Introduction to Basic Ultrasound Principles

Introduction to Basic Ultrasound Principles

• High-Quality Imaging: To produce high-quality images without artifacts, sonographers must understand the basic principles of ultrasound.

Acoustics

• Definition: Acoustics is a branch of physics dealing with sound; it involves the generation, propagation, and reception of sound waves.
• Ultrasound: Defined as sound frequencies greater than 20 ext{ kHz} (normal human hearing range: 20 ext{ Hz} to 20 ext{ kHz}).
• Sound Waves: Result from mechanical energy creating alternating compression and rarefaction in the medium. Diagnostic ultrasound uses short sound pulses with frequencies ranging from 1 ext{ MHz} to 20 ext{ MHz}.

Ultrasound Mechanism

• Transducer: Contains piezoelectric elements that convert electrical signals into mechanical vibrations, producing ultrasound images. The transducer directs sound beams into the body, with signals reflecting back, absorbing, or scattering.
• Velocity of Sound: Average velocity in soft tissues is 1540 ext{ m/sec}, determined by the stiffness and density of the medium.

Measurement of Sound

• Decibel (dB): Used to measure intensity, amplitude, and power of ultrasound waves, allowing for comparison.
• Frequency: Measured in Hertz (Hz); defined as the number of oscillations per second:
  • 1 ext{ Hz} = 1 ext{ cycle/sec}
  • 1 ext{ kHz} = 1000 ext{ Hz}
  • 1 ext{ MHz} = 1,000,000 ext{ Hz}
  • Important for determining resolution in ultrasound imaging.

Propagation of Sound in Tissue

• Reflection: Occurs at interfaces between different acoustic impedances. Larger impedance differences result in stronger reflections.
• Scattering and Refraction: Scattering occurs with small or rough interfaces; refraction is the change in direction of sound waves at interfaces with different transmission speeds.
• Absorption: Loss of sound energy as it converts to thermal energy, higher in bone than in soft tissue.

Image Resolution

• Three Types of Resolution:
  • Axial Resolution: Ability to distinguish objects at different depths, better with higher frequency (shorter wavelength) transducers.
  • Lateral Resolution: Ability to distinguish objects at the same depth; affected by the transducer's focal zone.
  • Azimuthal (Elevation) Resolution: Ability to resolve objects at equal distance perpendicularly to imaging plane, related to slice thickness.

Attenuation

• Definition: Sum of energy loss due to absorption, scattering, and reflection, reducing intensity as sound travels through a medium.
• Rate of Attenuation in Soft Tissue: 0.5 ext{ dB/cm} per 1 ext{ MHz}. Attenuation is significant in interfaces involving air and bone.

Transducer Selection in Clinical Imaging

• Selection depends on the type of ultrasound examination and the specific imaging requirements, considering factors like frequency, resolution, and depth penetration.