The X-ray Tube (Ch 5)

What makes up the x-ray tube?

Cathode, anode, and glass tube housing

What is 1 pointing to?

Stator electromagnets

What is 2 pointing to?

Armature

What is 3 pointing to?

+

What is 4 pointing to?

Rotating portion

What is 5 pointing to?

Molybdenum neck and base

What is 6 pointing to?

Electron beam

What is 7 pointing to?

Filament

What is 8 pointing to?

-

What is 9 pointing to?

Filament circuit

What is 10 pointing to?

Envelope

What is 11 pointing to?

Tungsten anode

What is 12 pointing to?

Bearing

What side is the negative side of the tube?

Cathode

What are the functions of the cathode assembly?

1. to produce a thermionic cloud

2. conduct the high voltage between the cathode and anode

3. focus the electron stream

What are the parts of the cathode assembly?

Filament, focusing cup, associated wiring

What is the filament?

A small, thin coil of thoriated tungsten

How thick in the filament?

0.1-0.2 mm

What is the atomic number of thorium?

90

Why is tungsten used for the filament?

• high melting point (3370 degrees C)

• hard to vaporize

What is the melting point of thoriated tungsten?

3420 degrees C

What do some tubes also use other than tungsten?

Molybdenum or rhenium

What is the atomic number of rhenium?

75

What is the melting point of molybdenum?

2620 degrees C

What is the melting point of rhenium?

3170 degrees C

What is the atomic number of molybdenum?

42

The length and width of the filament have a great affect on what?

Recorded detail (focal spot size)

How many filaments do most modern tubes have and what are they called?

2, dual focus tube

For dual focus tubes with a small focal spot, how big is it?

0.3 - 1 mm

For dual focus tubes with a large focal spot, how big is it?

1 - 3 mm

Why would you use a small focal spot on a dual focus tube?

For better detail

Why would you use a large focal spot on a dual focus tube?

For large techniques- so the filament does not become damaged

What is this?

Dual filaments in focusing cup

What is this?

Dual filament wire

What is the purpose of the filament?

It provides enough resistance to the flow of electrons that the heat produced will cause thermionic emission

When electrons leave the surface of the wire of the filament what forms?

An electron cloud

When high voltage is applied to the cloud it will be drawn towards the ______

Anode target

How many hours of tube life does the average filament have?

6-9 hours

How many exposures does the average filament life have?

10,000 to 20,000

What is the focusing cup?

A shallow depression in which the filament sits

How is the focusing cup charged?

Negatively to focus the electrons

What is the purpose of the focusing cup?

To narrow thermionic cloud as it is driven towards the anode

What is this?

Focusing cup

What happens when the focusing cup is not working properly?

The efficiency of the tube decreases— focal bloom

What causes the space charge effect?

When there are too many electrons around the filament

What makes up the anode assembly?

Anode, stator, rotor

What are the functions of the anode assembly?

1. serves as the target surface for the high-speed electrons from the cathode

2. the source of the x-ray photons

3. conducts the high voltage from the cathode

4. serves as the primary thermal conductor

What side of the x-ray tube do electrons travel from?

Cathode (filament)

Where are the x-rays actually produced?

Anode

What are the two types of anodes?

Stationary and rotating

What type of anode is this?

Stationary

What type of anode is this?

Rotating

Describe the stationary anode:

• a block of tungsten imbedded in the 45 degree angled end of a copper rod

• about 1.5 cm x 1 cm surface area

• only found in dental equipment and old x-ray units

Describe the rotating anode:

• provides a greater surface area

• dissipates heat better

• used in most modern x-ray units

How large is the focal spot for the stationary anode?

4 mm squared

How large is the focal spot for the rotating anode?

1835 mm squared

How large is the rotating anode disk?

5 to 13 cm in diameter

What is the rotating anode disk made of?

Molybdenum

What is the target focal track made of?

Tungsten-rhenium alloy

What makes with the rotating anode?

• rotating anode disk

• target focal track

• anode disk backing

What is this?

Rotating anode

What are the advantages of using tungsten as the target material?

1. high melting point (3370 degrees C)

2. high atomic number (74)

3. heat conducting ability

Why would you want to use rhenium?

Its used due to elasticity when focal track expands due to rapid heat

Why would you use graphite molybdenum backing?

Allows double heat loading capabilities

What is the stress relieved anode?

• majority of anodes

• dissipates heat more efficiently

• still needs warm up of tube

What is the target area also called?

Target, focus, focal point, focal spot or focal track

Where is the area of electron interaction?

The target area (where x-rays are created)

What is the actual focal spot?

The physical area of the focal track that is impacted by the electrons

What is the effective focal spot?

The area of the focal spot that is projected out of the tube toward the patient

What controls the effective focal spot?

The size of the actual focal spot (length of the filament) and the anode target angle

What is this?

Actual focal spot

What is the line focus principal?

When the target is angled (less than 45 degrees) the effective focal spot is smaller than the actual focal spot

The _____ the angle, the _____ the effective focal spot

Smaller

What doee anode angles range from?

7 to 17 degrees

What is the most common anode angle?

12 degrees

Due to the geometry of the angled anode target, the radiation intensity is greater on the _____ side

Cathode

The intensity of the x-ray beam can vary as much as ____ from anode to cathode

45%

What makes up the induction motor?

Stator and rotor

Describe the stator:

• outside the glass envelope

• series of electromagnets that rotate the rotor

• switch is pressed halfway sends current to stator

What turns the anode?

Rotator

If stator fails to rotor I twill not turn _____

Anode

Describe the rotor:

• located inside the stator

• electromagnetic field causes stator to turn causing rotor to turn

What speed does the rotor turn at?

3200-3600 RPM

High speed rotor revolve at ____ RPM

10000

What lines the shaft of the rotor?

Ball bearings

How long does ball bearings coast for?

60 seconds and slow down when exposure is taken

What is the glass envelope made of?

Heat-resistant Pyrex glass

What does the glass envelope do?

Encloses all of the cathode assembly and all of the anode assembly except the stator

What is the window of the glass envelope?

Thinner section where the primary beam exits the tube

The glass envelope maintains the ______

Vacuum

What is this?

Glass envelope

What are the two types of envelopes?

Glass and metal

Where are metal envelopes more common?

In CT

What do metal envelopes do?

• prolong tube life

• still have window segment

After envelope is constructed, _____ is removed from tube

Air

What does the vacuum do?

Allows electrons to flow without interference from gas atoms of air (increases efficiency of tube)

What are the functions of protective housing?

1. controls leakage and scatter radiation

2. insulates the high voltage

3. provides a means to cool the tube

How are x-rays produced?

Isotropically

The tube housing is designed to absorb most of the x-rays except those in the ______

Primary beam

What is the housing made of and lined with?

made with cast steel and lined with lead