Lecture 1

Question 1

What is electromagnetic radiation?

Answer 1

X rays travel in straight lines ignorer to form an image
velocity is constant = speed of light
energy is inversely proportional to wavelength
(shorter the wavelength the higher the energy)

Question 2

Where is electromagnetic radiation on the electromagnetic spectrum?

Answer 2

at the higher end past visible light so there are health and safety concerns

Question 3

What are the two categories of electromagnetic radiation?

Answer 3

X-rays and Gamma rays

Question 4

X-Rays?

Answer 4

Used in radiography
produced by the interaction of fast moving electrons with a metal target

Question 5

How are x-rays produced

Answer 5

By a small electrical circuit with a cathode and an anode
A tungsten wire filament is heated by the small electric current
This causes thermionic emission where heat is boiling off electrons forming a cloud of free electrons around the cathode
The cathode has a negatively charged focusing cup that holds the electrons there until they are focused into a beam

Question 6

mA control?

Answer 6

Milli amps= the current that runs through the cathode
Changing the mA will alter the small current heating the tungsten cathode filament

Question 7

What happens when the mA is increase

Answer 7

The higher the filament current, the higher the temp and the greater the number of electrons produced

Question 8

How do electrons move in an x-ray beam?

Answer 8

Towards the anode (target) through a tube by the high potential difference between the electrons
(Usually 50-100,00V)

Question 9

What is the anode?

Answer 9

The ‘target’
Made of solid tungsten
Has a anomic number of 74= efficient of producing electrons
High melting point of 3380*c so heat work become too much
Electrons are rapidly decelerated as they interact with the atoms of the anode- then producing x-rays

Question 10

kV control?

Answer 10

Will alter the potential difference across the tube between in cathode and anode

Question 11

X-ray spectrum?

Answer 11

maximum energy of x-ray emitted = kV applied across tube

Question 12

insert xray graph pic

Question 13

How are x-rays produced continued

Answer 13

Interactions of the electron beam from the cathode to the anode produces 99% heat and 1% x-ray
The x-rays focus on a small area of the anode called the focal spot
X-rays from a single point like the focal spot will give a better image but heat will be focused so will have to be removed

Question 14

Methods of heat removal from the focal spot on an anode/ different types of anode

Answer 14

Stationary anode
- used for a lower output machine, the tungsten target/ anode is embedded in a block of copper, will facilitate heat conduction away from the target

Rotating anode
- Higher output machine, the anode is a molybdenum disk with a tungsten track that runs round the edge, the disk rotates via electric motors, heat lost my convection from surface of disk as molybdenum is a poor conductor
heat is produced over a wider area

Question 15

Prep phase of X-ray

Answer 15

Pressing half way down on the button will put the x-ray machine in prep mode and start rotating the anode

Question 16

X-ray tube

Answer 16

Anode and cathode are inside and evacuated (vacuum inside so no air) pyrex tube
Tube is emerged in oil to help with heat conduction and electrical insulation
Tube in encased in lead except for a small window (where the x-ray emerges) - stops x-ray being out of control

Question 17

List the exposure factors

Answer 17

on/off button
kV control
mA control
timer

Question 18

On/ off

Answer 18

only active when connected to a mains power
when on area surrounding it is considered by law a ‘controlled area’ with access restrictions

Question 19

kV control

Answer 19

How much the electrons are accelerated towards the anode
Step-up transformer supplies a high voltage (kV) across the tube
higher the kV= greater potential difference across the tube- faster the electrons will travel
The electrons will have higher kinetic energy when they hit the anode resulting in the corresponding x-ray beam having a higher energy

Question 20

Selection of kV exposure factors

Answer 20

Thicker parts of the body = increase kV - so won’t be absorbed before
not liner - increase of 10kV doubles the exposure (approximately)

Question 21

mA control

Answer 21

Changes the small Current heating up the tungsten filament in the cathode
kV and mA may be linked to stop overheating
kV up mA drops to avoid overheating

Question 22

Selection of mA exposure factors

Answer 22

mA effects the number of X-rays produced from the anode
(higher mA leads to increased heating of the filament meaning more electrons accelerated across the X-ray tube)
liner relationship - double mA doubles the exposure (approximately)
mA and (exposure) time have similar effects = considered together as mAs

Question 23

mAs?

Answer 23

mA and exposure time together

Question 24

Timer

Answer 24

activates the high tension (kV) and filament (mA) circuits
increase time of exposure increases the number of x-rays produced
don’t want a long exposure time as things can move and look blurry - want as short as possible

Question 25

Thicker parts of the body

Answer 25

Requires an increase in mAs Doubling mAs doubles the exposure

Question 26

Exposure chart?

Answer 26

keeping a record of exposure factors helps to create one of these helps to guide future choices for animals of similar size

Question 27

Inverse square law

Answer 27

Distance will also affect exposure - distance usually fixed (between anode and cathode) the exposure is inversely proportional to the square of the distance from the x-ray x-rays produced from the focal spot will spread out more over a long distance but there will be the same number of x-rays

Question 28

Focal spot

Answer 28

Area on the anode that is hit by electrons Want to ideally keep as small as possible - want a 'point spot' of x-rays small focal spot= problems with heat dissapation some machines can give options of 'broad' or 'fine' focus - fine focus for smaller/ thinner areas of anatomy - won't be able to use high exposure Thicker anatomy - broad focus- high exposure factors due to tissue thickness

Question 29

Filtration

Answer 29

A thin layer aluminium is over the window of the x-ray tube to filter out low energy X-rays (as the beam will consist of many x-rays with a spectrum of energy) that wouldn't penetrate the patient so aren't useful for image formation

Question 30

Interaction of x-rays with matter - 3 possibilities

Answer 30

1) x ray photons pass through unchanged - travel in a straight line - don't loose energy - as don't have any interactions on the way - form the useful x-ray image 2) x ray photons are absorbed - depends on the material they are traveling through -will contribute to image formation as will be the are on image with less x rays (white) 3) x ray photons are scattered - photons are deflected to move in random directions -loose some energy - will interact and not take all energy away so will be scattered in a different direction - not useful for image formation - degrades image quality - radiation hazard - can limit the primary beam but not the scatter so need to stay far away from machine

Question 31

Factors affecting the absorption of X-rays

Answer 31

- atomic number - high = more absorption e.g. lead bone has higher atomic number than sort tissue as it has more minerals- will look opaque - higher density = more absorption - more change of interacting with nucleus - air won't absorb a significant amount of radiation - thicker tissue = more absorption

Question 32

How does bone, gas and soft tissue appear on x-rays

Answer 32

Bone = white - high atomic number good absorber - radiopaque Gas = black - radiolucent - low density so poor absorber Soft tissues = grey- intermediate atomic number and density - intermediate absorption

Question 33

5 different opacity detected

Answer 33

Gas, fat, soft tissues, bone, metal from darkest to lightest