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Radiology > Physics Basics > Flashcards

Physics Basics Flashcards

1
Q

What type of radiation are x-rays?

A

electromagnetic

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2
Q

What are the properties of em radiation?

A
  • No mass
  • No charge
  • Always travels at “speed of light”
  • 3x108 ms-1 = 671 million mph
  • Can travel in a vacuum
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2
Q

What are the 7 types of em radation?

A
  • gamma ray
  • x-ray
  • ultraviolet
  • visible
  • infrared
  • microwave
  • radio
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3
Q

How is the wavelength calculated?

A

Divide distance over cycle

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4
Q

How is the frequency calculated?

A

Divided the cycles by the seconds

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5
Q

How is the frequency calculated?

A

how many times the wave’s shape repeats per unit time

measured in hertz Hz

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6
Q

What is the wavelength?

A

the distance over which the wave’s shape repeats

measured in metres, m

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7
Q

How is speed calculated and how does this affect other measurements?

A

Speed = Frequency x Wavelength
* BUT speed of all electromagnetic radiation is constant
* 3x108 ms-1
* Therefore if frequency increases then wavelength must decrease (& vice versa)

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8
Q

What are photons?

A

EM radiation involves the movement of energy as “packets of energy”

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9
Q

What is the energy usually measured in?

A

electron volts, eV

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10
Q

What is 1 ev?

A

energy (in joules) gained by 1 electron moving across a potential difference of 1 volt

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11
Q

What are the values for x-ray photon energies?

range

A

~124eV – 124keV

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12
Q

What are the types of x rays?

A
  • Hard X-rays (higher energies)
  • Able to penetrate human tissues
  • Soft X-rays (lower energies)
  • Easily absorbed
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13
Q

What type of x-rays does medical imaging use?

A

hard >5kev

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14
Q

What are the properties of x rays?

A
  • Form of electromagnetic radiation
  • No mass, no charge, very fast, can travel in a vacuum, etc.
  • Undetectable to human senses
  • Man-made
  • Note: gamma rays are identical except that they occur naturally (& generally have higher energies)
  • Cause ionisation
  • ie. displacement of electrons from atoms/molecules
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15
Q

How are X-rays produced?

A

electrons fired at atoms at very high speed

on collision the kinetic enegry of these electrons is converted to EM and heat

the x-ray photons are aimed at a subject

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16
Q

What is the structure of the atom (bohr)?

A

Atoms are the “building blocks” of matter
* Consist of:
* Central nucleus
* Protons (+ve charge)
* Neutrons (neutral)
* Orbiting “shells”
* Electrons (-ve charge)

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17
Q

What is the relative charge, mass and location of neutrons, protons, and electrons?

A

neutron = 0, 1, nucleus
proton = +1, 1, nucleus
electron = -1, 0, shells

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18
Q

What is the atomic number and what is the mass number?

A
  • Atomic Number (Z) = number of protons
  • Unique to each element
  • Mass Number (A) = number of protons + neutrons
19
Q

What does the number of electrons equal in a ground state atom?

A

number of electrons = number of protons

20
Q

What is ionisation?

A

Ionisation = removing/adding electron(s) to an atom
* Atom - e- → positive ion
* Atom + e- → negative ion

21
Q

What is the innermost shell called?

A

K

22
Q

What are all the shells from innermost?

A

k, l, m, n, o

23
Q

What is the max number each shell can hold?

A
  • K=2, L=8, M=18, N=32
24
Q

What force holds orbiting electrons?

A

electrostatic force
-ve charge of electrons attracted to overall +ve charge of nucleus

25
Q

What is the binding energy?

A

To remove an electron from its shell, a specific amount of energy is required to overcome this attraction
* Binding energy = additional energy required to exceed electrostatic force

26
Q

What shells have the strongest binding energies?

A

The closer the electron is to the nucleus, the greater the electrostatic force (& therefore binding energy)
* K shell electrons have the highest binding energies
* Then L, then M,

27
Q

What increases the electrostatic force?

A

the more positively-charged the nucleus, the greater the electrostatic force

28
Q

What is the energy required to move an electron to a more outer shell equal to?

A

The specific amount of energy required to move an electron to a more outer shell (i.e. away from the nucleus) equals the difference in the binding energies of the 2 shells

conversely if an electron drops to a more inner shell then this specific amount of energy is released

29
Q

What are the components of an x-ray unit?

A

Tubehead
* Collimator
* Positioning arm
* Control panel
* Circuitry

30
Q

What is current and what is it measured in?

A
  • Flow of electric charge, usually by the movement of electrons
    *SI unit: amp (or ampere), A
  • Measure of how much charge flows past a point per second
31
Q

What are the two directions of current?

A
  • Direct current (DC) = constant unidirectional flow
  • e.g. batteries
  • Alternating current (AC) = flow repeatedly reverses direction
32
Q

What is AC?

How is the freqency found?

A
  • Flow periodically reverses direction
  • Number of complete cycles (reverse + reverse-back)
    per unit time is the frequency
  • SI unit: hertz, Hz (cycles per second)
33
Q

What current do x-rays require and what are they powered by?

A

X-ray production requires a unidirectional current
* But X-ray units are powered by mains electricity (AC)

34
Q

How is the AC current in x-ray machines rectified?

A

X-ray units have generators which modify the AC so that it mimics a constant DC
* Process known as rectification

35
Q

What is voltage and what is it related to?

A

Difference in electrical potential between 2 points in an electrical field
* Related to how forcefully a charge will be pushed through an electrical field
* SI unit: volt, V

36
Q

What is the voltage and current of mains supply?

A

Alternating current (≤13 amps)
* 220-240 volts

37
Q

What voltages does a dental x ray unit require?

A

One as high as 10s of thousands of volts

One as low as around 10 volts

38
Q

What do transformers do?

A

alter the voltage (& current) from one circuit to another

39
Q

What are the two seperate transformers required for x-ray unit?

A

x-ray tube (step up)
filament (step down)

40
Q

What is the step up transformer?

A

↑ potential difference across X-ray tube
* Usually 60,000-70,000 volts (60-70 kV)
* Current reduced to milliamps (mA)

41
Q

What is the step down transformer?

A
  • Step-down transformer
  • ↓ potential difference across filament
  • ~10 volts
  • ~10 amps
42
Q

How do photons travel from the x-ray beam?

A

Photons effectively travel in straight lines but diverge from the X-ray source (i.e. do not travel in parallel)

43
Q

What is the x-ray beam intensity?

A

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

44
Q

What is the x ray beam intensity proportional to?

A

current in filament (mA) & potential difference across X-ray tube (kV)

↑ number &/or energy of photons = ↑ intensity

45
Q

What is the inverse square law?

A

Intensity of X-ray beam is inversely proportional to the square of the distance between the X-ray source & the point of measurement

  • Intensity ∝ 1/distance2
  • Therefore, doubling the distance will quarter the dose
46
Q

What are alpha, beta and gamma particles produced by and what are x-rays produced by?

A

All produced by radioactive decay of unstable atoms

  • Unlike X-rays which are directly man-made

Radiology (15 decks)

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