Topic 2 - X-rays and ECGs Flashcards
Explain the process of thermionic emission and how this can lead to the production of X-rays.
- Filament (cathode, -vely charged electrode); heated so electrons have more energy
- With enough energy, ‘boil off’ (escape). Thermionic emission.
- Electrons accelerated towards anode (+ve electrode) by potential difference (voltage) between cathode and anode
- When electrons collide with metal target (anode), some kinetic energy converted to X-rays
In thermionic emission, why is the filament heated?
Gives more energy to its electrons
Why is thermionic emission done in a vacuum?
Prevent electrons colliding with air particles (knock them off target + decrease energy)
Why is a lead casing put around the tube in which thermionic emission takes place?
Absorb some of the X-rays so only aimed at one small area, e.g. in hospitals aimed at something specific like an arm
What is energy measured in?
joules, J
What is charge measured in?
coulombs, C
What is potential difference measured in?
Volts, V
What is the formula for the kinetic energy gained by each electron as it accelerates in thermionic emission?
kinetic energy = electronic charge (e) x accelerating potential difference (V)
What is current measured in?
amperes, A
What is the formula for current of the beam of electrons in thermionic emission?
Current (I) = number of particles per second (N) x charge on each particle (q)
What does the symbol e represent?
electrical charge, 1.6x10^-19
Why are X-rays potentially dangerous to our body?
- Very high frequency so high energy
- Enough energy to ionise molecules in living cells (removing electrons from atoms so they’re +ve charged ions)
What effect can the ionisation from X-rays have on cells?
Damaged or destroyed, leading to tissue damage or cancer
What is the relationship between intensity and distance from the source?
- Inverse square relationship
- e.g. if you move twice as far from source, radiation spread over 4x the area (1/2^2 = 1/4)
What kind of materials are used in hospitals to reduce people’s exposure to X-rays?
Lead and concrete
What factors affect how much X-ray radiation a material absorbs?
- Density, more dense = more radiation absorbed
- Thickness of material, thicker = more absorbed
What type of electromagnetic wave is used in a fluoroscope?
X-rays
How does fluoroscopy work?
- Patient placed between X-ray source + fluorescent screen
- Different amount of X-rays absorbed as pass through body
- Intensity of rays vary on what they pass through
- X-rays hit a fluorescent screen which absorbs them + fluoresces (gives off light) showing live image on screen by rays’ intensity (bright = more intense)
- Modern fluoroscopy: generally involves image intensifier, increaser brightness of image so lower dose can be given
- Attached to PC so image can be recorded
What is fluoroscopy used for?
- Diagnose problems in the way organs are functioning (e.g. movement through gastrointestinal tract or blood flow)
- Can easily pass through soft tissue, patient given ‘contrast medium’ (injection/ingestion). Improves contrast of image seen by enhancing soft tissue so more visible
What does CAT scan stand for?
computerised axial tomography scan
What do CAT scans produce?
2D slices through the body
How does CAT scanning work?
- X-ray rotates around body + picked up by thousands of detectors
- Computer works out how many X-rays absorbed + produce high quality image (especially useful to diagnose less obvious problems)
- Can make 3D image by stacking individual slices
- Often look for tumours/cancer
What are the advantages of using X-rays for imaging and treatment?
- Non invasive as no incisions or surgery
- Generally quite quick
- Usually higher quality images
- Hospitals try to limit exposure to minimise risk
What are the risks of using X-rays for imaging and treatment?
- Other safer methods e.g. ultrasound (although usually lower quality image produces)
- Ionising; cell mutation + cancer
