X-Ray Production

Question 1

How is an X-ray taken?

Answer 1

Electrons are accelerated towards the atoms at a very high speed
On collision, the kinetic energy of these electrons is converted to heat and electromagnetic radiation (X-rays)
The X-ray photons are aimed at a subject

Question 2

What are the components of an X-ray Tube?

Answer 2

Cathode (-ve)
–Filament
–Focusing Cup
Anode (+ve)
–Target
–Heat-dissipating block
Glass Envelope
–Vacuum inside

Question 3

What is a Cathode? What does it do?

Answer 3

Coiled metal wire
Low-voltage, high-current electricity passed through the wire
– heats up until incandescent and electrons are released from atoms in the wire by thermionic emission

Question 4

What is the Cathode filament made of? Why?

Answer 4

Tungsten
High atomic number, lots of electrons
High melting point

Question 5

What is the Cathode focusing cup made of? Why?

Answer 5

Molybdenum
High melting point

Question 6

Why is the focusing cup in the cathode negatively charged?

Answer 6

So it can repel the electrons towards the anode from those released by the filament

Question 7

What is the cathode-anode relationship?

Answer 7

Electrons are repelled from the cathode & attracted to the anode, they accelerate at a high speed and therefore have KINETIC ENERGY when colliding with the anode target

Question 8

What are electron volts?

Answer 8

Unit used to measure the kinetic energy gained by electrons as they pass from cathode to anode

Question 9

What is the Focal Spot?

Answer 9

Precise area on target where electrons collide & X-rays are produced (i.e. the X-ray source)

Question 10

What is the anode target made from? What does this produce?

Answer 10

Tungsten
Produces photons and heat when the block is bombarded with electrons

Question 11

What is the anode heat dissipating block made from?
What is embedded in this?
What is the purpose of this?

Answer 11

Copper
The target is embedded in a larger block of metal
Heat produced in the target dissipated into this block by thermal conduction and reduces risk of overheating (as the copper has high thermal conductivity)

Question 12

What is the Penumbra Effect?
How can you minimise this?

Answer 12

Blurring of radiographic images due to focal spot not being a single point but rather a small area
Minimised by shrinking the size of the focal spot

Question 13

How can we reduce the size of the focal spot?

Answer 13

Focal spot angulation

Question 14

What is Focal Spot Angulation?

Answer 14

Use an angled target (in the anode) to increase the surface area where electrons impact and reduce the apparent surface from where the X-ray beam is emitted (lowers the penumbra effect)

Question 15

What is the Glass Envelope?
What is it purpose?

Answer 15

It is an air tight enclosure to support the cathode and anode and maintain a vacuum
It is leaded glass to absorb X-ray photons except for an un-leaded window which only the X-ray photons travelling in the desired direction can escape through

Question 16

What surrounds the entirety of the X-ray tube?
What is the purpose of this?

Answer 16

Oil
Dissipates the heat produced by the X-ray tube by thermal convection

Question 17

What does the X-ray pass through as it is leaving the tubehead? Thickness of this?
What is the purpose of this?

Answer 17

Aluminium Filtration– 1.5mm for 70kV
Removes low energy (non-diagnostic) X-rays from the beam as these would be fully absorbed by the patient’s tissues and increase the dose but they would not contribute to the image
Results in the X-ray beam containing mostly diagnostic X-ray photons

Question 18

What is the Space Cone for?
How does the positioning of this affect the X-ray photons?

Answer 18

It dictates the distance between the focal spot (of target) & patient
If it is closer to the focal spot there is more divergence of the X-ray photons
If it is further away from the focal spot there is less divergence of the X-ray photons
A set distance helps ensure a consistent radiographic technique

Question 19

What is the Focus to Skin Distance for over 60kV?

Answer 19

200m

Question 20

What is the Collimator?
What is it for?

Answer 20

Lead diaphragm attached to the end of the spacer cone
It reduces the patients dose
–By cropping the X-ray beam to match the size & shape of X-ray receptor

Question 21

What does the rectangular collimation reduce?
What is a side effect of this?

Answer 21

Can reduce the effective dose to the patient by approximately 50%
However, can increase the chance of collimation errors (this is minimised with good radiographic technique)

Question 22

What is Continuous Radiation?

Answer 22

Produces a continuous range of X-ray photon energies
Maximum photon energy matches the peak voltage
Bombarding electron interacts with nucleus of target atoms

Question 23

What is Characteristic Radiation?

Answer 23

Produces specific energies of X-ray photon, characteristic to the element used for the target
Photon energies depend on the binding energies of electron shells
Bombarding electron interacts with inner-shell electrons of target atom

Question 24

Summary of the Beam leaving Dental X-Ray unit.

Answer 24

Focused stream of X-ray photons going in the same direction
–photons are diverging but near-parallel
There is a range of photon energies present
–lower energy, non-diagnostic photons have been removed by filtration
Photons in X-ray beam travel in a straight line at the speed of light until they interact with something