X-ray production

Question 1

How are x-rays produced?

Answer 1

Inside ‘x-ray tube’
Electric current passed through cathode filament
Electrons boil off surface of filament
Accelerated to anode by voltage difference
Electrons strike anode; produce x-rays by radiative interaction and sometimes collisional interaction

Question 2

Describe radiative interaction

Answer 2

Oncoming electron passes close to positive nucleus and slows as it bends around it- braking energy

Question 3

Describe collisional interation

Answer 3

Oncoming electron displaces inner shell electron. Outer shell electron fills vacancy, releasing energy as x-ray

Question 4

Describe the properties of the anode

Answer 4

Metal- usually tungsten as high melting point and high atomic no. (more +ve nuclei)
Rotates- inc. surface area struck by electrons

Question 5

Define the ‘focal spot’

Answer 5

The region of the target struck by x-rays. The smaller the focal spot, the better the radiographic detail (less penumbra)

Question 6

Describe two methods of reducing the focal spot

Answer 6

1. Angling the anode

2. Use smaller filament (+ lower mA values to prevent filament burning out)

Question 7

What does changing mA affect?

Answer 7

miliamperage:
controls no. of x-rays by changing amount of current passing through cathode filament (i.e. how many electrons can ‘boil off’)

Question 8

What does changing the exposure affect?

Answer 8

Changes the length of time the current passes through the filament and hence the no. of x-rays produced.

Question 9

What does changing kVp affect?

Answer 9

kilovoltage peak:
controls the energy of the x-rays by altering the potential difference between the cathode and anode, hence altering the velocity and therefore the energy of the electrons arriving at the anode

Question 10

List the three possible ways in which x-rays interact with matter

Answer 10

Coherent scattering
Compton scattering
Photoelectric effect

Question 11

Describe coherent scattering

Answer 11

Photon interacts with object and changes direction BUT object does not absorb photon and photon energy doesn’t change

Question 12

Describe Compton scattering

Answer 12

(Occurs for practically all scattered radiation).
X-ray photon ejects peripheral shell electron
Original photon scattered at different angle and reduced energy.
Norm = high kVp range. Low contrast image (as most photons reach film)

Question 13

Describe the photoelectric effect

Answer 13

Photon ejects inner shell electron and is absorbed.
Electron from higher shell drops into vacancy and emits x-ray energy termed ‘characteristic radiation’.
Norm = low kVp range (<30kVp)

Question 14

Define ‘attenuation’

Answer 14

X-ray beam intensity decreases as it passes through matter. Photons absorbed/scattered

Question 15

Describe how an image is formed on x-ray film

Answer 15

Photographic film w/ light sens. emulsion containing silver halide, becomes sensitised where x-rays/light hits it.
Sensitised crystals precipitate during development to form BLACK deposits.
Unexposed crystals remain CLEAR.
X-rays different attenuated by tissues to form shadowgram

Question 16

Describe ‘intensifying screens’

Answer 16

Sheets of phosphorescent crystals which emit light when struck by x-rays. Light then exposes film.
Lower mA values are needed.

Question 17

Describe ‘grids’

Answer 17

Alternating lead and aluminium strips which prevent photons from Compton scattering reaching the film and causing ‘fog’.
Higher mA values needed as some primary x-rays absorbed by grid.

Question 18

Define D.I.C.O.M.

Answer 18

Digital imaging and communication in medicine

Question 19

Define P.A.C.S.

Answer 19

Picture archiving and communication system

Question 20

Advantages of computed and digital radiography over conventional

Answer 20

More time efficient (so lower radiation dose)
Cost effective (no darkroom, film, processing)
Good quality image