P3.2 X-rays and ECGs Flashcards
What does ionising radiation do?
Turn atoms into ions by removing some of their outer electrons
What happens when the energy of an X-ray increases?
It becomes more ionising
How do you increase the energy of an X-Ray?
You increase its frequency and voltage
What is a negative electrode called?
A cathode
What is a positive electrode called?
An anode
What happens when you heat the cathode in an X-ray machine?
It becomes an electron gun
What is the process of an electron gun emitting electrons called?
Thermionic emission
What happens when there is a large potential difference between the anode and the cathode?
The electrons accelerate towards the anode
Why does an X-ray machine have an evacuated tube?
So that the electrons don’t collide with other particles as they cross the tube
What is an evacuated tube?
A tube from which the air has been removed so that there is a vacuum
What happens to the kinetic energy of electrons when they hit the anode?
Most of it is transferred to thermal energy, but some are transformed into X-rays
How do you increase the amount of X-rays?
You make the coil hotter (cathode) so that more electrons are emitted from the electron gun
What is another name for an X-ray machine?
X-ray tube
Why does the anode in an X-ray machine spin?
- to reduce wear
- to prevent it from overheating
At what angle are X-rays emitted to the electron beam?
90 degrees
What is current?
The rate of flow of electrons
How do you find current?
I = N x q Current = number of particles flowing each second x charge on each particle
How do you find the kinetic energy of an electron?
E = 1/2mv^2 or E =eV
What does m stand for in the formula used for finding kinetic energy?
Mass of an electron in kg
What does v stand for in the formula used for finding kinetic energy?
Velocity of the electron in m/s
What is kinetic energy measured in?
Joules or kgm^2/s^2
What does V stand for in the formula for finding kinetic energy?
Potential difference in volts
What does e stand for in the formula for finding kinetic energy?
Charge of an electron
What is the charge of an electron?
1.6 x 10^-19 C
When you double the distance away from a radioactive source, what happens to its strength?
It decreases to a quarter
How do you find out how the strength of a radioactive source changes in relation to distance?
By using the inverse square law
What does the inverse square law state?
The strength of a wave is inversely proportional to the square of the distance from the source
When the density of a material increases, what happens to the amount of X-rays it absorbs?
It increases
Why does bone show up lighter on an X-ray than organs or tissues?
- Bone is denser than organs or tissues so it absorbs more X-rays
- on X-rays dark areas are where a lot of X-rays have been detected
- because the bone has absorbed a lot of X-rays not many are detected by the scanner
- this means it shows up as white because of negative imaging
How does a computer build up a 3D image from a CAT scanner?
It builds up many cross-sectional views of the body gotten from the scanner
What do unusual areas of brightness or darkness indicate on a CAT scan picture?
Tumours or small areas of tissue that have died because their blood supply has been obstructed
What do fluoroscopes do?
Show a patient’s organs working
What can fluoroscopes be used for?
Detecting blocked blood vessels
How does a fluoroscopy get a moving image of the inside of the body?
It uses an X-ray detector attached to a digital video camera. The patient is put between this and the source
What are the benefits of CAT scans and fluoroscopes?
- painless for the patient
- non-invasive
- no biopsy
How is the risk of damaging other tissues minimised when treating cancer with X-rays?
They’re fired from several different directions so that only the tumour receives a high dose
What is an action potential?
An electrical signal that is sent to each muscle cell to tell it when to contract
Why does the human body conduct electricity?
It contains a high proportion of water with dissolved salt in it
What does ECG stand for?
Electrocardiogram
What is an ECG?
A picture of the heart’s electrical signals
What does an ECG show?
- how fast the heart is beating
- which part of the heart might not be working properly
How much time is one large square on an ECG equal to?
0.2 seconds
How do you find out the frequency of a heart beat in beats per second?
f = 1/t frequency = 1 / time period in seconds
How do you find the time period of a heart beat?
You work out the average length of one heartbeat
How does one action potential spread across the heart?
- the sino-ventricular node contracts
- the action potential spreads across both atria
- the atrio-ventricular node contracts
- the action potential spreads up the ventricle walls
- a contraction spreads up from the base of the ventricles
What is the frequency of a heart beat usually given as?
bpm, beats per minute
What is pulse oximetry used for?
- measuring the pulse rate of a person
- seeing how much oxygen their blood is carrying
What does a pulse oximeter consist of?
- a LED emitting red light
- a LED emitting infrared
- a detector
What does LED stand for?
Light emitting diode
What does oxygenated blood absorb more of in comparison to deoxygenated blood in a pulse oximeter?
Infrared
What does deoxygenated blood absorb more of in comparison to oxygenated blood in a pulse oximeter?
Red light
How does a pulse oximeter work out a pulse?
Each beat of the heart causes a surge of oxygenated blood, so the oximeter looks at the peaks of infrared absorbance
How does a pulse oximeter work out how much oxygen the blood is carrying?
By comparing the absorbance between the red light and infrared LEDs
How is the amount of oxygen the blood is carrying expressed in an oximeter?
As a percentage of its maximum
What does a pacemaker do?
Detect, amplify and transmit action potentials to other parts of the heart so that its chambers contract correctly
What can be used when someone’s heart doesn’t beat properly?
A pacemaker
Where do you put a pulse oximeter?
A finger or a toe
