X-ray production Flashcards
4 things involved in the production of a radiographic image
source of X-rays
object
image receptor
processing
source of X-rays e.g.
x-ray machine
production of the X-rays
object being X-rayed e.g.
teeth and jaws
interaction of X-rays with matter
processing of X-rays
conversion of latent image to permanent visible image by computer technology or chemical
how are X-rays made
when fast moving electrons are rapidly decelerated
electrons
negatively charged particle in an atom (-ve)
conceptually sited in orbits around the nucleus (+ve) - the Bohr model
X-ray machine features (4)
wall mounted
tubehead – contains X-ray tube
Jointed, positioning arm
control panel
siemens heliodent MD tubehead features
Tubehead
Spacer cone
Rectangular collimation – built-in or inserted
X-ray tubehead components (9)
filament – cathode
transformer
target – anode
target surround
evacuated glass envelope
shielding
filtration
collimator
spacer cone
filament -cathode
negative
tungsten
filament circuit (step down transformer): - low voltage, high current
tungsten
Symbol W
Z = 74 (no. of protons/electrons)
m.p. 3410 0C
filament function (3 stages)
- low voltage current passed through filament circuit
- filament heats up to incandescence
- electrons form a cloud around filament
transformer
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
operating potential for new equipment
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, ….
target - anode
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
target interactions - 2 main
heat production - 99%
(inefficient)
X-ray production - < 1%
how is heat produced by X-ray production
- 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
amount of X-ray production
<1%
- continuous spectrum
- characteristic spectrum
target for X-ray production - surround
Copper Cu
Z = 29
m.p. = 1080 0C
effective heat conductor
2 types of X-ray production
continuous spectrum
characteristic spectrum
X-ray production: continuous spectrum
Bremstrahlung/braking/white radiation
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
X-ray production: characteristic spectrum
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
characteristic radiation of Tungsten values (approx)
8 kV – L shell
58 kV – K shell
68 kV – K shell
evacuated glass envelope
evacuated glass
vacuum prevents risk of interaction of electrons with air atoms prior to meeting target
shielding
lead - Pb
Z = 82
to ensure dose rate in vicinity not > 7.5 µSvh-1
(Sv = Sievert)
shielding role
to ensure dose rate in vicinity not > 7.5 µSvh-1
Sv = Sievert
filtration
Aluminium - Al
Z = 13
- 5 mm < or = 70 kV
- 5 mm > 70 kV
collimater
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
better shape collimater
Rectangular collimation should be provided on new equipment, and retro fitted to existing equipment
beam size - collimation
circular area - 2828 sq. mm
- no greater than 60 mm. diameter at patient end of spacer cone
rectangular area - 2000 sq. mm
- 30% reduction
space cone/ Beam Indicating device BID
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)
spacer cone
controls focus-skin distance (fsd)
measure from external marker to patient end of cone
focus -skin distance
use long X-ray focus - skin distance (fsd) to reduce magnification - at least 20 cm
N.B. “long spacer cone” = long fsd
