Basic Physics

Question 1

What range of energy can be carried by x-ray photons?

Answer 1

12eV-124keV

Question 2

What are the two types of x-rays?

Answer 2

• hard
  
  • higher energies
  • able to penetrate human tissues
• soft
  
  • easily absorbed
  • lower energies

Question 3

What is the threshold energy for medical imaging?

Answer 3

• over 5keV

Question 4

What is meant by x-rays causing ionisation?

Answer 4

• they are able to displace electrons from atoms/molecules

Question 5

How are x-rays produced?

Answer 5

• electrons are fired at atoms at a high speed
• on collision the kinetic energy is converted to EM radiation and heat
• x-ray photons aimed at a subject

Question 6

What are the basic components of a dental x-ray unit?

Answer 6

• tubehead
• collimator
• positioning arm
• control panel
• circuitry

Question 7

What is current?

Answer 7

• flow of electric charge
  
  • movement of electrons
  • measured in amps
• direction
  
  • DC: direct current
    - unilateral
  • AC: alternating current
    - repeatedly reversing
    - cycles per second measured in Hertz (Hz)

Question 8

By what process is a DC current turned into an AC current?

Answer 8

• rectification
• x-ray units contain generators to modify AC

Question 9

What is voltage?

Answer 9

• difference in electrical potential between 2 points in an electrical field
  
  • related to how forcefully a charge will be pushed through a field
• measured in volts (V)

Question 10

What is the electrical supply to dental x-ray unit?

Answer 10

• mains supply
  
  • alternating current (<13 amps)
  • 220-240 volts
• rectified for unit
• requires two different voltages
  
  • high as >10,000 volts
  • low as 10 volts

Question 11

What are transformers?

Answer 11

alter the voltage and current from one circuit to another

Question 12

How many transformers are present in an x-ray unit and what are they for?

Answer 12

• 2 transformers
• mains
  
  • converted to be appropriate for x-ray tube
  • step up transformer
  • increases potential difference cross X-ray tube
    - 60-70kV
  • reduces current to milliamps
• mains
  
  • converted to be appropriate for filament
  • step down transformer
  • decreases potential difference across filament
  • ~10V
  • ~10A

Question 13

What is x-ray beam intensity?

Answer 13

• quantity of photon energy passing through a cross-sectional area of the beam per unit time

Question 14

How is x-ray beam intensity increased?

Answer 14

• increasing number of photons
• increasing energy of photons
• proportional to current in filament (number)
• proportional to voltage across X-ray tube (energy)

Question 15

What is the inverse square law?

Answer 15

• intensity of x-ray beam is inversely proportional to the square of the distance between the x-ray source and the point of measurement
• doubling the distance will quarter the dose

intensity ∝ 1/distance^2