorption Example: Imaging over bone (e.g., rib) results in a bright anterior echo and a dark shadow behind due to energy absorption.
  • Scattering Example: Liver tissue appears grainy because small structures scatter the sound, allowing visualization of the parenchyma.
  • Specular Reflection Example: The diaphragm appears as a bright, smooth line due to strong, organized reflection, especially when the beam is perpendicular.
  • Diffuse Reflection Example: The surface of the liver or spleen, which is not perfectly smooth, causes weaker, scattered echoes.
  • Incidence Angle Example: Adjusting the transducer to achieve a more perpendicular angle to a vessel wall increases the strength of the returning echo.

Action Items / Next Steps

  • Complete the unit 6A workbook, including practice problems and open-ended study questions.
  • Review and memorize the rules for 3 dB and 10 dB changes.
  • Practice using attenuation, decibel, and half-value layer thickness formulas with sample questions.
  • Be able to identify and explain the different types of attenuation and provide examples for each.
  • Practice recognizing how incidence angle affects image quality and echo strength in different scenarios.

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