The X-Ray TubeThe X-Ray Tube

S. GuilbaudS. Guilbaud

Education DirectorEducation Director

School of Radiologic TechnologySchool of Radiologic Technology

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X-Ray TubeX-Ray Tube

• Electrical device used for the generation Electrical device used for the generation of x-rays.of x-rays.

• This is accomplished by the acceleration This is accomplished by the acceleration of electrons and then suddenly of electrons and then suddenly decelerating them.decelerating them.

• The energy of the x-rays is dependent The energy of the x-rays is dependent on the kinetic energy of the electrons.on the kinetic energy of the electrons.

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X-Ray tube componentsX-Ray tube components

• Glass envelopeGlass envelope• Cathode Cathode • Anode Anode • Protective housingProtective housing

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Glass envelopeGlass envelope

• Made of Pyrex glassMade of Pyrex glass– Able to withstand tremendous heatAble to withstand tremendous heat

• Tube maintains a vacuum.Tube maintains a vacuum.• Tube windowTube window

– A segment of glass that is thinner A segment of glass that is thinner than the rest of the glass envelope.than the rest of the glass envelope.

– Contributes to inherent filtration.Contributes to inherent filtration.• 0.5mm Al equivalency. 0.5mm Al equivalency.

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Cathode Cathode

• Negatively charged electrode.Negatively charged electrode.• Two primary parts:Two primary parts:

– Filament Filament – Focusing cupFocusing cup

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Cathode

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Focusing cupFocusing cup

• Metallic shroud containing the two Metallic shroud containing the two filaments.filaments.– Usually made from nickel.Usually made from nickel.

• Contains a negative charge.Contains a negative charge.– Designed to repel electrons.Designed to repel electrons.– Designed to condense electron beam Designed to condense electron beam

to small area on on focal track.to small area on on focal track.

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Focusing cupFocusing cup• Four factors Four factors

determine the determine the effectiveness of the effectiveness of the cup.cup.– Size & shape.Size & shape.– Charge Charge – Filament size & shape.Filament size & shape.– Position of filament Position of filament

w/in cup.w/in cup.

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Filament Filament • Small coil of thoriated tungsten.Small coil of thoriated tungsten.• Modern x-ray tubes contain two filament.Modern x-ray tubes contain two filament.

– They correspond to the focal spot sizes.They correspond to the focal spot sizes.

• When machine is turned on, small amount of When machine is turned on, small amount of current flows through to heat filament.current flows through to heat filament.

• Tube current is adjusted by controlling the Tube current is adjusted by controlling the filament current.filament current.

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Anode Anode

• Positively charged electrode.Positively charged electrode.• Two types.Two types.

– Stationary anode.Stationary anode.– Rotating anode.Rotating anode.

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Stationary Anode Stationary Anode

• Made of tungsten Made of tungsten target embedded target embedded in a large copper in a large copper bar.bar.

• Usually used in Usually used in dental x-ray dental x-ray machine.machine.

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Rotating anodeRotating anode• Constructed of Constructed of

tungsten target tungsten target (focal track).(focal track).– High melting point High melting point

3400340000 Celsius. Celsius.

• MolybdenumMolybdenum– Surrounds tungsten Surrounds tungsten

target area.target area.– Assists in dissipating Assists in dissipating

heat.heat.

• Graphite Graphite – Serves as mount for Serves as mount for

molybdenum and molybdenum and tungsten target.tungsten target.

– Also assists in Also assists in dissipating heat.dissipating heat.

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Rotating anodeRotating anode

• Provides greater Provides greater target area and target area and greater heat greater heat dissipation.dissipation.

• Affords the ability to Affords the ability to attain greater attain greater exposure loads by exposure loads by providing a larger providing a larger area for the electron area for the electron beam to interact with beam to interact with the target.the target.

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Rotating anodeRotating anode

• The heating The heating capacity is further capacity is further enhanced with an enhanced with an increased RPM increased RPM (3400).(3400).

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Induction motorInduction motorResponsible for driving the rotating anode.Responsible for driving the rotating anode.Consists of two parts separated by the glass envelope.Consists of two parts separated by the glass envelope.

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Induction motorInduction motor

• Works on the principle similar to the Works on the principle similar to the transformer.transformer.– Electromagnetic induction.Electromagnetic induction.– Current flowing in the stator develops a Current flowing in the stator develops a

magnetic field.magnetic field.– Stator windings are sequentially energized Stator windings are sequentially energized

so that the induced magnetic field rotates on so that the induced magnetic field rotates on the axis of the stator.the axis of the stator.

– This causes the rotor to rotate.This causes the rotor to rotate.

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Line focus principleLine focus principle

• The area of the x-The area of the x-ray tube anode ray tube anode from which the x-from which the x-ray photons are ray photons are emitted.emitted.

• This is called the This is called the actual focal spotactual focal spot

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Line focus principleLine focus principle

• The projection The projection perpendicular to perpendicular to the central ray, the central ray, which is its which is its apparent area apparent area from the position from the position of the film, is the of the film, is the effective focal effective focal spot.spot.

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Line focus principleLine focus principle

• Was incorporated Was incorporated into x-ray tube into x-ray tube targets to allow a targets to allow a large area for large area for heating while heating while maintaining a small maintaining a small focal spot.focal spot.

• The effective focal The effective focal spot is the area spot is the area projected onto the projected onto the patient and film.patient and film.

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Line focus principleLine focus principle

• Focal spot sizes Focal spot sizes always make always make reference to the reference to the effective focal effective focal spot.spot.

• The lower the The lower the target angle, the target angle, the smaller the smaller the effective focal effective focal spot size.spot size.

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Line focus principleLine focus principle

• The advantage of The advantage of the line-focus the line-focus principle is that it principle is that it provides the provides the detail of a small detail of a small focal spot while focal spot while allowing a large allowing a large amount of heat amount of heat dissipation.dissipation.

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Line focus principleLine focus principle

• The unfortunate bi-product of the line-The unfortunate bi-product of the line-focus principle is the “anode heel focus principle is the “anode heel effect”effect”

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Anode heel effectAnode heel effect

• Construction Construction phenomenon that phenomenon that causes the x-ray causes the x-ray photons exiting the photons exiting the tube on the cathode tube on the cathode side to have a side to have a greater energy value greater energy value than those exiting than those exiting the tube on the the tube on the anode side.anode side.

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Anode heel effectAnode heel effect

• More energy More energy absorption occurs absorption occurs at the anode heel at the anode heel resulting in less resulting in less energy value from energy value from the incident the incident photons at the photons at the anode heel.anode heel.

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Anode heel effectAnode heel effect

• This is used to This is used to advantage when advantage when imaging imaging anatomical parts anatomical parts that are unequal that are unequal in thickness and in thickness and densities densities throughout their throughout their respective respective lengths.lengths.

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Using the anode heel effectUsing the anode heel effect

• The following anatomical parts may The following anatomical parts may be imaged using the anode heel be imaged using the anode heel effect:effect:– Thoracic vertebraeThoracic vertebrae– Humerus Humerus – Femur Femur – Tibia & fibula Tibia & fibula – Forearm Forearm

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Using the anode heel effectUsing the anode heel effect

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Anode heel effectAnode heel effect

• The thicker portion of the The thicker portion of the anatomical part is placed beneath anatomical part is placed beneath the cathode end of the x-ray tube.the cathode end of the x-ray tube.

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Protective housingProtective housing

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Protective housingProtective housing

• X-ray tube is always mounted X-ray tube is always mounted inside a lead-lined protective inside a lead-lined protective housing that is designed to:housing that is designed to:– Prevent excessive radiation exposure.Prevent excessive radiation exposure.– Prevent electric shock to the patient Prevent electric shock to the patient

and operator (technologist).and operator (technologist).

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Protective housingProtective housing

• Incorporates specially designed high-Incorporates specially designed high-voltage receptacles.voltage receptacles.

• Provides mechanical support for the x-Provides mechanical support for the x-ray tube and protects it from damage.ray tube and protects it from damage.

• Some tube housings contain oil in which Some tube housings contain oil in which the tube is bathed.the tube is bathed.

• Some tube housings contain a cooling Some tube housings contain a cooling fan to air-cool the tube.fan to air-cool the tube.

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Protective housingProtective housing

• When properly designed, they When properly designed, they reduce the level of leakage reduce the level of leakage radiation to radiation to less than 100 mR/hr at 1 meter when when operated at maximum conditions.operated at maximum conditions.

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Tube rating chartsTube rating charts

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Tube rating chartTube rating chart

• A graph that indicates the A graph that indicates the maximum exposure values that maximum exposure values that may be made w/o damage to the may be made w/o damage to the tube.tube.

• Each chart contains a family of Each chart contains a family of curves representing the various curves representing the various tube currents in mA.tube currents in mA.

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Tube rating chartTube rating chart

• The X axis and Y axis show scales The X axis and Y axis show scales of the two radiographic parameters of the two radiographic parameters of kV and mA.of kV and mA.– For a given mA, any combination of For a given mA, any combination of

kVp and time that lies below the mA kVp and time that lies below the mA curve is safe.curve is safe.

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Tube rating chartsTube rating charts

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Anode cooling chartAnode cooling chart

• Provides the Provides the thermal capacity thermal capacity of the anode and of the anode and its heat its heat dissipation dissipation characteristics.characteristics.

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Anode cooling chartAnode cooling chart

• Thermal energy is Thermal energy is measured in British measured in British Thermal Units Thermal Units (BTU’s) where x-ray (BTU’s) where x-ray thermal energy is thermal energy is measured in Heat measured in Heat Units (HU).Units (HU).

• Thus:Thus:– 1 kVp, 1 mA, 1 s = 1 1 kVp, 1 mA, 1 s = 1

HU.HU.

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Calculating Heat UnitsCalculating Heat Units

• For a single phase unit,For a single phase unit,– HU = kVp x mA x sHU = kVp x mA x s

• For a 3 phase 6 pulse unit,For a 3 phase 6 pulse unit,– HU = 1.35 x kVp x mA x s HU = 1.35 x kVp x mA x s

• For a 3 phase 12 pulse unit,For a 3 phase 12 pulse unit,– HU = 1.41 x kVp x mA x s HU = 1.41 x kVp x mA x s

• For a high frequency unit,For a high frequency unit,– HU = 1.44 x kVp x mA x sHU = 1.44 x kVp x mA x s

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Anode cooling chartAnode cooling chart

• Determines the Determines the maximum heat maximum heat capacity of the capacity of the anode.anode.

• Determines the Determines the length of time length of time required for required for complete cooling complete cooling following any level of following any level of heat input.heat input.

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ReferencesReferences

Bushberg, et al. Bushberg, et al. The Essential Physics of Medical ImagingThe Essential Physics of Medical Imaging, , Williams & Wilkins, 1994.Williams & Wilkins, 1994.

Bushong, S. Bushong, S. Radiologic Science for Technologists, Physics, Radiologic Science for Technologists, Physics, Biology, and ProtectionBiology, and Protection, 7, 7thth Edition, Mosby, 2000. Edition, Mosby, 2000.

Carlton et al. Carlton et al. Principles of Radiographic Imaging an Art and Principles of Radiographic Imaging an Art and a Sciencea Science, 3, 3rdrd Edition, Delmar, 2001. Edition, Delmar, 2001.

Selman, J. Selman, J. The Fundamentals of X-Ray and Radium PhysicsThe Fundamentals of X-Ray and Radium Physics, , 88thth Edition, Charles Thomas, 1994. Edition, Charles Thomas, 1994.