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78 Cards in this Set
- Front
- Back
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Imaging Process: 5 steps |
1. Image Acquisition 2. Image processing 3. Image archiving 4. Image display 5. Image analysis |
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Image Acquisition |
Beam created Beam passes through patient Beam picked up by IR
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mAs |
Milliamperage/second
controls AMOUNT of radiation emitted -number of photons Double mAs= double amount of exposure Half mAs= half amount of exposure
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kV |
Killovoltage
controls STRENGTH of radiation emitted
increases kV by 15% means double amount of radiation
Decrease kV by 15% means half amount of radiation |
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Image Processing |
Film: processed to make invisible image visible
Digital: processed to improve image quality
CR: 3 Components: IR, IRD, workstation DR: Image goes directly from IR to computer 2 Types: Direct Direct, Indirect Direct |
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Image Archiving |
digital Images go to PACS |
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Image Display |
post processing viewed on the computer monitor
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Image analysis
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radiation exposure brightness contrast resolution/detail distortion |
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Wave theory |
wave= disturbance of a medium x-rays= no medium required frequency= number of cycles(waves)/second wavelength+ distance between 2 corresponding points on a wave period= time required to complete one cycle |
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Particle Theory |
-x-rays behave like particles when interacting with matter -photon or quantum= bundle of energy -each photon carries a specific amount of energy, depends on frequency -energy and frequency are directly proportional |
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Binding energy
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electron in motion around nucleus= shells
k shell is closest to nucleus than LMNOP&Q
set max e- in each shell
shell can begin filling before previous shell is finished |
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Excitation |
x-rays transfer energy to an atom
electrons remain with atom, but x-rays can move an e- to a higher energy level within the atom
e- will drop back to original energy level naturally to maintain atom stability |
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ionization |
occurs within an atom when an e- is removed or added to or from an atom |
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radiation: non-ionizing & ionizing |
Non-ionizing: excites (microwaves, heat, radar) Ionizing: excites and ionized ( xrays, gamma, alpha etc) |
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Electro-magnetic Radiation |
no mass no charge X-rays: man made in a tube Gamma: emitted from radioactive nuclei |
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Particulate Radiation |
Alpha -mass -positive charge -cant travel far (5cm) Beta -small mass -negative charge -travels far (10-100cm) -emitted from radioactive nuclei |
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Radiation Production |
-x-rays produced in an x-ray tube -occur when high speed electrons are stopped abruptly at target -Bremsstrahlung & Characteristic |
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Bremsstrahlung Radiation Production |
-"braking" or slowing -99% of energy produced is HEAT 1. incident e- interacts with force field of nucleus 2. e- slows rapidly and changes direction 3. cause lost energy from electron= X-rays 4. variable energy x-rays= one e- may have many brems interactions
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Characteristic Radiation Production |
1. incident e- interact with inner shell and e- is removed (ionized) 2. e- from higher shell fills hole (energy difference between shells) 3. excess energy emitted as x-rays 4. characteristic cascade occurs, not all x-rays are diagnostic |
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X-ray Beam |
-diagnostic x-ray range= mostly Brems target interactions (80-90%) and (10-20%) characteristic -characteristic interactions will be seen more above 70kV because 69.5kV is needed to remove k shell from atom of tungsten
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Generators |
-convert mechanical energy into electrical energy -made from conductor and array of magnets -make the power required by the x-ray machine provided from the power grid
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Auto-transformers |
-varies incoming voltage for other transformers -transformers have primary and secondary coils |
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Transformers |
-step-up: increased incoming voltage to kV -step-down: decreases incoming voltage for filament (wall:110kV x-ray:55kV) |
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Phases of Power |
-voltage comes in alternating current -during the negative cycle, current will flow from anode to cathode, but anode doesn't emit electrons, so no x-rays are formed -rectification: converts AC-->DC |
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Single Phase: unrectified |
unrectified because it goes below the line |
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3 phase unrectified |
voltage more constant across tube than single phase
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3 phase rectifed |
less loss of energy because it is rectified (above line) |
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3 phase generators (2 types) |
3 phase 6 pulse -produce a voltage ripple of 13-25% -V never falls below 75-87% of peak kV setting
3 phase 12 pulse -produce VR of 4-10% -V never falls below 90-96% -produces 40% more photons than single phase |
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High frequency generator |
-produces almost constant potential voltage to x-ray tube -VR: 3-4% -achieves peak kV in 10% of the time needed for 3 phase generators -has small transformer -allows for compact x-ray unit
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Voltage Ripple |
expressed as a percent of max kV |
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Falling Load generator |
-specially designed for 3 phase or high frequency -exposure starts at highest mA possible -advantages: shorter exposure time -disadvantages: shorter x-ray tube life due to high mA values which increase wear, cannot set mA, cannot do tomography |
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Purpose of Falling Load Generator |
-to allow x-ray images to be taken in shortest time possible
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Tube Components |
-power source -source of electrons (filament) -target for electrons/focal spot (anode) -vacuum -focusing cup (wraps around filament and makes electrons travel in straight line) |
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X-ray tube needs |
source of electrons target material high voltage vacuum |
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Protective Housing |
-x-ray tube inside housing -controls leakage and scatter radiation -composed of cast steel and lined with lead -isolates high voltage components -provides a mean to cool tube (oil) |
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Inside the Housing |
Envelope Vacuum Cathode Assembly Filament Grid-based Tube Anode assembly Stator Rotor Rotating Anode |
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Envelope |
Glass casing that surrounds cathode and anode, excluding stator
maintains a high vacuum
x-ray photons leave envelope through a window segment |
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Vacuum |
increase efficiency of X-ray tube
removal of all air per its electrons to flow from cathode to anode without encountering gas atoms |
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Cathode Assembly |
-negative charge -made of filament, focusing cup and wiring -conducts high voltage across gap between cathode and anode -focuses electron stream towards anode |
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Filament |
-provides resistance -rhenium, molybdenum, usually tungsten wire -high melting point -hard to vaporize -usually two filaments= DUAL SELECTION -smaller: less heat at anode, increased image detail -larger: more heat, decreased detail, fuzzy image |
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Grid-based Tube |
-used in angiography and pulsed fluoroscopy -applying a negative charge to the focusing cup repels electrons, applying a positive charge attracts e- -can regulate flow of electrons to anode exactly, and quickly |
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Anode Assembly |
kV positive charge target surface conducts high voltage back into the circuitry primary thermal conductor consists of: anode, stator, rotor |
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Stator |
series of electromagnets
only part of assembly outside of the glass envelope
EM effects causes rotor to turn by creating a magnetic field
outside glass envelope |
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Rotor |
inside envelope within stator hollow copper cylinder rotating anode (RA) -diameter 5-13cm -faster it rotated better heat dissipation middle layer= molybdenum -high melting point -poor heat conductor -lighter than tungsten target layer is rhenium alloyed tungsten -90% tungsten+10% rhenium |
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Why tungsten? |
high atomic number of 74 high melting point |
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Why rhenium? |
provides better elasticity when focal track expands rapidly to extreme heat |
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Anode Disc |
focal track -circular path on the anode struck by the electrons target or focal spot -the area of the focal track being struck by electrons at a given time during exposure -where x-rays are created |
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Line Focus principle |
-used to reduce the effective area of focal spot, permitting the best resolution of detail possible while having the largest actual FS -principle that viewing a sloped surface at an angle reduces its apparent size -line focus principle applies in only ONE direction producing a rectangular shaped focal spot |
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Focal Spot: actual and effective |
Actual: physical area on focal track -size controlled by length of filament
Effective: area of focal spot projected out of tube toward object and IR -smaller than actual |
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Anode Target Angle |
-less that 45 degrees, most common is 12 degrees
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focal spot affected by |
actual -dimension of filament (length) -focusing cup -technique selection -distance between cathode and anode
Effective -same factors as actual -plus anode target angle
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Focal spot blooming |
as increase mA, FS size increases due to -increases number of e- given off -increased beam size -resulting larger actual FS and larger effective FS -doesn't significantly affect detail
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Anode Heel Effect |
-Caused by the line focus principle -radiation intensity is greater on cathode side of tube |
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Tube Placement |
Anode placed at head of table Cathode at foot (this is because of body thickness) |
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Anode Heel Effect Advantages |
use more intense radiation to penetrate thicker portion of body
-thoracic vertebra, humerus, femer, tibia, fibula, radius, ulna, foot |
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Extra-focal radiation |
(off-focus radiation) -x-ray photons that are not produced at the focal spot -these photons produce a ghost image on film -does NOT contribute to detail |
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Anode Heating |
-99% of energy produced is due to e- braking= heat
-tubes cool off using oil in the cooling system
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3 Types of Anode heating charts |
1. radiographic tube rating charts 2. anode cooling charts 3. housing coolant charts |
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Power Ratings: Generators |
-power rating of tube is determined at 0.1 seconds -defines maximum mA and kV at 0.1 seconds
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Tube Rating Charts |
used to determine exposure factor combos allowed for a single exposure without overloading the tube -different charts for different units -different charts for each focal size |
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Accumulation of Heat |
maximum heat units (HU) or kW-s of kJ allowed |
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Tube over heat indicator |
if the tube over heat indicator light appears, the system has over-heated. The system will not allow any exposures to be taken until the tube is properly cooled down |
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Housing Cooling Charts |
calculates the time necessary for the housing to cool enough for additional exposures
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Tube aging and failure |
Filament Thinning -Damage/Aging: due to evaporation, reduces life of filament by 60%
Vaporization -particles deposit on inner surface of tube |
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Recomendations Filament |
operate rotor at a minimum
use lowest mA possible
handle tube gently |
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Envelope Damage |
-metal from filament can deposit on the inside of glass over time -cracking of envelope due to arcing -decrease in quality of photons exiting tube -increase quality of beam exiting tube |
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Focal track damage |
-becomes pitted -decreased AHE (anode heel effect) -decreased photon output from focal spot |
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Anode disc |
-can warp, buckle, or crack -bearing wear |
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Bearings Damage |
distorted in shape, roughened surface, rotor itself can be distorted in shape
increased rotor noise, slower rotor speed, changes in thermal characteristics |
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Stator/Rotor Failure |
-e- overheat target area on anode
-exceeds melting point of tungsten
-tungsten drips onto glass envelope
-immediate tube destruction (crack) |
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Overall tube failure |
-single excessive exposure over tube limit -excessive exposure on cold anode -use lower mA station -don't make repeated exposures near tube limit -don't use tube if you hear loud rotor bearings |
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Isolation and cooling in Tube |
-between glass envelope and tube housing is dielectric oil -isolates high voltage components from tube housing -oil absorbs heat produced during x-ray production |
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Attenuation |
the reduction in the number of x-ray photons in the beam and subsequent loss of energy as the beam passes through matter |
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Five possible interactions between radiation and matter |
-photo-electric absorption -Compton scattering -coherent scattering -pair production -photo-disintegration |
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Photon Interactions |
1. transmission 2. absorption 3. scatter |
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Photo-electric Absorption |
1. interaction with the inner shell e- in matter 2. incident photon ejects the e- and is totally absorbed 3. ionized atom 4. the vacancy is filled by another e-, creating secondary radiation |
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3 Rules of Photo-electric absorption |
-incident photon must have more energy than binding energy of atoms inner shell e- -PE interaction more likely if x-ray photon energy is slightly higher than the atom binding energy -PE interactions more likely to occur with e- with more tightly bound in orbit |
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Compton Scattering |
1. photon interacts with a loosely bound outer shell electron 2. removes the electron 3. photon proceeds in a different direction 3. photon has less energy once scattered 5. photon will continue to scatter until it is absorbed photoelectrically or transmitted |
