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