3 July July July Conventional X-rays Generator Basic components of an X-ray machine:  Electron source.  Vacuum where electrons were accelerated.  Energy source that caused electrons to be accelerated.  Target made of metals of high atomic number and high melting point.

3 July July July In conventional X-ray generators, These basic components are:  Thermionic emission from heated electrode.  Short hollow glass envelope ( m).  Potential difference, i.e. voltage applied from the transformer.

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

3 July July July  3 principle generator control variables (exposure factors):  Tube kilovoltage  Tube current  Exposure time

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

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

3 July July July  In order to produce X-rays, the electrons’ kinetic energy must be above the threshold value.  For clinical purposes, this range will usually lie between 40 kV and 130 kV.

3 July July July  K ilovoltage’s role as provider of the X-ray beam’s energy. There are 2 principal reasons the need for kV range:  X-ray beam’s penetrating ability (Quality)  X-ray beam intensity The need for kilovoltage variation

3 July July July  X-ray beam’s quality (or X-ray beam’s penetrating ability)  If the most occuring photon energy within an X-ray beam is raised, for example, by kilovoltage selection, the beam’s quality is higher.  The more opaque the structure, the higher the selected kilovoltage.

3 July July July  X-ray beam intensity  Intensity is defined as the the rate of flow of X-ray energy through a unit area lying at 90º to the path of the beam.

3 July July July X-rays beam’s quality  When the electrons interact with the X-ray tube target, the electrons from the filament may have their kinetic energy converted into photons of X-rays. Production of a photon causes an electron to lose some of its kinetic energy.

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

3 July July July  The more opaque the structure, the higher the selected kilovoltage.  If the kilovoltage (most commonly occuring photon energy within an X-ray beam) is raised, the beam’s quality is higher.

3 July July July X-rays beam’s intensity  Intensity is defined as the rate of flow of X-ray energy through a unit area perpendicular to the path of the beam.

3 July July July Voltage Transformation

3 July July July  An increase up to the required kilovoltage is achieved quite easily using a transformer.  A conventional transformer has 3 principal components:  A primary winding  A secondary winding  A central magnetic core

3 July July July  Primary winding The coiled length of wire across which the primary voltage is applied.  Secondary winding The coiled length of wire across which the secondary voltage is induced.  Central magnetic core Around which both windings are arranged.

3 July July July  The voltage relationship between primary and secondary is established by the phenomenon of electromagnetic induction.  EM (Electromagnetic) Induction: is the production of an electric current across a conductor moving through a magnetic field.

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

3 July July July The high tension transformer  A transformer which converts a relatively low voltage into a higher value is said to step up the voltage. Conversely, a voltage reduction is achieved by a step down transformer.

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

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

3 July July July Self-rectification  The disadvantage to operate an X-ray tube from an alternating voltage supply is the fact that during alternate half-cycles, when the anode is –ve and the cathode +ve, there is no tube current.

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

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

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

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

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

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

3 July July July High Frequency Generator  This type of generator produces high output and accuracy, based on the conversion of the standard mains voltage frequency, e.g. from 50 Hz (UK) up to values in the thousands cycle per second.

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

3 July July July Figure: Principle of the high frequency X-ray generator. The input voltage (A) is rectified and smoothed with capacitors (B). It is then fed to a circuit which reconverts it to an alternating voltage by the action of an inverter, but now at a high frequency (C). This high frequency voltage is transformed up to the required kilovoltage (D), rectified and smoothed (E) for application across the X-ray tube(F).