physics

Question 1

What is a photon?

Answer 1

Packet of energy
Single particle of light/x ray
X-ray beam made up of many photons

Question 2

What atomic physics should be known?

Answer 2

Transition of electron to higher energy orbit= photon input

Transition of electron to lower energy orbit= emitted photon

Question 3

What is ionisation?

Answer 3

If an electron has enough energy it can be freed from the pull of the nucleus= positive ion and a free electron

Question 4

What is electromagnetic radiation?

Answer 4

Light, UV, microwave, radio waves, X-rays and gamma rays

Not charged
Beams of many single photons
Travels at speed of light
Energy determines frequency and wavelengths
E=hv 
(h is Planck constant)
(v is frequency)

Question 5

What is particle radiation?

Answer 5

Alpha and beta particles from radioactive decay
Many particles are charged
Beams on many individual particles
Speed and energy dependent on initial emission conditions

Question 6

What are alpha particles?

Answer 6

2 neutrons and 2 protons (helium nucleus w charge of 2+)
Unstable nucleus can emit this to reduce the proton no and become more stable

Relatively high mass and charge= highly ionising
Stopped by ~10mm air or less than mm of Aluminium

Question 7

What are beta particles?

Answer 7

Electron
Unstable nucleus ejects electron by internal conversion or when neutron converts to proton and electron
(n—>p + e- + y)
Emitted particle= -1 charge
Highly ionising and faster
More penetrating- few mm of Aluminium and metres of air

Question 8

What is the electromagnetic spectrum?

Answer 8

As energy increases, frequency increases but wavelength decreases

Question 9

What are gamma rays?

Answer 9

Excited nucleus emits gamma ray in order to return to ground state

Alpha/beta p. emit gamma rays- daughter (Y) nucleus produces are in excited states as a result of decay

No charge
Less ionising
Can be v penetrating- 100mm lead

Question 10

What is the difference between x rays and gamma rays?

Answer 10

Identical except manner of production

Question 11

What is an X-ray?

Answer 11

Produced by accelerating electrons towards metal target in high voltage, evacuated tubes

Stopped by 1-2mm lead

2 processes- Bremsstrahlung (braking radiation) and characteristic formation

Question 12

What is a diagnostic X-ray tube?

Answer 12

Generates a lot of heat
Anode needs high Z to to absorb electrons and high melting point

Tungsten melting point= 3410 degrees
Tungsten atomic/proton no= 74

Question 13

What is the glass envelope for?

Answer 13

Keeps vacuum in (generated by electrons interaction w air)

Question 14

What is the copper block for?

Answer 14

Transmits heat away from target so it doesn’t melt and crack

Question 15

What is the focusing device for?

Answer 15

Electrons are focused into small area before they’re accelerated

Question 16

What is the lead shield for?

Answer 16

Protection

Question 17

What is rectification?

Answer 17

Stable power supply- maximum intensity and quality
If not- lower quality beam
(Reduced by ripple)

Question 18

What is Bremsstrahlung (braking radiation?

Answer 18

Continuous spectrum produced by rapid deceleration of electrons passing close to target nuclei
(Loses some energy [photon])

High no of photons w low energy
Low no of photons w high energy

Question 19

What is characteristic X-ray formation?

Answer 19

Electron is knocked out of K shell leaving gap
L shell electron falls into gap and emits energy as an X-ray photon

Energy of photon emitted= (energy of electron in L shell) - (energy electron now has in K shell)

Question 20

How are radiations compared via spectra?

Answer 20

Line spectra- alpha/beta particles (nuclear medicine)

Continuous spectra- x rays

Question 21

How are radiations compared via fluorescence on a phosphor?

Answer 21

Alpha- yes

Beta- less

Gamma- v little but observed w some eg. Sodium iodine

X-ray- yes

Question 22

How are radiations compared via speed?

Answer 22

Alpha- ~x10^7m/s

Beta- ~x10^8m/s but variable

Gamma- 3x10^8m/s

X-ray- 3x10^8m/s

Question 23

How are radiations compared via no of ion pairs per mm in air?

Answer 23

Alpha- ~10^4

Beta- 10^2

Gamma- ~1

X-ray- <1

Question 24

What is the tube current (mA)?

Answer 24

No of electrons hitting target
Intensity proportional to mA
Beam quality unaffected
Shape of spectrum unaltered

Question 25

What is the tube voltage (kVp)

Answer 25

Peak tube voltage= kVp- maximum photon energy Voltage between cathode and anode (60-70kV) Intensity is proportional to kVp^2 Beam quality is increased

Question 26

What is the target atomic number?

Answer 26

Intensity proportional to Z | Beam quality is unaffected

Question 27

What is filtration?

Answer 27

Intensity decreases w filtration Beam quality increases w filtration Removes low energy radiation

Question 28

What is attenuation?

Answer 28

I=I0e^-ut ``` u= linear attenuation coefficient (higher for lower energy photons) t= kg/m^2 u/p= mass attenuation coefficient, m^2kg^-1 ``` X-ray beams are polyenergetic

Question 29

What is filtration?

Answer 29

Result of attenuation Beam hardening Low energy photons- no use in image formation but contribute to patient dose Legal requirement- 1.5mm Aluminium

Question 30

What is photoelectric absorption in tissue?

Answer 30

Photon interacts w tightly bound inner electron Photons energy absorbed If energy absorbed higher than electron binding= electron emitted (photo electron) Gap in shell filled by electron from higher shell- emits characteristic radiation

Question 31

What is Compton scatter absorption in tissue?

Answer 31

``` If binding energy small= ignored As binding energy increases, interaction w whole atoms Scattering Fogging on film Patient receives dose No energy lost Wavelength remains same ```

Question 32

What is the appropriate kV?

Answer 32

Most tissue similar density and Z so low kV optimises contrast Transmission increases w kV so less mA or time which decreases patient dose So need compromise between contrast and dose