Chapter 5: The X-ray Tube

line-focus principle

A principle that states by angling the face of the anode target we can maintain a large actual focal spot size and create a small effective focal spot size.

X-Ray Tube Housing:

Contains the x-ray tube. Its what the technologist sees. Made of metal. Its purposes: 1. holds the x-ray tub. 2. lead-lined to prevent leakage radiation from exiting (no more than 100mR/hr) 3. Insulated against electrical hazards from high voltages. 4. Cools with oil bath and cooling fans to dissipate heat away from the tube.

Anode heel effect

Phenomenon resulting from the angling of the target face that causes the intensity of the x-ray beam to be less not he anode side because the "hell" of the target is into path of the beam.

leakage radiation

Photons produced int he x-ray tube that are traveling in directions other than toward the patient.

What are the 4 major parts of the x-ray tube?

1. Cathode 2. Anode 3. Envelope 4. Tube housing.

The cathode has 2 primary parts. What are they?

1. Focusing cup- made of nickel. Surrounds the filament. Its purpose is to focus the stream of electrons. It has a negative charge to keep the electron cloud spreading. it helps to focus the electrons towards the anode. 2.Filament- a coil of tungsten wire embedded int he focusing cup. its purpose is that it is the source of electrons for x-ray production. The outer shell electrons of the filament atoms are "boiled off" and ejected from the filament. Thermionic emission boiling off of electrons. Its dual focus- there is a large and small, only one gets heated up. This focal spot size affects recorded detail sharpness. The smaller the size the better the detail, ex: 100S. The larger the size the less detail, ex: 100L.

What are the 3 main parts to the anode?

1. Target- The area on the anode that is struck by the electrons from the cathode. Made of tungsten. Focal Spot-they physical area of the target that is bombarded by electrons. angle on anode directs x-rays downward towards window (envelope exit). 2. Rotor- (made of copper) Rotates the target rapidly during x-ray production and is located inside the class envelope. 3. Stator- electric motor that turns the rotor at high speed and is located outside the glass envelope.

List the order of energy in the x-ray tube.

1. electrical 2. thermal 3. kinetic 4. thermal and electromagnetic. electromagnetic = x-ray photons

The purposes of the rotating anode:

1. immediate deceleration of the electrons of the tube current. 2. Spreads the heat produced over a larger area (can use higher exposures without damage to tube) 3. Conducts current to the rectifier and other parts of the closed circuit. 4. Transmits amounts of heat to the oil surround the envelope.

__% of energy produce at the anode is heat. ___% of the energy produced is x-ray.

99. 1.

space charge

A cloud of electrons formed by the focusing cup as electrons are boiled off of the filament.

filament

A coil of wire, usually 7-15 mm long, 1-2 mm wide, and usually made of tungsten with 1%-2% thorium added.

heat units (HUs)

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

focusing cup

A metal shroud that is made of nickel and surrounds the x-ray tube filaments on their back and sides, leaving the front open and facing the anode target.

rotor

A part of an induction motor made up of an iron core (iron bars embedded in the copper shaft) surrounded by coils and located int he center of the stators.

stator

A part of an induction motor made up of electromagnets arraigned in paris around the rotor.

target window

A thinned section of the x-ray tube enclosure that is the desired exit point for the x-rays produced.

induction motor

An electric motor in which the shaft is rotated through mutual induction. It is the Rotor and Stator.

Where are x-rays created?

At the target. X-rays are isotropic-produced in all directions.

Envelope

Enclosure that contains the cathode and anode. there are 2 types: glass and metal The purposes: 1. allows efficient flow of electrons from cathode to anode (no air molecules to bump into). 2. Provides insulation from electric shock (cathode and anode are electrical charges). 3. Dissipates heat by conducting it to insulating oil that surrounds it.

what is the physical area of the target that is bombarded by electrons called? what is it called on a rotating anode?

Focal spot. Focal track for rotating.

Glass vs Metal envelopes:

Glass: made of pyrex. tolerates high heat. vacuum- contains no air so that nothing will interfere with the travel of the electrons towards anode. Immersed in oil- to disperse the heat created. Window (port) where the x0rays exit the envelope, designed to allow the primary beam to exit the tube (made of thinner glass) Metal: More commonly used today. Longer tube life span than glass envelopes.

thermionic emission

The literal boiling off of electrons from a filament by a flow of electrical current.

Cathode

The negative end of the x-ray tube and source of electrons. Negatively charged electrode in the x-ray tube. The purpose: source of negative charged electrons that are used to produce an x-ray. Has two primary parts: focusing cup and filament.

space-charge effect

The self-limiting factor caused by the space charge reaching a size commensurate with the current used and making it difficult for additional electrons to be emitted.

Effective focal spot

The x-ray beam area as seen from the perspective of the patient.

protective housing

a lead-lined metal structure that provides solid, stable mechanical support and serves as an electrical insulator and thermal cushion for the x-ray tube.

What are used to dissipate the heat?

anode, envelope, oil bath, and cooling fans.

Actual Focal Spot

area of x-ray tube target actually bombarded with filament electrons. exact point where the x-ray beam derives from. on a rotating anode it is called a focal track. Angle on anode directs x-ryas downward towards window (envelope exit).

Why is tungsten used?

it has a very high melting point and its thermal conductivity is almost equal to that of copper. Furthermore, it has a high atomic number (74), improving the efficiency of x-ray production. more efficient heat dissipation (thermal conductivity).

X-ray Tube

the part of the radiographic system that produces the x-radiation. Converts electrical and kinetic energy into electromagnetic (x-ray) and thermal energy. Electrical energy is supplied to the tube through two large electrical cables.

Anode

the positive end of the tube that provides the target for electron interaction to produce x-ryas; also an electrical and thermal conductor. It is made of molybdenum (stem), copper (rotor), tungsten (target), and graphite. There are 2 types: stationary and rotating. Stationary is less common and is used in places like a dental office. The disadvantage is that it heats up rapidly in a small target area so it damages the tube faster. The rotating -anode is made of molybdenum coated with tungsten and mounted on a copper shaft with a molybdenum core. Immediate deceleration of the electrons of the tube current. spreads heat produced over a larger area ( can use higher exposures without damage to table). Has 3 main parts: Target, stator, rotor

Off-Focus Radiation

x-rays produced outside of the focal spot. Less off-focus radiation occurs with a metal envelope.