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)
control panel
- on/off switch (closes electrical current)
- line voltage compensator (built in, may not be seen on control panel)
- kilovoltage selector (selects power of rays)
- milliamperage selector (selected quantity of rays)
- timer (selects time of exposure)
- warning light (shows exposure being made and x-rays being emitted)
large animal portable units
need to be able to travel
- less powerful and less mA
- good for legs + feet
- generator, transformers, and x-ray tube all miniature
summary
1) circuit receives power
2) step up transformer increases power and supplies it to cathode
3) filament circuit steps down power to filament itself
4) cathode filament heated, e- boil off. anode begins to rotate
5) exposure switch closed
6) e- drawn from cathode to anode target
7) e- energy transferred to tungsten atoms in anode -> produce x-rays
