Basic physical principles of ultrasound

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








Fetal Echocardiography

 * Transposition




 * A Descriptive Case Presentation




 * Fetal Cardiac Myxoma



The Guide to Pediatric Echocardiography
