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Advanced Image Aquisition > Xray Tube > Flashcards

Xray Tube Flashcards

1
Q

Conditions necessary for X-ray production

A
  1. Source of electrons
  2. High voltage
  3. Target material
  4. Vacuum
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2
Q

X-ray tube construction

A

Cathode
Anode
Glass envelope
Protective housing

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3
Q

The cathode contains:

A

Filament

Focusing cup

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4
Q

The anode contains:

A 
Target
Stator
Rotor
Glass envelope
Protective housing
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5
Q

What is the filament made out of and why?

A

Thoriated tungsten
For: low vaporization so tube won’t get gassy
High melting point so it won’t burn out the bulb
And high thermionic emission
Longer tube life

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6
Q

What is the significance of a dual focus filament?

A

Small filament for detail, large filament for heat

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8
Q

When does thermionic emission occur

A

After the filament has reached incandescence, glows orange or red

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9
Q

What does a biased focusing cup do?

A

Charges more negative than normal to narrow electron stream for detail. Ex: mammo tube

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10
Q

Grid controlled tube

A

Has 3 charged electrodes:
Anode
Cathode
Grid/focusing cup
Focusing cup charges negative which closes off electron flow then charges positive to allow exposure
PURPOSE: allow precise rapid exposures. Pulse

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11
Q

What is a stationary anode used for?

A

Dental unit.
Made out of tungsten rhenium allow embedded in copper
Low heat dissipation

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12
Q

What is a rotating anode made out of?

A

Tungsten rhenium on molybdenum disk.

Has a larger target area for higher heat dissipation

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13
Q

What is the target made out of and why?

A

Tungsten bc:
Higher atomic number (74) gives high efficiency x Ray production and high energy X-rays
High melting point allows high tube current without bubbling or pitting
High heat conducting ability to dissipate heat
Rhenium provides elasticity

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14
Q

Stator

A

Only part outside of the glass envelope

Electromagnets

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15
Q

Rotor

A

Copper cylinder/cuff attached by molybdenum stem

Inside the rotor is silver plated ball bearings which serve as a lubricant between cuff and core

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16
Q

How fast does a common anode spin?

A

3200-3600 rpm

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17
Q

How fast does a high speed anode spin?

A

10,000-12,000 rpm

Used in cath lab and Fluoro

18
Q

What is the glass envelope?

A

Purpose: maintain the vacuum
Contains cathode and anode
Window is where primary beam exits

19
Q

What can the envelope be made out of and which is better?

A

Heat resistant Pyrex
Can be metal: metal is better for:
Less likely to fail, high output of X-ray tubes, less off focus radiation

20
Q

What does the protective housing do?

A

Provides mechanical support for tube, cushions tube from rough handling, cools the tube, isolated high voltage, controls leakage(which may not exceed 1 mGya/hr or 100 mR/ hr at 1 meter)

21
Q

What is off focus radiation?

A

Radiation not produced at the focal spot but in the tube

If too much leaks, can cause ghosting of structures

22
Q

What happens when tungsten is vaporized?

A

Gassy tube
Deposit on envelope cause increased filtration of the primary beam and can cause high voltage arcing
Breaking of the filament from overuse of the rotor

23
Q

Space charge limited region

A

Too low of kV ranges not all electrons are driven to the target.
Below 70 KV space charge compensator comes in and adjusts filament amps to make up for electrons that didn’t get pushed over.
At about 70 kV, all electrons are pushed over called saturation current

24
Q

What is the focusing cup made out of?

A

Molybdenum or nickel

25
Q

Line focus principle

A

Want small effective focal spot for detail and large actual focal spot for heat dissipation
Smaller angle=smaller effective focal spot

26
Q

What makes an anode heel effect more or pronounced?

A

Small effective focal spot size
Short SID
Large cassette size

27
Q

What can cause tube failure in the anode?

A

Melting, putting, cracking of anode
Tungsten vaporization
Damage to rotor bearings (most common cause of tube failure)

28
Q

What causes tube failure in cathode end?

A

Filament vaporization

High voltage arcing

29
Q

Production of X-rays includes

A
  1. Low volts/high amps to warm filament
  2. Apply rotor to give proper thermionic emission
  3. Push exposure button-high volts/low amps to drive electrons to targets that the speed of light.
  4. X-ray/ heat production

Advanced Image Aquisition (7 decks)

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