P3 Revision - Topic 6 (Medical Physics)

Refraction
Waves change direction as they enter a different medium
Caused by change in density - changes the speed of the waves

Waves slow down and bend towards normal
When light enters glass or plastic - slows down
If a wave hits a boundary - will not change direction, but slows down
Hits a different medium, some light will pass through, some will be reflected

-If light is leaving something denser than air, you get total internal reflection
-Above certain angle of incidence, all the light will be reflected back into the material

Optical Fibres
Carry pulses or light or infrared radiation
Light is reflected off the sides of the inner core repeatedly until it's at the other end
Fibre must be narrow enough and not bend too sharply
-This keeps the angle above the critical angle

Endoscopes
Have two bundles of optical fibres,one to carry light and the other to carry image
Image seen through the eyepiece,or TV screen
Advantage = this makes it easier for smaller operations with 'key-hole' surgery

Pulse oximeter has
-a transmitter- emits two beams of red light.
-a photo detectore to measure light.

How it works
-Placed on the side of a thin part, (finger or ear lobe)
-Beams of light pass through the tissue - some light is absorbed by blood
-Amount of light absorbed depends on the colour of the blood which depends on the oxyhaemoglobin content
-Reflection pulse oximetry reflects light off rd blood cells instead of shining light through someone's body

When a force moves an object, energy is transferred and work is done.

Work Done = Force(F) x Distance(s)
-Work is not a type of energy - it is an energy transfer!
-It is measured in Joules, or J for short
A body builder lifts a barbell weighing 2000N. Calculate the work done by the body builder in lifting the barbell a distance of 0.75m.

Power is the energy transferred per second.
Power = Energy Transferred(W) / Time Taken(s)
-One Watt = 1 Joule of energy
-It is measured in Watts, or W for short
A toy rocket with a mass of 2.5kg, accelerating at 20m/s2, travels 40m in 2seconds. Calculate the power of the rocket motor.

Answer = 2000 X 0.75 = 1500J
Answer = (2.5 x 20) x 40 = 2000J  then divide by 2 = 1000J

Metabolic rate is the rate at which your body uses energy
-The more active you are, the more energy that you're transferring
-Chemicals in food give the body it's energy needed.

Basal Metabolic Rate (BMR) is the rate you burn energy at rest
-BMR tells you the minimum amount of energy needed to keep you alive
-To measure someone's BMR, they can't eat for 12 hours and they lie down and the amount of heat energy generated by the body is measured and used to find the BMR.

Factors that might affect BMR
age - children have a higher basal metabolic rate - energy is needed to grow.
body fat % - lower your body fat %, higher the BMR
gender - men generally have a lower % of body fat than women
exercise - muscle building and fat burning lowers body fat % = higher BMR
body surace area - larger body surface area = higher BMR
diet - starvation or suden calorie reduction lowers BMR
external temperature - exposure the cold increases the BMR

Muscle cells can generate potential differences
Between inside and outside of a muscle cell, there's a potential difference.
Potential different across the cell membrane at rest is called resting potential.
These differences can be measured with needle electrodes.
Resting potential of a muscle is about -70mV (millivolts)

EMG machine detects small electrical signals in muscles, identifies problems and helps stroke victims learn to use their muscles again.

Electrocardiographs mesause the action potentials of the heart
When <3 beats, action potential passes though atria, making them contract, after a fraction of a second, a second action potential passes through ventricles - they contract too.
The muscle relaxes afterwards
These produce weak electrical signals on the skin
Electrocardiographs record the action potentials 
The results are displayed on a secreen or printed out (ECG)

frequency (hertz) = 1 / time period (s)

Radiation is energy emitted from a source

More intense the radiation - the more energy it carries.
A big object is going to catch more radiation than a small one.

Intensity is the rate at hich energy arrives at 1 square metre of surface

Intensity = Power(P) x Area(A)

The intesity of radiation decreases as the distance from the source increases
-If you move twice as far from the heater, the same radiation is being spread over 4 times the area!
-This is known as the inverse square  relationship

Intensity also depends on what it is passing through
Dark glass absorbes some of the light, reducing the intensity

The splitting of U-235 Needs Neutrons
used in some nuclear reactors.
it's quite stable, and to split it it needs to be made unstable first
this is done by firing slow - moving neutrons at the nucleus.
low - energy neutrons are called thermal neutrons.
when thermal neutrons joins uranium nucleus U-236 which is unstable is created.
U-236 THEN SPLITS INTO TWO SMALLER ATOMS AND 2/3 FAST MOVING NEUTRONS
these fissions products have a nuclei which are unstable and radioactive

Proton Enrichment Forms Isotopes That Emit Positrons
-A proton is absorbed by a nucleus - increasing proton number - new element is made
-To be absorbed into the nucleus - the proton needs a lot of energy - particle accelerator is used.
-proton enrichment makes radioisotopes which are usually positron emitters
-positron emitters are used in PET scanning, monitors blood flow & metabolism
-these isotopes have short half-lives

total momentum after is equal to the total momentum before.
Momentum is Always Conserved
-momentum = mass x velocity

Positron/Electron Annihilation
- particle +antiparticle = annihilation.
-All the mass is converted to energy, given off in the form of gamma rays.
- positrons and electrons collide head on at the same speed.
-Particles have the same mass and oppsite velocity - total momentum before the collison is zero.
-Two gamma rays are produced - same energy - opposite velocities
-Mass/Energy is conserved too, so all mass is converted to energy.

E=mc2

PET Scanning Involves Positron/Electron Annihilation
-positron emission tomography.
-scanning technique in hospitals - shows tissue and organ function
-it's better than x rays because they show much more than just structure.
-x rays are cheaper.

1)inject patient with a sustance the body uses, containing a positron-emitting radioactive isotope with a short half life.
2)positrons emitted by the radioisotope collide with electrons in the organs.
3)detectors around the body record these gamma rays, and a computer builds up a map of radioactivity in the body.
4)distribution of radioactivity matches up with metabolic activity.

Radiation can destroy a cell and damage it so it can't divide.
Radiation alter gentic material - causing mutations.

Examples:
Physics
-endoscopes - keyhole surgery
-ECGs - see how the heart's working

Biology
-gene therapy
-treatment for infertility

Chemistry
-research into new drugs