X-ray Flashcards
principles for x-ray
beam of high energy electrons
interact with a target material
interact with patient tissue, some pass through
interact with image to produce grey scale
why is bone very absorbant
due to its high density and atomic no.
white parts of image
more energy absorbed
what is xray energy measured in
kVp
peak kilovolts
what is the role of aluminum filter
absorb low energy photons
what is the effects of a higher kVp
lower contrast - more grey
what is the effect of a lower kVp
high contrast
what is the amount of xrays measured in
mAs
milli amps x second
what is the relationship between density and absorption
linear
if you double density of tissue, you double absorption and scattering
role of image receptor
convert xrays energy to image
different types of receptor
film screen - chemical reaction
computed - sacnning laser
direct-direct - computing
how does direct - direct radiography work
as xrays pass through crystals, release electrons, they ionise and form current which is collected at the capacitor.
thin film tranistor is switched so the current is released
what should you look at to get a good image
amount of collimnation
centering - where the middle ray will be
limit diverging beam
generator
creates high voltage
made up of coils
primary and secondray (has more coils =higher current)
alternating current, changes direction of a magnetic field, generates voltage
order of electromagnetic spectrum
radio microwaves infared ultra violet xrays gamma (highest)
useful characteristics of xrays
penetrate matter
interact with matter
can cause certain materials to fluoresce
can cause certain materials to produce electric charge
how are xrays generated
cathode produces electrons
filaments are heated, removing electrons
creates a high potenial difference between cathode and anode
electrons accelerate to anode
why is the glass tube in oil
heat disserpation
electrical insulation
properties of anode
good conductor
good heat disapator
made of tungston
electricity
flow of electrons in a material and the rate of flow of electrons is a meaure of the electrical current
potential force
is the ‘driving force’ which moves the electrons along a conductor, has an effect on kinetic energy, measured in kVp
electric current
amount of current, determines the amount of thermionic emission, measured in milliampes mA
3 ways which electrons collide with target
- interaction between them and outer electrons in the atom (no heat or xray)
- interaction between them and the nuclei of the atom (heat and xray)
- interaction between them and individual electron in the inner orbits (heat and xray)
how much energy is produced as heat in xray production
95-99%
bremstrahlung radiations
energetic electrons from the filament interact with the nuclei of target atoms and are slowed down, thus giving off energy in the form of xray
characteristic radiations
energentic electrons remove electrons from the inner orbitals and the vacanies created are filled by the electrons from outer orbitals, giving off energy in the form of x-ray
closer orbital is to nucleus - higher binding energy but lower potential energy
x-ray spectrum
bremsstrahlung radiations form a continous spectrum
characteristics radiations form line spectra as the shells have the same amount of discrete energy
tungsten atomic number and melting point
74
3370 C
quality of xray beam
measure of its penetraing power
quanity of xray beam
measure of the number of photons in the beam
why is funneling of xray important
saftey
only hits part of the body you want
less radiation exposure
newtons law of conservation of energy
energy neither created nor desroyed, only changed in form
why is tungsten used
heat stable
good electrical conductor
has alot of electrons
- positive protons in nucleus
why does characteristic xrays have two spikes on the spectrum
first spike if for first shell in the atom
second spike for second shell
each has 2 spikes due to the different spin orbitals in the paired electrons in the shells
why do we have rotating andoes
heat diserpation
preserves the anode
more high energy eletrons
andoe angle
makes source of xray as close to point source as possible
small angle = closer to point source = more effective source
role of filtration
enriches beam with high energy photons by absorbing lower energy xrays
higher average energy
- greater penetration
- reduce skin dose
ionisation chamber
gas filled
a propotion of incident photons cause ioniastion which is collected at the charged anode of the chambre
measure dose area product in cG
generators
cathode needs low voltage so mulitple circuits
high voltage tranformer
voltage rectifier
what does voltage rectifier do
converts oscilaiting voltage to constant voltage
intensity across the beam
varies because electrons travel into deeper layers of the anode, deeper they go = lower intensity
increase as angle is reduced, side closest to andoe is lower
fine focus filament
smaller
used when geometric focus limit image quality
energy focused on smaller area
reduce tube loading to prevent damage
when is the fine focus filament used
high detail
low patient attenutaion
extremities
broad focus filament
image limited by patient attentuation
increased heat dissipation
permits higher tube loading
when is broad focus filament used
less detail required
high patient attenuation
chest, abdomen, l.spine
