Week 2 Flashcards
X-rays are produced when…
highly energetic electrons interact with matter;
there is a conversion of some of their kinetic E into electromagnetic radiation (99% of energy is lost, converted to heat)
A device that produces X-rays in the diagnostic energy range contains (4):
- an electron source
- an evacuated path for electron acceleration
- a target electrode
- an external power source to provide a high voltage to accelerate the electrons
X-ray tube insert contains: (2)
- the electron source
- the target within an evacuated glass or metal envelope
tube housing provides (2):
- protective radiation shielding
- cooling of the X-ray tube insert
what do X-ray beam filters do?
shape the X-ray energy spectrum
what do the Collimators do?
define the size and shape of the X-ray field
The X-ray generator (functions, 2):
- supplies the voltage to accelerate the electrons
- permits control of the X-ray beam characteristics
Characteristics of X-ray photons:
- well defined intensity
- penetrability
- spatial distribution
- photons = have ability to ionize other substances
- invisible
- can not be heard
- have no odour
- are not affected by electric or magnetic fields
ability of a photon to ionize substances means:
causing the atoms through which the photon passes to eject electrons from their electron shells
it accounts for imaging properties and their potential harmful effect
X-rays can be detected by (4):
- their ability to ionise other substances
- cause fluorescence
- give rise to colour changes in several substances (barium platinocyanide)
- produce changes which can be made visible in photographic film
An X-ray tube consists of two components:
- The insert which is evacuated and is where the X-rays are produced
- The tube shield which supports the insert and is responsible for electrical and radiation safety
The function of the X-ray tube is to: (6)
- Provide a beam of X-rays from as near a point source as possible (focus).
- Dissipate the heat produced effectively to prevent damage to the X-ray tube.
- Provide a consistent quality (kVp) and quantity (mAs) of radiation.
- Allow X-rays to emerge only from the window (port) of the housing of the tube.
- Provide an electrically safe environment for the practitioner.
- The tube is securely supported but capable of easy movement into any position and then
being maintained in that position.
Materials used in the construction of tube housing:
steel construction lined with lead (except port)
Materials used in the construction of port:
- plastic
or - Beryllium
Insulation between housing and insert:
mineral oil
Materials used in the construction of Insert (Tube envelope):
- metal or ceramics (modern),
- borosilicate glass
Materials used in the construction of Filament (cathode) assembly/focusing cup:
- nickel
or - stainless steel
Materials used in the construction of Filament:
tungsten
Materials used in the construction of Anode disc:
- molybdenum alloy
- graphite disc
- tungsten (90 per cent) and rhenium (10 per cent) alloy (prevents crazing effect)
Materials used in the construction of Anode stem:
molybdenum
Materials used in the construction of stator windings:
copper
Materials used in the construction of Additional filtration:
Aluminium and copper
Insert of X-ray tube functions (7):
- maintains a vacuum f/ X-ray production
- contains the anode assembly and cathode assembly fixed within the envelope, which supports these two electrodes in correct alignment at the correct distance
- metal components can be earthed so that there is no build-up of static
- All seals and metal poles are carefully chosen to match the expansion coefficients (heat) of the different parts, which reduces the risk of damage to the insert during operation
ANODE ASSEMBLY
The anode assembly consists of:
- Anode disc and focal track.
- Anode molybdenum stem (has a small cross-section and is as long as possible to restrict the conduction of heat to the bearing assembly).
- Copper rotor assembly (rotors use induction to rotate the anode at speeds of 3000–9000 r.p.m. during exposures).
- HT connection for the positive side of the tube circuit
rotating anode consists of a tungsten rhenium disc, which is typically 90–150 mm in
diameter (A large rotating disc increases tube rating and thermal capacity)
