Lecture 1: intro & fundamental physics Flashcards
what are the 10 characteristics of x-rays?
1- Travels in a straight line (essential) 2- Penetrates matter (essential) 3- obeys inverse square law (sometimes) 4- travels at the speed of light 5- excites atom 6- ionises atoms 7- produces chemical effects 8- produces biological effects 9- produces photographic effects 10- produces fluoroscopic effects
What is the inverse square law?
What is the equation for inverse square law?
- The intensity of the rays decreases with distance from the source because the rays are spread over greater areas as the distance increases.
- 1/D2 (D is the distance from the source of radiation)
What are the 2 types of energy?
1- potential
2-kinetic
What does an atom consists of?
- Nucleus: protons and neutrons
- Electrons
How small is an electron?
Very small, approximately 1840X smaller than the size of a proton.
Where can electrons be found in an atom?
They are not part of the nucleus but orbit around it, in a number of electron shells/energy bands
In radiation what are the electron energy bands labelled as?
K, L, M
K being the innermost shell
What is the formula for the number of electrons in an energy band?
N = 2(n2)
N: number of electrons
n: band number
What is the rule of electrons filling energy bands?
The inner bands must be completely filled first. (first K then L then M)
What is a proton?
- Subatomic particle that is found in the nucleus of an atom
- It is large and determines the atomic number (element)
- Has a positive charge
- protons spin and each moving proton creates a magnetic field
- for each direction of movement there is an opposing proton
What is a neutron?
- subatomic particle that is found in the nucleus of an atom
- slightly smaller than a proton
- has no charge, is neutral
- in smaller elements there is an equal number of neutrons and protons. But as the elements get bigger there are more neutrons than protons present.
What are electrons?
- subatomic particles that orbit the nucleus of an atom in energy shells
- very small, about 1840X smaller than a proton
- it has a negative charge
- The electrons orbit the nucleus in all different directions, if they all moved in the same direction they would create a very strong magnetic field
How do electrons move?
In a sinosoidal form, this is how they maintain energy to stay within the energy level
What is needed to produce X rays?
1- two electrical supplies (one low voltage, one high voltage)
2- perfect vacuum (evacuated glass tube)
3-a means of converting kinetic energy of movement into kinetic energy of electromagnetic radiation
Why is an image fixed?
What 2 functions does the fixer have?
-To make the image permanent and the protect it from damage
Functions:
1-The fixer dissolves the unexposed silver bromide crystals so no further exposure can take place: image is made permanent
2- The fixer hardens the emulsion to protect it and prevent scratching during storage
-after fixing, the image is washed and dried
What does the developer do?
How is an image developed?
-The developer is a reducing agent: it donates electrons
1- The crystals in the film emulsion are surrounded by a negative charge shield, this repels the electrons of the developer.
2- After x-ray exposure, the crystals with dissociated molecules will have a defect in this negative charge shield. This defect is known as the sensitivity spec. This will now allow electrons from the developer to enter the crystals.
3- The developer only enters exposed crystals and donates electrons.
4-The donated electrons exert a force of attraction on the positive charged silver atoms, causing them to clump together and this darkens the exposed area.
can developer enter unexposed crystals?
No, only exposed crystals
what is a latent image?
It is an image that already exists, but is invisible.
It needs to be developed.
How is a latent image formed?
What happens to the silver-bromide crystals?
- Upon interaction with an x-ray, the silver or bromine atom of the crystal is ionised and this weakens the bonds between silver and bromine causing them to dissociate.
- The number of molecules dissociating is proportional to the number of x-ray photons hitting that part of the emulsion. The more x-rays hit the emulsion, the more particles dissociate.
What is the active part of the x-ray film made of?
It is a suspension of silver halide crystals in a gelatinous base (called the emulsion).
The halide used is silver-bromide (AgBr) this is light sensitive.
What is a molecule?
Two or more atoms joined together
What is the valency band?
- The last permanently occupied band. (The outermost electron band)
- This governs the chemical properties of the atom, if you change the number of electrons in the valency band, you change the chemical properties of the atom
- The electrons in this band make/break connections with other atoms
What does an X-ray film pack consist of?
1- outer cover: water and light proof
2- black card: primary function is to add physical strength, so card doesn’t bend in patients mouth. Secondary function is to absorb light.
3- lead foil: to maintain a clear image by preventing any ‘backscatter’ from other tissues (tongue, teeth) affecting the image
4- film with photographic emulsion: this produces the image.
What is the smallest amount of a compound?
molecule
what is the smallest amount of an element?
atom
What does an overexposed radiograph look like?
It is very dark (opposite to photography)
What is the radiographic density?
The degree of film darkening, or how black it is
What does high subject density mean?
What will it result in/look like?
give an example of the type of tissue
- It means the subject has absorbed a lot of radiation
- It results in low radiographic density of the image (it appears very white)
- example: enamel
What does low subject density mean?
What will it result in/look like?
give an example of the type of tissue
- It means the subject hasn’t absorbed a lot of radiation
- It results in high radiographic density of the image (it appears very black)
- example: pulp
what is radiographic contrast?
How will this be observed in a healthy tooth?
- The difference in density between areas.
- A healthy tooth will demonstrate good radiographic contrast
What is sharpness?
The level of fine detail that can be seen
What is radiography?
general term used to describe the actual production of the radiographic image
What is radiology?
the interpretation and diagnostic conclusions as to what the radiograph shows
What imaging steps are involved in producing radiograph images?
1- latent image formation
2- developing the latent image
3- fixing the image
What are the components of an X-ray machine?
What is their purpose?
1- Cathode filament (-ve): produces electrons
2- supply of low voltage (12V): causes electrons to gather around filament
3- supply of high voltage (65-70kV): accelerates electrons
4- evacuated glass tube (vacuum): removes obstructions
for electrons so they have a clear path
5-Anode block (+ve) This is a block of copper with a Tungsten target: converts KE of electrons into x-ray photons and heat
5- Lead shielding: absorbs radiation
6- aluminium filter: absorbs low energy photons
What is the purpose of the cathode filament in an X-ray machine?
To produce electrons
What is the cathode made of in an X-ray machine?
filament of metal with a high melting point
What happens at the cathode of the X-ray machine?
- A low voltage supply at the cathode produces a cloud of electrons around the metal filament.
- The filament becomes negatively charged
- When a high voltage is applied (70kV) the electrons accelerate towards the Tungsten target
What is thermionic emission?
Thermally induced flow of charge carriers (electrons) from a surface.
What controls the rate of x-ray production in an X-ray machine?
The filament current:
The more electrons per unit of time = the more x rays per unit of time
What happens to the electrons once they hit the tungsten target?
The electrons penetrate the surface of the tungsten target but in doing so they are slowed down.
The electrons lose a lot of kinetic energy because of this deceleration
What 2 types products are created from the kinetic energy lost by the electrons?
It is converted in:
- X ray photons
- heat
How much of the kinetic energy is converted into X ray photons?
only 1-5%
The process is not very efficient
The remaining energy produces heat
How high is the temperature created by the energy lost by the electrons at the Tungsten target?
2000°C
What is also attached to the Tungsten target?
A large copper block
Why is a copper block used at the target end of the X-ray machine?
- Good electrical conductor: as the electrons give up kinetic energy, they can move through the block of copper and continue the circuit
- Good thermal conductor
- Acts as a heat sink: copper has a high melting point and can quickly move the heat away from the Tungsten
How many photons does each dose of x-ray contain?
720 million photons
What is lead shielding used for?
to contain any stray electrons or x-ray photons
Why do we filter low energy photons?
- Because they are ionising and therefore harmful
- They can be absorbed by soft tissue
- It adds to the patient dose but has no diagnostic purposes
What happens to the electrons moving through the Tungsten target?
What would happen if one electron was closer to the nucleus than the other?
What would happen if one electron was further away from the nucleus than the other?
The negative charge of the electrons will be attracted to the positive charge of the nucleus in the Tungsten target
- The electron closer to the nucleus will experience a greater pull and be diverted through a tight curve, giving up more of its’ kinetic energy
- The electron further from the nucleus will experience a weaker pull and be diverted through a gentle curve, losing a little of its’ kinetic energy
Will all x-ray photons produced have the same amount of energy?
No, not all the electrons will give up all their kinetic energy- it depends on how much deceleration they experienced.
Instead, a range of energies is produced
What is the highest energy an x-ray photon can have?
The same as the high voltage energy applied: 70kV
What are the 6 stages in full processing of radiographs?
1- Development 2- Rinse 3- Fixing 4- Rinse 5- Wash 6- Dry
Why is automatic processing of radiographs quicker than manual?
- The chemical concentration is higher than manual
- The temperature is higher than manual
What are the 7 geometry conditions to produce an accurate radiographic image?
1- X ray photons should be emitted from an infinitely small point source
2- Object should be parallel to the image recording plate (film)
3- The object and the film should be as close to possible as each other (short OFD)
4- The focus should be as far from the object as possible (long FOD)
5- Focus should be as far from the film as possible (long FFD)
6- The central ray of the x-ray beam should be directed to the centre of the object and recording plate
7- The central ray should be perpendicular to the image recording plate
In imaging geometry, what is the focus?
The point of origin of the x-ray beam
What is the minimum requirement for the film to skin distance?
20cm
