Physics - Chapter 5

X-ray Tube

A lead-lined mechanical structure / metal support that serves as an electrical insulator and thermal cushion for the x-ray tube

Protective Housing

Which 2 structures dissipate heat away from the tube and protect it from thermal damage?

Oil Bath - draws heat away from the tube

Cooling Fans - circulate air around the assembly

What type of radiation does the protective housing design reduce?

Leakage Radiation

The housing protects the x-ray tube and serves as

an electrical insulator and thermal cushion

What 3 parts of the tube are encased in a glass or metal envelope (enclosure)?

Anode, Cathode, Induction Motor

What is the main purpose of the envelope / enclosure?

To maintain a vacuum within the tube

What is the target window?

Desired exit point for the x-rays produced

Provides the target for electron interaction to produce x-rays and is an electrical and thermal conductor

Anode

A Tungsten button embedded in a copper rod; the target does not move

Stationary Anode

What is the primary disadvantage of the stationary anode?

Electrons are always hitting the same small target area; heat builds up rapidly, damaging tube life; limited exposure factors

Which type of anode design is used in general purpose radiography today?

Rotating Disc Anode - made of Molybdenum

Target Material - Tungsten

Shaft - Copper

Added to increase thermal capacity and tensile strength - Rhenium

Electromagnets arranged in pairs; located outside of the tube

Stator

Made of an iron core located inside the tube (in the center of the stator)

Rotor

How are the rotor and the stator operated?

Through mutual induction

The operation of opposing pairs of stators in sequence that causes the rotation of the rotor

Induction Motor

The anode rotates at speeds of?

3400 revolutions per minute

What is the purpose of rotating the anode?

To spread the tremendous amount of heat produced during x-ray production over a larger surface area

What is the line - focus principle?

By angling the face of the anode target, a large actual focal spot (area actually bombarded with filament electrons) can be maintained and a small effective focal spot size (x-ray beam as seen from the patients perspective) can be created.

True or False: The size of the electron stream depends on the size of the filament

True

True or False: The larger the effective focal spot, the sharper the image will be

False - the Smaller effective focal spot

What are the target angles for a general purpose tub?

7-18 degrees

12 most common

Describe the Anode - Heel Effect

Caused by photons produced on the anode side, which must penetrate the “heel” of the target before exiting the tube thus reducing the intensity of the beam on the anode side.

“the angle causes the intensity of the beam to be less on the anode side”

Provides the source of electrons needed for x-ray production

Cathode

True or False: The filaments are connected to the primary circuit and the focusing cup is connected to the filament circuit

False - Filament = Filament Circuit

Focusing Cup = Secondary Circuit

What is a dual - focus tube?

2 filaments

True or False: Filaments are made of Tungsten

True

What element is added to increase thermionic emission and extend the filament life?

Thorium - radioactive metallic element

What is thermionic emission?

The boiling off of electrons

What is the focusing cup made of?

Nickel

What does the focusing cup do?

Receives a strong negative charge from the secondary circuit that forces electrons together into a “cloud” (called a space charge) as they are boiled off of the filament.

What are the 3 things needed to produce x-rays?

A large potential difference to give kinetic energy to the filament electrons (kVp)

A vehicle on which kinetic energy can ride; a quantity of electrons (mAs)

A place for interaction (target of the anode)

Major causes of tube failure

1. Frequent use of very high or max. exposure factors

2. Use of lower, but very long exposure factors (maintaining the tube at high temperatures)

3. Overload of the filament (prolonged excessive heating caused by prepping the rotor or arcing from the filament).

3 process of heat transfer

1. Conduction of heat by heat-tolerant materials

2. Radiation of heat energy from the anode to the oil bath

3. Convection of heat into the room by cooling fans

X-ray machines also have built in protective circuits that prevent the use of unsafe exposure techniques.

What can be done to extend tube life?

• Warm-up steps should be followed

• Do not prep the rotor excessively

• Press both the rotor and exposure buttons almost simultaneously

• Do not consistently use extremes of high exposure factors.

A measure of the amount of heat stored in a particular device?

Heat Units

Generator Types - Heat Numbers

Single Phase = 1

3 phase, 6 pulse = 1.35

3 phase, 12 pulse = 1.41

High Frequency = 1.45