X-RAY PRODUCTION
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X-ray tube is the most important part of
the x-ray machine – tube is where the xrays are produced.
Radiographers must understand how the xray tube is constructed and how it works
Kilovoltage peak (kVp), milliamperage
(mA) and exposure time.
Awareness of heat being produced as
excessive heat can damage the tube.
X-RAY TUBE
CATHODE
• Production of x-rays require a
rapidly moving stream of electrons
that are suddenly decelerated or
stopped.
• https://youtu.be/HDjdU0bdJV0
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• Source of electrons:
CATHODE
• Also known as the negative electrode.
• The negative electrode is heated:
electrons are emitted.
• The electrons are attracted to the
anode.
• Causes the electrons to move rapidly
towards the positive electrode.
• Suddenly stopped- decelerated.
• Kinetic energy transferred to the anodex-rays and heat are produced.
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CATHODE
• Negatively charged electrode in an x-ray tube.
• Comprises a filament and a focusing cup.
• Filament is a coiled tungsten wire that is a
source of electrons during x-ray production.
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FILAMENT:
• FIGURE 1: (FAUBER, 2013: 13).
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FILAMENT CONT:
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Most x-ray tubes are referred as dual
focus:
They have two filaments: a large and a
small.
Only one filament is energized at a time.
Large focal spot- large filament is
energized.
Small focal spot- small filament is
energized.
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FOCUSING CUP
• Made up of nickel.
• Nearly surrounds the filament.
• Open at one end- allows electrons to flow
freely across from cathode to anode.
• Negatively charged- cloud of electrons
emitted from filament: they don’t spread
apart.
• Purpose: to focus the stream of electrons
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ANODE:
• Positively charged electrode.
• Composed of :
• Molybdenum.
• Copper.
• Tungsten.
• Graphite.
• The above are used for their thermal and
electrical conductive properties.
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ANODE…cont.
• Anode consists of a target.
• Stator and rotor- in rotating anode tubes.
Function of target:
• Abruptly decelerates and stop electrons in the
tube current- X-rays are produced.
• Can be either stationary or rotating.
• Rotating is more common.
• Rotates at a set speed 3000-10000 revs per
min(RPM).
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ROTATING AND STATIONARY
ANODES
STATIONARY
ROTATING
FIGURE 2: (FAUBER, 2013:14).
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ANODE…
• Target of rotating anode is made of:
• Tungsten and rhenium alloy.
• This layer or track is embedded in a base
of molybdenum and graphite.
FIGURE 3: (FAUBER, 2013: 15).
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ANODE….
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Tungsten- 90% of the rotating target.
Rhenium- 10%.
Face of anode is angled: assist x-ray photons to exit the tube.
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• Rotating targets -5-20degrees angulation.
• Why is tungsten used?
• High atomic number 74- efficient x-ray
production.
• High melting point 34000 C.
• Most of energy that is produced by an xray tube is heat, so melting of target
does become a problem- especially during
high exposures.
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FIGURE 4: (FAUBER, 2013: 15).
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ROTATING ANODE…
• Turn rotating anode: a stator and rotor are
required.
• Stator: electric motor that turns the rotor at a
very speed.
• Rotor, made of copper: rigidly connected to
target through anode stem.
• Anode stem, made of molybdenum: causes
target to rapidly rotate during x-ray production.
• High- strength ball bearings: located in rotor –
facilitates rotation that is smooth at high speeds.
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HEAT IS MAINLY PRODUCED
• X-ray energy constitutes only a small
percentage.
• Heat can cause a problem if allowed to build up.
• Therefore transferred to the envelope –
insulating oil surrounding the tube.
• Many have fans that blow air over the tube to aid
cooling.
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FOCAL SPOT AND HEAT DISSIPATION
• Actual focal spot- the physical area of the target
that is bombarded by electrons during x-ray
production.
FIGURE 5 (FAUBER, 2013: 16)
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FOCAL SPOT CONT..
• Stationary targets: focal spot is fixed.
FIGURE 6 (FAUBER, 2013: 16)
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• Rotating target: focal spot is represented by a
focal track.
FIGURE 6(FAUBER, 2013: 16)
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bbb
• Rotating anode is capable of with standing
higher heat loads – Because bombardment
occurs over a larger area.
• Rotating anodes: all applications of radiography.
• Stationary anodes: limited to small anatomic
structures, e.g.. Teeth.
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X-RAY TUBE HOUSING- envelope
FIGURE 7 (FAUBER, 2013: 17).
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• Glass or metal envelope.
• Metal envelope are more common- improved electrical
properties, more robust.
• Disadvantage of glass:
Tungsten evaporated from the filament during exposure
deposits on the inside of the glass- middle portion of the
envelope.
Evaporation affects the flow of electrons- tube fails.
Replacement from glass to metal- optimal functioning and
extended tube life.
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FURTHER ADVANTAGE OF METAL ENVELOPE..
• Reduction of OFF- FOCUS RADIATION.
• Off- focus radiation: projectile electrons are
reflected and x-rays are produced from outside
the focal spot.
• Metal tube envelope collects these electrons
and conduct them away from anode.
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FUNCTION OF ENVELOPE
• Allows air to be completely evacuated from tubeefficient flow of electrons.
• Provides insulation from electrical shockcathode and anode contain electrical charges.
• Dissipates heat – conducts heat into the
insulating oil that surrounds the tube.
• Insulating oil- provide more insulation from
electrical shock and assist in heat dissipation.
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• Tube is enclosed in a metal tube housing.
• This is the housing that radiographers moves
when positioning for x-ray beam.
• Has a port that allows the exit of the primary
beam.
• Lined with lead to prevent leakage radiation.
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LEAKAGE RADIATION.
• Any x-rays other than the primary beam that escapes the
tube housing.
• Maximum upper limit- 100mR/hr- measured ta 1m from
source during maximum output.
• Electrical current is supplied to the x-ray tube by two high
voltage cables.
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• Who remembers this
• (clinic- 9/2/2016)/
• (clinic-6th/2/2018)
• (clinic- 5th/2/2019)
• FIGURE 8 (MULLA, 2016)
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PRODUCTION OF X-RAYS
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https://youtu.be/3_bZCA7tlFQ
https://youtu.be/4zrw9RIQJGY
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