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
What type of cells in the body produce a potential difference (voltage) and how?
- Muscle cells
- Between inside of muscle cell and outside
- Potential difference across cell membrane of muscle cell at rest is called resting potential
What happens to the potential difference of a muscle cell when stimulated by an electrical signal?
- The potential difference changes from -70mV to about +40mV.
- This increased potential called action potential.
- Action potential passes down length of the cell, making the muscle cell contract
What do electrocardiographs measure?
The action potentials of the heart
What is the heart made up of?
- Muscle
- Split into 4 chambers (atria at top + ventricles at bottom)
How does a heart beat occur (using action potential)?
- Action potential passes through atria making them contract
- Fraction of second later: another action potential passes through ventricles making them contract
- Once action potential passed, muscles relax
- Action potentials produce weak electrical signals on skin
How does an electrocardiograph work?
- Records action potentials of heart using electrodes stuck on chest, arms + legs
- Results displayed on screen/printed as graph called electrocardiogram (ECG)
- Used to look at action of the heart
How would you work out heart rate from an ECG?
frequency (hertz) = 1 / time period (seconds)
Multiplying by 60 turns this to heart rate in beats per minute
What is a pacemaker?
A device used to regulate heart beat
What is an example of a natural pacemaker and how does it work?
In the heart - groups of cells in wall of right atrium that produce electrical signals that pule about 70 times a minute. Signals spread over the atria + make them contract by action potential. Signals pass to ventricles which contract too.
What does the heart’s natural pacemaker control and what happens when this doesn’t work as well?
- Heart rate
- May not be fast enough
- Irregular pulses
- Other problems with electrical signals being sent
What happens when the heart’s natural pacemaker when it cannot do its job properly and how does this help?
- Artificial pacemaker fitted
- Keeps heart beating steadily using small electric impulses to stimulate heart to beat
- Small electric impulses sent via electrodes (thin wires attached to heart)
How is a pacemaker fitted?
- Minor surgery + only small incision to insdert it
- Powered by batteries so need replacing when battery loses power
What are some features of modern pacemakers?
- Can be programmed externally (settings can be changed without surgery again)
- Monitor breathing/temperature, use this to match heart rate to activity being done
What is the role of haemoglobin in the body?
- Carries oxygen from lungs to cells around body
- Pigment which makes blood red
- Changes colour depending on oxygen content: lots of oxygen = bright red (oxyhaemoglobin), without oxygen = purply (reduced haemoglobin)
How does a pulse oximeter work?
- Has a transmitter, emitting 2 beams of red light. Has photo detector to measure it
- Placed on either side of thing part of body e.g. finger/ear lobe
- Beams of light pass through tissue. Some absorbed by blood, reducing light detected by detector
- Amount of light absorption depends on blood colour, which depends on oxyhaemoglobin content
- Arteries of healthy people: at least 95% oxyhaemoglobin + no more than 5% reduced haemoglobin
- Reflection pulse oximetry uses similar technique; reflects light off red blood cells instead of shining light through part of body
What is meant by the term ‘bradycardia’?
An abnormally slow heart rate
What is meant by the term ‘tachycardia’?
An abnormally rapid heart rate
