8: Ultrasound

Physics of Sound

What is Sound?

  • A sound is a pressure wave, specifically a form of mechanical energy that moves as a longitudinal wave.
  • Wave Characteristics:
    • Compression: High-pressure regions in the sound wave.
    • Rarefaction: Low-pressure regions in the sound wave.

Key Concepts of Sound Waves

  • Pressure Wave: Sound is a type of pressure wave created by vibrating objects which set molecules of a medium (e.g., air) into motion.
  • Wave Structure Representation:
    • Compression is denoted by orange rings in diagrams.
    • Rarefaction is represented by white spaces between the orange rings.

Sound Wave Characteristics

  1. Frequency

    • A measure of pitch, indicating how many cycles occur in a given time frame, measured in Hertz (Hz) or cycles/sec.
    • Higher frequencies lead to higher pitches, while lower frequencies lead to lower pitches.
    • Human hearing range: 20 Hz to 20,000 Hz.
    • Clinical ultrasound frequencies range: 1 MHz to 20 MHz (1 - 20 million Hz).

  2. Wavelength

    • The distance between two identical points on adjacent cycles of waves.
    • Relationship to frequency: Higher frequency produces shorter wavelengths, lower frequency produces longer wavelengths.

  3. Amplitude

    • Indicates loudness of sound, measured in decibels (dB).
    • Higher amplitude signifies greater pressure change and louder sound; lower amplitude indicates softer sound.

Speed of Sound Propagation

  • Propagation Velocity: Affected by the medium's stiffness.
  • Average speeds:
    • Air: 343 m/sec
    • Soft Tissue: 1,540 m/sec (reference average value)
    • Bone: 3,000 - 5,000 m/sec

Application in Ultrasound Technology

  • Understanding sound propagation speeds has significant applications in ultrasound technology.

Ultrasound Fundamentals

The Ultrasound Device

  • An ultrasound device emits ultrasound waves into the body and listens for echoes, allowing for dynamic imaging.
  • Echo Imaging Cycle Process:
    1. Electric current applied to piezoelectric elements inside the transducer, causing vibration.
    2. Waves generated enter the body.
    3. Waves encounter boundaries and reflect back.
    4. Reflected waves cause vibrations in the piezoelectric elements, converting mechanical energy back into electrical signals.
    5. An algorithm processes the signals, plotting images on the screen based on echo duration (time delay) and location of crystal reception.

Types of Views in Ultrasound Imaging

  • Short-Axis View: Cross-section of the structure.
  • Long-Axis View: Longitudinal section of the structure.

Image Interpretation and Echogenicity

  • Brightness correlates directly to the amplitude of returning signals.
  • Definitions of Echogenicity:
    • Hyperechoic: Structures appear bright (e.g., bone), producing strong echoes.
    • Hypoechoic: Structures appear darker (e.g., liver), producing weak echoes.
    • Anechoic: Structures appear black with no echo (e.g., vascular structures).

Visualization Techniques in Ultrasound

  • Vascular Structures:
    • Arteries are pulsatile, while veins are not.
    • Techniques for identifying vessels include assessing pulsatility and using color Doppler for flow visualization.

Ultrasound Physics and Methodology

Resolution and Attenuation

  • Resolution: Ability to distinguish two structures as separate.

    • Types of Resolution:
    • Axial Resolution: Along the beam's length, improved by higher frequency.
    • Lateral Resolution: Across the beam's width, enhanced by focusing.
    • Elevational Resolution: Thickness of the beam, determined by transducer design.

  • Attenuation: Reduction of sound wave strength as it travels, influenced by medium density and frequency.

    • Attenuation effects: absorption, reflection, scattering, refraction.
Artifact Considerations
  • Understanding the properties of sound waves also includes recognizing common ultrasound artifacts such as:
    • Air artifacts: Caused by improper contact between the transducer and skin.
    • Shadowing and Enhancement: Alterations in image quality due to varying tissue densitometry.

Clinical Applications of Ultrasound

  • Point-of-Care Ultrasound (POCUS): Provides quick bedside assessments, especially in emergency settings.
  • Cardiac Imaging Windows:
    • Imaging Windows: Parasternal, apical, and subcostal windows to assess cardiac function.
    • Each window provides distinct views for evaluations of chamber size, function, and valvular issues.
Gastric Ultrasound
  • Rapid pre-operative evaluations to assess gastric content and volume, valuable for intubation decisions.
  • Measurement techniques involve assessing the gastric antrum to measure volume accurately.

Conclusion

  • Mastery of these principles equips healthcare professionals with essential skills for utilizing ultrasound technology effectively in various clinical settings, ultimately improving patient care.