Ultrasound Imaging

• 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