Basic physical principles of ultrasound

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Sound waves

• 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)

Ultrasound and tissue interaction

• 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

Ultrasound set-up

• Energy source
• Transducer: converts electrical pulses to vibrations and returned vibrations back to electrical pulses
  • piezoelectric crystals
    • found inside the transducer
    • compress 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

Resolution

• axial- along the length of the ultrasound beam
• lateral- side to side resolution of the 2D image
• elevational- the thickness of the tomographic slice

Basics of Echocardiography

Basics of Echocardiography-Part 1 {{#ev:youtube|QdkJ6qzMfNE}}

Basics of Echocardiography-Part 2 {{#ev:youtube|h4dZmbfXKJQ}}

Basics of Echocardiography-Part 3 {{#ev:youtube|SaORcN29CtM}}

Basics of Echocardiography-Part 4 {{#ev:youtube|zZVBN2cdqSo}}

Fetal Echocardiography

• Transposition

<Youtube v=Rkm8e9q1GSc/>

• A Descriptive Case Presentation

<Youtube v=ZIk9GhWqqfE/>

• Fetal Cardiac Myxoma

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The Guide to Pediatric Echocardiography

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