X-ray production

Question 1

4 things involved in the production of a radiographic image

Answer 1

source of X-rays

object

image receptor

processing

Question 2

source of X-rays e.g.

Answer 2

x-ray machine

production of the X-rays

Question 3

object being X-rayed e.g.

Answer 3

teeth and jaws

interaction of X-rays with matter

Question 4

processing of X-rays

Answer 4

conversion of latent image to permanent visible image by computer technology or chemical

Question 5

how are X-rays made

Answer 5

when fast moving electrons are rapidly decelerated

Question 6

electrons

Answer 6

negatively charged particle in an atom (-ve)

conceptually sited in orbits around the nucleus (+ve) - the Bohr model

Question 7

X-ray machine features (4)

Answer 7

wall mounted

tubehead – contains X-ray tube

Jointed, positioning arm

control panel

Question 8

siemens heliodent MD tubehead features

Answer 8

Tubehead

Spacer cone

Rectangular collimation – built-in or inserted

Question 9

X-ray tubehead components (9)

Answer 9

filament – cathode

transformer

target – anode

target surround

evacuated glass envelope

shielding

filtration

collimator

spacer cone

Question 10

filament -cathode

Answer 10

negative

tungsten

filament circuit (step down transformer): 
- low voltage, high current

Question 11

tungsten

Answer 11

Symbol W

Z = 74 (no. of protons/electrons)

m.p. 3410 0C

Question 12

filament function (3 stages)

Answer 12

• low voltage current passed through filament circuit
• filament heats up to incandescence
• electrons form a cloud around filament

Question 13

transformer

Answer 13

step-up

240 eV domestic input

60 - 70 keV high voltage output

huge attraction of -ve electrons (mA) from cathode towards positive anode (target)

Flow of electrons ~ 7 to 15 mA

Question 14

operating potential for new equipment

Answer 14

New equipment should operate within the range 60 to 70 kV

Attention is drawn to dental sets that have an effectively constant potential (DC) output, ….

Question 15

target - anode

Answer 15

positivve

tungsten

effective area 0.7 mm2
- 20 degree slope (i.e. not parallel to filament), greater area

referred to also as focus or focal spot

Question 16

target interactions - 2 main

Answer 16

heat production - 99%
(inefficient)

X-ray production - < 1%

Question 17

how is heat produced by X-ray production

Answer 17

• incoming electron (e-)
  deflected by cloud of outer-shell tungsten electrons, or
  collides with an outer shell electron, displacing it
• small loss of energy (E)
• loss of energy in form of HEAT
• removed through copper block, oil, then air

99% - inefficient

Question 18

amount of X-ray production

Answer 18

<1%

• continuous spectrum
• characteristic spectrum

Question 19

target for X-ray production - surround

Answer 19

Copper Cu

Z = 29

m.p. = 1080 0C

effective heat conductor

Question 20

2 types of X-ray production

Answer 20

continuous spectrum

characteristic spectrum

Question 21

X-ray production: continuous spectrum

Bremstrahlung/braking/white radiation

Answer 21

incoming e- passes close to nucleus of a target atom

e- rapidly decelerated and deflected

amount of deceleration and deflection proportional to E loss

E loss in form of electromagnetic radiation as a continuous spectrum of energies

maximum E is applied kV (e.g. 70 kV)

straight down angled line

Question 22

X-ray production: characteristic spectrum

Answer 22

incoming e- collides with an inner shell (orbit) target e-

target e- displaced to an outer shell or completely lost from atom

target atom unstable

orbiting e- s re-arranged to fill vacant orbital slots to return atom to neutral state

difference in E between orbits is released as characteristic radiation, of known E values

same mechanism as photoelectric absorption – covered in lecture on Interaction with matter

vertical spikes on graph at certain points

Question 23

characteristic radiation of Tungsten values (approx)

Answer 23

8 kV – L shell
58 kV – K shell
68 kV – K shell

Question 24

evacuated glass envelope

Answer 24

evacuated glass

vacuum prevents risk of interaction of electrons with air atoms prior to meeting target

Question 25

shielding

Answer 25

lead - Pb
Z = 82

to ensure dose rate in vicinity not > 7.5 µSvh-1
(Sv = Sievert)

Question 26

shielding role

Answer 26

to ensure dose rate in vicinity not > 7.5 µSvh-1

Sv = Sievert

Question 27

filtration

Answer 27

Aluminium - Al
Z = 13

1. 5 mm < or = 70 kV
2. 5 mm > 70 kV

Question 28

collimater

Answer 28

lead

circular or rectangular diaphragm

max. beam diam - 60mm at patient end of spacer cone

Rectangular collimation should be provided on new equipment, and retro fitted to existing equipment

Question 29

better shape collimater

Answer 29

Rectangular collimation should be provided on new equipment, and retro fitted to existing equipment

Question 30

beam size - collimation

Answer 30

circular area - 2828 sq. mm
- no greater than 60 mm. diameter at patient end of spacer cone

rectangular area - 2000 sq. mm
- 30% reduction

Question 31

space cone/ Beam Indicating device BID

Answer 31

direction indicating device or beam-indicating device (BID)

circular or rectangular

controls target (focus) -skin

distance
100 mm < 60kV
200 mm = or > 60kV

Not to be confused with the beam-aiming device of film holders (the ring)

Question 32

spacer cone

Answer 32

controls focus-skin distance (fsd)

measure from external marker to patient end of cone

Question 33

focus -skin distance

Answer 33

use long X-ray focus - skin distance (fsd) to reduce magnification - at least 20 cm

N.B. “long spacer cone” = long fsd