The X-ray Tube Flashcards
What does the anode do? (simple answer)
Converts e- into X-rays and heat
What does the filament do? (simple answer)
Heats up producing free e-
What does the glass insert do? (simple answer)
Holds anode and filament, preserves vacuum
What does the tube oil do? (simple answer)
Surrounds insert, helps to cool tube with convection current
What does the tube housing do? (simple answer)
Contains oil and shields against extraneous radiation
What does the tube port do? (simple answer)
Directs X-ray beam to patient
How much energy is converted to X-rays?
99% heat, 1% X-rays
What are some of the requirements of an X-ray tube?
Efficient
Reduce risk of damage from heat
Near a point source as possible
Adjust quality and intensity
Electrically safe
What do both stationary and rotating anodes have in common?
Filament
Anode
Glass tube
Tube oil
Metal tube housing
Explain the structure and materials of the anode
BLOCK AND TARGET
Block- copper, ability to store heat as it has a large mass
Target- converts electrons into X-rays, made of Tungsten
Advantages of Tungsten in an anode
High atomic number of 74 means it makes it more efficient at producing X-rays
Reasonable heat conduction
High melting point of 3,420 degrees Celsius
Low vapour point of 5000 kPa
Malleable
Explain the Filament
Negative terminal provides electrons
Consists of coiled Tungsten filament wire
Surrounded by a Nickel focusing cap (Nickel doesn’t release free electrons)
Filament wires release electrons during thermionic emission (filament becomes hot and electrons escape from the surface and float in a cloud around the wire)
Most X-ray tubes have 2 filaments…broad and fine focus
What does the vacuum do?
No gas molecules present so it stops ionisation during the X-ray process. Stops thermionic electrons colliding with gas electrons to stop wear and breakdown
What does the focusing cup do?
Achieves a point source of X-ray to minimise an un-sharpness
Prevents electron stream from diverging as similar charges repel
The edges of the focusing cup have a higher negative electrical charge
What does the glass insert do?
Acts as an electrical insulator between filament and anode
Made from Pyrex glass as it withstands heat without damage
Preserves the internal vacuum
What does tube oil do?
Surrounds glass insert
Provides means of cooling as heat builds up on the glass by convection currents
On higher output tubes, the oil may be pumped through a heat exchanger to provide a more effective cooling mechanism
What does tube housing do?
Surrounds glass insert
Contains tube oil
Where high tension cables plug into to connect x-ray tube to generator
Has a conductive sheath which provides a safe and efficient connection to earth
Encloses the glass insert in a lead lining creating an effective radiation barrier
What does the tube port do?
Directs X-ray beams towards patient
An area of low beam attenuation provided by a cup shaped plastic
Photons can lose energy as they pass through the port
Explain the rotating anode
Rotating anode tubes offer higher tube loadings (focal track, mass of anode disc and speed of rotation)
Shorter exposure times, higher kV and mA selections and multiple exposures
Spreads heat over a larger surface area
Typically 3500 rotations per minute
TWO filaments offset from midline, broad/fine focus
More durable
Explain the materials that comprise the rotating anode disc
90% Tungsten and 10% Rhenium alloy to reduce crazing as Rhenium has little effect on X-ray conversion
Large surface area improves heat loss
Molybdenum has a lower density to avoid excess movement whilst it spins (thinner anode stem can be used)
Graphite coating reduces friction
Explain the ‘line focus principle’
To have both as small target as possible to have sharpness whilst having a larger target to evenly distribute heat to avoid damage.
Achieved by angling the anode, which can be adjusted to determine the overall size of the X-ray beam
Explain the anode stem
Connects the disc to the moving components
Made of Molybdenum (lightweight, reduces heat flow)
Narrow nature of stem also reduces heat flow
How does the rotor work?
Copper rotor
Works by induction currents, rotor is surrounded by copper windings that have high electrical conductivity, enhancing the efficiency induction by stator windings
How do stator windings work?
Act as a pair at 90degrees to one another, supplied from an alternating current source passing through coils creating a rotating magnetic field which in turn induces currents into the rotor
Light beam diaphragm
Attaches around tube port, contains 2 pairs of lead sheets at 90 degrees of one another which can change size and shape X-ray beam
Also has cross hairs to allow accurate centring of area of interest
