1 - Physics basics

Question 1

What is a radiograph?

Answer 1

Image created by x-rays projected through an object and interacted with a receptor

Question 2

What are radiographs used for?

Answer 2

• visualising structures within the body, particularly mineralised structures
• aids diagnosis, treatment planning and monitoring

Question 3

What are the common intra-oral radiographs?

Answer 3

• periapical
• bitewing
• occlusal

Question 4

What are the common extra-oral radiographs?

Answer 4

• panoramic
• lateral cephalograms

Question 5

What are properties of EM radiation?

Answer 5

• no mass
• no charge
• travel at 3x10^8 m/s
• travel in a vacuum

Question 6

What is the EM spectrum?

Answer 6

• 7 groups with different properties based on energy, wavelength and frequency
• gamma ray, x-ray, UV, visible, infrared, microwave and radio

Question 7

What is frequency?

Answer 7

• how many times a waves shape repeats per unit time
• measured in hertz
• 1Hz = one cycle per second

Question 8

What is wavelength?

Answer 8

• distance over which the wave’s shape repeats
• measured in metres

Question 9

How is energy proportional to frequency?

Answer 9

• directly proportional
• higher frequency = higher energy

Question 10

How are frequency and wavelength related?

Answer 10

Higher frequency = shorter wavelength

Question 11

What are photons?

Answer 11

• “packets of energy”
• measured in electron volts, eV
• 1 eV = energy gained by an electron moving across a potential difference of 1 volt

Question 12

What are the different types of x-rays?

Answer 12

• photon energy varies between 124eV - 124keV
• hard x-rays (>5keV)
• soft x-rays

Question 13

Which type of x-rays are used in medical imaging?

Answer 13

Hard x-rays >5keV

Question 14

How are x-rays formed?

Answer 14

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

Question 15

What is an atom made up of?

Answer 15

• protons
• neutrons
• electrons

Question 16

What is the atomic number?

Answer 16

• Z
• number of protons
• unique to each element

Question 17

What is the mass number?

Answer 17

• A
• number of protons and neutrons

Question 18

What are the atomic and mass number of tungsten?

Answer 18

Z = 74
A = 184

Question 19

What is the ground state of an atom?

Answer 19

When number of electrons = protons

Question 20

What is ionisation?

Answer 20

Removing or adding of electrons to change the charge of an atom

Question 21

What is the innermost shell of an electron called?

Answer 21

K

Question 22

What is the maximum number of electrons in a K shell?

Answer 22

2

Question 23

What is the maximum number of electrons in a L shell?

Answer 23

8

Question 24

What is the maximum number of electrons in a M shell?

Answer 24

18

Question 25

What is the maximum number of electrons in a N shell?

Answer 25

32

Question 26

How do you calculate the maximum number of electrons in a shell?

Answer 26

2n^2

Question 27

How are electrons held in the shells?

Answer 27

- negative charge of electrons attracted to positive nucleus - to remove electrons, binding energy must be broken

Question 28

What is binding energy?

Answer 28

- energy required to exceed the electrostatic force of the attraction of the electrons to the nucleus - closer the electron is to the nucleus, the higher the binding energy

Question 29

What is the K shell binding energy of a tungsten atom?

Answer 29

69.5keV

Question 30

How does electrons moving between shells release energy?

Answer 30

- an electron moving to a more outer shell releases binding energy - energy released is equal to difference in energy shells

Question 31

What is current?

Answer 31

- flow of electric charge - movement of electrons - measured in amps, A

Question 32

What is direct current?

Answer 32

Constant unidirectional flow of electrons (eg batteries)

Question 33

What is alternating current?

Answer 33

- electron flow repeatedly reverse direction (eg mains) - number of complete cycles per unit time = frequency - main frequency = 50Hz

Question 34

What type of current is used in x-ray machines?

Answer 34

- DC - powered by AC source, therefore must be rectified

Question 35

What is voltage?

Answer 35

- difference in electrical potential between 2 points - how forcefully a charge is push through an electrical field - measured in volts, V

Question 36

What voltage is required in an x-ray machine?

Answer 36

- 2 different voltages - one as high as 10s of thousands of volts - one around 10V

Question 37

What is a transformer?

Answer 37

Alters voltage and current from one circuit to another

Question 38

What transformers are present in an x-ray machine?

Answer 38

- step up transformer, mains to X-ray tube (cathode/anode) - step down transformer, main to filament

Question 39

How do x-ray beams travel?

Answer 39

Photons travel in straight lines but diverge from the source

Question 40

What is the inverse square law?

Answer 40

- intensity of x-ray beam is inversely proportional to square of the distance between the source and point of measurement - "double the distance, quarter the dose"