Fundamentals of X-ray Production

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Fundamentals of X-ray Production
By Professor Jarek Stelmark
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X-rays were not developed they were discovered,
and quite by accident. During the 1870s and
1880s, many university physics laboratories were
investigating the conduction of cathode rays, or
electrons, through a large, partially evacuated
glass tube known as a Crookes tube.
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On November 8, 1895, Roentgen was working in his
physics laboratory at Würzburg University in
Germany. In late 1895, a Roentgen was working
with a cathode ray tube in his laboratory. He was
working with tubes similar to our fluorescent
light bulbs. He evacuated the tube of all air,
filled it with a special gas, and passed a high
electric voltage through it. When he did this,
the tube would produce a fluorescent glow.
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Roentgen shielded the tube with heavy black
paper, and found that a green colored fluorescent
light could be seen coming from a screen setting
a few feet away from the tube. He realized that
he had produced a previously unknown "invisible
light," or ray, that was being emitted from the
tube a ray that was capable of passing through
the heavy paper covering the tube.
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The discovery of x-rays is characterized by many
amazing features, and this causes it to rank high
among the events in human history. First, the
discovery was accidental. Second, probably no
fewer than a dozen contemporaries of Roentgen had
previously observed x-radiation, but none of
these other physicists had recognized its
significance or investigated it. Third, Roentgen
followed his discovery with such scientific vigor
that within little more than a month, he had
described x-radiation with nearly all the
properties we recognize today.
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Crookes tubes generated the electrons needed to
create x-rays by ionization of the residual air
in the tube, instead of a heated filament, so
they were partially but not completely evacuated.
The Crookes tube was improved by William Coolidge
in 1913. The Coolidge tube, also called hot
cathode tube, is the most widely used. It works
with a very good quality vacuum. In the Coolidge
tube, the electrons are produced by thermionic
effect from a tungsten filament heated by an
electric current. The filament is the cathode of
the tube. The high voltage potential is between
the cathode and the anode, the electrons are thus
accelerated, and then hit the anode.
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An x-ray tube is an electronic vacuum tube with
components contained within a glass or metal
enclosure. The x-ray tube, however, is a special
type of vacuum tube that contains two electrodes
the cathode and the anode. It is relatively
large, perhaps 30 to 50 cm long and 20 cm in
diameter. The glass enclosure is made of Pyrex
glass to enable it to withstand the tremendous
heat generated.
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The cathode is the negative side of the x-ray
tube it has two primary parts a filament and a
focusing cup.
Filament The filament is a coil of wire similar
to that in a kitchen toaster, except much
smaller. The filament is approximately 2 mm in
diameter and 1 or 2 cm long. In the kitchen
toaster, an electric current is conducted through
the coil, causing it to glow and emit a large
quantity of heat. An x-ray tube filament emits
electrons when it is heated. When the current
through the filament is sufficiently high, the
outer-shell electrons of the filament atoms are
boiled off and ejected from the filament. This
phenomenon is known as thermionic emission.
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Focusing Cup The filament is embedded in a metal
cup called the focusing cup (Because all the
electrons accelerated from cathode to anode are
electrically negative, the electron beam tends to
spread out owing to electrostatic repulsion. Some
electrons can even miss the anode completely.
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The focusing cup is negatively charged so that it
electrostatically confines the electron beam to a
small area of the anode. The focusing cup forms
them into a cloud. This cloud is called a space
charge The effectiveness of the focusing cup is
determined by its size and shape, its charge, the
filament size and shape, and the position of the
filament in the focusing cup.
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Most x-ray tubes have two filaments mounted in
the cathode assemble side by side, creating
large and small focal spot size
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Selection of one or the other focal spot is
usually made with the mA station selector on the
operating console. Normally, either filament can
be used with the lower mA stationapproximately
300 mA or less. At approximately 400 mA and up,
only the larger focal spot is allowed because the
heat capacity of the anode could be exceeded if
the small focal spot were used.
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The anode is the positive side of the x-ray tube
it conducts electricity and radiates heat and
contains the target.
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• The anode serves three functions in an x-ray
  tube.
• The anode is an electrical conductor. It receives
  electrons emitted by the cathode
• The anode also provides mechanical support for
  the target.
• The anode also must be a good thermal
  dissipator. When the projectile electrons from
  the cathode interact with the anode, more than
  99 of their kinetic energy is converted into
  heat. This heat must be dissipated quickly

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Target The target is the area of the anode
struck by the electrons from the cathode. In
stationary anode tubes, the target consists of a
tungsten alloy embedded in the copper anode. In
rotating anode tubes, the entire rotating disc is
the target
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In stationary anode tubes, the target consists of
a tungsten alloy embedded in the copper anode
The rotating anode x-ray tube allows the electron
beam to interact with a much larger target area
therefore, the heating of the anode is not
confined to one small spot, as in a stationary
anode tube. Rotational speeds of the anodes
ranges from 3,000 -4,000 rpms for the general
purpose x-ray tubes and 10,000 to 12, 000 rpms
for the special applications x-ray tubes (ex. CT.)
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The rotating anode is powered by an
electromagnetic induction motor.
An induction motor consists of two principal
parts separated from each other by the glass or
metal enclosure. The part outside the enclosure,
called the stator, consists of a series of
electromagnets equally spaced around the neck of
the tube. Inside the enclosure is a shaft made of
bars of copper and soft iron fabricated into one
mass. This part is called the rotor.
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• The three things needed to produce x-rays are now
  present
• The large potential difference to give kinetic
  energy to the filament electrons (provided by the
  kVp setting)
• A quantity of electrons provided by mAs
• A place for interaction (the target of the anode).

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The electron cloud is attracted to the anode
target because of the huge potential difference.
In fact, these filament electrons will reach
speeds of about half the speed of light in the
short 1 to 3 cm between the focusing cup and
anode target when 70 -80 kVp is utilized. As
the electron cloud flows from cathode to anode,
it is a continuation of the flow of electricity
through the x-ray circuit.
As they penetrate the target surface these
filament electrons interact with the atoms of the
target (Tungsten). These electrons are suddenly
decelerated. Their loss of kinetic energy
resultys in the production of mostly heat and
small amount of x-rays.
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Potential difference kVp
Electrons mA
X-ray photons
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The primary exposure technique factors the
radiographer selects on the control panel are
milliamperage, time of exposure, and kilovoltage
peak (kVp). Depending on the type of control
panel, milliamperage and exposure time may be
selected separately or combined as one factor,
milliamperage/second (mAs)
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Milliamperage Milliamperage (mA) is a measure of
the rate of current flow across the x-ray tube,
that is, the number of electrons flowing from
filament to target each second. The number of
available electrons is determined by the filament
heat.
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When filament heat is increased, more electrons
are available each second to cross the tube.
Thus, increasing the filament heat increases the
mA in the x-ray tube circuit. When more electrons
strike the target, more x-rays are produced,
so mA controls the volume of x-ray
production. High mA settings produce more x-rays
per second, and low mA settings produce x-rays at
a slower rate. Stated differently, mA controls
the intensity of the x-ray beam, determining the
number of photons that will strike the film
during 1 second. If the mA is doubled, the x-rays
are doubled, and if the mA is halved, the x-rays
are reduced by 50.
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Exposure time refers to the length of time that
the x-rays are turned on. It is the duration of
the x-ray exposure. Exposure time is measured in
units of seconds (sec). Most x-ray exposure times
are less than 1 second, and therefore milliseconds
(msec) are used 1 millisecond equals 0.001
second. A timer in the x-ray circuit terminates
the exposure after a preset length of time. The
quantity of x-rays produced is directly
proportional to the exposure time. For example,
it is apparent that if the exposure time is
doubled, twice as many x-rays will be produced.
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When the kVp is increased at the control panel, a
larger potential difference occurs in the x-ray
tube, giving more electrons the kinetic energy to
produce x-rays and increasing the kinetic energy
overall. The result is more photons (quantity)
and higher energy photons (quality).
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In radiography it is often useful to know the
total quantity of an exposure. The quantity of
x-ray photons in an exposure cannot be determined
by either the mA or the exposure time alone.
Although mA determines the rate of x-ray
production, it does not indicate the total
quantity, because it does not indicate how long
the exposure lasts. Exposure time does not
indicate the total quantity either, because it
does not measure the rate of x-ray production. To
determine the quantity of radiation involved in
an exposure, both mA and time must be
considered. The unit used to indicate the
quantity of exposure is milliampere-seconds,
abbreviated mAs. This unit is the product of mA
and exposure time
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