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Conventional X-rays GeneratorConventional X-rays GeneratorBasic components of an X-ray Basic components of an X-ray machine:machine:Electron source. Electron source. Vacuum where electrons were Vacuum where electrons were

accelerated.accelerated.Energy source that caused electrons Energy source that caused electrons

to be accelerated.to be accelerated.Target made of metals of high atomic Target made of metals of high atomic

number and high melting point.number and high melting point.

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In conventional X-ray generators,In conventional X-ray generators,These basic components are:These basic components are:Thermionic emission from Thermionic emission from

heated electrode.heated electrode.Short hollow glass envelope Short hollow glass envelope

(0.1-0.5 m).(0.1-0.5 m).Potential difference, i.e. voltage Potential difference, i.e. voltage

applied from the transformer.applied from the transformer.

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An X-rays requires 2 or 3 An X-rays requires 2 or 3 different voltage supplies different voltage supplies which must also be which must also be adjustable, to control the adjustable, to control the X-rays output, suiting it to its X-rays output, suiting it to its various clinical purposes.various clinical purposes.

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3 principle generator 3 principle generator control variables control variables (exposure factors):(exposure factors):Tube kilovoltageTube kilovoltageTube current Tube current Exposure timeExposure time

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The need for The need for kilovoltage kilovoltage

The kilovoltage applied across The kilovoltage applied across the X-rays tube gives these the X-rays tube gives these electrons their potential energy. electrons their potential energy. As they begin to move across to As they begin to move across to the tube’s anode, it becomes the tube’s anode, it becomes their kinetic energy. their kinetic energy.

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X-rays is produced caused by X-rays is produced caused by the conversion of electron the conversion of electron kinetic energy across an X-ray kinetic energy across an X-ray tube to X-rays. tube to X-rays.

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In order to produce X-rays, In order to produce X-rays, the electrons’ kinetic energy the electrons’ kinetic energy must be above the threshold must be above the threshold value. value.

For clinical purposes, this For clinical purposes, this range will usually lie range will usually lie between 40 kV and 130 kV.between 40 kV and 130 kV.

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KKilovoltage’s role as provider of ilovoltage’s role as provider of the X-ray beam’s energy. There the X-ray beam’s energy. There are 2 principal reasons the need are 2 principal reasons the need for kV range:for kV range:

X-ray beam’s penetrating ability X-ray beam’s penetrating ability (Quality)(Quality)

X-ray beam intensityX-ray beam intensity

The need for The need for kilovoltage variation kilovoltage variation

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X-ray beam’s quality (or X-ray X-ray beam’s quality (or X-ray beam’s penetrating ability)beam’s penetrating ability)If the most occuring photon If the most occuring photon

energy within an X-ray beam energy within an X-ray beam is raised, for example, by is raised, for example, by kilovoltage selection, the kilovoltage selection, the beam’s quality is higher. beam’s quality is higher.

The more opaque the The more opaque the structure, the higher the structure, the higher the selected kilovoltage.selected kilovoltage.

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X-ray beam intensityX-ray beam intensityIntensity is defined as the the Intensity is defined as the the

rate of flow of X-ray energy rate of flow of X-ray energy through a unit areathrough a unit area lying at 90º lying at 90º to the path of the beam.to the path of the beam.

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X-rays beam’s qualityX-rays beam’s qualityWhen the electrons interact with When the electrons interact with

the X-ray tube target, the the X-ray tube target, the electrons from the filament may electrons from the filament may have their kinetic energy have their kinetic energy converted into photons of converted into photons of X-rays. Production of a photon X-rays. Production of a photon causes an electron to lose some causes an electron to lose some of its kinetic energy. of its kinetic energy.

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Higher tube kilovoltage can Higher tube kilovoltage can enable electrons (possess enable electrons (possess more kinetic energy) to more kinetic energy) to produce photons of greater produce photons of greater X-ray energy, thus capable X-ray energy, thus capable of greater penetration: can of greater penetration: can pass through structures pass through structures which are more radiopaque.which are more radiopaque.

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The more opaque the The more opaque the structure, the higher the structure, the higher the selected kilovoltage.selected kilovoltage.

If the kilovoltage (most If the kilovoltage (most commonly occuring photon commonly occuring photon energy within an X-ray energy within an X-ray beam) is raised, the beam) is raised, the beam’s quality is higher. beam’s quality is higher.

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X-rays beam’s intensityX-rays beam’s intensityIntensity is defined as the Intensity is defined as the

rate of flow of X-ray energy rate of flow of X-ray energy through a unit area through a unit area perpendicular to the path of perpendicular to the path of the beam.the beam.

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Voltage TransformationVoltage Transformation

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An increase up to the required An increase up to the required kilovoltage is achieved quite kilovoltage is achieved quite easily using a transformer.easily using a transformer.

A conventional transformer has A conventional transformer has 3 principal components:3 principal components:A primary windingA primary windingA secondary windingA secondary windingA central magnetic coreA central magnetic core

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Primary windingPrimary windingThe coiled length of wire across The coiled length of wire across which the primary voltage is applied.which the primary voltage is applied.

Secondary windingSecondary windingThe coiled length of wire across The coiled length of wire across which the secondary voltage is which the secondary voltage is induced.induced.

Central magnetic coreCentral magnetic coreAround which both windings are Around which both windings are arranged.arranged.

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The voltage relationship The voltage relationship between primary and secondary between primary and secondary is established by the is established by the phenomenon of electromagnetic phenomenon of electromagnetic induction. induction.

EM (Electromagnetic) Induction: EM (Electromagnetic) Induction: is the production of an electric is the production of an electric current across a conductor current across a conductor moving through a magnetic moving through a magnetic field.field.

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The ratio between the applied, The ratio between the applied, primary voltage and the induced primary voltage and the induced secondary voltage is determined secondary voltage is determined by the transformer’s by the transformer’s turns ratioturns ratio, , between the number of turns between the number of turns (around the core) forming the (around the core) forming the respective primary and respective primary and secondary windings.secondary windings.

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The high tension transformerThe high tension transformerA transformer which converts a A transformer which converts a

relatively low voltage into a relatively low voltage into a higher value is said to higher value is said to step upstep up the voltage. Conversely, a the voltage. Conversely, a voltage reduction is achieved by voltage reduction is achieved by a a step downstep down transformer. transformer.

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The transformer used for The transformer used for generating a kilovoltage across generating a kilovoltage across the X-ray tube is fed by a the X-ray tube is fed by a relatively low voltage which it relatively low voltage which it increases or increases or steps upsteps up. Due to . Due to the magnitude of its output, this the magnitude of its output, this particular transformer is usually particular transformer is usually known as the generator’s high known as the generator’s high tension (HT) transformer. tension (HT) transformer.

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To operate efficiently, an To operate efficiently, an X-ray tube should be fed with X-ray tube should be fed with a kilovoltage which has a a kilovoltage which has a fixed polarity, so that its fixed polarity, so that its anode is consistently at a anode is consistently at a high positive potential, and high positive potential, and its cathode is negative.its cathode is negative.

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Self-rectificationSelf-rectificationThe disadvantage to operate The disadvantage to operate

an X-ray tube from an an X-ray tube from an alternating voltage supply is alternating voltage supply is the fact that during alternate the fact that during alternate half-cycles, when the anode half-cycles, when the anode is –ve and the cathode +ve, is –ve and the cathode +ve, there is no tube current.there is no tube current.

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Self-rectified equipment may Self-rectified equipment may still be used for dental still be used for dental radiography; it offered a cheap radiography; it offered a cheap and compact arrangement for and compact arrangement for low powered production of low powered production of X-rays. X-rays.

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Three Phase generatorThree Phase generatorThe problem of filling the The problem of filling the

intervals between the X-ray intervals between the X-ray pulses from a two-pulse pulses from a two-pulse generator was solved when an generator was solved when an x-ray generator was invented x-ray generator was invented which employed the whole of the which employed the whole of the mains electricity supply, not just mains electricity supply, not just one or two of its phases.one or two of its phases.

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The full output from an The full output from an electricity supply service electricity supply service comprises 3 identical, sine-comprises 3 identical, sine-wave supplies. The 3 wave supplies. The 3 phases do not rise and fall phases do not rise and fall simultaneously: they are out simultaneously: they are out of phase with each other by of phase with each other by an interval of a third of a an interval of a third of a cycle, i.e. 120cycle, i.e. 120°°. .

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When rectified, in a manner When rectified, in a manner similar to the two-pulse similar to the two-pulse generator, each phase generator, each phase supplies 2 pulses per cycle, supplies 2 pulses per cycle, giving a total of 6 per cycle, giving a total of 6 per cycle, i.e. a six-pulse generator.i.e. a six-pulse generator.

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Figure: Figure: Three-phase Three-phase Voltage rectification.Voltage rectification.Alternate half-cycles Alternate half-cycles of each waveform of each waveform are inverted, to are inverted, to produce 6 forward produce 6 forward or positive voltage or positive voltage pulses per cycle. pulses per cycle.

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The use of a 3-phase The use of a 3-phase supply, while achieving an supply, while achieving an almost constant potential almost constant potential across the tube, still results across the tube, still results in an X-ray beam which in an X-ray beam which contains low energy contains low energy photons.photons.

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High Frequency GeneratorHigh Frequency GeneratorThis type of generator This type of generator

produces high output and produces high output and accuracy, based on the accuracy, based on the conversion of the standard conversion of the standard mains voltage frequency, mains voltage frequency, e.g. from 50 Hz (UK) up to e.g. from 50 Hz (UK) up to values in the thousands values in the thousands cycle per second.cycle per second.

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Conversion of the primary Conversion of the primary voltage to a high frequency voltage to a high frequency supply before it is fed to supply before it is fed to the HT transformer the HT transformer enables it to generate enables it to generate kilovoltages with greatly kilovoltages with greatly increased efficiency. increased efficiency.

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Figure: Figure: Principle of the high frequency X-ray generator.Principle of the high frequency X-ray generator.The input voltage (A) is rectified and smoothedThe input voltage (A) is rectified and smoothedwith capacitors (B). It is then fed to a circuitwith capacitors (B). It is then fed to a circuitwhich reconverts it to an alternating voltage bywhich reconverts it to an alternating voltage bythe action of an inverter, but now at a highthe action of an inverter, but now at a highfrequency (C). This high frequency voltage is frequency (C). This high frequency voltage is transformed up to the required kilovoltage (D),transformed up to the required kilovoltage (D),rectified and smoothed (E) for application across rectified and smoothed (E) for application across the X-ray tube(F).the X-ray tube(F).