The X-ray Tube

Question 1

What does the anode do? (simple answer)

Answer 1

Converts e- into X-rays and heat

Question 2

What does the filament do? (simple answer)

Answer 2

Heats up producing free e-

Question 3

What does the glass insert do? (simple answer)

Answer 3

Holds anode and filament, preserves vacuum

Question 4

What does the tube oil do? (simple answer)

Answer 4

Surrounds insert, helps to cool tube with convection current

Question 5

What does the tube housing do? (simple answer)

Answer 5

Contains oil and shields against extraneous radiation

Question 6

What does the tube port do? (simple answer)

Answer 6

Directs X-ray beam to patient

Question 7

How much energy is converted to X-rays?

Answer 7

99% heat, 1% X-rays

Question 8

What are some of the requirements of an X-ray tube?

Answer 8

Efficient
Reduce risk of damage from heat
Near a point source as possible
Adjust quality and intensity
Electrically safe

Question 9

What do both stationary and rotating anodes have in common?

Answer 9

Filament
Anode
Glass tube
Tube oil
Metal tube housing

Question 10

Explain the structure and materials of the anode

Answer 10

BLOCK AND TARGET
Block- copper, ability to store heat as it has a large mass
Target- converts electrons into X-rays, made of Tungsten

Question 11

Advantages of Tungsten in an anode

Answer 11

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

Question 12

Explain the Filament

Answer 12

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

Question 13

What does the vacuum do?

Answer 13

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

Question 14

What does the focusing cup do?

Answer 14

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

Question 15

What does the glass insert do?

Answer 15

Acts as an electrical insulator between filament and anode
Made from Pyrex glass as it withstands heat without damage
Preserves the internal vacuum

Question 16

What does tube oil do?

Answer 16

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

Question 17

What does tube housing do?

Answer 17

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

Question 18

What does the tube port do?

Answer 18

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

Question 19

Explain the rotating anode

Answer 19

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

Question 20

Explain the materials that comprise the rotating anode disc

Answer 20

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

Question 21

Explain the ‘line focus principle’

Answer 21

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

Question 22

Explain the anode stem

Answer 22

Connects the disc to the moving components
Made of Molybdenum (lightweight, reduces heat flow)
Narrow nature of stem also reduces heat flow

Question 23

How does the rotor work?

Answer 23

Copper rotor
Works by induction currents, rotor is surrounded by copper windings that have high electrical conductivity, enhancing the efficiency induction by stator windings

Question 24

How do stator windings work?

Answer 24

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

Question 25

Light beam diaphragm

Answer 25

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