Topic 2 - X-rays and ECGs Flashcards
What is thermionic emission
releasing electrons from a surface
how do you produce x-rays
a filament acts as a cathode and is heated which gives more energy to its electrons, Once they have enough energy, they escape, this is called thermionic emission. The electrons are then accelerated towards the anode by a potential difference between the cathode and the anode. Electron beams can be used to produce x-rays when the electrons collide with a metal target (the anode) some of their kinetic energy is converted into x-rays. This is done in a glass tube that contains a vacuum
Why is producing x-rays done in a vacuum with lead casting.
Vacuum- to stop electrons colliding with air particles that would knock them off target and decrease their energy Lead casing - to absorb some of the x-rays so they’re only aimed at the thing in question
How can x-rays harm the body
They have a very high frequency and so high energy. They have enough energy to be able to ionise molecules in living cells.
What happens when something is ionised
removing electrons from atoms, leaving them as positively charged ions. Ionisation means that cells might be damaged or destroyed which can lead to tissue damage or cancer.
What does the intensity of radiation depend on?
The distance from the source because the same amount of radiation is hitting a smaller area.
What is inverse square relationship?
If you move twice as far from the source the same radiation is being spread over 4 times the area so you only receive 1/22 of the intensity.
How does the type of material affect the amount of x-ray absorption
The more dense/thicker a material the more radiation it absorbs.
Which materials are mainly used to reduce peoples exposure to x-rays
lead and concrete
What are fluoroscopes
Moving images of the inside of a patients body
How do fluoroscopes work?
Place a patient between an x-ray source and a fluorescent screen. Different amounts of x-rays are absorbed as they pass through the patients body. Intensity depends on what they’ve been passed through. X-rays hit a fluorescent screen which absorbs the x-rays and fluoresces (give off light) to show a live image on the screen. The higher the intensity of x-rays the brighter the screen.
How is modern fluoroscopy better?
An image intensifier is used to increase the brightness of the image so a lower dose of x-rays can be given to a patient. the screen is attached to a computer so the images can be recorded.
What is fluoroscopy used for?
Used to diagnose problems in the way organs are functioning.
What is a contrast medium
because x-rays pass through soft tissues easily the patient is given a contrast medium (by injection or ingestion). This is a substance which improves the contrast of the image seen by enhancing the soft tissue, making it more visible
What are CAT scans
Computerised axial tomography. 2d image of slice through the body
How do CAT scans work
An x-ray beam rotates around the body and is picked up by thousands of detectors. A computer works out how many of the x-rays are absorbed and produces a high quality image which is useful for diagnosing less obvious problems
How can CAT scans be used to make 3d image
Stack the individual slices
What are CAT scans used to check for
tumours and cancer
What are the benefits of x-rays
Better to diagnose someone properly rather than give wrong treatment.
Hospitals limit x-ray exposure and reduce risk to patient as much as possible
Non invasive procedure/quick
What part of the body can generate potential difference
muscle cells. There is a potential difference between the inside of a muscle cell and the outside. When its at rest its called resting potential
How is resting potential measured
with small needle electrodes
What is the normal resting potential
-70 mV (millivolts)
What happens to the potential difference when a muscle cell is stimulated
It changes to +40 mV. The increased potential is called action potential, it passes down the length of the cell making the muscle cell contract
What is an electrocardiograph
records the action potentials of the heart using electrodes stuck onto the chest, arms, and legs. For accurate readings patient should lie still and relax
What happens to the heart when it beats in terms of potential difference
An action potential passes through the atria making them contract. A fraction of a second later another action potential passes through the ventricles making them contract. Once the action potential has passed the muscle relaxes
What is an ECG
a graph of the results from electrocardiograph. Electrocardiogram
Describe it
The horizontal line is the resting potential The ‘blip’ at P shows contraction of atria The QRS blip shows the contraction of ventricles, weird shape because of relaxation of atria as well. T is relaxation of ventricles
How do you convert from Hz to bpm
Hz x 60
What is a pacemaker?
Device used to regulate heart rate
Why do some people need pacemakers?
heart rate isnt fast enough
Pulses irregularly
Problems with electrical signals being sent
How do artificial pacemakers work
produces small electric impulses to stimulate the heart to beat. Impulses sent via electrodes (thin wires attatched to the heart)
What are the problems with pacemakers?
Involves surgery (but only minor)
Powered by batteries so will need replacing when it loses power
What are the advantages of modern pacemakers?
Can be programmed externally so settings can be changed without surgery
Some can monitor things such as breathing and temperature and then adjust heart rate accordingly.
What is a pulse oximeter
Check % oxygen in blood
How do pulse oximeters work?
Haemoglobin changes colour depending on the amount of oxygen in the blood. When theres lots of oxygen its bright red (oxyhaemoglobin), when it has given up its oxygen to cells its purple (reduced haemoglobin).
Pulse oximeter has a transmitter which emits two beams of red light and a photo detector to measure light. It is placed on thing part of body (finger/ear lobe). Beams of light pass through tissue and some light is absorbed by blood reducing the amount of light detected. The amount of light absorbed depends on the colour of the blood so the oxyhaemoglobin content.
What % oxyhaemoglobin do healthy people have in arteries?
at least 95%
What is reflection pulse oximetry?
reflects light off red blood cells instead of shining light through part of the body
