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Medical Radiation Sciences 1 > Unit 2 > Flashcards

Unit 2 Flashcards

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USMLE® Step 1 flashcards
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
Q

What happens when you increase the kVp?

A

Increases energy of photons
Increases the number of photons
Does NOT change the number of electrons

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26
Q

What does the anode do?

A

X-ray production
supports the target
conducts current
dissipates heat
stationary or rotating

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27
Q

What is a stationary anode?

A

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

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28
Q

What is the Tungsten target?

A

made of tungsten (W)
High atomic number - good at producing X-rays
High melting point
High thermal conductivity
Low vapourization

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29
Q

What is target damage?

A

excessive heat causes damage
heat concentrated onto small area
unequal thermal expansion and concentration causes pitting
Factors (tube ratings): mA, time, kVp

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30
Q

What is a rotating anode?

A

has better heat dissipation because of rotation
Components: disc, focal track, stem, rotor, stator
target made of tungsten and rhenium (10%)

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31
Q

What are the elements of the anode disc backing?

A

molybdenum and carbon
Lessening thermal conductivity

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32
Q

What is the stem of the anode?

A

connects disc to rotor
made of molybdenum - poor heat conductor
thin

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33
Q

What is the rotor of the anode?

A

made of copper - high conductivity
rotates on bearings

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34
Q

What is the stator of the anode?

A

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

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35
Q

what type of motor is the rotor and stator?

A

induction motor
generator and motor principle stimultaneously
electromagnetic induction

36
Q

What is tube aging?

A

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
Q

What causes filament wear?

A

caused by: high mA and longer prep
Consequences: tungsten plating on tube can cause arcing (may shatter the tube)
filament may break

38
Q

What causes anode wear?

A

causes: uneven thermal expansion and high exposure ratings (mA and kVp)
Consequences: pitting and unequal x-ray intensity

39
Q

Difference of rotating vs stationary targets for heat dissipation?

A

rotating has 500x heat capacity

40
Q

What causes bearing wear?

A

causes: heavy use and longer prep
Consequences: slower rotation (more pitting) and “grinding noise”

41
Q

What causes housing damage?

A

causes: physical damage and overheating
Consequences: oil leakage - things get hotter and then the tube can get more damaged

42
Q

How to prevent tube wear?

A

lower techniques, shorter preps
warm up the tube
- help prevent the anode damage
- procedure varies between manufacturers

43
Q

What factors affect tube ratings?

A

focal spot size
rotation speed
disk diameter (larger = better ratings)
waveform
segmented/grooved anode
metal tube (no fear of tungsten evaporation)

44
Q

Heat storage capacity

A

how many exposure you can do without cooling
- important for bearings

45
Q

What are heat units?

A

HU = kVp x mAs

46
Q

Heat units/second

A

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
Q

What is an anode cooling chart?

A

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
Q

how does the tube cool down?

A

tube is in contact with oil to dissipate heat
- usually just the end of the rotor

49
Q

What is a rotating envelope tube?

A

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
Q

What is the purpose of beam restriction?

A

patient safety
image quality

51
Q

how does beam restriction impact patient safety?

A

protects patients from radiation
smaller field size - less tissue irradiated
less energy absorbed by patient
lowers dose

52
Q

How does beam restriction impact image quality?

A

helps image contrast
less scatter
less tissue irradiated
higher subject contrast
improves image quality

53
Q

What was the start of the aperture diaphragm?

A

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
Q

use of cones and cylinders

A

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
Q

What is a variable aperture or light beam collimator?

A

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
Q

what do the different sets of shutters do?

A

first set - reduce off focus radiation
second set - allow field size/shape selection

57
Q

correlation between shutter distance and blur

A

lowe shutters = less blur

58
Q

what angle must the mirror be at inside the tube??

A

45 degrees

59
Q

what happens if the bulb and focal spot are not equal distance from the mirror?

A

bulb too close - xray beam would be smaller than light field
bulb too far - xray beam would be larger than the light field

60
Q

what percent of SID can misalignment be?

A

2%

61
Q

What is positive beam limitation?

A

limits the size of the collimation to the size of the IR

62
Q

What is a collimation scale?

A

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
Q

how is the diverging beam created?

A

by isotropic emission

64
Q

How does the diverging beam affect the image?

A

magnification
distortion
- focal spot blur
inverse square law for intensity
anode heel effect
these affects are not always detrimental

65
Q

Magnification

A

caused by diverging beam
outer portion of beam makes the object appear larger
affected by SID and SOD/OID

66
Q

magnification factor

A

how much larger the object appears in the image
Magnification factor = SID/SOD

67
Q

why is magnification useful

A

can be used to increase object size
better than zooming in
- more pixels per area of anatomy

68
Q

How is distortion caused

A

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
Q

how is foreshortening caused

A

object is angled but IR is flat

70
Q

how is elongation caused

A

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
Q

when do you use your angle to your advantage

A

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
Q

focal spot blur

A

caused by diverging beam AND multiple point sources
edges of object hit by x-rays from multiple sources
creates penumbra
- geometric unsharpness

73
Q

blur prevention

A

longer SID
shorter OID
smaller focal spot size
- less point sources

74
Q

Inverse square law

A

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
Q

Large focal spot pros and cons

A

pros
- more area to be hit by electron beam
- less pitting of target surface
- higher ratings
cons
- more point sources
- more blur

76
Q

small focal spot pros and cons

A

pros
- sharper image
- less blurring
cons
- lower ratings
- more easily damaged

77
Q

what is the line focus principle

A

best of both worlds
use angle to achieve large actual focal spot
- high ratings
AND small effective focal spot
- less blur

78
Q

actual focal spot

A

large area to receive mA
- can take more heat and dissipate it better
rectangular in shape
point sources are farther apart
- larger penumbra

79
Q

effective focal spot

A

focal spot as seen from the POV of IR
square shape
point sources are closer together
- better spacial relation

80
Q

Anode angle

A

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
Q

Same size, different angles

A

large angle
- shallow slope
- larger effective size
smaller angle
- steeper slope
- smaller effective size

82
Q

field size

A

smaller angles give us smaller field sizes

83
Q

On what images would a small angle be detrimental?

A

larger pieces of anatomy

84
Q

Anode heel effect

A

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
Q

when do you use the anode heel effect

A

can act like a filter
align anode to thinner portion of anatomy
T-spine
abdomen

86
Q

how to reduce the anode heel effect

A

longer SID
smaller field sizes

87
Q

biangular anodes

A

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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