Xray production Flashcards
What do Xray tubes contain
Heated filament
Briefly, how are xrays produced
Electrons are drawn off in a cathode and are accelerated with high velocity to the metal anode target, this collision produces xray photons which form the image
Why are electrons held in a vacuum
to avoid collision with air molecules
how is the image formed
interaction of photons with patient
Cathode is positive/negative?
negative
Anode is positive/negative?
Positive
Types of xray generator
- single phase generator (2 pulse)
- 3 phase generator (6 pulse)
- 12 pulse generator
- medium frequency generator
Where are xrays generated
xray tube
where are electrons produced
cathode
what does a generator do in an xray tube
applies high voltage across the tube, which accelerates the electrons across the tube through the vacuum
where do the accelerated electrons go in the xray tube
they hit the target anode and interact with the tungsten metal to produce xrays which are then emitted from the tube
Components of the xray tube
- cathode = negative heated filament
- anode = positive surface of metal with high atomic number (tungsten)
- evacuated glass tube
- tube voltage 30-150kV applied between cathode and anode to accelerate electrons
characteristics of xray filament
tungsten wire 0.2mm diameter coiled into a spiral
temperature above 2200 C
size of focal spot increased with tube current
2 sizes: broad and fine focus
what is ‘blooming’
small filament will take less/more current, produce more/fewer xrays and require longer/shorter exposure times
small filament will take less current, produce fewer xrays and require longer exposure times
what is the line focus principle
if electrons are made to strike a sloping target, the apparent length of the source is smaller –> effective focal spot
target angle of anode
12-14 ‘
Smaller angle gives higher/lower gain, smaller/larger focal spot, but angular width of useful cone of radiation is reduced/increased
Smaller angle gives higher gain, smaller focal spot, but angular width of useful cone of radiation is reduced
2 types of anode
stationary and rotating
Properties of a stationary anode
thick plate tungsten bonded to copper block
properties of tungsten
high atomic number, high melting point, acceptable thermal conductivity and thermal capacity
where are stationary anodes used
dental xray sets
describe a rotating anode
anode is at the edge of a rapidly rotating disc
made of tungsten
which element is bonded to tungsten in a rotating anode and why
molybdenum - increases thermal capacity
what is the purpose of additional metal bonded to tungsten in anodes?
to dissipate heat
prep stage in a rotating anode
motor is energised before xray exposure is initiated to allow anode to reach operating speed
2 types of rotor?
high speed - 9000 rpm
standard speed - 3000 rpm
maximum angle yield to cathode?
5-10 ‘
decreasing anode target angle allows for greater/lesser loading for focal spot size, but reduces/increased useful area of beam
decreasing anode target angle allows for greater loading for focal spot size, but reduces useful area of beam
when can the Heel effect be used
where there are differences in body thickness within xray beam e.g. breast imaging
What is the anode heel effect
how can damage to the anode be reduced
stray non-focal radiation from bombarding stray electrons can be minimised by covering the parts of the anode not used to generate xrays with a layer of carbon
what is the glass envelope
highly evacuated glass vessel
glass has a low/high coefficient of expansion
low
what is tube rating
defines maximum input of electrical energy (load) that may be applied to an xray tube without causing damage through overheating
Energy (J) = ?
Energy (J) = kV x mAs
why does heat need to be dissipated at the anode
energy arrives within a fraction of a second at the focal spot, if not dissipated/removed, the temp rise would melt the focal spot
Factors affecting tube loading
- type of anode
- composition and construction of anode
- focal spot size
- anode target angle
- diameter of anode disc
- rotation speed of anode
- electrical power
- rate of heat dissipation
how to prolong xray tube life
- short exposure with high tube current deposits heat more quickly and produces a greater temp raise than lower current for a longer time
- heat radiated away more quickly
- if temp rise not great enough to cause damage, better to use short exposures despite temp rise
- gradually warm up xray tube
differences in mammography xray tube
filtration makes beam more penetrating
in a mammography tube, the cathode-anode distance is…
<1cm to ensure close control
in a mammography tube, the focal spot is…
smaller to improve resolution
in a mammography tube, the window is made of …
beryllium instead of glass
Ceramic xray tube properties
- better insulation
- more compact design
- larger anode, greater heat capacity and more efficient heat transfer
- liquid metal lubricant to with low vapour pressure to aid cooling
- better tube lifetime (less arcing)
- earthed metal enclosure
- off focus electrons attracted to grounded metal enclosure
- rapid series of exposure
what is arcing
the sign of the end of an xray tube’s life
Detector dose (+ units)
certain level of dose required at image receptor to form an image (uGy)
Entrance surface dose (ESD) (+units)
used to describe dose to the patient
absorbed dose in air back scatter
mGy
Effective dose (+units)
used to estimate the risk associated with an exposure to radiation
- summation of absorbed tissue doses x weighting factor
- estimated using ESD and other exposure factors
uSv / mSv
what can be changed about xray equipment
- distance from xray focus to patient / detector
- applied potential i.e. potential difference between cathode + anode kV
- electron beam current - controlled by filament current
- exposure time - controls total energy fluence
- focal spot size
- filtration
assuming a constant dose and patient/detector distance, increasing the distance from the xray tube to the detector will result in…
- increased mAs due to inverse square law
- decreased ESD
increased kVp (voltage) across the cathode and anode, the maximum energy…
increases
increasing kVp does what to the beam
harder, more penetrating beam
for a constant detector dose, increasing kV reduces/increases the ESD
reduces
mA = ?
current through cathode filament
increasing mA, exposure time or current-time product (mAs) will decrease/increase quantity of xrays
increase
(quality not affected)
what can reduce patient dose and how
filtration - removing lower energy xrays (soft radiation)
inherent filtration
- target
- exit window
- collimator assembly
added filtration
- aluminium
what is collimation
method to limit irradiated area to anatomy of interest
light beam diaphragm (LBD)
see diagram
