XRAY TUBE, PRODUCTION AND IMAGING SYSTEM

X-ray tube

Device that produces x-rays by converting electrical energy into x-ray photons.

Cathode and Anode

Two major components of the x-ray tube.

Protective housing

Glass or metal container enclosing the x-ray tube.

Provide mechanical support, prevent radiation leakage, protect from shock.

Purpose of the protective housing.

1 mGy/hr at maximum output

Maximum permissible x-ray leakage radiation at 1 meter.

Vacuum enclosure

Gas-free tube enclosure allowing electron flow without interference.

Pyrex glass

Type of glass used in x-ray tubes due to its heat resistance and low expansion.

Metal enclosure

Material sometimes used instead of glass to prolong tube life.

Filament

Component of the tube that emits electrons when heated.

Tungsten

Material used for the filament in x-ray tubes.

High melting point, high atomic number, low vaporization rate.

Reason tungsten is used in filaments.

Thorium

Element added to tungsten filament to extend life.

Filament support wires

Part that supports the filament and conducts current to it.

Focusing cup

Device that narrows the stream of electrons from the filament to the anode target.

Nickel

Material used for the focusing cup.

Concentrate electron beam onto the anode target.

Purpose of the focusing cup.

Space charge effect

Effect of space charge on tube current at low kVp.

Saturation current

Condition when further increase in filament current does not increase tube current.

Cathode

Negative terminal of the x-ray tube.

Anode

Positive terminal of the x-ray tube.

Stationary and rotating anodes

Two types of anodes.

Tungsten or tungsten-rhenium alloy

Material used for the anode target surface.

High atomic number, high melting point, good thermal conductivity.

Reason tungsten is used for the anode target.

Anode stem

Component that supports the rotating anode target.

Molybdenum

Material used for the anode stem to reduce heat conduction to bearings.

Molybdenum or graphite

Material used for the rotating anode disk base.

Lightweight, heat storage capacity.

Advantage of graphite anode disk base.

Induction motor

Mechanism used to rotate the anode.

Stator

Stationary part of the induction motor.

Rotor

Rotating part of the induction motor.

Increase heat capacity by spreading heat over a larger surface.

Purpose of anode rotation.

7° to 20°

Angle of the anode target surface in most diagnostic tubes.

Line-focus principle

Principle that effective focal spot is smaller than the actual focal spot.

Actual focal spot

Area on the anode target bombarded by electrons.

Effective focal spot

Projected x-ray beam source size.

Smaller angle → smaller effective focal spot.

Effect of anode angle on effective focal spot size.

Anode heel effect

Variation in x-ray intensity across the beam caused by anode angle.

More intense on cathode side, less on anode side.

Intensity difference due to heel effect.

Place thicker body part under cathode side.

Use of anode heel effect in imaging.

Target interactions

Process of x-ray production in the anode.

Less than 1%

Percentage of kinetic energy converted to x-rays in the anode.

Heat

Form of energy for the majority of kinetic energy in the anode.

Bremsstrahlung radiation

Radiation produced when electrons are decelerated by the nucleus.

Characteristic radiation

Radiation produced when an inner shell electron is ejected and outer electron fills vacancy.

69 kVp

Minimum tube voltage required for tungsten K-shell characteristic radiation.

Tube current (mA), Tube voltage (kVp), Exposure time

Primary factors affecting x-ray production.

Directly proportional

Relationship between mA and x-ray quantity.

Increases quantity and quality

Effect of increasing kVp on x-ray beam.

Insulation and heat dissipation

Purpose of the tube housing oil.

Radiation, conduction, convection

Method of heat dissipation in the x-ray tube housing.

Anode cooling chart

Device used to monitor anode heat loading.

Tube rating chart

Device used to monitor tube heat load.

Housing cooling chart

Device used to monitor housing heat load.

Repeated high exposures

Cause of pitting in the anode target.

Reduced x-ray output and roughened target surface

Result of anode pitting.

Prolonged use and high filament current

Cause of filament vaporization.

Coating of inside glass envelope, causing tube arcing

Result of filament vaporization.

Use lower mA and avoid long exposure times

Method to prolong filament life.

Prevent thermal shock to anode

Reason for warm-up procedure for x-ray tubes.

3,400 rpm

Typical anode rotation speed in diagnostic tubes.

10,000 rpm

High-capacity anode rotation speed.

Stationary anode

Type of anode used in dental and portable imaging units.

Tungsten embedded in copper

Material of stationary anode target.

Good heat conduction

Function of the copper anode body in stationary anodes.

Light weight, heat capacity

Reason for choosing molybdenum or graphite for rotating anode body.

X-ray production

Conversion of electrical energy into x-ray photons inside the x-ray tube.

Heated filament

Source of electrons in the x-ray tube.

Thermionic emission

Process of electron emission from a heated filament.

Focusing cup

Focusing device that directs electrons toward the anode target.

Electron acceleration

Process of accelerating electrons toward the anode by applying high voltage.

Bremsstrahlung and Characteristic

Two types of target interactions producing x-rays.

Bremsstrahlung radiation

Radiation produced when a projectile electron is slowed or deflected by the nucleus.

Difference between entering and exiting electron kinetic energy

Radiation energy in Bremsstrahlung interaction.

Equal to the electron’s kinetic energy (tube kVp)

Maximum x-ray photon energy in Bremsstrahlung radiation.

Characteristic radiation

Radiation produced when an inner shell electron is ejected and outer electron fills vacancy.

Difference between binding energies of involved shells

Photon energy in characteristic radiation.

69 kVp

Minimum tube voltage for tungsten K-shell characteristic radiation.

Less than 1%

Fraction of kinetic energy converted into x-rays in the anode.

Heat

Form of energy for the majority of kinetic energy in the anode.

Directly proportional

Relationship between mA and x-ray quantity.

Directly proportional

Relationship between exposure time and x-ray quantity.

Increases quantity and quality

Effect of increasing kVp on x-ray beam.

Higher kVp → higher photon energy

Relationship between kVp and Bremsstrahlung photon energy.

X-ray emission spectrum

Spectrum representing x-ray photon energies produced.

Continuous spectrum

Type of x-ray spectrum representing Bremsstrahlung photons.

Discrete spectrum

Type of x-ray spectrum representing characteristic photons.

Increases amplitude only

Effect of increasing mA on x-ray emission spectrum.

Increases amplitude and shifts peak to higher energy

Effect of increasing kVp on x-ray emission spectrum.

Reduces amplitude, shifts average energy higher

Effect of filtration on x-ray emission spectrum.

Glass or metal envelope

Material used for inherent filtration in the tube.

Aluminum

Material used for added filtration.

2.5 mm Al equivalent

Total filtration requirement for diagnostic x-ray tubes above 70 kVp.

Remove low-energy photons

Purpose of filtration in x-ray beam.

Photoelectric effect

Interaction between x-ray photon and matter involving complete absorption.

Compton effect

Interaction between x-ray photon and matter involving scattering with partial energy transfer.

Coherent scattering

Interaction where an x-ray photon interacts with an atom and excites it without ionization.

Pair production

Interaction where photon energy is converted into matter–antimatter pair.

1.02 MeV

Minimum photon energy required for pair production.

Photodisintegration

Interaction where photon is absorbed by nucleus and nuclear fragment is emitted.

10 MeV

Minimum photon energy required for photodisintegration.