Medical Physics

Question 1

What is the difference between x-rays and gamma waves

Answer 1

X rays are formed by decelerating electrons whereas gamma waves are emitted from a nucleus with too much energy

Question 2

What are the 4 types of x-ray attenuation

Answer 2

Photoelectric effect
Compton scattering
Simple scattering
Pair production

Question 3

Features of the photoelectric effect

Answer 3

Less than 100 KeV
When a photon overcomes the work function of an electron, transferring its energy and releasing it

Question 4

Features of Compton scattering

Answer 4

Between 0.5 MeV and 5 MeV
High energy photons collide with loosely bound electrons ejecting it from the atom. The x ray is then scattered with reduced energy and a longer wavelength

Question 5

Features of pair production in terms of x-ray attenuation

Answer 5

Greater than 1.02 MeV
When a high energy x-ray interacts with the nucleus of an atom and is converted into matter, specifically one electron and one positron

Question 6

Features of simple scattering

Answer 6

X rays with less than 20 KeV
When an x ray photon hits and electron in an atom but does not free it and deflects with reduced energy

Question 7

What is computerised axial tomography

Answer 7

A thin fan shaped x ray beam is fired at a patient with a detector on the other side. The x-ray emitter revolves around the patient creating a slice image of the patient,after one complete revolution it moves 1 cm relative to the patient to create the next slice

Question 8

Advantages and disadvantages of a CAT scan compared to an x-ray image

Answer 8

Advantages: 3D image can be created,more detail and soft tissue

Disadvantages: Expensive, Slow, High x-ray exposure

Question 9

Describe how a gamma camera works

Answer 9

A lead collimator ensures only gamma waves directly beneath the camera are detected.
A scintillator emits visible light photons upon being struck by a gamma photon.
A photomultiplier amplify this photon to produce more electrons via the photoelectric effect
The electrical pulses are fed to a computer which constructs an image

Question 10

Describe how a positron emission tomography (PET) scan works

Answer 10

A positron emitting tracer is ingested.
Positrons are emitted in certain areas from this tracer, collide with electrons and annihilate
This produces two gamma photons in opposite directions, which is then picked up by an array of gamma cameras around the patient
A processing unit calculated where the annihilation occurred and maps it to see activity

Question 11

Compare a PET scan to a CAT scan

Answer 11

PET scans: more expensive, more time consuming, shows biological function of the body
CAR scans: higher doses to radiation, detailed images of tissue and bone structure