X-Ray Production Flashcards
x-ray closed circuit criteria
- must have enough power to eventually produce x-rays
- must have selections where the power can be increased/decreased
- power must travel in same direction through tube
- must always be a way to produce free electrons and w/enough energy to produce x-rays
4 factors that help transmit electricity
1) current (I): milliamperage/unit
2) voltage (V): speed electrons transfer energy along circuit
3) resistance (R): factor that slows the current
- large diameter = low resistance
- small diameter = high resistance
4) time: time x-rays are produced, determined by setting on control panel
power supply
- standard power supply not enough to activate most units, require transformers (auto, filament, and step up) in both facility and within unit
- North America power companies supply an alternating current
- power supply to x-ray unit must not fluctuate - need rectifiers to change AC to direct current
- rectifiers will keep current positive and allow proper flow in one direction (cathode -> anode)
transformers
- either increase/decrease power
- autotransformer: provides voltage requested w/ kVp selection switch
- high-voltage transformer (step-up): increases current to amount requested w/ mA selection switch
- filament transformer (step down): reduces current to right amount to heat cathode filament and produce electron cloud
rectifiers
Half wave: negative half of a wave taken out (period of time with nothing). Works for portable/smaller units only. Also not as powerful
Full wave: same as half wave (negative taken out) but time at baseline (AKA time of nothing) drastically shorter
generators
single-phase: 2 pulses per 1/60 second full wave
three-phase: 6 or 12 pulses per 1/60 second full wave
- can be thought of as 3 single-phase generators superimposed 120 degrees from each other
- presents a nearly constant potential across the x-ray tube
components
1) x-ray tube + table
2) x-ray generator (control panel)
3) high-voltage transformer
line voltage compensator
standard on every unit
- older units will have compensator mounted on control panel
- older units require compensator to be checked before every exposure
- newer units compensate automatically internally
- compensator can stabilize incoming power line to x-ray unit (needed in areas where incoming power lines are limited)
- connected through kilovolt meter and is method of in/decreasing incoming power line voltage
x-ray tube
main component of circuit
- cathode
- anode
consists of:
- glass enclosure (special heat-resistant glass) housing cathode + anode
- tube shielded w/metal covering (restricts radiation from exiting the tube other than at port)
when exposure switch is closed electrons are drawn from cathode -> anode by electromagnetic force
- density of anode metal causes electrons to lose speed
- forward motion energy converted to 99% heat and 1% x-rays
failure:
- damage to glass envelope from arcing/gassy tube results in deposits acting as a ‘secondary anode’ -> air molecules interact w/ e- stream -> inconsistent pictures with identical exposures
cathode
provides source of electrons
- directs them to anode
- negative side of tube
- consists of coiled filaments + focusing cup
- 2 filaments: large and small (older machines often only 1)
- large for dense body parts, small for detail (extremities, pocket pets)
- filament used depends on mAs setting
- made of tungsten (withstands high temps, high potential electron availability)
- filaments held in focusing cup aimed at anode
- focusing cup positioned opposite outer rim of anode
- has slightly negative charge to help focus boiled off e- towards anode
- beveled edges focus on very small areas on anode where x-rays are produced
ionization:
- when circuit is activated, exposure switch is closed
‘space charge effect’ =
- filament transformer sends electricity to cathode -> increases temp of cathode and e- ‘boiled off’ filament (‘thermionic emission’) -> cloud of e- produced (‘space charge’)
failure:
- filament evaporation (inappropriate technique and using pre-exposure setting). use the correct pre-exposure switch!!
- result = no e- cloud forms, film is blank, no movement on mA meter
anode
attracts e- stream -> produces x-rays
- positive side of x-ray unit
- bevelled target on cylindrical base
- target made of tungsten (high atomic # lets it absorb e- and heat)
- e- focused on this target are now ‘photons’
focal spot = small area where e- are focused and x-rays produced + sent down to patient
2 types:
1) rotating
- entire structure rotates so e- not always focused on same spot
- target on beveled front edge
- outer edge beveled at an angle to direct ray down to patient (fixed angle - can’t adjust)
- stem dissipates heat emitted
2) stationary
- portable units, dental units, fluoroscopy C-arms
- needs heat dissipation, read lights on unit warning of time needed between exposures
- composed of copper + tungsten
- targeted end of anode also end that is angled down to patient
line focus principle
how e- interact w/anode and change direction so x-ray goes toward patient
- …. AKA angle of the bevel of anode + resulting change of direction
- narrow beam (smaller focal area) = high res, more heat produced
- wide beam (large focal area) = low res (‘penumbra’ aka halo effect), less heat
- actual focal spot: area on target where e- contact
- effective focal spot: area projected onto patient + film
anode heel effect
intensity of radiation is greater on cathode side than anode side (due to target angulation)
- important when patient is thicker on one side of anatomy than other
- thicker end of animal should be placed on cathode end to take advantage of greater radiation
x-ray unit switches
- on/off switch (on control panel) + wall switch (located at eye level within reach of generator) + line voltage compensator
exposure switch - 2 stages:
1) activates rotating anode and boosts filament circuit
2) activates exposure and x-rays produced
- waiting for a ‘ready’ signal important to avoid overheating tube
- foot pedal switches used to allow restraint of animal during exposure
timer switch:
- controls time voltage is applied across the tube -> determines duration of x-ray production
- exposure 1/30 sec or less needed to prevent motion from appearing
collimator
- prevents unnecessary irradiation of patient and technologist
- decreases scatter radiation
- wide variety w/adjustable collimators being most common
- often contain a light bulb directed onto a mirror which reflects down onto tabletop (but must be tested to make sure the tabletop illumination matches radiographic field)
- cross hairs painted on plexiglass window to identify center of radiographic field
- knobs enable to adjust size of radiographic field (smaller = less scatter produced)
