Imaging with EM

• Imaging Using EM Waves

  • Sound waves can be used to analyse structures that are hidden from direct observation
  • Examples of the use of sound waves
  • Echo sounding used by shipping to detect the ocean floor
  • Ultrasound used to look inside the human body
  • Ultrasound crack detection to find cracks in rail tracks
  • Reflection seismology to detect oil and gas underground
  • Seismic activity (Earthquakes) can be used to investigate the structure of the Earth
• The properties of a substance that allow the detection of hidden structures are:

  • Reflection
  • Absorption
  • Transmission
  • The speed of sound in the substance
• Each type of substance will produce different amounts of reflection, absorption and transmission

  • Sounds will travel faster in solids than liquids and sound travels faster in liquids than gases
  • Each type of substance will also transmit a sound wave at a specific speed
• • Certain structures will reflect a proportion of the sound wave and transmit the rest
  • Some substances will absorb sound waves with very little reflection
  • By detecting the amount of sound reflected and the speed of the wave the hidden structure can be identified
  • Electromagnetic waves such as infra-red, X-rays and gamma rays are also used as alternatives to medical imaging
  • This is to explore structures which are hidden from direct observations (e.g in organs)
• Ultrasound in Medicine

  • When ultrasound reaches a boundary between two media, some of the waves are partially reflected
  • The remainder of the waves continue through the material and are transmitted
  • Ultrasound transducers are able to:
  • Emit ultrasound
  • Receive ultrasound
  • The time taken for the reflections to reach a detector can be used to determine how far away a boundary is
  • This is because ultrasound travels at different speeds through different media
  • This is done by using the speed, distance, and time equation
• • This allows ultrasound waves to be used for both medical and industrial imaging
  • In medicine, ultrasound can be used:
  • To construct images of a foetus in the womb
  • To generate 2D images of organs and other internal structures (as long as they are not surrounded by bone)
  • As a medical treatment such as removing kidney stones
• • An ultrasound detector is made up of a transducer that produces and detects a beam of ultrasound waves in the body
  • The ultrasound waves are reflected back to the transducer by boundaries between tissues in the path of the beam
  • For example, the boundary between fluid and soft tissue or tissue and bone
• • When these echoes hit the transducer, they generate electrical signals that are sent to the ultrasound scanner
  • Using the speed of sound and the time of each echo’s return, the detector calculates the distance from the transducer to the tissue boundary
  • By taking a series of ultrasound measurements, sweeping across an area, the time measurements may be used to build up an image
  • Unlike many other medical imaging techniques, ultrasound is non-invasive and is believed to be harmless
• Ultrasound in Industry

  • In industry, ultrasound can be used to:
  • Check for cracks inside metal objects
  • Generate images beneath surfaces
  • A crack in a metal block will cause some waves to reflect earlier than the rest, so they will show up as pulses on an oscilloscope trace
  • Each pulse represents each time the wave crosses a boundary
  • The speed of the waves is constant, so measuring the time between emission and detection can allow the distance from the source to be calculated