Basic physical principles of ultrasound

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Ahmed Zaghw, M.D. [2]

• Ultrasound uses high frequency energy to create sound waves, characterized by
  • frequency: number of cycles per second (Hz)
  • wavelength: distance between excitations (mm)
  • amplitude: height of sound wave (decibels)

• Reflection

• • occurs when an ultrasound wave hits tissue. Part of the wave is reflected back to the transducer
  • tissues with greater density reflect a greater portion of the beam
  • tissues at a 90 degree angle to the beam reflect a greater portion of the beam

• Acoustic impedance

• • tissues can be defined by their individual acoustic impedance
  • AI = density x acoustic velocity

• Scatter

• • small structures lead to scattering of the ultrasound signal
  • scattering radiates the beam in all directions, very little reaches back to the transducer

• refraction: ultrasound waves can be deflected as they pass into a medium of different acoustic impedance

• Attenuation
  • when sound travels through a medium, intensity diminishes with distance
  • caused by absorption of the energy by the tissues, and conversion to heat
  • also affected by acoustic impedance and mismatch in impedance between adjacent structures

• Energy source
• Transducer

It converts the electrical pulses to vibrations and returned vibrations back to electrical pulses

• Piezoelectric crystals

• It is found inside the transducer
• It compresses and expand the electric current that is applied, generating a sound wave
• Same crystals also act as receivers. An electric current is generated when a reflected wave hits a crystal.

Image formation: based on the time interval between the ultrasound transmission and the arrival of the sound wave back to the transducer

• Axial- along the length of the ultrasound beam
• Lateral- side to side resolution of the 2D image
• Elevational- the thickness of the tomographic slice

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