X-Rays

X-Rays

1. Electron Beams

   • To produce X-rays you need a cathode, an anode, an evacuated gas tube and some thermionic emission.
   • 1) A filament acts as a cathode (negatively charged) and it is heated to give off more energy to the electrons. Once they have enough energy they boil off (escape) which is called thermionic emission.
   • 2) These electrons are then accelerated towards the anode (positively charged) by a voltage between the cathode and anode.
   • 3) Electron beams can be used to produce X-Rays so when the electrons collide with a metal target (anode) some of their kinetic energy is converted into X-Rays.
   • 4) The glass tube contains a vacuum to prevent electrons colliding with air particles as this would knock them off target and decrease their energy.
   • 5) A lead casing is put around the tube to absorb some of the X-Rays so they're only aimed at the specific thing.

2. Intensity and Absorption

   • X rays have a very high frequency and energy so can be dangerous to use. They have enough energy to ionise molecules in living cells which removes electrons from atoms leaving them as positively charged ions. This can leave damaged or destroyed cells which can lead to tissue damage or cancer.
   • Intensity of radiation depends on the distance from the source. If you move twice as far from the source the same radiation is being spread four times the area so you only receive 1/4 (1/2 squared) of the intensity. This is known as an inverse squared relationship.
   • X-Rays are absorbed more by some materials than others, generally the more dense the material the more radiation it absorbs. Absorption is also affected by the thickness of the material through which the rays are travelling. The thicker the material, the more rays will be absorbed which is why lead and concrete are used t reduce people's exposure (e.g. in hospitals).

3. X-Ray Imaging

   • Fluroscopes
     • 1) Basic fluoroscopy works by placing a patient between an X-ray source and fluorescent screen. Different amounts of x-rays will be absorbed as they pass through the patient's body so the intensity of x-ray will vary depending on what they've passed through in the body.
     • 2) The x-rays then hit a fluorescent screen which absorbs and give off light to show a live image on the screen which is down to the difference in x-ray intensity which exposes different bits of the screen to different amounts of x-rays. The higher the intensity, the brighter the screen.
     • Modern fluoroscopy now generally involves using an image intensifier to increase the brightness of the screen so a lower dose of x-rays can be given to the patient.
     • The screen is attached to a computer so images can be recorded.
     • Fluoroscopy is used to diagnose problems in the way organs are functioning (e.g. looking at movement through the gastrointestinal tract or blood flow). As x-rays pass easily through soft tissue the patient is given a 'contrast medium' by injection or ingestion which improves the contrast of the image by enhancing soft tissue and making it more visible.
   • CAT Scans
     • Computerised Axial Tomography scans produce an image of a 2D slice through the body.
     • 1) An x-ray beam rotates around the body and is picked up by thousands of detectors.
     • 2) A computer works out how many of the x-rays are absorbed and produces a very high quality image which is useful for diagnosing less obvious problems.
     • CAT scans can also be used to make 3D images by stacking the individual slices.
     • CAT scans are often used to look for tumors and cancer.
   • Risks and Benefits
     • X-rays can be harmful but they are often the best choice to treat or diagnose a patient because the benefits outweigh the risks (e.g. it is better to be able to properly diagnose a patient's injury rather than using the wrong treatment).
     • Hospitals try to limit x-ray exposure and reduce risk to the patient as much as possible.
     • It is an non-invasive procedure as there are no incisions or surgery so the procedures are generally fairly quick.
     • Safer non-ionising radiation techniques are used for imaging and treatment when possible (e.g. ultrasound) but the quality of image isn't usually as good so is less useful when diagnosing medical conditions.