X-ray tubeX-ray tube

The X-Ray Tube DevelopmentThe X-Ray Tube Development

Dr. Roentgen used a Dr. Roentgen used a Crookes-Hittorf tube Crookes-Hittorf tube to make the first x-ray to make the first x-ray image.image.

There was no There was no shielding so x-rays shielding so x-rays were emitted in all were emitted in all directions.directions.

The X-Ray Tube DevelopmentThe X-Ray Tube Development

This is the variety of This is the variety of tube designs tube designs available in 1948.available in 1948.

The Coolidge tube , The Coolidge tube , the first to use a the first to use a heated filament to heated filament to obtain electrons was obtain electrons was still available. still available.

The X-Ray Tube DevelopmentThe X-Ray Tube Development

Two major hazards plagued early Two major hazards plagued early radiography.radiography.– Excessive radiation exposureExcessive radiation exposure– Electric ShockElectric Shock

Moderns tubes are designed to overcome Moderns tubes are designed to overcome these problemsthese problems

The modern tube is based on the Coolidge The modern tube is based on the Coolidge tubetube

The X-Ray Tube DevelopmentThe X-Ray Tube Development

This is a modern This is a modern rotating anode x-ray rotating anode x-ray tube. tube.

It is encased It is encased completely in a metal completely in a metal protective housing.protective housing.

The housing provide The housing provide electrical and electrical and radiation safety radiation safety

The The Modern X-ray Tube Modern X-ray Tube

There are two principle parts:There are two principle parts:– The rotating anodeThe rotating anode– The cathodeThe cathode

Any tube that has two electrodes is called a Any tube that has two electrodes is called a diode.diode.

Inside the tube is a high vacuumInside the tube is a high vacuum

Parts of the X-ray TubeParts of the X-ray Tube

Protective HousingProtective Housing

The tube is housed in The tube is housed in a lead lined metal a lead lined metal protective housing.protective housing.

The x-ray photons are The x-ray photons are generated generated isotropically or in all isotropically or in all directions. directions.

The housing is The housing is designed to limit the designed to limit the beam to window.beam to window.

Protective Housing-leakage Protective Housing-leakage radiationradiation

The housing is designed to minimize the The housing is designed to minimize the radiation outside the portradiation outside the port

The tube can not have more than 100 The tube can not have more than 100 mR at 1 m (26 µ C/kg) / Hour when mR at 1 m (26 µ C/kg) / Hour when operated at its maximum output.operated at its maximum output.

Protective Housing – prevent Protective Housing – prevent damagedamage

The housing also provide mechanical support The housing also provide mechanical support and protection from damage.and protection from damage.

On some tubes, the housing also contains oil On some tubes, the housing also contains oil that provides more insulation and a thermal that provides more insulation and a thermal cushion.cushion.

Never hold the tube during an exposure.Never hold the tube during an exposure.

Never use the cables or terminals as Never use the cables or terminals as handles.handles.

Protective Housing- supply of high Protective Housing- supply of high voltagevoltage

The housing incorporates specially designed The housing incorporates specially designed high voltage receptacles to protect against high voltage receptacles to protect against electrical shock.electrical shock.

Some housing have a fan for cooling.Some housing have a fan for cooling.

The X-Ray Tube Glass EnvelopeThe X-Ray Tube Glass Envelope

The glass envelope The glass envelope is made of Pyrex to is made of Pyrex to withstand the withstand the tremendous heat tremendous heat produced during x-produced during x-ray.ray.

The window is a 5 The window is a 5 cm square with a cm square with a thin section of glass thin section of glass where the useful where the useful beam is emitted.beam is emitted.

The CathodeThe Cathode

The cathode is The cathode is the negative the negative side of the tube side of the tube and contains and contains two primary two primary parts:parts:

The filamentsThe filaments

The focusing The focusing cupcup

The FilamentsThe Filaments

Most tube have two filaments which Most tube have two filaments which provide a large focal area and a small provide a large focal area and a small focal area focal area

Two foci provide the choice of quick Two foci provide the choice of quick exposures or high resolution.exposures or high resolution.

The filaments are made of thoriated The filaments are made of thoriated tungsten.tungsten.

The FilamentsThe Filaments

Tungsten is used in x-ray tube filaments Tungsten is used in x-ray tube filaments because of it’s because of it’s – High melting point of 3410°C.High melting point of 3410°C.– Comparaivelly Low work function (emits Comparaivelly Low work function (emits

electrons at lower temperatures )electrons at lower temperatures )– High tensile strengthHigh tensile strength– Flexibility to be turned to a thin spiralFlexibility to be turned to a thin spiral

Electrons are produced by thermionic Electrons are produced by thermionic emission when a 4 A or higher current is emission when a 4 A or higher current is applied.applied.

Focusing CupFocusing Cup

This is made of This is made of nickel and it houses nickel and it houses the filaments the filaments

The focusing cup The focusing cup has a negative has a negative charge so that it charge so that it can condense the can condense the electron beam to a electron beam to a small area of the small area of the anode.anode.

Filament CurrentFilament Current

The current that flows through the The current that flows through the Filament is called the filament currentFilament is called the filament current

This is resulted from applying a low This is resulted from applying a low voltage across the filamentvoltage across the filament

When the x-ray machine is turned on, a When the x-ray machine is turned on, a low current flows through the the filament low current flows through the the filament to warm it and prepare it for the big to warm it and prepare it for the big thermal jolt necessary for x-ray production.thermal jolt necessary for x-ray production.

Filament CurrentFilament Current

When the machine is ‘stand by’, the When the machine is ‘stand by’, the filament is not hot enough for thermionic filament is not hot enough for thermionic emission. emission.

So, it is boosted to a higher value just So, it is boosted to a higher value just before the exposurebefore the exposure

Once the current is high enough for Once the current is high enough for thermionic emission electrons are emitted thermionic emission electrons are emitted from the filamentfrom the filament

X-ray tube currentX-ray tube current

This is the current that flows across the x-This is the current that flows across the x-ray tube (flowing electrons from filament to ray tube (flowing electrons from filament to anode)anode)

This is resulted when the high potential This is resulted when the high potential difference (kV) is applied across the tube difference (kV) is applied across the tube & if emitted electrons are available at the & if emitted electrons are available at the filamentfilament

Filament Current & Tube Filament Current & Tube CurrentCurrent

The x-ray tube current is The x-ray tube current is adjusted by controlling the adjusted by controlling the filament current.filament current.

A small rise in filament A small rise in filament current will result in a current will result in a large rise in tube current.large rise in tube current.

The relationship between The relationship between tube and filament current tube and filament current is dependent upon the is dependent upon the tube voltage.tube voltage.

Space ChargeSpace ChargeWhen emitted by the When emitted by the filament, the filament, the electrons form a electrons form a cloud near the cloud near the filament momentarily filament momentarily before being before being accelerated to the accelerated to the anode. This is called anode. This is called a a space charge.space charge.

Space Charge EffectSpace Charge Effect

It prevents further emission of electrons and It prevents further emission of electrons and hence limit the tube current for a particular hence limit the tube current for a particular anode voltage (kV).anode voltage (kV).

Because of that the tube current (mA) Because of that the tube current (mA) increases when the anode voltage (kV) is increases when the anode voltage (kV) is increasedincreased

This is against the basic requirement in This is against the basic requirement in radiography ; which requires independent radiography ; which requires independent varaition of kV and mAvaraition of kV and mA

At At veryvery high high mAmA and and very very low kVplow kVp, the , the thermionic thermionic emission can emission can be space be space charge charge limited.limited.

Saturation CurrentSaturation CurrentThe highest tube current The highest tube current for a given filament for a given filament current is called the current is called the saturation currentsaturation current

After saturation the tube After saturation the tube current is independent of current is independent of the kVthe kV

Above 1000 mA space Above 1000 mA space charge limited exposure charge limited exposure can be a major problem. can be a major problem.

The AnodeThe Anode

The anode is the The anode is the positive side of positive side of the tube.the tube.

X-ray tubes are X-ray tubes are classified by the classified by the type of anode:type of anode:– Stationary ( top)Stationary ( top)– Rotating Rotating

(bottom)(bottom)

The Stationary AnodeThe Stationary Anode

Stationary anodes are used in dental Stationary anodes are used in dental x-ray and some portable x-ray x-ray and some portable x-ray machine where high tube current and machine where high tube current and power are not required.power are not required.Tube output is lowTube output is low– 80kv80kv– 15 mA15 mA– Long exposure times over 100msLong exposure times over 100msLarger focal spots –low resolutionLarger focal spots –low resolutionHeat is confined to a small areaHeat is confined to a small area

The Rotating AnodeThe Rotating Anode

The Rotating Anodes are used for all the The Rotating Anodes are used for all the other applications where high tube out put other applications where high tube out put and high tube rationgs are requiredand high tube rationgs are required– 125 – 150 kVp125 – 150 kVp– 25 – 1200 mA25 – 1200 mA– Short exposure times upto less than 1msShort exposure times upto less than 1ms

Smaller focal spots with high ratings – high Smaller focal spots with high ratings – high resolutionresolution

The Rotating AnodeThe Rotating Anode

The rotating The rotating anode allows the anode allows the electron beam to electron beam to interact with a interact with a much larger much larger apparent target apparent target area.area.

The heat is not The heat is not confined to a confined to a small area.small area.

The functions of Rotating AnodeThe functions of Rotating Anode

The anode serves three major functions:The anode serves three major functions:– Receives the electrons emitted from the Receives the electrons emitted from the

cathode.cathode.– It is a electrical conductor.It is a electrical conductor.– Mechanical support for the target.Mechanical support for the target.

The Rotating AnodeThe Rotating Anode

The Anode must also The Anode must also be a good thermal be a good thermal conductor.conductor.

When the electron When the electron beam strikes the beam strikes the anode more than 99% anode more than 99% of the kinetic energy of the kinetic energy is converted to heat.is converted to heat.

The TargetThe Target

Tungsten-rhenium is used as the target for the Tungsten-rhenium is used as the target for the electron beam.electron beam.

Tungsten is used for three reasonsTungsten is used for three reasons– High atomic numberHigh atomic number– Heat conductivityHeat conductivity– High melting point..High melting point..

Rhenium is used to increase the surface Rhenium is used to increase the surface properties to minimise the pitting and cracking properties to minimise the pitting and cracking of the targetof the target

The Rotating Anode - speedThe Rotating Anode - speed

When the exposure button is depressed, current When the exposure button is depressed, current is applied to the tube that produces a magnetic is applied to the tube that produces a magnetic field that starts the rotation of the anode.field that starts the rotation of the anode.

The rotor is the rotating part of an electromagnetic The rotor is the rotating part of an electromagnetic induction motor.induction motor.

It spins at 3000/3400 rpm.It spins at 3000/3400 rpm.

High speed anodes spin at 9,000/10,000 rpm.High speed anodes spin at 9,000/10,000 rpm.

Breaking The Rotating AnodeBreaking The Rotating Anode

When the anode is spinning at the When the anode is spinning at the correct speed, the exposure can be correct speed, the exposure can be made.made.

After the exposure is completed, it After the exposure is completed, it slows by reversing the motor. slows by reversing the motor.

This is necessary to avoid excessive This is necessary to avoid excessive wear and tear of the bearingswear and tear of the bearings

Dissipation of heat from the targetDissipation of heat from the target

Even with the anode rotating, some melting Even with the anode rotating, some melting occurs. occurs.

The heat must be rapidly dissipated from The heat must be rapidly dissipated from the target. the target.

Molybdenum and copper are used to Molybdenum and copper are used to rapidly transfer the heat from the target.rapidly transfer the heat from the target.

The anode dissipates heat by radiating The anode dissipates heat by radiating towards the glass enveloptowards the glass envelop

The anode stemThe anode stem

The anode stem is made from The anode stem is made from molybdenummolybdenum

It is made appropriately thin as to minimize It is made appropriately thin as to minimize the heat conduction towards the rotorthe heat conduction towards the rotor

Tube coolingTube cooling

The x-ray tube uses all three forms of cooling.The x-ray tube uses all three forms of cooling.– RadiationRadiation– ConductionConduction– ConvectionConvection

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