Lecture 1 Flashcards
What is electromagnetic radiation?
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)
Where is electromagnetic radiation on the electromagnetic spectrum?
at the higher end past visible light so there are health and safety concerns
What are the two categories of electromagnetic radiation?
X-rays and Gamma rays
X-Rays?
Used in radiography
produced by the interaction of fast moving electrons with a metal target
How are x-rays produced
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
mA control?
Milli amps= the current that runs through the cathode
Changing the mA will alter the small current heating the tungsten cathode filament
What happens when the mA is increase
The higher the filament current, the higher the temp and the greater the number of electrons produced
How do electrons move in an x-ray beam?
Towards the anode (target) through a tube by the high potential difference between the electrons
(Usually 50-100,00V)
What is the anode?
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
kV control?
Will alter the potential difference across the tube between in cathode and anode
X-ray spectrum?
maximum energy of x-ray emitted = kV applied across tube
insert xray graph pic
How are x-rays produced continued
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
Methods of heat removal from the focal spot on an anode/ different types of anode
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
Prep phase of X-ray
Pressing half way down on the button will put the x-ray machine in prep mode and start rotating the anode
X-ray tube
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
List the exposure factors
on/off button
kV control
mA control
timer
On/ off
only active when connected to a mains power
when on area surrounding it is considered by law a ‘controlled area’ with access restrictions
kV control
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
Selection of kV exposure factors
Thicker parts of the body = increase kV - so won’t be absorbed before
not liner - increase of 10kV doubles the exposure (approximately)
mA control
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
Selection of mA exposure factors
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
mAs?
mA and exposure time together
Timer
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
Thicker parts of the body
Requires an increase in mAs
Doubling mAs doubles the exposure
Exposure chart?
keeping a record of exposure factors helps to create one of these
helps to guide future choices for animals of similar size
Inverse square law
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
Focal spot
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
Filtration
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
Interaction of x-rays with matter - 3 possibilities
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
Factors affecting the absorption of X-rays
- 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
How does bone, gas and soft tissue appear on x-rays
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
5 different opacity detected
Gas, fat, soft tissues, bone, metal
from darkest to lightest
