Ultrasound: physics and knobology

Nature of Sound Waves

• How far the sound waves can travel depend on
  • how strong the signal is
  • the medium that the waves have to travel though; underwater is greater medium compared to brick wall.

Wave Properties

• Penetration
• Attenuation
• Reflection

Penetration

• Ultrasound waves, depending on amount of energy, will pass through a medium to a certain depth.

Attenuation

• As ultrasound waves travel through medium they continually losing energy to medium that they pass.

Reflection

• The rest of waves' energy after travelling through medium is reflected back toward their source.
• Equals to "echo" of submarine sonar.
• Enable image generation on ultrasound machine.
• Eventually energy will not be enough to reflect back to machine, therefore no image.

Effect of Different Tissue Densities

• Ultrasound waves have different abilities to travel through different mediums.
• This quality of mediums or tissues is called resistance or impedance.
• Travel well though liquid: blood, urine.
• Travel less well through solid organs: liver, spleen.
• Not at all though bone (high resistance).
• The more a substance reflect energy back towards the probe the brighter the image appears on screen. On the other hand the less it reflects, the darker the image.
• If the probe is not at right angle to surface of interest scatter (deflection) of ultrasound waves occurs, especially when scanning aorta.

Liquid

• Appears black. No resistance therefore no echo and no energy.
• Blood filled structures: heart or blood vessels.
• Urine filled bladder.
• Cystic structures: gallbladder.
• In normal location fluid is contained and surrounded by echogenic membrane.
• Free fluid (internal haemorrhage or ascites) has no membrane and has irregular shapes.

Solid Organs

• Can function as "acoustic window", which allow deeper penetration into the body. Fluid filled bladder has the same function as well.

Bone

• Bone reflect back almost 100% of the ultrasound waves. It appears as bright white.
• No waves traverse beyond bone therefore no energy after that. Areas behind the bone appears black.
• Bone can impede the view. Also it can be useful serving as landmark, i.e.spine.

Gas

• Almost always bowel gas.
• Subcutaneous emphysema and pathologic gas-forming process in structure can also be the case.
• Appear as grey, "snow storm" appearance, which provide no information.
• Gas or air between the probe and the body can cause problem. Apply generous amount of gel to create better acoustic contact.

Tissue Interface

• The greater the different in echogenicity the easier it is to identify structures.
  • Fluid in pericardial space.
  • Free fluid in hepatorenal or splenorenal pouch.
  • Gestational sac in uterus.
  • Blood-filled vessels.

Modes of Transmission

• B: Brightness, most commonly used.
• A: Amplitude
• M: Motion, often used for cardiac.
• D: Doppler; "Colour" and "Pulse Wave" types.

Frequency

• Different types of probes have different range of frequency.
• Frequency is inversely related to penetration.
• Frequency is directly related to axial resolution (the ability to distinguish between two objects at different depth).
• Low frequency probe is good for looking deep into the body cavity (better penetration).
• High frequency probe is good for look at small objects near the surface.

Probes or Transducers

• Format is the field of view produced by the probe which appear on the screen. There are two formats; linear and sector.
  • Linear format has rectangular field of view. Usually used for looking at objects close to the surface.
  • Sector format has pie-wedge shape field of view. Usually used for looking at deep objects.
• Array is the way crystals are arranged at the transducer head. There are three types which are commonly used; phased, flat-linear and convex- or curved-linear.
  • Curved-linear array probe is the one most commonly used in general for beginners.
  • Flat-linear probe is the one usually used looking at objects close to the surface and produce rectangular field of view.

Understanding Ultrasound Images

• 2-D image of 3-D structure.
• Planes of view: the way the probe is placed along the line in different direction on the body.
  • Longitudinal view: the probe is placed along the line which is running from head to toe (along the body).
  • Transverse view: the probe is placed along the line which is running from left to right (across the body).

See Also

• Ultrasound (main)