X-RAY production Flashcards
What are features that make an x-ray?
- high energy light
- must have high enough energy to overcome electronic bond of electron in an atom (remove electron from atom)
What does MAC stand for and what does it mean?
mass attenuation coefficient = measure of how many x-rays get through the patient to form the image
What 2 things does MAC depend on?
- material
- photon energy
give examples of high and low MAC
bone = high MAC
tissue = low MAC
What does HVT stand for and what does it mean?
half value thickness = thickness at which the number of photons drops by half
draw an image of the basic components in an X-ray tube and label
cathode, focusing cup, filament, anode, tungsten target, beryllium window, evacuated enclosure
Between the anode and cathode in X-ray tube, which is =ve or -ve?
anode = +ve
cathode = -ve
What is the tube potential/kVp?
voltage difference between anode and cathode
why is X-ray tube made of glass?
prevents the entrance of air molecules that would inhibit the x-ray formation in the tube
The tube must cope with lots of heat, what helps remove heat?
oil surrounding the tube
Explain the basic operation of an X-ray tube
- electric current runs through filament
- filament gets white hot
- electrons boiled off filament known as THERMIONIC EMISSION
- hot electrons form cloud above filament and repel due to like charges
- focusing cup is negatively charged which counteracts this effect
- large voltage set between anode and cathode
- electrons get accelerated to anode (TUBE CURRENT mA)
- electrons at 100000km/s hit target
What is the efficiency between x-rays and heat formation in X-ray tube?
x-ray = 2% efficiency
heat = 98% efficiency
Define bremsstrahlung (when electrons hit anode)
- electrons slam into anode
- electrons attracted to =ve and -ve parts of atom in target
- causes huge DECELERATION and change in direction of electron in target material (change in acceleration)
- electrons have to give up energy in the form of radiation = x-rays
- energy of x-ray is continuum
What is an x-ray spectrum?
range of x-ray energies produced in anode shown on a graph
What is beam hardening and how is it shown in the x-ray spectrum?
when additional filtration added causes more low energy x-rays being attenuated and high energy x-rays remain only being slightly lower.
beginning of graph lowers then goes back to same gradient towards end as high energy x-rays remain similar.
What is mAs and how does this link to dose? + how does it affect the x-ray spectrum?
mAs is tube current multiplied by exposure time in seconds
dose is proportional to mAs and the higher the mAs the higher the peak intensity on x-ray spectrum.
what is thermionic emission?
heating of filament leading to the emission of electrons
what can the electron cloud formed in X-ray tube be reffered to as?
space cloud
How is the beam in an X-ray controlled?
beam shuts off after selected exposure time or under AEC
What does AEC stand for?
automatic exposure control
How does mAs and heating relate?
How does exposure time relate to image formed?
How does mAs relate to x-rays generated?
increased tube current increase tube heating
increase exposure time increases chances patient will move = blurry image
increase mAs increases x-rays generated
Explain bremsstrahlung production in anode
- accelerated electron comes close to dense +ve atomic nucleus and is deflected
- loses energy which is emitted as x-ray photon
How does bremsstrahlung produce a range of x-ray energies?
the more deflected the electron is, the more energy it has to emit and hence gives higher energy x-ray.
What is bad about low keV x-rays?
it will be absorbed by patient adding to their radiation dose but not contributing to enhanced contrast in image formed
What does the window made out of beryllium do in X-ray tube?
filters out lowest energy x-rays
Changing mAs has no effect on the shape of the x-ray spectrum
How does changing kV affect maximum energy of x-ray spectrum and the number of x-rays produced across spectrum?
increased kv increases maximum energy of spectrum and increases amount of x-rays produced across spectrum
Maximum energy of x-ray in keV is equal to kV across the tube
Whats the effect of filtration
- increases average energy of beam/beam hardening
- reduces amount of x-rays in beam
What must you do to compensate for adding additional filtration?
increase mA or s or both
why does copper attenuate more than aluminium (filter)
due to its higher atomic number
How does mAs link to production of noise?
- higher mAs = more x-rays produced
- more reach the detector
- noise is REDUCED
- patient dose increases as more x-ray interaction
-lower mAs = less x-rays made
- less reach detector
- noise is INCREASED
- patient does decr4eases as less x-ray interaction
How does kV link to production of noise?
what is a negative this carries?
- increased kV = more of generated x-rays passing through patient and reach detector
- less noise
BUT also less contrast as increased average energy of x-rays means more passes through patient
How does kV link to contrast?
- decreased kV = less x-rays reaching detector and more x-ray absorbed by patient
- increased contrast but higher noise
define focal spot
area of the anode which receives the beam of electrons from the cathode
How does focal size affect image production?
the finer the focus, the smaller the umbra (shadow) and the smaller the penumbra (partial shadow) (which is the zone of unsharpness).
hence the image becomes sharper (as there is less of a shadow around the object) vice versa
How does increased focal spot size affect image blurring?
- BIGGER focal spot (broad focus) gives lower spatial resolution
- spreads heat generated in anode over LARGER area
- mA increases
- decreasing the exposure time for same mAs
- less chance patient will move
- lower resolution with less chance of blurring
How does decreased focal spot size affect image blurring?
- smaller focal spot gives HIGER spatial resolution
- spreads heat generated in anode over smaller area
- smaller mA required
- so increase exposure time for same mAs
- more chance patient moves
- higher resolution with more chance of blurring
What is the anode heel effect?
- tilting of the anode means the some x-rays are produced at the surface (closer to cathode) and dont travel through much of the anode, producing higher energy x-rays
- some x-rays are produced further within the anode and travel through quite a bit of the anode producing x-rays with lower energy
- hence x-ray intensity is increased as you move along the anode-cathode axis (towards the cathode)
when might the anode heel effect be useful?
when you are x-raying AP ranging from the cervical neck down to the chest, cervix requires lower energy x-rays and chest requires higher energy x-rays simultaneously
What is the half value layer?
the thickness of material that reduced the number of x-rays to half the original value
Why would the second half value of a mixed energy beam be thicker than that of the first?
the lower energy x-rays will have been filtered out in the first, causing beam hardening and the increase in the average beam energy making it more penetrating.
Hence the 2nd half value layer will be thicker as more material is required to get the x-ray beam down to half its original value.
Why is tungsten wire used?
strong
high melting point
low vaporisation
thermionic emission above 2200 degrees Celsius
What is the focusing cup made from and its function?
nickel
it focuses electrons on anode
Why is molybdenum used in a rotating anode?
prevents the heat produced by the electron interactions from traveling down the stem of the anode and warping the bearings that allow the anode to rotate.
protects bearings
What is a stationary anode made out of?
tungsten plate embedded in copper anode
What does the line focus principle do?
reduces the apparent size of the focal spot
Whats the difference between the actual and apparent focal spot?
The actual focal spot describes the physical area of the focal track that is impacted by the electrons. The effective focal spot is the area of the focal spot that is projected toward the object being imaged and the image receptor.
What is a rotating anode made of?
large disc of tungsten fixed to molybdenum/graphite base
What degree is a rotating anode angled at?
20-60 degrees
what speed does induction motor rotate the anode at?
3600 rpm
How are x-rays ‘shaped’ ?
x-rays are produced in all directions but are attenuated by shielding in undersired directions
resulting x-ray beam is COLLIMATED to the desired shape and size and filtration eliminates low energy beams.
What is the general rule of thumb for thickness of filtration according to kV + what material?
aluminimum
1.5mm Al for <= 70kV
2.5mm Al for > 70kV
