Edexcel P3 Topic 6 – Medical Physics
Revision
Seeing inside the body
Refraction in waves: The speed of a wave changes with the density of the material it
enters. This change in speed may cause a change in direction – this is known as
refraction.
Light waves are transverse waves. They can be refracted.
Sound waves are longitudinal waves. They can be refracted.
Light rays and infrared rays
change speed when they pass
from glassair or perspexair
This is because air and
glass/perspex - have different
densities. If they hit the boundary
of the material at an angle other
than 90º their path changes
direction.
Beyond a certain angle, called
the critical angle, all the rays
reflect back into the glass or
Perspex. This is called total
internal reflection, TIR.
Edexcel P3 Topic 6 – Medical Physics
Revision
Total internal reflection:
An optical fibre is a thin rod of high-quality glass. Very little light is absorbed by
the glass. Light getting in at one end undergoes repeated total internal reflection,
even when the fibre is bent, and emerges at the other end.
Edexcel P3 Topic 6 – Medical Physics
Revision
An endoscope is an instrument used by Doctors and Surgeons.
A bundle of very thin optical fibres is used with lenses to see inside a body.
Only a small hole in the skin is necessary to insert the endoscope.
Some of the optical fibres take light down to the end of the endoscope which shines
inside the body.
Other optical fibres in the bundle collect the reflected light using lenses.
The reflected light is sent along the
fibres to a computer which displays
the information as a picture on a
monitor.
It is sometimes possible to perform
medical operations inside people by
using an endoscope, rather than
making a large cut in the skin.
Edexcel P3 Topic 6 – Medical Physics
Revision
Pulse oximeters use light to check the % of oxygen in the blood.
This is useful for monitoring a patient’s health before and after surgery.
Haemoglobin carries oxygen around the body. Haemoglobin is the pigment that
makes the blood red.
Haemoglobin changes colour depending on its oxygen content:
 When it is rich in oxygen it is bright red in colour (called oxyhaemoglobin)
 After oxygen is released to cells its purply/red (called reduced haemoglobin)
In a normal, healthy person around 95% of the blood is oxyhaemoglobin, and no
more than 5% is reduced haemoglobin.
A pulse oximeter emits two beams of red
light through a thin part of the body
(e.g. ear lobe or finger).
The light beams pass
through the tissue and are
partially absorbed by the
blood.
This reduces the amount of light detected
on the other side.
The amount of light absorption depends on the colour of the blood:
Hb absorbs more red and less infrared
HbO2 absorbs less red and more infrared
Edexcel P3 Topic 6 – Medical Physics
Revision
Metabolic rate is the rate at which your body uses energy
The body is doing ‘work’ all the time, e.g. moving muscles, digestion, heart beating
All of these mechanisms involve transferring energy, and the more active you are the
more energy is transferred
When a force moves an object, energy is transferred and work is done
work done = force x distance
power = work done
time taken
The amount of energy used by the human body per second, minute, hour or day is
called the metabolic rate
We get our energy in the form of chemical energy from food – most is transformed to
heat and kinetic energy
Basal Metabolic Rate (BMR) is the rate at which the body transfers energy at rest
(i.e. not doing anything at all!)
BMR is the minimum amount of energy needed to keep you alive
To measure someone's BMR, they must fast for 12 hours. Then lie down so they are
fully at rest. The amount of heat energy generated by the body is measured and
used to find the BMR.
BMR is measured as the energy (kJ) needed per hour per m 2 of body surface area
Factors that can affect BMR:
 Age - children have a higher basal metabolic rate as energy is needed to grow
 Body fat % - lower body fat % = higher BMR
 Gender - men generally have a lower % of body fat than women = higher BMR
 Exercise - muscle building and fat burning lowers body fat % = higher BMR
 Body surface area - larger body surface area = more heat transfer = higher BMR
 Diet - starvation or sudden calorie reduction lowers BMR to preserve energy
 External temperature - exposure the cold increases the BMR to keep warm
Edexcel P3 Topic 6 – Medical Physics
Revision
Muscle cells can generate potential difference. Between the inside of a muscle cell
and the outside, there is a potential difference (voltage).
The p.d. across the cell membrane of a muscle cell at rest is called the
resting potential.
The p.d. can be measured with a needle electrode – the resting potential of a muscle
cell is approx. -70mV (millivolts).
When the muscle is stimulated by an electrical signal the p.d. changes from -70mV
to +40mV. The increased potential is called an action potential. The action potential
passes down the length of the cell, which causes the muscle cell to contract.
An electromyograph (EMG) is used to detect small electrical signals in muscles.
The results can be used to identify problems like muscular dystrophy and motor
neurone disease. EMG is also used to help stroke victims to learn to use their
muscles again.
The heart is a pump controlled by electrical signals.
An electrocardiograph measures the action potentials of the heart.
When the heart beats, an action potential passes through the atria causing them to
contract. Then, another action potential passes through the ventricles, causing them
to contract too.
Once the action potential has passed, the muscle relaxes. These action potentials
produce weak electrical signals on the skin.
An electrocardiograph records the action potentials of the heart using electrodes
stuck on the chest, arms and legs. It can detect problems with heart rhythm. For
accurate readings the patient must sit or lie down.
Edexcel P3 Topic 6 – Medical Physics
Revision
The results can be displayed on a screen or printed off as a graph called an
electrocardiogram (ECG).
Horizontal line = resting
potential
P = contraction of atria
QRS = contraction of
ventricles, relaxation of atria
T = relaxation of ventricles
You can work out the heart rate from an ECG using the equation:
frequency (hertz) = 1
time period (seconds)
From peak to peak = 1 second
frequency = 1
1 = 1Hz
1Hz x 60 (convert into beats per minute) = 60bpm
Edexcel P3 Topic 6 – Medical Physics
Revision
Momentum conservation
When particles collide or are emitted from nuclei momentum is always conserved
(Momentum = mass x velocity)
So total momentum before = total momentum after
1. Collision: bouncing off
This is similar to when a fast-moving neutron hits a nucleus and bounces off again
2. Collision: joining together
This is similar to when a neutron or proton collides with an atom and is absorbed
into the nucleus (nuclear bombardment)
3. Explosion: shot and recoil
This is like a particle being emitted from a nucleus. The nucleus recoils like a fired
gun.
Edexcel P3 Topic 6 – Medical Physics
Revision
Example: The diagram represents an iron nucleus capturing a neutron. Find the final
velocity.
Before
m1 = 1
v1 = 2km/s
After
m2 = 58
v2 = 0.5km/s
momentum before = momentum after
(1x2) + (58 x 0.5) = (59 x v3)
31 = (59 x v3)
v3 = 0.53km/s to the right
m1 + m2 = 59
v3 = ?
___________________________________________________________________
Example: The diagram represents an alpha decay. Find the velocity of the nucleus
after the decay.
Before
m1 + m2 = 146
v1 = 0km/s
After
m1 = 4
v2 = -15,000km/s
momentum before = momentum after
(146 x 0) = (4 x -15000) + (142 x v3)
0 = -60000 + (142 x v3)
60000 = (142 x v3)
v3 = 423km/s to the right
m2 = 142
v3 = ?
Edexcel P3 Topic 6 – Medical Physics
Revision
When electrons and positrons collide, the positrons are annihilated.
All of the mass of both particles is converted into energy, which is given off in the
form of gamma rays.
When positrons and electrons meet, they collide head on at the same speed, and
moving in opposite directions.
The particles have the same mass and opposite velocities, so the total momentum
before collision is zero. Momentum is always conserved so that gamma rays
produced need to have a total momentum of zero.
This happens through the production of two gamma rays which have the same
energy, but opposite velocities.
Mass/energy is also conserved in this reaction – all of the mass has been converted
into energy - (E = mc2)
Edexcel P3 Topic 6 – Medical Physics
Revision
Medical uses of radiation
PET scanning (positron emission tomography) is used to show tissue or organ
function. It shows more than X-Rays but is expensive.
PET scans show areas of damaged tissue in the heart by detecting decreased
blood flow. This can reveal coronary artery disease and damaged or dead heart
muscle caused by heart attacks.
PET scans can also identify active cancer tumours by showing metabolic activity in
tissue. Cancer cells have a much higher metabolism than healthy cells as they are
growing rapidly out of control.
PET scans can record blood flow and activity in the brain. This helps research
and treat illnesses like Parkinson’s, Alzheimer’s, epilepsy, depression etc.
How PET works:
Step 1: The patient is injected with a substance used by the body (e.g. glucose), that
has a positron-emitting radioactive isotope with a short half-life (eg. 11C, 13N, 15O,
18F). This is called a radiotracer. It moves through the body to the organs.
Step 2: Positrons emitted by the radioisotope collide with electrons in the organs.
They are annihilated and high-energy gamma rays are emitted.
Step 3: Detectors around the body map these gamma rays, and a computer builds
up a map of radioactivity in the body.
Step 4: The distribution of radioactivity matches up with metabolic activity. This is
because more of the glucose injected is taken up and used by cells that are doing
more work (i.e. have an increased metabolism).
Risks
Radiation can destroy a cell completely. Radiation can damage a cell so it can’t
divide. Radiation can alter the genetic material in a cell. This can cause mutations.
It can also make the cell divide and grow uncontrollably (cancer).
This is why it is important to limit exposure to radiation.
Edexcel P3 Topic 6 – Medical Physics
Revision
X-Rays and PET scans only use small doses of radiation, but any exposure
increases the risk of cancer so it is not recommended to scan patients too often and
only do it if necessary.
Sometimes radiation treatment is the best choice. Radiotherapy can be used to treat
cancer. High-energy X-rays or gamma rays are used to destroy cancer cells.
Radiation damages cancer cells much more than it damages normal cells.
Radiotherapy doesn’t always lead to a cure, but it can reduce suffering when a
patient is close to death. Treatment that reduces suffering without curing an illness is
called palliative care.
Ethical issues in medical research:
New medical techniques can help us treat illness. For example:
Endoscopes help with keyhole surgery,
X-Rays diagnose broken bones, lung conditions etc,
High-energy X-Rays treat cancer,
ECGs show us how the heart is working,
Gene therapy,
Treating infertility,
Embryo selection,
Genetically modifying animals and humans,
Researching new drugs,
New diagnosis tests by analysing chemicals in the body
However, new techniques have to be tested on people or animals, this raises moral
and ethical dilemmas.
There may be harmful side effects, places on medical trials can be limited and it can
be years before medicines are available to the public.
Environmental factors: Unused medical radioisotopes must be disposed of. Some
are high-level nuclear waste with long half-lives that produce dangerous levels of
radiation.
Economic factors: Companies are unlikely to pay for research unless they can
profit somehow. Drug research is often funded by companies that will sell the drugs
for a profit.