How is ultrasound attenuated in different body tissues? What do you understand by ‘half-power distance’?

  • Different tissues attenuate ultrasound waves dramatically differently
  • Results in different abilities to transmit sound
  • Often represented by the depth at which the energy of the sound wave has been halved – the “half-power distance”
Tissue
Half Power Distance (mm)
Water
3800
Blood
150
Most soft tissue
10 to 50
Muscle
6 to 10
Bone
2 to 7
Air, Lung
under 1
  • Significant attenuation through bone and air explains why ultrasound is poor at visualising these tissues
  • The degree of attenuation through muscle explains why transoesophageal echocardiography can produce superior images to transthoracic echocardiography