X-ray Production

Question 1

3 stages of X-ray production

Answer 1

1. Electrons accelerated towards atom at very high speed
2. On collision, kinetic energy of electrons converted to heat and EM radiation (X-ray photons)
3. X-ray photons aimed at subject

Question 2

Main components of X-ray unit

Answer 2

Tubehead, collimator (cap to aim X-ray beam), positioning arm, control panel and circuitry

Question 3

Components of X-ray tube

Answer 3

Glass envelope (vaccum inside)
Cathode (negative)
Anode (positive)

Question 4

Describe 2 main parts of cathode (negatively charged side of X-ray tube)

Answer 4

Filament- Coiled metal wire points toward anode, heats up until thermionic emission where electrons are released. Made of Tungsten
Focusing cup- metal plate around filament. Negatively charged to focus electrons to anode. Made of molybdenum.

Question 5

Describe cathode-anode relationship

Answer 5

High voltage passes through, electrons released at filament are repelled, attracted to anode creating big potential difference.
Greater PD, greater acceleration, greater kinetic energy

Question 6

How does the X-ray tube get the voltage it needs

Answer 6

Transformers take mains electrical supply (220-240V) and convert by changing voltage and current

Question 7

Name two types of transformer in X-ray tube head

Answer 7

Step up- takes mains and increases PD across X-RAY TUBE to 60-70 thousand volts
Step down- Decreases PD across FILAMENT to about 10V

Question 8

Define electron volts (eV)

Answer 8

1 eV= Kinetic energy gained by one electron moving across PD, from cathode to anode, of 1V
(PD is 70kV, each electron will have 70keV of kinetic energy)

Question 9

Main features of target of anode

Answer 9

Metal block bombarded by electrons
Produces photons and heat
Made of tungsten (high melting point)
Off angle ( not facing cathode straight on)
Has focal spot (made by focusing cup) where electrons hit and X-RAYS ARE PRODUCED

Question 10

Describe heat-dissipating block of anode

Answer 10

Where target is embedded
Heat produced by target dissipates into block by thermal conduction (REDUCES RISK OF OVERHEATING)
Made of copper (high melting point and thermal conductivity)

Question 11

What is penumbra effect

Answer 11

Blurring of radiograph due to focal spot not being a single point but rather a small area
Small focal spot= reduced penumbra effect

Question 12

What is the adaptation within the X-ray tube to reduce penumbra effect

Answer 12

Angled target of anode

Question 13

Main features of glass envelope

Answer 13

Air tight-contains cathode and anode
Maintains a vaccum
Made of leaded glass
APART from un-leaded window-> X-ray photons travel in desired direction

Question 14

X-ray tubehead main features

Answer 14

Metal (Lead) shielding, absorbs X-rays -> window where X-ray beam exits
Aluminium filtration
Oil surrounds X-ray tube for dissipation
Space cone

Question 15

Need for aluminium filtration?

Answer 15

Remove lower energy X-ray beams (low energy photons increase dose but do not contribute to image)

Question 16

Minimum thickness of aluminium required for aluminium filtration

Answer 16

<70kV= 1.5mm
>70kV= 2.5mm
N.B most modern equipment operates from 60-70kV

Question 17

Function of spacer cone

Answer 17

Maintains distance between focal spot and patient
Known as FOCUS TO SKIN DISTANCE
Also indicates direction of beam

Question 18

Focus to skin distance requirements

Answer 18

< 60 kV 100mm
>60 kV 200mm (MODERN EQUIPMENT)
Usually dot on X-ray tube head to indicate focal point

Question 19

3 fates of X-ray photons in xray tubehead

Answer 19

Attenuated by lead shielding (of glass, tube head, spacer cone)
Attenuated by aluminium filtration (too low energy)
Exit tubehead to form X-ray beam

Question 20

Main features of Collimator

Answer 20

Lead attached to end of spacer cone
Reduces pt dose
Crops X-ray beam to match shape and size of receptor
Standard in new equipment and fitted in old

Question 21

FUNCTION of collimator

Answer 21

Reduces pt dose up to 50% of original area
Changes from 60mm diameter circular beam to 45x35mm rectangular beam (size 2 receptor)
Also reduces SCATTER

Question 22

Features of control panel

Answer 22

On/off
Timer
Exposure time selector and presets
Warning light/noise
Kilovoltage selector

Question 23

What are the two X-ray producing electron interactions

Answer 23

Continuous radiation interactions (most)
Characteristic radiation interactions

Question 24

Describe continuous radiation interaction

Answer 24

Electron passes close to nucleus (tungsten nucleus in target of anode)
Electron decelerates
Kinetic energy lost
This is released as X-ray photons

Question 25

In continuous radiation interactions, why are there X-ray photons produced at different energies?

Answer 25

Closer to nucleus> more deceleration and deflection> more energy released> higher energy photon
MORE LOWER ENERGY PHOTONS AS MORE ELECTRONS DO NOT COME CLOSE TO NUCLEUS OF TUNGSTEN ATOM

Question 26

Describe characteristic radiation interaction

Answer 26

Electron from cathode displaces electron in tungsten atom or removes it completely
Electrons rearrange and as an electron drops from outer shell to inner shell, releases a photon of a specific energy