4.1 & 4.2 - The X-Ray Tube

Primary purpose of the X-ray tube

To generate an x-ray photon beam with both the quantity and quality desired by the operator.

Key requirements of an X-ray tube

- Generate photons unidirectionally (beam)

- Be durable (most expensive part to replace)

Main components of the X-ray tube

Housing, Diaphragm, Oil, Envelope, Induction motor, Anode (+), Cathode (-).

What is the function of the tube housing in an x-ray machine?

It holds the oil.

What does the tube housing do to leakage radiation?

It stops leakage radiation.

What feature does the tube housing contain for x-rays?

It contains a port that allows x-rays to leave as the primary beam.

Role of the oil in the X-ray tube

- Electrically insulates the anode and cathode

- Absorbs and dissipates heat produced in the tube.

What is the purpose of the diaphragm in a bellows?

Allows for expansion of oil as it heats up.

How does the diaphragm in a bellows prevent damage?

Prevents rupture of the solid tube housing by accommodating oil expansion.

What is the historical material used for the envelope in an X-ray tube?

Glass

What is the modern material often used for the envelope in an X-ray tube?

Metal

What is the primary function of the envelope in an X-ray tube?

Maintains a vacuum by keeping air out

How does the envelope in an X-ray tube affect electrical insulation?

It electrically insulates the anode and cathode

What is one of the functions of the envelope in an X-ray tube related to heat?

Dissipates heat to the surrounding oil

What is the negatively charged side of the tube called?

Cathode

What is the electron source?

Cathode

What are properties of the tungsten filament?

- High melting point

- Small and large focal point

What is the focusing cup made of in the cathode?

Nickel

What charge does the focusing cup have?

Negatively charged

Thermionic emission

The process by which high filament current heats tungsten, causing outer shell electrons to be released.

What is the space charge effect?

The cloud of freed electrons around the filament repels further electron release.

What causes the space charge effect?

The negative charge of the cloud of freed electrons.

What is the function of the focusing cup?

It keeps the electron cloud together.

What material is the focusing cup made of?

Negatively charged nickel.

How does a smaller focusing cup affect the electron cloud?

It results in a smaller focal spot.

Sequence of exposure preparation

1. Prep

2. Filament heats

3. Electrons form a cloud

4. Rotor spins up

5. Expose

6. High voltage (kVp) is applied

7. Electrons are accelerated toward the anode.

Anode properties

- The positively charged end of the tube

- Site of x-ray photon production.

What material has a high melting point and is used in anodes?

Tungsten

Which material increases ductility and toughness in anodes?

Rhenium

What material is known for being a good heat conductor in anodes?

Molybdenum

Which material conducts and stores heat while reducing rotor mass in anodes?

Graphite

Types of anodes

- Stationary anode: Fixed focal spot

- Rotating anode: Spinning focal track, increases area to spread heat.

Induction motor function

Rotates the anode; Designed to work without physical contact with the rotor.

What happens during outer shell electron excitation?

Produces heat (infrared), but no ionization.

What is produced by inner shell electron ejection?

Characteristic radiation (x-ray production)

What type of radiation is produced by atomic nucleus interaction?

- Electron is slowed and redirected

- X-ray production - Bremsstrahlung radiation.

Percentage of interactions that result in heat

99%

What happens to the anode during operation?

The anode gets very hot.

What material is used for heat dissipation in the anode due to its high melting point?

Tungsten

What material is used for heat conductivity in the anode?

Copper

What is Bremsstrahlung radiation?

'Braking radiation' created when electrons slow down near the nucleus.

What type of spectrum does Bremsstrahlung radiation produce?

A spectrum of photon energies.

What happens to photons less than 15 kVp and higher than 85 kVp in Bremsstrahlung radiation?

They don't leave the tube.

What shape does the energy distribution of Bremsstrahlung radiation form?

A bell curve.

Percentage of interactions that result in Bremsstrahlung

0.85%

What is characteristic radiation?

Radiation produced when an incoming electron ejects an orbital electron, resulting in specific energies.

What determines the specific energies of characteristic radiation?

The energy difference between electron shells.

What is required for an incoming electron to produce characteristic radiation?

The incoming electron must have enough energy to eject an orbital electron.

Which electrons must be ejected to produce useful x-ray photons?

Only the innermost electrons.

What is the minimum kVp required for characteristic radiation to be useful?

70 kVp

What percentage of emitted radiation does characteristic radiation make up?

Approximately 0.15%

What is the general shape of the x-ray emission spectrum?

Bell curve from Bremsstrahlung radiation

What is the average peak energy of the x-ray emission spectrum?

30-40 keV

What is the energy of the sharp peak in the x-ray emission spectrum for characteristic radiation?

70 keV

Sequence of events from prep to exposure

1. Prep

2. Filament circuit on and electron cloud forms

3. Rotor begins spinning

4. Expose

5. kVp applied

6. Cathode repels electrons anode attracts them

7. Electrons hit anode, and x-rays produced.

What is the anode heel effect?

X-rays do not exit the tube with uniform intensity.

Where is the X-ray beam more intense?

On the cathode side.

What is the relationship between anode target angles and the heel effect?

Smaller anode target angles result in a greater heel effect.

In clinical practice, where should thicker body parts be placed in relation to the X-ray tube?

Under the cathode side.