Unit 2 Flashcards
What are the 3 types of support for X-ray tube?
ceiling
floor
C-arm
What is the ceiling support system?
most frequently used
2 perpendicular sets of ceiling mounted rails
telescoping column
What is the floor mounted support system?
single column with rollers
allows for limited vertical, transverse and longitudinal movement
can be used with either table or wall Bucky
makes cross table difficult
What is the C-arm support system?
interventional radiography suites
image receptor and tube connected
flexible movement with limited SID
U-arm variant for X-ray
What does X-ray housing provide?
radiation protection
electrical protection
thermal protection
physical protection
How does the housing provide radiation protection?
lead lined
window allows useful radiation to escape
How does the housing provide electrical protection?
high voltage receptacles
glass and oil - makes sure electricity is contained
How does the housing provide thermal protection?
oil
cooling fan
Why is the physical housing around the X-ray tube important?
glass is fragile
casing protects against bumps
What is the glass tube?
contains cathode and anode
Pyrex - high heat capacity and an insulator
Vacuum - no collisions and prevents oxidation of electrodes
What is a metal tube used for?
part or all of the is replaced by metal
on newer high capacity x-ray units
less tube wear, longer lifespan
reduces the chance of arcing
What is the filament of the cathode?
a tungsten coil - good e emitter
1-2% thorium - better e emitter but radioactive
approx 2mm diameter
1-2cm length
different length for different focal spot
Why use Tungsten?
melting point of 3410C
prevents the wire from melting
low vapour pressure
does not easily evaporate
less vaporization occurs at high temperature
What is thermionic emission?
current creates heat
heat gives more energy to electrons
electrons can escape binding forces
1st stage of the switch - hot current starts making an electron cloud
How does thermionic emissions work?
needs to reach threshold to emit 2200C
after threshold, small filament current increase causes a large tube current increase
what is the danger of thermionic emission?
too much heat can cause evaporation
What is a space charge?
electrons form cloud around filament
space charge is held in place
- negative electrons repel each other
- filament becomes “positive”
- equilibrium
what is a compensation circuits?
Ensures mA is what we chose
mA can change depending on what kVp we select
How does compensation circuit work with high kVp?
- easy to pull mA across tube
- lowers filament heating current
- lowers space-charge size
How does compensation circuit work with low kVp?
- much harder to pull mA across tube
- increase filament heating current
- increase space charge size
What is the difference between the filament current and tube current?
Filament current - creates space charge and prep button
Tube current - allows mA to flow across tube and exposure button
What is the focusing cup?
supports filaments
made of nickel because it is a good conductor but not a good thermionic emitter
negatively charged
focuses electrons to target focal spot
What are dual focal spots?
focusing cup holds 2 filaments
one for each focal spot size
- small: 0.1-1mm, lower mA, higher resolution
- large: 0.3-2mm, higher mA, lower resolution
Selecting mA
mA has to be selected in “steps” and proportional to filament current
small focal spot: uptown 300 mA
large focal spot: over 400 mA
What happens when you increase the kVp?
Increases energy of photons
Increases the number of photons
Does NOT change the number of electrons
What does the anode do?
X-ray production
supports the target
conducts current
dissipates heat
stationary or rotating
What is a stationary anode?
Target does not move
need low parameters
energy localized to one area
stem is copper
- high thermal capacity
- high conductivity
- low melting point
target is tungsten
What is the Tungsten target?
made of tungsten (W)
High atomic number - good at producing X-rays
High melting point
High thermal conductivity
Low vapourization
What is target damage?
excessive heat causes damage
heat concentrated onto small area
unequal thermal expansion and concentration causes pitting
Factors (tube ratings): mA, time, kVp
What is a rotating anode?
has better heat dissipation because of rotation
Components: disc, focal track, stem, rotor, stator
target made of tungsten and rhenium (10%)
What are the elements of the anode disc backing?
molybdenum and carbon
Lessening thermal conductivity
What is the stem of the anode?
connects disc to rotor
made of molybdenum - poor heat conductor
thin
What is the rotor of the anode?
made of copper - high conductivity
rotates on bearings
What is the stator of the anode?
causes rotor to spin (no wires)
>3000 rpm, upto 12000 rpm - higher rotation has better heat dissipation
coast time of around 60s - coasting to a stop, no braking
what type of motor is the rotor and stator?
induction motor
generator and motor principle stimultaneously
electromagnetic induction
What is tube aging?
Over time the different parts of the x-ray tube wear down
- filament
- anode
- bearings
higher exposure ratings and longer prep times cause wear faster
What causes filament wear?
caused by: high mA and longer prep
Consequences: tungsten plating on tube can cause arcing (may shatter the tube)
filament may break
What causes anode wear?
causes: uneven thermal expansion and high exposure ratings (mA and kVp)
Consequences: pitting and unequal x-ray intensity
Difference of rotating vs stationary targets for heat dissipation?
rotating has 500x heat capacity
What causes bearing wear?
causes: heavy use and longer prep
Consequences: slower rotation (more pitting) and “grinding noise”
What causes housing damage?
causes: physical damage and overheating
Consequences: oil leakage - things get hotter and then the tube can get more damaged
How to prevent tube wear?
lower techniques, shorter preps
warm up the tube
- help prevent the anode damage
- procedure varies between manufacturers
What factors affect tube ratings?
focal spot size
rotation speed
disk diameter (larger = better ratings)
waveform
segmented/grooved anode
metal tube (no fear of tungsten evaporation)
Heat storage capacity
how many exposure you can do without cooling
- important for bearings
What are heat units?
HU = kVp x mAs
Heat units/second
amount of electrons emitted per second
energy of electrons hitting target per second
more important for filament and target
HU/s = kVp x mA
What is an anode cooling chart?
shows the heat capacity of the anode
shows the length of time to completely cool
- cools very rapidly at first then slows down
only one chart per machine
how does the tube cool down?
tube is in contact with oil to dissipate heat
- usually just the end of the rotor
What is a rotating envelope tube?
entire tube spins, not just anode
anode in contact with oil - better heat dissipation
external bearings and motor
don’t have to worry about envelope tube
What is the purpose of beam restriction?
patient safety
image quality
how does beam restriction impact patient safety?
protects patients from radiation
smaller field size - less tissue irradiated
less energy absorbed by patient
lowers dose
How does beam restriction impact image quality?
helps image contrast
less scatter
less tissue irradiated
higher subject contrast
improves image quality
What was the start of the aperture diaphragm?
first used around 1899 by Dr. William Rollins (father of radiation protection)
Lead plates with holes cut in them
- designed for specific film sizes
- constricted the size of the beam
use of cones and cylinders
simple lead shapes inserted into diaphragm
reduce more of the beam because of increased length
further improved image sharpness
downside: make the xray field circular while the plates were rectangular
What is a variable aperture or light beam collimator?
what we use today
multiple levels of lead shutters to restrict beam more effectively (2-3 shutters)
lightbulb and mirror to represent x-ray field
what do the different sets of shutters do?
first set - reduce off focus radiation
second set - allow field size/shape selection
correlation between shutter distance and blur
lowe shutters = less blur
what angle must the mirror be at inside the tube??
45 degrees
what happens if the bulb and focal spot are not equal distance from the mirror?
bulb too close - xray beam would be smaller than light field
bulb too far - xray beam would be larger than the light field
what percent of SID can misalignment be?
2%
What is positive beam limitation?
limits the size of the collimation to the size of the IR
What is a collimation scale?
for when positive beam limitation is not available and you can’t see the IR
the dials have notations for different sizes at different SID’s
how is the diverging beam created?
by isotropic emission
How does the diverging beam affect the image?
magnification
distortion
- focal spot blur
inverse square law for intensity
anode heel effect
these affects are not always detrimental
Magnification
caused by diverging beam
outer portion of beam makes the object appear larger
affected by SID and SOD/OID
magnification factor
how much larger the object appears in the image
Magnification factor = SID/SOD
why is magnification useful
can be used to increase object size
better than zooming in
- more pixels per area of anatomy
How is distortion caused
by diverging beam
- unequal magnification of object
changes the shape of the object
- elongation
- foreshortening
affected by SID, OID, object position and thickness of object
how is foreshortening caused
object is angled but IR is flat
how is elongation caused
object is parallel to IR, but central ray is angled
object is parallel to central ray, but IR is angled
causes object to be stretched out
when do you use your angle to your advantage
bisecting angle method
- reduce distortion
- angle of central ray to IR = 1/2 angle of object to CR
superimpositions
- two objects on top if each other
- use angle to seperate
focal spot blur
caused by diverging beam AND multiple point sources
edges of object hit by x-rays from multiple sources
creates penumbra
- geometric unsharpness
blur prevention
longer SID
shorter OID
smaller focal spot size
- less point sources
Inverse square law
caused by diverging beam
as you get farther from the source, more and more of the diverging beam misses the object
I1/I2 = (d2/d1)^2
Large focal spot pros and cons
pros
- more area to be hit by electron beam
- less pitting of target surface
- higher ratings
cons
- more point sources
- more blur
small focal spot pros and cons
pros
- sharper image
- less blurring
cons
- lower ratings
- more easily damaged
what is the line focus principle
best of both worlds
use angle to achieve large actual focal spot
- high ratings
AND small effective focal spot
- less blur
actual focal spot
large area to receive mA
- can take more heat and dissipate it better
rectangular in shape
point sources are farther apart
- larger penumbra
effective focal spot
focal spot as seen from the POV of IR
square shape
point sources are closer together
- better spacial relation
Anode angle
angle is from vertical
usually 12-17%
- 45 degrees = incident electron beam size
- <45 degrees = effective size is smaller than incident
- >45 degrees = effective size is larger than incident
Same size, different angles
large angle
- shallow slope
- larger effective size
smaller angle
- steeper slope
- smaller effective size
field size
smaller angles give us smaller field sizes
On what images would a small angle be detrimental?
larger pieces of anatomy
Anode heel effect
caused by angled target
some photons travel through the target itself
- some attenuation happens
- lower intensity
happens more with steeper angles
lowers intensity specifically at the anode side
intensity is unaffected at the cathode side
when do you use the anode heel effect
can act like a filter
align anode to thinner portion of anatomy
T-spine
abdomen
how to reduce the anode heel effect
longer SID
smaller field sizes
biangular anodes
some anodes have 2 different angles
chooose either one depending on if your need the sharper image or larger field
