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Ultrasound Echoes and Reflection Principles

Sep 8, 2025

Overview

This lecture explains how echoes are created and used in ultrasound imaging, focusing on the physical principles of sound interaction with tissue interfaces. It covers reflection, transmission, scattering, refraction, and the key rules and equations that govern these processes.

What Are Echoes and How Are They Created?

  • An echo is a sound wave that reflects off a tissue interface and returns to the transducer.
  • In ultrasound, high-frequency sound waves are sent into the body; echoes from tissue interfaces are processed to create images.
  • A tissue interface is where two different tissues or media meet, causing changes in how sound behaves.
  • At each interface, sound can:
    • Be absorbed (energy lost as heat)
    • Be reflected (echoes)
    • Be scattered (sent in many directions)
    • Be refracted (change direction)
    • Be transmitted (continue forward)
  • The grayscale in ultrasound images depends on the strength of the reflected echoes:
    • Strong reflectors = bright (white)
    • Weak/no reflection = dark (black)
    • Intermediate strengths = shades of gray

Types of Reflection and Scattering

  • Specular Reflection:
    • Occurs at large, smooth interfaces (e.g., diaphragm, vessel walls, valve leaflets, fetal bones).
    • Produces strong, bright, linear echoes.
    • Most effective when the sound beam strikes the surface perpendicularly (90°).
    • If the angle is not 90°, the echo may not return to the transducer.
  • Near-Specular Reflection:
    • Surfaces are mostly smooth but slightly rough.
    • Still produce strong echoes, but are less angle-dependent than true specular reflectors.
  • Diffuse Reflection:
    • Occurs at large, rough, or irregular surfaces (e.g., chamber walls, kidney collecting system).
    • Produces weaker, less organized echoes that are not as angle-dependent.
    • Most echoes still return toward the transducer, but some are lost to the sides.
  • Scattering:
    • Happens when sound interacts with small, non-specular structures (smaller than a wavelength).
    • Echoes are sent in many random directions.
    • Responsible for the "acoustic speckle" or grainy texture seen in images of organ parenchyma, muscle, fat, and connective tissue.
    • Scattering is not angle-dependent and forms the background texture of most ultrasound images.
  • Rayleigh Scattering:
    • Special case of scattering with very small reflectors (e.g., red blood cells).
    • Scattering is omnidirectional (equal in all directions).
    • Highly dependent on frequency:
      • Rayleigh scattering ∝ (frequency)^4
      • Doubling frequency increases Rayleigh scattering by 16×; tripling increases it by 81×.
    • Important in Doppler imaging; high frequencies cause excessive attenuation due to increased scattering.

Transmission and Refraction

  • Transmission (Transmit Wave):
    • Most sound energy continues forward beyond an interface as the transmit wave.
    • Without transmission, imaging deeper structures would not be possible.
  • Refraction:
    • Occurs when the transmitted wave enters a new medium at an oblique angle (not 90°) and the propagation speeds of the two media are different.
    • Causes the wave to change direction (bend).
    • Can result in imaging artifacts, such as side-by-side duplication of anatomy.
    • The amount and direction of refraction depend on the difference in propagation speeds and the angle of incidence.

Key Rules & Concepts for Reflection and Transmission

  • Impedance (Z):
    • Resistance to sound propagation in a medium.
    • Calculated as:
      Z = density × propagation speed
      (Z in Rayls)
    • A difference in impedance at an interface is required for reflection (at normal incidence).
  • Normal Incidence (90°):
    • Reflection only occurs if Z₁ ≠ Z₂.
    • If Z₁ = Z₂, all energy is transmitted; no reflection.
  • Oblique Incidence (≠ 90°):
    • Reflection and transmission cannot be predicted by impedance alone.
    • The angle of reflection equals the angle of incidence:
      θ_reflection = θ_incidence
    • Refraction occurs only if both:
      • The incidence is oblique.
      • The propagation speeds of the two media are different (c₁ ≠ c₂).

Calculating Intensities and Coefficients

  • Incident Intensity (I₀):
    • Total energy per area before the wave hits the interface.
    • Usually considered as 100% for calculation purposes.
  • Reflection Intensity (I_R):
    • Portion of energy reflected back toward the transducer.
  • Transmission Intensity (I_T):
    • Portion of energy transmitted forward into the next medium.
  • Intensity Reflection Coefficient (IRC):
    • Fraction (or percentage) of incident intensity that is reflected.
    • IRC = (I_R / I₀) × 100%
  • Intensity Transmission Coefficient (ITC):
    • Fraction (or percentage) of incident intensity that is transmitted.
    • ITC = (I_T / I₀) × 100%
  • Energy Conservation:
    • IRC + ITC = 100%
    • I_R + I_T = I₀
  • Normal Incidence (90°) and Impedance:
    • If Z₁ = Z₂:
      • IRC = 0%, ITC = 100%
    • If Z₁ ≠ Z₂:
      • Partial reflection and transmission occur.
  • IRC Formula for Normal Incidence:
    • IRC = [ (Z₂ – Z₁) / (Z₂ + Z₁) ]² × 100%
    • ITC = 100% – IRC
  • Example Calculations:
    • If I₀ = 60 mW/cm², I_R = 2 mW/cm²:
      • IRC = (2 / 60) × 100% = 3.3%
      • ITC = 100% – 3.3% = 96.7%
      • I_T = I₀ – I_R = 58 mW/cm²
    • If IRC = 20%, I₀ = 50 mW/cm²:
      • I_R = 0.20 × 50 = 10 mW/cm²
      • ITC = 80%
      • I_T = 0.80 × 50 = 40 mW/cm²

Physics of Normal vs. Oblique Incidence

  • Normal Incidence (90°):
    • Reflection and transmission are determined by impedance mismatch.
    • Predictable using the IRC formula.
    • Key relationships:
      • If Z₁ = Z₂, all energy is transmitted.
      • Small impedance mismatch → small reflection, most energy transmitted.
      • Large impedance mismatch (e.g., tissue to air) → large reflection, little energy transmitted.
  • Oblique Incidence (≠ 90°):
    • Reflection and transmission cannot be predicted by impedance.
    • Reflection may or may not occur; transmission may or may not occur.
    • If reflection occurs, θ_reflection = θ_incidence.
    • If transmission occurs:
      • If c₁ = c₂, θ_transmission = θ_incidence (no refraction).
      • If c₁ ≠ c₂, refraction occurs (θ_transmission ≠ θ_incidence).

Refraction and Snell’s Law

  • Refraction:
    • Change in direction of the transmitted wave at an interface due to different propagation speeds and oblique incidence.
    • Two conditions required:
      1. Oblique incidence (not 90°)
      2. Different propagation speeds (c₁ ≠ c₂)
  • Snell’s Law:
    • Relates the angles and speeds of sound in two media:
      • sin(θ₁) / sin(θ₂) = c₁ / c₂
      • Or, sin(θ_transmission) / sin(θ_incidence) = c₂ / c₁
    • θ₁ = angle of incidence, θ₂ = angle of transmission, c₁ = speed in medium 1, c₂ = speed in medium 2
  • Scenarios:
    1. c₁ = c₂:
      • θ_transmission = θ_incidence (no refraction)
    2. c₁ > c₂:
      • θ_transmission < θ_incidence (transmitted wave bends toward the normal)
    3. c₁ < c₂:
      • θ_transmission > θ_incidence (transmitted wave bends away from the normal)
  • Clinical Note: In soft tissues, propagation speeds are similar, so refraction is usually minimal. Large differences (e.g., soft tissue to bone) cause significant refraction and strong reflections.

Key Terms & Definitions

  • Echo: Reflected sound wave returning to the transducer.
  • Interface: Boundary between two different media or tissues.
  • Reflection: Redirection of sound back toward the transducer.
  • Specular reflector: Large, smooth surface causing strong, angle-dependent reflection.
  • Diffuse reflector: Rough, irregular surface causing weaker, scattered reflection.
  • Scattering: Deflection of sound in many directions from small structures; creates tissue texture.
  • Rayleigh scattering: Scattering from very small reflectors (e.g., red blood cells); highly frequency-dependent.
  • Transmission (Transmit Wave): Sound energy that continues through the boundary into the next medium.
  • Refraction: Change in direction of sound as it enters a new medium at an oblique angle with a different speed.
  • Impedance (Z): Resistance to sound, measured in Rayls; Z = density × speed.
  • Incident intensity (I₀): Power per area before hitting an interface.
  • Reflection intensity (I_R): Power per area reflected back.
  • Transmission intensity (I_T): Power per area transmitted forward.
  • IRC: Percentage of incident intensity reflected.
  • ITC: Percentage of incident intensity transmitted.
  • Normal incidence: Sound strikes boundary at 90° (perpendicular).
  • Oblique incidence: Sound strikes boundary at any angle except 90°.
  • Incidence angle (θ_incidence): Angle between the incident beam and the normal (perpendicular) to the interface.
  • Reflection angle (θ_reflection): Angle between the reflected beam and the normal.
  • Transmission angle (θ_transmission): Angle between the transmitted beam and the normal.

Action Items / Next Steps

  • Practice calculating IRC, ITC, and intensity values using the provided equations and examples.
  • Review and memorize key vocabulary and the six main rules for reflection and transmission:
    1. Energy cannot be created or destroyed (conservation of energy).
    2. With normal incidence, reflection only occurs if impedances differ.
    3. With normal incidence and equal impedances, 100% transmission occurs.
    4. With oblique incidence, reflection and transmission cannot be predicted by impedance.
    5. For oblique incidence, the reflection angle equals the incidence angle.
    6. Refraction requires oblique incidence and different propagation speeds.
  • Complete assigned practice activities and questions to reinforce understanding of these principles and calculations.

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