DDT (CA1)

• X ray images are produced when a body is placed between a source of x rays and a detector.

  • this can be used in conjunction with contrast media to highlight specific areas of the body
  • different tissues absorb different amounts of X-ray based on their density
  • X-rays blacken photographic films
• X-ray formation:

  • cathode is heated and electrons are essentially boiled off (thermionic emission)
  • electrons are accelerated by high voltage in the X-ray tube
  • electrons hit an anode and produce X-rays and heat
• functions of the anode:

  • converts electron energy to X-radiation
  • dissipate heat
• digital subtraction angiography (DSA) is used to study blood supply to various organs
• the k-edge is the abrupt increase in photoelectric absorption of X-ray photons observed at an energy level just beyond the binding energy of the k-shell electrons of the absorbing atoms

  • they are specific to each element
  • they increase with atomic number (so does photon energy and k-shell binding energy)
  • binding energy is expressed in kiloelectronVolts (keV)
• contrast media is carefully selected so that k-edge matches x-ray energies to be used

  • the contrast media is then introduced to blood vessels to be studied and they become highly absorbing of x-rays and hence highly visible in the x-ray image
• process of creating a DSA:

  1. a normal x-ray is taken
  2. contrast media is introduced and another x-ray is taken
  3. both images are digitised and one is digitally subtracted from the other
• tomography is a technique for displaying a representation of a cross section through a human body or other solid object using x-rays or ultrasound

  • the basic principle of CT is that the structure of an object can be reconstructed from a series of x-ray projections taken uniformly around the object
• how a CT is formed:

  • many pencil thin beams of x-rays are passed through a section of tissue from many different directions so that all the beams have a common crossing point
  • the common crossing point yields more detailed information
• modern CT scanners use multiple x-rays projections to obtain detailed information on thousands of tiny volumes within a subject within a matter of seconds
• linear attenuation coefficient:

  • when a thin x-ray beam is passed through a section of the body, the intensity of the beam on exit depends on the linear attenuation coefficient
  • the intensity will depend on the cumulative attenuation along the path of the x-ray beam (lambert-beer law)
• lambert-beer law is a relationship between the attenuation of radiation through a substance and the properties of that substance

  • it explains the exponential decrease in the intensity of the x-ray beam with the distance it travels in the body
• how are x-ray CT scans are constructed:

  • the linear attenuation coefficients of all sections of the tissue are mathematically manipulated to calculate the actual absorption as a function of position within the subject
• how are x-ray CT scans are constructed:

  1. a 3d array is constructed by a computer
  2. each element of the array corresponds to a specific location within the patient
  3. the numerical value assigned to each element corresponds to the calculated absorption at that location
  4. colour scheme is developed kinda like paint by numbers

  need to add absorption values at crossover points to represent total absorption at that point
• ionising radiation is radiation with a frequency greater than 1x10^15 Hz that will break bonds and cause electrons to be freed from the atom
• for biological tissues, ionising radiation has:

  • wavelengths less than 300 nm (UV)
  • photon energies greater than 4 eV
• ionising radiation can be electromagnetic or particle radiation.

  • electromagnetic radiation e.g. UV, X-ray and gamma radiation
  • particle radiation e.g. alpha, beta and neutron
• biological effects of ionising radiation principally result from damage to cellular DNA, either directly or indirectly:

  • direct - radiation damages DNA
  • indirect - radiation causes the formation of free radicals which damage DNA
• • microwave radiation can cause molecular rotation and torsion
  • infrared radiation causes molecular vibration
  • visible radiation causes electron level changes
  • UV radiation causes compton scattering which can cause secondary damage
  • X-rays, gamma rays and particle radiation causes photoionisation
• radiotherapy is the use of ionising radiation to stop cancer cells from growing and dividing, by damaging their genetic material

  • while both normal and cancer cells are damaged, healthy cells are given a chance to repair themselves and function normally
  • this is due to fractionation of radiotherapy, so radiation is delivered in fractions (smaller doses) over time
• x-rays, gamma rays or electrons can be used in radiotherapy

  • the type of radiation depends on the patient and the application
• radiotherapy is used as a localised treatment for solid tumours such as skin, tongue, larynx, brain, breast and uterine cancer

  • it can also be used as a treatment for leukaemia and lymphoma
• several beams are used in radiotherapy to concentrate the intensity of the beam on the specific region of the tumour

  • this provides the maximum cumulative dose to the tumour and minimal damage to the surrounding tissue
• side effects of radiotherapy include body aches, swelling, metastases (other cancers), fatigue and skin burning
• brachytherapy:

  • this is another mode of delivering radiation
  • it is administered internally as radioactive sources can be implanted into the body in the region of malignancy
  • they can be implanted as a high dose for a short time or a low dose for a long time

  temporary brachytherapy catheters may be strategically inserted into the breast and radioactive material is injected into them at treatment
• treatment planning for radiotherapy:

  • the precise location of tumour is found by medical imaging
  • mathematical models are used to predict the pattern of irradiation that should be used to avoid damage to healthy tissues
  • need to take into consideration the beam direction, beam strength, the number of beams and the positioning of the patient
• film badges indicate the cumulative exposure to radiation by the degree of blackening of photographic film

  • a single badge contains a series of filters of different thicknesses and of different materials
  • this allows an estimation of the approximate energy or wavelength of the incident radiation

  it detects the radiation itself rather than ionisation caused by the radiation
• a geiger-muller tube is a glass tube that that encases a smaller metal tube filled with a gas (e.g. helium, oxygen)

  • a wire electrode through the metal tube has a high voltage
  • radiation then enters through a window, causing an avalanche of ionisation within the tube
• how does a geiger-muller tube work:

  1. radiation enters the tube and causes ionisation of some gas molecules
  2. resultant electrons are accelerated towards the high voltage wire electrode (+ve), which ionises more molecules
  3. the avalanche of charged particles cause a short, intense pulse of current which is detected
  4. output signal is measured which indicates an event has taken place (usually indicated as a click or high pitched sound)
• after an event in the geiger-muller tube, the discharge has to be stopped so as to detect another ionisation event

  • this time is called the dead time and lasts 100 to 500 milliseconds
• scintillation is the production of small flashes of visible light from certain materials as a result of the absorption of high energy radiation

  • a scintillator consists of a transparent crystal which flouresce when struck with with ionising radiatio
• the visible flash produced by the scintillation crystal is detected and amplified by a photomultiplier tube (PMT)

  • since the number of emitted photons per meV of incident energy is fairly constant, the intensity of the scintillation flash can be used to determine the original photon energy