MV photon production

Question 1

what are the three challenges at high energy

Answer 1

1) electron production- need high initial energy electrons
2) electron acceleration- high MV p.d is dangerous
3) x-ray production- most xrays are produced in forward direction

Question 2

what is used to generate electrons and provide some initial acceleration

Answer 2

electron gun

Question 3

what does acceleration of these electrons use

Answer 3

positive potential difference (like kV tube)

Question 4

how does the electron gun work (3 points)

Answer 4

1) thermionic emission- beam of electrons which repel eachother
2) focussing uses a ring of high negative potential to compress the beam
3) high positive charge (anode) ‘pulls’ electrons to inject them into the waveguide

Question 5

how does electron acceleration work (4 points)

Answer 5

1) final energy required is 6 to 20 MeV
2) kilovoltage acceleration technique would cause arcing and danger
3) you NEVER get 6MV of potential difference
4) Instead we use waveguides

Question 6

what is 1MV

Answer 6

a MegaVolt which is a potential difference.

MV is NOT a unit of energy

Question 7

what is 1MeV

Answer 7

Mega Electron-Volt.
This is the amount of energy an electron gains in 1MV of potential difference.
MeV IS a unit of energy

Question 8

what is a waveguide

Answer 8

rectangular cross-section pipe acts as a motorway for electromagnetic waves

Question 9

what does the accelerating waveguide have

Answer 9

a unique internal structure that changes how these waves are contained

Question 10

what is the accelerating wave-guide made of (2 things)

Answer 10

1) hollow tube (round or square) made from conducting material e.g. copper
- must be efficient at transmitting the waves with minimal heat loss
2) contains internal irises of different apertures and spaced at varying differences

Question 11

how does the waveguide acceleration work (2 points)

Answer 11

1) microwaves pass through the waveguide and the electrons ‘hitch a ride’ on these
2) electrons gain kinetic energy safely- as much as from a MV potential difference

Question 12

kilovoltage x-ray distribution

Answer 12

1) x-rays are generated in all directions but with high intensity perpendicular to the incoming electrons
2) target is used is a ‘reflection target’ where photons appear to be reflected out of the target at right angles

Question 13

Megavoltage x-ray distribution

Answer 13

1) x-rays are distributed predominantly in the forward direction
2) a reflection target would be inefficient
3) a transmission target is used

Question 14

what is a transmission target

Answer 14

• 6mm thick ‘foil’

- Gold or tungsten

Question 15

4 key components of the linear accelerator

Answer 15

1) stand
2) gantry
3) head
4) couch

Question 16

3 functions of the stand

Answer 16

1) supports the gantry and allows rotation- complex series of motors and conducting rings
2) generates microwaves for acceleration- passes them to the gantry via a waveguide
3) contains the water cooling system

Question 17

4 functions of the Gantry

Answer 17

1) rotates to direct the x-rays
2) contains the electron gun
3) houses the acceleration waveguide
4) bending magnet- turns the electron beam through 90 degrees

Question 18

3 functions of the head

Answer 18

1) contains the targets and filters- can have multiple energies and modalities
2) collimation structure
- primary cone-shaped collimator
- moving ‘jaws’ of tunsten or steel (secondary)
3) houses detection and dosimetry systems

Question 19

4 functions of the couch

Answer 19

1) ‘isocentrically’ mounted- rotates around the same axis as the treatment head
2) capable of precision motion:
- translation in 3 directions
- rotation about the isocentre (some couches can rotate in 2 planes)
3) radiolucent flat-top surface
4) indexed for connection of immobilisation devices

Question 20

‘ram’ design couches

Answer 20

have a pillar support rising from a pit under the couch

Question 21

‘scissors’ design couches

Answer 21

have a larger base but do not require a pit