X-ray imaging

• What are the two methods of X-ray production?

  Bremsstrahlung

  Characteristic Radiation
• Describe the Bremsstrahlung method for producing X-rays.

  When high-speed electrons collide with tungsten, they undergo steep deceleration.

  Some of the kinetic energy of the electrons is converted into X-ray photons - due to the conservation of energy.

  The enery of the x-ray photons can take any value up to the kinetic energy of the incoming electron.
• How does the energy of Bremsstrahlung radiation (X-ray photons) vary as wavelength increases?

  As wavelength increases, the energy of the X-ray photons decreases.
• What is the maximum energy of an X-ray photon in Bremsstrahlung radiation?Why is this?

  $E_(max)=$$eV$

  
  

  The accelerated electron gains energy equal to the product of its charge and accelerating potential and since the x-ray photons can have a maximum energy = to the incoming electrons, this is the maximum energy of an X-ray photon.
• How do you calculate the maximum X-ray frequency?

  $f_(max)=$$eV/ h$
• How do you calculate minimum wavelength?

  $λmin=$$hc/eV$
• Describe how X-rays are produced by the characteristic radiation method.

  • Incoming electrons ionise inner-shell electrons which are replaced by outer electrons.
  • When outer electrons fill gaps left by inner electrons, they release x-rays of discrete energies.
• What is the relationship beyween minimum wavelength and maximum energy?

  They are inversely proportional.
• What is an x-ray tube?


  a device that converts an electrical input into x-rays.
• What is the role of the heated cathode in an X-ray tube?

  Causes thermionic emission.

  The heat liberates electrons from the cathode.
• What is the role of the anode in an X-ray tube?

  The anode is connected to a high voltage supply, this allows electrons to be accelerated up to a voltage of 200kV.
• Why does the anode rotate in an X-ray tube?

  To avoid overheating as only 1% of the kinetic energy is converted to x-rays meaning the rest is converted to heat energy.
• What is the function of the metal target in an x-ray tube?

  The electrons emitted from the cathode hit the metal target at high speed causing them to lose some of their Kinetic energy. This is reemitted as X-ray photons.
• What is the function of the high voltage power supply in an x-ray tube?

  It creates a large potential difference between the cathode and the target. The high voltage power supply causes te electrons from the cathode to be accelerated to a high velocity towards the target.
• What is the purpose of the lead shielding surrounding an x-ray tube?

  Ensures the safety of the operators and recipients of the X-rays.
• What is the purpose of the grating on an x-ray tube?

  allow a concentrated beam of x-rays to escape and be controlled safely.
• Why are the anode and cathode housed in a vaccuum chamber?

  to ensure the electrons do not collide with any particles on their way to the metal target.
• How does controlling the intensity of the x-ray beam reduce exposure and minimise the risk to the patient?

  To minimise exposure, the beam intensity should be reduced by lowering the cathode current. Lowering cathode current means there is less thermionic emission so less electrons reach the anode each second meaning less x-ray photons are emitted each second. This reduces the risk to the patient as less ionising photons are passing through the body each second.
• What is a beam definer?

  A beam definer consists of 2 pairs of lead sheets with a narrow opening in the centre which is placed close to the x-ray tube.
• How does a beam definer help to minimise EXPOSURE to the patient?

  They produce a collimated beam which minimises the risk to the patient as it the narrow beam is used to investigate a specific area of the body only. This means the lead plates absorb scattered photons so the body is not exposed to ionising photons.
• What are metal filters?

  A thin sheet of metal, usually aluminium, placed between the x-ray tube and the patient.
• How does an aluminium (metal) filter minimise exposure to the patient?

  Reduces the intensity of low-energy photons menaing they are less likely to be absorbed by the body. Because of this, it minimises risk to the patient because it reduces the amount of ionising photons the body can absorb whilst still allowing high-energy photons to pass straight through.
• How can exposure time be reduced using more sensitive equipment?

  Intensifying the image and using an electronic detector instead of photographic detection.#
• Define Contrast

  the difference in the degree of blackening between structures.
• How can image contrast be improved in X-rays?

  Using hard X-rays for bones and Soft X-rays for tissue.

  Using a contrast media
• Define sharpness of an image?

  How well-defined the edges of structures are.
• How can image sharpness be improved?

  • Using a narrower X-ray beam.
  • Reducing X-ray scattering by using a collimator or lead grid.
  • Smaller pixel size.
• What is a contrast medium?

  A substance which is a good absorber of X-rays. This ensures a bigger contrast can be obtained on an x-ray image.
• Why are contrast media good absorbers of X-rays?

  They have a large attenuation coefficient in comparison with other tissues meaning they enhance the image as the substance is opaque in x-rays.
• When are barium and iodine used as contrast mediums?

  Iodine is used in liquids ie to observe blood flow and is injected.

  Barium sulphate is used in the digestive system and is ingested.
• Why do contrast materials have high attenuation coefficients?

  They have large atomic numbers
• What are the layers of flat-pannel detectors?

  The scintillator layer

  The photodiode pixel layer

  The electronic scanner layer
• How are X-rays detected using FTP?

  The electrons in the scintillator absorb high energy x-rays and emit visible photons.

  The photodiode pixels absorb the visible photons releasing electrons.

  The electrons generate a p.d. in the electronic scanner which is processed and transmitted as a digital image to a computer.
• What are the uses for FTP detectors?

  high-quality images of the bones and joints and are used in airport security.
• How is photographic film detection used to detect x-rays?

  An x-ray intensifying cassette decreases the overall exposure time of x-rays to be shortened.
• Why is the intensifying screen beneficial to the patient?

  It reduces the exposure time reducing the ionising dose of radiation received by the patient.

  The patient does not have to be stationary for long.
• How does an image intensifier work?

  • An image forms on the fluorescent screen as incident X-rays are absorbed and re-emitted as visible photons.
  • Visible photons cause electrons to be emitted from the photocathode
  • The emitted electrons are ACCELERATED through a large p.d. towards anode which focus them on an output window.
  • The intensified image is formed on the fluorescence viewing screen at the end of the evacuated tube.
• Why does the intensifier increase the brightness of the image?

  • Electrons are focused on a smaller area for a given power output. I=P/A, therefore, the intensity increases.
  • The electrons are given a large amount of energy at the anode due to the acceleteration meaning several photons are emitted for every electron arriving at the fluorescent screen.
• What is the image intensifier used for?

  real-time images.
• What is the disadvantage of the image intensifier?

  A higher radiation dose is exposed to the patient because there is a continuous beam of X-rays on the area throughout the procedure.

  However, this is minimised compared with taking several images of the same region.