CHAPTER 6

External Structures of X-Ray Tube

The Support Structure, metal/glass enclosure, protective housing

Preferred Detent Position

X-ray tube is centered above the examination table at the standard SID

Floor-to-Ceiling Support

Single column with rollers at each end

Floor Support System

Floor mounted with rails

Isotropic

Equal intensity in all directions

Useful Beam

X-Rays emitted through the window

Mechanical Support

Protective housing

Protects the tube

Insulator

A material that does not conduct heat well

Cushion

Dissipates heat

Enclosure

(Glass or Metal)

X-Ray tubes are designed with an __

Leakage Radiation

X-rays that escape through the protective housing other than the primary rays

All radiation coming from within the x-ray tube housing except the useful beam

Cathode

Negative side of the x-ray tube

Filament

Focusing Cup

Filament

Coil of wire

Thermionic Emission

Emission of electrons from a heated surface

Thoriated Tungsten

Filaments are usually made of __

Focusing Cup

A recessed area where the filament lies, directing the electrons toward the anode

Grid-Controlled Tubes

Designed to turn on & off very rapidly

Grid

Element in the tube that acts as a switch

Filament Current

The x-ray tube is adjusted by controlling the __

Space Charge

Cloud of electrons emitted from the filament before they are accelerated to the anode

Space Charge Effect

Electrostatic repulsion

Cloud of electrons that forms around the filament

Space Charge Limited

Thermionic effect at low kVp

kVp

Kilovoltage peak

Measures the power of penetrability (quality) of the x-rays produced

mA

Milli-Ampere

Focal spot size is dependent on this

Saturation Current

Further increasing in kVp does not result in higher mA beyond maximum value

Anode

Positive side of the x-ray tube

Conducts electricity & radiates heat

Contains the target

Stationary Anode

Anode that stays in a fixed position

A nonmoving anode, usually found in dental and small portable radiography units

Rotating Anode

An anode plate that rotates around a stem made of molybdenum to aid in heat dissipation

Powered by an electromagnetic induction motor

Electrical Conductor

A material through which electric charge can flow easily

Target

Area of the anode struck by the electrons from the cathode

Higher

Shorter

tube currents and exposure times are possible with the rotating anode

Focal Spot

__ is the actual x-ray source

Line-Focus Principle

__ results in an effective focal spot size much less than the actual focal spot size

Smaller

Larger

The the anode angle, the the heel effect

Off-Focus Radiation

Occurs when projectile electrons are reflected and x-rays are produced from outside the focal spot

Excessive Heat

__ results in reduced x-ray tube life

Radiation

Energy that is radiated or transmitted in the form of rays or waves or particles

Conduction

Heat transfer by direct contact

Cold Anode

Maximum radiographic techniques should never be applied to a __

Vaporized Tungsten

The most frequent cause of abrupt tube failure is electron arcing from the filament to the enclosure because of __

Open Filament

Due to prolonged periods of mA operation

Housing Cooling Chart

Graph chart showing the cooling rate of the x-ray tube housing

Heat Unit

Thermal energy unit to express capacity

1 HU = 1 J

Anode Rotation Speed

3,400 or 10,000 RPM

X-Ray Tube Current

Cathode to anode electron flow

Open Filament

Induction Motor Failure

Cracked Anode

3 causes of x-ray tube failure

Thorium + Tungsten Filament Wire

Prolongs x-ray tube life

Filament inside Focusing Cup

Shapes the electron beam electrostatically

Higher Atomic Number

= Efficient x-ray production

Thermal Conductivity

= Heat dissipation

High Melting Point

= Greater heat capacity

Heel Effect

Causes radiation intensity on the cathode side of the x-ray beam to be higher than on the anode side

Variation in radiation intensity result due to the target's angle

Ceiling Support System

Primary support structure for the x-ray tube

Allows the greatest ease of movement & range of position

Protective Housing

• Covers the x-ray tube

• Reduces leakage radiation

• Provide mechanical support

• Serves as a way to conduct heat away from the x-ray tube target

Glass or Metal Enclosure

Surrounds the cathode (-) and the anode (+)

Tungsten Filament

Inside cathode

Source of electrons