Ultrasound Imaging

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  • Ultrasound is sound with frequency above the audible range - >20,000 cycles/second
  • Frequencies used in clinical imaging are in the 1 to 20 MHz range
  • Velocity of sound is determined by the bulk modulus and density of the medium
  • velocity = frequency x wavelength
  • Attenuation occurs in waves due to absorption - water absorbs less than soft tissue, bone absorbs more
  • A reflective wave is generated at the interface of two media with different acoustic impedances, given by impedance = density x velocity
  • Lower frequencies produce less resolution but image deeper into the body
  • Higher frequency sound waves have a smaller wavelength and thus are capable of reflecting or scattering from smaller structures
  • Higher frequency sound waves also have a larger attenuation coefficient and thus are more readily absorbed in tissue, limiting the depth of penetration of the sound wave into the body

Ultrasound Modes:

  • A mode - amplitude - displayed as a chart of echo vs time - old fashioned ultrasound
  • B mode - brightness - assigns gray scale values to pixels from received echo amplitudes - normal modern ultrasound mode
  • M mode - motion - only gives one spatial dimension but gives very high resolution

Uses of Ultrasound

  • Anaesthesiology: Line placement, nerve blockade, neuraxial block landmark identification
  • Diagnostics: echocardiography, obstetric, abdominal/pelvic, chest
  • Urology, musculoskeletal, vascular
  • Lithotripsy of kidney stones, phacoemulsification
  • There are no confirmed adverse biologic effects of diagnostic ultrasound
  • Absorption of sound waves can contribute to heating, however this effect is small compared to changes in body temperature in daily living
  • Cavitation occurs when ultrasound causes dissolved gases to become microbubbles, and can cause extravasation of blood cells in the lung
  • Spatial compound imaging combines multiple lines of insonation within a plantar field to produce a 2 dimensional image. This is produced by sending out ultrasound beams at a series of adjacent angles

Doppler Ultrasound

  • Doppler shift occurs when a wave source and receiver are moving relative to each other. This produces a change in frequency such that the frequencies of the transmitted and received sound sources are not the same frequency
  • Colour doppler encodes areas with mean frequency shifts - blue indicating flow away and red indicating flow towards the transducer

Ultrasound Safety

  • Being able to visualise the needle at all times during peripheral nerve blocks is vital
  • Intra-vascular injection and intraneural injection are the greatest complications