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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

18
Q

what is a compensation circuits?

A

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

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
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
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

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

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

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

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

26
Q

What does the anode do?

A

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

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

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

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

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%)

31
Q

What are the elements of the anode disc backing?

A

molybdenum and carbon
Lessening thermal conductivity

32
Q

What is the stem of the anode?

A

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

33
Q

What is the rotor of the anode?

A

made of copper - high conductivity
rotates on bearings

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

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

Medical Radiation Sciences 1 (2 decks)

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