notes 4 Ultrasound Attenuation and Imaging Techniques

Attenuation

• Definition: Attenuation is the reduction in intensity and amplitude of a sound wave as it travels through a medium. This occurs because some of the energy is absorbed, reflected, or scattered.
• Attenuation represents the sum of acoustic energy loss resulting from:
  • Absorption
  • Scattering
  • Reflection
• Visual Representation:
  • As sound travels through the abdomen, it becomes attenuated due to reflection, scattering, and absorption.

Attenuation in Human Soft Tissue

• In human soft tissue, sound attenuates at a rate of 0.5 \,\text{dB/cm} per million hertz.
• Factors Increasing Attenuation:
  • Air coupled with soft tissue
  • Bone coupled with soft tissue
• Example: Attenuation through a solid calcium interface, like a gallstone, produces a shadow with sharp borders on the ultrasound image.
• Acoustic Impedance Differences:
  • Differences in acoustic impedance in biologic tissues are generally slight.
  • Only a small component of the ultrasound beam is reflected at each interface.
  • Exception: Air-tissue and bone interfaces.
  • Anatomy beyond the lung and bowel cannot be imaged because of air interference.

Acoustic Impedance

• Acoustic impedance increases if density or propagation speed increases.
• Definition: Acoustic impedance is the product of the velocity of sound in a medium and the density of that medium.
  • Z = \rho v where:
    • Z is acoustic impedance,
    • \rho is density,
    • v is the velocity of sound.
  • Bone conducts sound at a significantly faster speed than soft tissue.

Real-Time Ultrasound

• Much of the sound beam is absorbed or scattered as it travels through the body.
• Real-time compound imaging allows sound to be steered at multiple angles, including perpendicular to the body, to produce the best image.
• Signals are averaged from multiple angles.
  • Accentuation of high-level reflectors occurs over weaker reflectors and noise.

Harmonic Imaging

• Sound waves contain many component frequencies.
• Harmonics are components whose frequencies are integral multiples of the lowest frequency (the fundamental or first harmonic).
  • Harmonic imaging involves transmitting at frequency f and receiving at frequency 2f (the second harmonic).
• Filtering out the fundamental frequency and creating images from echoes of the second harmonic should result in an image relatively free of noise formed during the passage of sound through distorting layers of the body wall.