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X rays are part of the electromagnetic spectrum
and are really a form of light. Like visible light, X-ray photons travel at the
speed of light, and they can produce a latent image on film. Unlike visible light, X rays can penetrate most
opaque matter, make
some materials fluorescent, and produce ionization of some materials.
Bremsstrahlung radiation results when an
electron passes near the nucleus of an atom. The close passage of the
electron to the nucleus causes the electron to change its course thus losing
much of its energy in the process. In the world of quantum particles, energy is
always exchanged in discreet particles of light known as photons. The
loss of energy by the electron as it is deflected by the heavy nuclei in the
anode target produces a very high energy photon of light called
an x-ray. The
dental x-ray tube produces Bremsstrahlung radiation.
Kilovoltage and Milliamperage
Electricity flows through a wire like water flows
through a pipe. If the water is under low pressure, it will flow
slowly. If it is under high pressure, the water flows faster and more
water flows through the pipe each second. In Electricity, the "pressure"
in the wire is measured in volts, or Kilovolts (thousands of
volts), and the amount of electricity, called the current, flowing
through the wire each second is measured in amperes or in milliamperes
(a milliampere is one thousandth of an ampere).
Electricity under high voltage (great pressure)
flows so fast that it can jump across gaps in the wire. Think of water
under high pressure rushing out of a fireman's hose. The nozzle on the
fireman's hose constricts the flow of water to make the water shoot out further.
This constriction in the water line can be thought of as resistance to
the flow of water. In electricity, wires come with various degrees of
resistance depending on their composition and diameter.
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When current flows through a high
resistance wire, the wire will heat up, just like the filament in a light
bulb.
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When high voltage pushes current across a low
resistance gap, the electrons that make up the current can jump the gap with
great speed. Both of these principles are operating in an x-ray tube.
The x-ray machine takes energy from an electrical
source (usually a 220V outlet) and converts it to two separate voltage streams.
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One stream is a low voltage source which can
be varied so that different amounts of current can flow through the high
resistance wire that composes the filament in the cathode. The current that flows
through the filament is measured in milliampres (mA).
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The second stream is very high voltage,
measured in kilovolts (thousands of volts at peak voltage -- kVp). This
voltage is applied across the gap between the anode and the cathode.
How KVP and MA come together in an
x-ray tube
The schematic of the x-ray tube is reproduced
here for the reader's convenience.
The low voltage stream (measured in milliampers--mA)
flows through the high resistance wire in the heating element of the cathode pictured on the left side of
the diagram above. The heating element resides inside a metal covering called a filament
focusing cup. Whenever a piece of metal gets red hot, electrons become
excited and tend to "boil" off the surface. The higher the
temperature of the metal, the larger the number of electrons that boil off. As the electrons boil off
the filament, it would normally become progressively more positively charged, and
the negatively charged electrons would simply fall back into filament.
But that's not what happens in an x-ray tube. That's where the high
voltage stream comes in.
The high voltage stream is applied across the anode and cathode connections at either end of the
tube. This applies thousands of volts (generally 70 thousand) across the gap between the filament on the cathode
(negative) side, and the
tungsten target on the anode (positive) side of the system. This voltage
causes the negatively charged electrons that boil off the filament to be
attracted to the positively charged anode, just like a magnet attracts iron
filings. The gap is filled with a
vacuum so nothing interferes with the flow of electrons across the gap (low
resistance). This is a bit like
applying thousands of pounds of pressure to the water in a pipe.
Obviously, water under very high pressure would shoot out of the open end of the pipe
like the stream of water through the fireman's hose.
In the X-ray tube, the
huge kVp across the gap supplies so much pressure to the stream of electrons
generated by the hot filament that the electrons go speeding across the gap with
tremendous velocity. The focusing cup is negatively charged and repels the
negatively charged electrons so they are all pushed to the center of the cup and
end up focused in a tight beam. The electrons hit the tungsten target so
hard that they "explode" into a shower of high energy
photons (see the explantaion of bremsstrahlung radiation
above) . These photons are the x-rays. X-ray photons are like
the photons in visible light except that they contain so much energy that they
can penetrate opaque objects. But even with their great energy, objects of
varying density can block some of them, casting
shadows on whatever screen is there to stop the photons that get through, just like
regular light. In the case of medical x-rays, the "screen" is generally an
x-ray film or a digital sensor.
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The energy of the x-rays is controlled by the
high voltage kVp. When the kVp is increased, the x-ray photons that
are produced have a shorter wavelength, and thus have higher energy and pack more of a punch.
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The number of x-ray photons produced
(i.e. the intensity of the beam) depends
on the number of electrons that boil off the filament. This depends on
the temperature of the heating element which in turns depends on the current
flowing through the filament. The amount of current flowing through
the filament is controlled by the mA setting.
Kilovoltage and milliamp settings
In dental x-ray
units, the kVp and mA are set by the manufacturer and are rarely changed by the
end user. (Some machines allow such changes, but practitioners very rarely
make changes from factory default settings.) The only variable that is
normally adjusted by the operator is the
time that the kVp is applied across the x-ray tube. Modern tubes use
tenths and hundredths of a second as a standard measure of time, but older
machines used "pulses", each pulse being 1/60 of a second. Many of these
machines are still in use, so if you see a unit with a dial labeled in whole
numbers, it is probably measuring time in pulses. To get the actual time,
multiply the number on the dial by 1/60 of a second. 6 pulses would be a
tenth of a second.
The most common
settings for dental x-ray units are 70 kVp (kilovoltage peak) or 90 kVp. If the
kVp were to be changed on a dental x-ray unit, a 15%
increase in kilovoltage would double the density on the radiograph. In this
case, an operator would have to cut the
exposure time in half to keep the same density on the film. The kilovoltage is
responsible for the quality of the x ray beam. Milliamperage (mA) is responsible
for the quantity or number of rays produced. For dental use, the normal range
for milliamperage is between 7 and 15 mA.
According to federal guidelines, a chart with the
settings for time (seconds or impulses), kVp, and mA for the techniques most
commonly used must be posted near the control panel of each x-ray unit in the
office.
An x-ray beam with the lowest possible
kilovoltage should be used, but not less than 60 kVp. Filtration equivalent to 2.5 mm of
aluminum should be used for 70 kVp or more. Those units operating below 70 kVp
should have the equivalent of 1.5 mm of aluminum.
Filtering
The photons produced by the x-ray tube come in a
range of energies. It is desirable to limit the output of the x-ray tube
to only the most energetic photons. Low energy photons are more easily
absorbed by soft tissue and would generally not reach the film. On the
other hand, they represent an increased absorbed dose of radiation to the
patient. Since aluminum is transparent to high energy x-rays, but more
opaque to low energy x-ray photons, the low energy photons are filtered out by
placing a flat aluminum disk in the path of the radiation beam. Most
modern machines are factory set to produce 70 kVp radiation, so most x-ray tubes
come with a 2.5 mm thick aluminum filter.
Controlling x-rays
The x-ray radiation wavelengths and penetration
characteristics are controlled according to three variables:
1. Filament Temperature: The higher the
temperature of the filament, the more electrons are released. We measure the
level of current as milliamperes (mA). Increasing the mA increases the number of
electrons emitted from the cathode. This, in turn, increases the number of
x-rays produced.
2. Kilovoltage: The voltage between the
negatively charged cathode and the positively charged anode is expressed in peak
kilovolts (kVp). Increasing the kVp increases the speed of the electrons that
strike the target. Higher kVp settings produce shorter wavelength (higher
energy) x-rays. These
have more penetrating power than longer wavelength x-rays.
3. Time: A timer on the x-ray tube controls the
number of seconds that electrons are produced by the cathode. This also
influences the number of x-rays produced.
Penetration and the x-ray image
When x-ray beams enter an object, they have a
uniform distribution of high energy wavelengths. The x-rays are then absorbed to a
greater or lesser degree depending on what tissue they encounter before they
strike the x-ray film. In this way, the patient’s tissues form a pattern of
x-ray radiation (called differential attenuation.)[i]
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Dentists and allied dental professionals often
seek CE courses from ADA CERP recognized providers to fulfill their
CE requirements for re-licensure. Most state and
provincial licensing boards will accept CE credits issued by ADA
CERP recognized providers. In the spring of 2003, the FDI World
Dental Federation became the first internationally based CE provider
to be granted ADA CERP recognition.
Please contact your state board directly for their specific rules
and regulations. Most states approve supervised self-study courses
that are ADA CERP accredited.
Those dentists, hygienists, dental assistants
and radiographers interested in receiving 3 continuing
education credits for this course may take a 10 question test at a
cost of $35 and receive their certificate immediately by clicking
here.
Those dentists, hygienists, dental assistants
and radiographers interested in receiving 8 continuing
education credits for this course may take a 25 question test at a
cost of $66 and receive their certificate immediately by clicking
here.
Note: There are no questions on tables or
Glossary. |
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[i]
http://www.kodak.com/US/en/health/dental/focus/dentHyXct.jhtml
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