Imaging Using EM Waves (OCR GCSE Physics A (Gateway)) : Revision Note

Imaging Using EM Waves

Higher Tier Only

• 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

• Where:

  • v = speed in metres per second (m/s)

  • s = distance in metres (m)

  • t = time in seconds (s)

• 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 can be used to construct an image of a foetus in the womb

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

Ultrasound is partially reflected at boundaries, so in a bolt with no internal cracks, there should only be two pulses (at the start and end of the bolt)