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Medical Radiation Sciences 1 > Unit 2 > Flashcards

Unit 2 Flashcards

(87 cards)

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1
Q

What are the 3 types of support for X-ray tube?

A

ceiling
floor
C-arm

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2
Q

What is the ceiling support system?

A

most frequently used
2 perpendicular sets of ceiling mounted rails
telescoping column

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3
Q

What is the floor mounted support system?

A

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

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4
Q

What is the C-arm support system?

A

interventional radiography suites
image receptor and tube connected
flexible movement with limited SID
U-arm variant for X-ray

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5
Q

What does X-ray housing provide?

A

radiation protection
electrical protection
thermal protection
physical protection

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6
Q

How does the housing provide radiation protection?

A

lead lined
window allows useful radiation to escape

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7
Q

How does the housing provide electrical protection?

A

high voltage receptacles
glass and oil - makes sure electricity is contained

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8
Q

How does the housing provide thermal protection?

A

oil
cooling fan

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9
Q

Why is the physical housing around the X-ray tube important?

A

glass is fragile
casing protects against bumps

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10
Q

What is the glass tube?

A

contains cathode and anode
Pyrex - high heat capacity and an insulator
Vacuum - no collisions and prevents oxidation of electrodes

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11
Q

What is a metal tube used for?

A

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

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12
Q

What is the filament of the cathode?

A

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

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13
Q

Why use Tungsten?

A

melting point of 3410C
prevents the wire from melting
low vapour pressure
does not easily evaporate
less vaporization occurs at high temperature

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14
Q

What is thermionic emission?

A

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

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15
Q

How does thermionic emissions work?

A

needs to reach threshold to emit 2200C
after threshold, small filament current increase causes a large tube current increase

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16
Q

what is the danger of thermionic emission?

A

too much heat can cause evaporation

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17
Q

What is a space charge?

A

electrons form cloud around filament
space charge is held in place
- negative electrons repel each other
- filament becomes “positive”
- equilibrium

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18
Q

what is a compensation circuits?

A

Ensures mA is what we chose
mA can change depending on what kVp we select

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19
Q

How does compensation circuit work with high kVp?

A
  • easy to pull mA across tube
  • lowers filament heating current
  • lowers space-charge size
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20
Q

How does compensation circuit work with low kVp?

A
  • much harder to pull mA across tube
  • increase filament heating current
  • increase space charge size
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21
Q

What is the difference between the filament current and tube current?

A

Filament current - creates space charge and prep button
Tube current - allows mA to flow across tube and exposure button

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22
Q

What is the focusing cup?

A

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

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23
Q

What are dual focal spots?

A

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

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24
Q

Selecting mA

A

mA has to be selected in “steps” and proportional to filament current
small focal spot: uptown 300 mA
large focal spot: over 400 mA

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25
What happens when you increase the kVp?
Increases energy of photons Increases the number of photons Does NOT change the number of electrons
26
What does the anode do?
X-ray production supports the target conducts current dissipates heat stationary or rotating
27
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
28
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
29
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
30
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%)
31
What are the elements of the anode disc backing?
molybdenum and carbon Lessening thermal conductivity
32
What is the stem of the anode?
connects disc to rotor made of molybdenum - poor heat conductor thin
33
What is the rotor of the anode?
made of copper - high conductivity rotates on bearings
34
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
35
what type of motor is the rotor and stator?
induction motor generator and motor principle stimultaneously electromagnetic induction
36
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
37
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
38
What causes anode wear?
causes: uneven thermal expansion and high exposure ratings (mA and kVp) Consequences: pitting and unequal x-ray intensity
39
Difference of rotating vs stationary targets for heat dissipation?
rotating has 500x heat capacity
40
What causes bearing wear?
causes: heavy use and longer prep Consequences: slower rotation (more pitting) and "grinding noise"
41
What causes housing damage?
causes: physical damage and overheating Consequences: oil leakage - things get hotter and then the tube can get more damaged
42
How to prevent tube wear?
lower techniques, shorter preps warm up the tube - help prevent the anode damage - procedure varies between manufacturers
43
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)
44
Heat storage capacity
how many exposure you can do without cooling - important for bearings
45
What are heat units?
HU = kVp x mAs
46
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
47
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
48
how does the tube cool down?
tube is in contact with oil to dissipate heat - usually just the end of the rotor
49
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
50
What is the purpose of beam restriction?
patient safety image quality
51
how does beam restriction impact patient safety?
protects patients from radiation smaller field size - less tissue irradiated less energy absorbed by patient lowers dose
52
How does beam restriction impact image quality?
helps image contrast less scatter less tissue irradiated higher subject contrast improves image quality
53
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
54
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
55
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
56
what do the different sets of shutters do?
first set - reduce off focus radiation second set - allow field size/shape selection
57
correlation between shutter distance and blur
lowe shutters = less blur
58
what angle must the mirror be at inside the tube??
45 degrees
59
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
60
what percent of SID can misalignment be?
2%
61
What is positive beam limitation?
limits the size of the collimation to the size of the IR
62
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
63
how is the diverging beam created?
by isotropic emission
64
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
65
Magnification
caused by diverging beam outer portion of beam makes the object appear larger affected by SID and SOD/OID
66
magnification factor
how much larger the object appears in the image Magnification factor = SID/SOD
67
why is magnification useful
can be used to increase object size better than zooming in - more pixels per area of anatomy
68
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
69
how is foreshortening caused
object is angled but IR is flat
70
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
71
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
72
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
73
blur prevention
longer SID shorter OID smaller focal spot size - less point sources
74
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
75
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
76
small focal spot pros and cons
pros - sharper image - less blurring cons - lower ratings - more easily damaged
77
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
78
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
79
effective focal spot
focal spot as seen from the POV of IR square shape point sources are closer together - better spacial relation
80
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
81
Same size, different angles
large angle - shallow slope - larger effective size smaller angle - steeper slope - smaller effective size
82
field size
smaller angles give us smaller field sizes
83
On what images would a small angle be detrimental?
larger pieces of anatomy
84
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
85
when do you use the anode heel effect
can act like a filter align anode to thinner portion of anatomy T-spine abdomen
86
how to reduce the anode heel effect
longer SID smaller field sizes
87
biangular anodes
some anodes have 2 different angles chooose either one depending on if your need the sharper image or larger field

Medical Radiation Sciences 1 (4 decks)

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