Clarius: Fundamentals of Ultrasound 1 (Physics)

Key Concepts:

• Ultrasound transducer (probe)
• Piezoelectric effect
• Sound wave propagation
• Attenuation (absorption, reflection, scatter)
• Time Gain Compensation (TGC)
• Acoustic impedance
• Frequency (Hertz, MHz)
• Wavelength
• Resolution
• Penetration
• Acoustic shadowing
• Speckle

1. Ultrasound Imaging Basics:

• Ultrasound imaging uses a transducer to send sound waves into the body.
• Sound waves bounce off tissues and return to the transducer as echoes.
• The ultrasound engine processes these echoes to create a black and white image.
• The image represents a cross-section inside the patient, formed line by line.

2. How Ultrasound Works:

• The transducer contains piezoelectric elements that expand and contract when an electric current is applied, generating pressure waves (sound waves).
• These waves propagate through the body.
• Echoes from tissues return to the transducer elements and are processed to create images.
• The system interprets the signal based on the known speed of sound in tissue (approximately 1,540 m/s) to display structures at their correct location.
• Shallow structures appear at the top of the image, and deeper structures at the bottom.
• This process occurs rapidly, allowing for real-time moving images.

3. Sound Beam Interactions:

• Attenuation: The weakening of the ultrasound wave as it travels through tissue.
  • The major cause of attenuation is absorption, where acoustic energy is converted into heat.
  • Deeper structures are harder to see due to attenuation.
• Time Gain Compensation (TGC): Ultrasound systems compensate for attenuation by increasing the gain at deeper levels, brightening deeper structures.
• Reflection: Occurs when a sound wave encounters a boundary between two different media (e.g., tissue and bone).
  • Some of the wave bounces back toward the source.
  • The angle of incidence equals the angle of reflection.
  • Strong reflectors like bone create acoustic shadows, preventing imaging behind them.
• Scatter: Occurs when ultrasound waves encounter a non-homogeneous surface.
  • The sound wave is scattered in random directions.
  • Scatter results in speckle, the grainy appearance in ultrasound images.
• Acoustic Coupling: Sound needs a medium to travel through the body. Air reflects ultrasound, so gel is used to ensure proper contact between the transducer and the patient. Air trapped between the transducer and skin creates shadowing.

4. Sound Frequencies:

• A sound wave cycle consists of a complete positive and negative pressure change.
• Wavelength: The distance traveled during one cycle.
• Frequency: Measured in cycles per second, or Hertz (Hz).
  • Audible range for humans: 16-20,000 Hz.
  • Ultrasound: Frequencies higher than 20 kHz.
  • Typical ultrasound imaging range: 1-20 MHz.
• Smaller piezoelectric elements vibrating faster generate higher frequencies. Larger elements vibrating slower create lower frequencies.
• Lower frequencies penetrate deeper but have lower resolution. Higher frequencies have better resolution for shallow structures but are more attenuated.

5. Image Clarity and Frequency Selection:

• Frequency is a critical parameter affecting image clarity.
• Lower frequencies provide better penetration for deeper structures.
• Higher frequencies provide better resolution for shallow structures.
• Selecting the transducer with the right frequency is crucial for optimal imaging of the anatomy being examined.

6. Conclusion:

Ultrasound images are created by listening to echoes from structures reflecting an ultrasound beam. Attenuation weakens the signal in deeper structures, which is compensated for by TGC. Higher frequencies offer better resolution for shallow anatomy, while lower frequencies enable deeper imaging. Absorption and reflection can prevent imaging behind solid structures or air. Understanding these principles is essential for effective ultrasound use.