Week 1 Lecture 1 Flashcards
what are the three components of x-ray imaging systems that influence the flow of electrons?
- operating console
- generator
- x-ray tube
what does the operating console consist of?
specialized keyboards that control the intensity and quality of the radiation used during imaging
why are high voltage generators preferred?
- production of near constant-potential
- produces greater x-ray quantity and quality
- compact in size
where do you find the generators on new CT equipments?
built into the scanners because they are so small
why is the production of greater x-ray quantity and quality important?
it improves the image quality and decreases patient dose
x-ray tube
- electronic vacuum that produces x-ray photons
- focuses useful beams of radiation through a window
- help with heat dissipation
what are the 2 components that make up the x-ray tube?
- anode
- cathode
protective housing
contains the x-ray tube
what are the 3 functions of the protective housing?
helps to prevent:
1. electric shock
2. exposure to leaking radiation
3. damage to internal components
what is the role of metal and glass envelopes?
to maintain the vacuums inside the tube as well as providing structural support
why is the vacuum important for the tube?
- maintains the tube’s current
- allows for more efficient x-ray production and longer tube life
in modern ct tubes, which is preferred? metal or glass envelopes?
metal
why is oil found between the housing?
to dissipate heat
what causes arcing?
deposits from tungsten vaporization
why are metal envelopes preferred over glass ones?
metal envelopes:
- prevent arcing
- enables higher tube currents
- improves heat dissipation
window on the envelope
- emits useful x-ray photos from the tube
- enables maximum emission of x-ray photons with the least amount of of absorption
where do you find the window?
near the target – on the anode side
cathode
negatively charged portion of the x-ray tube
what is the purpose of the cathode?
- produces electrons
- directs the electrons to a specific location on the anode
how does the cathode produce electrons?
thermionic emission
what are the two primary parts to the cathode?
- filament
- focusing cup
filament
thoriated tungsten coils that boils off electrons when it is heated
why are there two filaments in an x-ray tube?
difference in the filament size used affects spatial resolution and impacts the heat capacity of the anode
coil size of the filament correlates to what?
focal spot sizes
both filaments can be used at once. t/f
false
larger filaments are _____ and _____.
longer and wider
focusing cup
- metal canal that surrounds the filament to electrostatically confine electrons
- ensures the electrons will strike the anode target at the right spot
why is the focusing cup negatively charged?
to prevent the electrons from spreading out
anode
positively charged portion of the x-ray tube
what are the 3 purposes of the anode?
- produces x-rays
- conducts electricity
- provides mechanical support for the target
what are the 3 components of the anode?
- bearing assembly
- rotor
- target/disk
where is the bearing assembly found?
it is covered by the rotor + covered in oil
what is the purpose of the bearing assembly?
- to provide and ensure smooth rotation of the anode disk
- conducts heat away from the x-ray tubes
liquid bearing method vs. silver ball-bearing technology
liquid bearing method improves anode disk rotation
reduces the vibration and noise
what is the purpose of the rotor?
prevents transmission of heat from disk to the bearings
what is the stator?
- tube-like structure that surrounds the rotor
- makes the target rotate by electromagnetic induction
where do you find the stator?
outside the metal envelope
what is the purpose of the target?
collides with the electrons from the cathode
how are x-rays created?
when fast moving electrons strike the metal target
targets are designed to improve heat capacity and spatial resolution. how?
by the angles that are found on the targets
how does the target move?
by the stator due to electromagnetic induction
focal spot(s)
area of electron collision therefore site of x-ray emission
where do you find focal spots?
on the angled portion of the targets
larger or steeper angles = ?
larger or steeper angles of target will result in a larger surface areas
larger surface areas will mean larger area of interaction which reduces the concentration of heat
steeper angles also = ability to irradiate wider sections of anatomy = decrease spatial resolution
does CT imaging or general radiography generate more heat?
CT imaging
to manage heat production, what do CT tubes have in terms of FSS, target angles + size, and rotation speed?
larger fss
larger angles on larger and thicker targets
faster target rotation speeds
what are the three designs for CT targets?
- all-metal disk
- brazed graphite
- chemical vapour deposition (CVD) graphite disk
what are limitations for all-metal disks?
- heavy
- fixed targets
- poor heat dissipation
- short tube life
*which target design was used in old/conventional scanners?
all-metal disks
*which two target designs are more commonly found in helical/spiral ct scanners?
brazed graphite and chemical vapour deposition graphite disks
*what is the difference between the brazed graphite targets and the CVD targets?
both have graphite bases, but the difference is brazed graphite’s focal track is made of tungsten rhenium while the CVD’s focal track just has the layer of tungsten r. over it
what are the two advantages to brazed graphite?
- capacity to withstand more heat (higher thermal capacity)
- longer tube life
what are the three advantages to CVD?
- allows for disks to be large and lightweight
- large heat-storage capacity
- fast cooling rates
what are the steps to x-ray production?
operating console to engage generator
generator sends current of electricity to x-ray tube
cathode’s filament is being heated which creates electrons
potential difference develops (e- from cathode to anode)
target on the anode gets bombarded with electrons
production of heat (99%) and x-ray photons (1%)
steps to getting a CT image.
- perform scout
- scan sequence
- check for quality/accuracy - repeat scans if necessary
- post-procedural care + dismiss patient
- perform image post-processing
protocols =
“how to scan” guide
what are the four key considerations to achieve a ct image?
- scan type (axial or helical)
- patient positioning
- use of contrast media
- scan paramaters (technique selections - kVps, mAs)
smaller filaments
increases spatial resolution
decreases heat capacity
larger filaments
used when imaging longer lengths of anatomy or hypersthenic patients
hypersthenic
obese
sthenic
“average” body
hyposthenic
thin - below average
asthenic
frail - “very thin”
why does CT use a thicker target than GR x-ray tubes?
exposures are longer and produce more heat
what is an important consideration in CT imaging?
overheating reduces maximum exposure times
when would a large focal spot be preferred?
imaging larger body parts
