6.5 - Medical imaging

• How are X-rays produced 


  Charged particles are accelerated or decelerated. The kinetic energy is transferred into high energy photons
• How are gamma rays produced 


  Released from radioactive decay
• Simple elastic scattering 


  X-ray photon with low energy interacts with an electron from an atom. Photon is deflected and the energy doesn't change

  
• Photoelectric effect 


  X-rays can cause the emission of electrons from a surface of a metal
• Pair production


  High energy X-ray photons interacts with nucleus of an atom. It can be absorbed and its energy is released as a positron and a electron
• Compton scattering 


  X-ray photons with energy between 0.1 - 10 Mev are directed towards an atom. An electron is released and a photon with less energy
• Soft X-rays 


  X-rays used in medical imaging that have lower energies than gamma rays
• Impedance matching gel



  Used between a transducer and skin to minimise reflection, as the impedances of air and skin are very different
• Acoustic impedance



  The product of a sound wave's density and the speed of sound in the medium, unit: kg m-2 s-1
• Ultrasound B-scan



  series of A-scans that are stitched together to create a 2D image, the transducer is moved across the patient's skin and uses the time and speed to calculate distance to a boundary at each point
• Ultrasound A-scan



  Uses a single transducer to emit a signal and then detect the reflected signal, used to determine the distance from the device to the point of reflection (usually a boundary between media) by using the time and the speed of sound in the medium
• Ultrasound 


  A longitudinal sound wave with a frequency higher than 20 kilo hertz
• How an ultrasound transducer works


  1.It has an alternating potential difference that causes a piezoelectric crystal to contract and expand at a resonant frequency of the crystal to maximise intensity 2. Once the ultrasound has been created, the potential difference is turned off and the reflected signal is detected by the transducer
• How piezoelectric crystals work


  1.Applying a potential difference to a piezoelectric crystal will cause it to produce ultrasound waves, and a piezoelectric crystal absorbing ultrasound waves will produce an alternating potential difference 2. They tend to be made from quartz, polymeric or ceramic materials
• Piezoelectric effect



  piezoelectric material either generates a potential difference when it is contracted or expanded, or will contract or expand when a potential difference is applied
• X-ray attenuation


  When a material absorbs X-rays, decreasing the intensity exponentially
• Attenuation coefficient


  The greater the attenuation (absorption) coefficient, the more the material will absorb incident X-rays
• What does CAT stand for

  Computerised axial tomography
• What does PET stand for 


  Positron emission tomography
• What is meant by the piezoelectric effect

  The material expands or contracts when a pd is applied across its opposite faces
• Explain how ultrasound is used to measure the speed of blood flow

  Pulses of ultrasound are aimed at the blood

  Transducer placed at an angle

  The frequency of reflected wave is different to the emitted wave

  The ratio of frequency equal ratio of speed (dappled effect)
• Ethical problems with pet scans 


  Some patients may not be able to have one because of their location
• Describe the structure of an x Ray tube and how x rays are produced

  Tube with a cathode and anode

  High voltage connected between cathode and anode

  Accelerated electrons shot at metal and their Ek is transformed into x rays
• Collimator


  only allows photons travelling a certain direction through
• Photomultiplier tube


  amplifies the signal
• Scintillator


  Scintillation crystal which emits many visible light photons for every incident high-energy photon Photocathode which produces an electron for every incident visible photon
• Structure of a gamma camera

  Collimator which only allows photons travelling a certain direction through Scintillation crystal which emits many visible light photons for every incident high-energy photon Photocathode which produces an electron for every incident visible photon Photomultiplier tube which amplifies the signal Computer which detects the signal and displays the image on a screen