Physics Basics And X Ray Production

Question 1

What is binding energy?

Answer 1

Additional energy required to exceed electrostatic force (remove an electron from its shell)
This increases as you get closer to the nucleus

Question 2

What are the 5 elements of a dental x ray unit?

Answer 2

Tube head
Collimator
Positioning arm
Control panel
Circuitry

Question 3

What type of currents do X ray units require and how is this achieved?

Answer 3

X ray units require a unidirectional current so have generators which modify the AC to mimic a constant DC (rectification).

Question 4

What is the purpose of a transformer in an X ray unit?

Answer 4

Alters the voltage and current from one circuit to another.
One alters mains -x ray tube (step up)
Another alters mains - filament (step down)

(Need 2 different voltages)

Question 5

What is the inverse square law?

Answer 5

Intesity of the x ray beam is inversely proportional to the square of the distance between the x ray source and point of measurement

Eg. Doubling the distance will quarter the dose

Question 6

What are the components of the x ray tube?

Answer 6

Glass envelope- vacuum inside to prevent entry of air, leaded glass absorbs photons travelling in undesired direction
Cathode (+ve) - filament and focussing cup
Anode (-ve) - target and heat dissipating block

Question 7

What is the filament made of?

Answer 7

Tungsten

Question 8

What is the focusing cup made of?

Answer 8

Molybdenum

Question 9

What is the relationship between cathode and anode?

Answer 9

Cathode is negatively charged, electrons are repelled away from cathode and attracted to anode- hit target with high kinetic energy

Question 10

What is the anode made of?

Answer 10

Tungsten

Question 11

What is the heat dissipating block made of?

Answer 11

Copper

Question 12

Why is the target at an angle to the filament?

Answer 12

Increases the actual surface area where electrons impact
Reduces the apparent surface area from where the x ray beam is emitted (reduced penumbra effect- slight blurring due to more than one focal point)

Question 13

What is the purpose of aluminium filtration and what is the minimum thickness required?

Answer 13

Removes lower energy x rays (non-diagnostic) from the beam (this would increase pt dose with no added effect)
<70 kV- 1.5mm

Question 14

What is the purpose of a spacer cone?

Answer 14

Dictates the distance between focal spot and patient (focus to skin distance- fsd)
This is usually 200mm

Question 15

What is a collimator?

Answer 15

Lead diaphragm attached to end of spacer to reduce patient dose (crops beam to match size and shape of receptor)

Use rectangular with size 2 receptors

Question 16

Why is rectangular collimation recommended?

Answer 16

Can reduce patient dose by approx 50%

Question 17

What are the 2 interactions taking place at the tungsten target?

Answer 17

Heat production - bombarding electrons reach tungsten outer electrons and are deflected. This loss of kinetic energy produces heat which is dissipated out of the tube. This is 99% of the interactions

X ray production - continuous radiation interactions (mostly) and characteristic radiation interactions

Question 18

What is continuous radiation?

Answer 18

Bombarding electrons pass close to target nucleares, causing it to be rapidly decelerated and deflected (lost kinetic energy released as X ray photons) - the amount of energy of the x ray photons depends on how close it came to the electron.

Eg. If collides directly with electron - 70KeV photon produced.

Some are of low energy and are removed via aluminium filter

Question 19

What is characteristic radiation?

Answer 19

Bombarding electron collides with inner shell electron and either displaces it into more peripheral shell (excitation) or removes it completely (ionisation).
Remaining orbiting electrons rearrange themselves to fill space - when an electron drops to a lower shell, it loses energy (difference in binding energy) which is emitted as a photon of specific energy (depends on element)