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.
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In conventional X-ray generators,
These basic components are:
Thermionic emission from
heated electrode.
Short hollow glass envelope
(0.1-0.5 m).
Potential difference, i.e. voltage
applied from the transformer.
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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.
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3 principle generator
control variables
(exposure factors):
Tube kilovoltage
Tube current
Exposure time
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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 tubes anode, it becomes
their kinetic energy.
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X-rays is produced caused by
the conversion of electron
kinetic energy across an X-ray
tube to X-rays.
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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.
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The need for
kilovoltage variation
Kilovoltages role as provider of
the X-ray beams energy. There
are 2 principal reasons the need
for kV range:
X-ray beams penetrating ability
(Quality)
X-ray beam intensity
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X-ray beams quality (or X-ray
beams penetrating ability)
If the most occuring photon
energy within an X-ray beam
is raised, for example, by
kilovoltage selection, the
beams quality is higher.
The more opaque the
structure, the higher the
selected kilovoltage.
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X-ray beam intensity
Intensity is defined as the the
rate of flow of X-ray energy
through a unit area lying at 90
to the path of the beam.
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X-rays beams 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.
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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.
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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
beams quality is higher.
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X-rays beams intensity
Intensity is defined as the
rate of flow of X-ray energy
through a unit area
perpendicular to the path of
the beam.
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Voltage Transformation
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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
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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.
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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.
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The ratio between the applied,
primary voltage and the induced
secondary voltage is determined
by the transformers turns ratio,
between the number of turns
(around the core) forming the
respective primary and
secondary windings.
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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.
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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 generators high
tension (HT) transformer.
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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.
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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.
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Self-rectified equipment may
still be used for dental
radiography; it offered a cheap
and compact arrangement for
low powered production of
X-rays.
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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.
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The full output from an
electricity supply service
comprises 3 identical, sinewave 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.
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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.
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Figure:
Three-phase
Voltage rectification.
Alternate half-cycles
of each waveform
are inverted, to
produce 6 forward
or positive voltage
pulses per cycle.
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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.
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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.
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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.
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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).
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