After
reading this section you will be able to do the following:
Identify
the main parts to the basic x-ray generator.
List the
three things that an X-ray generator must supply in order to
produce x-rays.
In this section the basic construction
of X-ray equipment and some different types of x-ray systems will
be introduced. Most standard x-ray systems have three main components
which are a x-ray tube, a high voltage power supply, and a control
unit. Working together, these components are common to all standard
systems.
From our introductory discussion on
the generation of x-rays you may recall that there were three
principle requirements to generate X-radiation. These three requirements
include a source of electrons, a means of acceleration, and a
target for interaction. You should recognize that electrical power
is necessary for X-ray generation.
Where do
the electrons come from?
You already know that
matter is made up of atoms, and atoms have electrons that orbit
around the nucleus in shells. All we need to do is get the electron
free of their orbit. How do we do this? The answer is fairly simple.
If we take a piece of conductive wire and pass a current through
it, the wire will heat up due to the resistance in the wire. The
heat of the wire excites the electrons and they will break away
(boil off) from the wire to expend the energy picked up from the
heat of the current. When the energy of the electron is expended,
it will return to the wire to become heated again. So this heated
wire serves as our source of electrons.
Why
do the electrons need to be accelerated and how is it done?
Our second requirement
is to get the electrons traveling at high speeds. The reason we
need to propel the electrons at high speeds is because the energy
that the electron possesses and can transfer is dependent on its
velocity. The higher the velocity of the electron when it interacts
with an atom, the greater the energy of the radiation that will
be produced. Propelling the electron is fairly simple. Since unlike
charges (positive and negative) attract, and electrons posses
a negative charge, all we need is a positive charge nearby to
attract the electron. We can accomplish this by placing a piece
of metal (anode) a short distance away from the wire filament
(cathode).
When we apply a voltage
to this anode, we place a high positive charge on it. This high
positive charge acts much like a magnet, only it is attracting
free electrons. The positive charge will possess a strong attractive
force to the negative charge of the electrons that are boiling
off of the filament. This attractive force pulls the electrons
towards the anode at high speeds. By increasing the voltage applied
to the anode we can increase the speed of the electrons.
What does the target
material do?
The third and final requirement is
to have a target material for the electrons to interact with.
By placing some sort of matter between the electrons (filament)
and the positive charge (anode) we meet our need. Also, the anode
itself can be used as the target. In high voltage X-ray generators
a special target material (Tungsten) is usually embedded into
the anode. This gives the electrons a suitable material to interact
with and produce x-rays. When the electron hits the target material,
several things can happen. The electron can be absorbed by an
atom and its energy transferred to the atom, the energy of the
electron can cause another electron to be knocked out of its energy
shell, or the electron may just slightly interact with other atomic
particles. Radiation will be produced in all of these cases, but
the energy of the radiation will be different.
The following illustration is a basic
schematic representing an x-ray tube:
Modern x-ray tubes come in many shapes
and sizes, normally they are of the glass or metal-ceramic tube
(envelope) style. As compared to early gas filled x-ray tubes,
modern tubes are of the high vacuum style. The modern techniques
of tube design have allowed for smaller tubes, extended tube life,
and more efficient and stable operation.
The means of acceleration
of the electrons is provided by applying a potential difference
(voltage) across the tube anode and cathode and is independent
of the voltage and current across the filament.
The x-ray tube is technically
referred to as an envelope. Typical construction may be from blown
glass or metal-ceramic styles. Glass envelope tubes are still
common today, although they have definite disadvantages to the
newer metal-ceramic designs. Due to the tremendous amount of heat
generated during x-ray production, glass suffers from thermal
and mechanical shock. Metal-ceramic materials do not suffer damage
from the excessive heat to the degree that glass does and are
rapidly replacing the glass style tube.
The
Cathode
From the above illustration let's
look at each of the components separately beginning with the cathode.
The cathode is the negative terminal of the tube assembly
and includes the filament, which is a small-coiled wire that is
commonly made from tungsten. The filament provides the electrons
for acceleration to the target (anode). Tungsten is metal
with the desired properties for filaments, you have probably seen
a tungsten filament in a light bulb before. The filament is normally
powered by an alternating current that is supplied to it by a
separate transformer.
In many of the x-ray
tubes, the current supplied to the filament ranges from a few
hundred micro-amperes (symbol 109 \f "Symbol" \s 12mA)
to several milli-amperes (mA). Filament current may be varied
or fixed to maintain a constant tube current. Remember from our
earlier discussion that the filament supplies the electrons. Adjustments
in current to the filament varies the number of electrons that
will boil off the filament. This in turn controls the number of
x-rays that the tube is generating. Filament current controls
the x-ray intensity.
The
Anode
The positive terminal of an x-ray
tube is called the anode, it serves three important functions,
(1) it provides a complete circuit for purposes of accelerating
the electrons, (2) it houses the target material, and (3) it helps
to cool the tube. We already mentioned before that the generation
of X-rays generates a tremendous amount of heat. If the heat in
a tube was ignored, the target material that is embedded in the
anode would be destroyed in a short period of time. The anode
is typically made from materials with good thermal properties
to dissipate heat. Copper is a common anode material. In addition
to using thermally conductive materials for the anode, alternate
means of cooling that may be employed are gas, oil, water, or
air.
Does
the density of the target material matter?
As previously mentioned,
the anode also houses the target material. As an integral part
of the tube, the target requires special consideration. The target
provides the means for electron interaction (bombardment). The
target is commonly made from tungsten and other materials like
cobalt, iron, or copper. Another important characteristic of the
target material is its density. The material must be of high atomic
mass for electron interaction. Remember that when the electron
interacts with the target atoms the result is the generation of
x-rays. Low density materials do not provide sufficient density
for interaction.
The
High Voltage Power Supply
A high voltage power supply is an
important component of an X-ray generation system. When we say
high voltage supply, we need to differentiate from that of commercial
electricity. Keep in mind that the filament uses a relatively
small voltage supply to cause small currents (mV) in the filament,
while the anode of the tube requires a large voltage supply to
maintain a high positive charge for acceleration of the electrons.
Commercial power is commonly available as 110 volts, 220, or 440
volts. X-ray systems require very high voltages commonly in the
range from 5 kilovolts (Kv) to as much as 400 Kv or more. So how
can we supply low voltage to the filament, and high voltage to
the anode? This is accomplished by using a transformer. A transformer
will allow us to supply the proper voltages to the filament and
anode. The next question we need to answer would be what is a
transformer and how does it work?
What are
transformers?
Transformers are electromagnetic
devices that allow a voltage of alternating current to be changed;
the voltage may be increased or decreased. Two common types of
transformers which are of importance to x-ray generation are step-up
and step-down. Transformers are comprised of two sets of windings
(coiled conductors) that are electrically isolated from each other.
One set of windings is connected to a power supply and is known
as the primaries. The other set of windings is connected
to a load (in this case the x-ray tube) and is referred to as
the secondary windings.
The principle operation
of a transformer is based on induction. If you have studied electricity,
you should know that when you pass current through a conductor,
a magnetic field is established in and around the conductor. This
magnetic field can be used to induce a voltage and current flows
in a conductive material that is placed close by.
The
Control Unit
The third essential component
to a standard x-ray system is the control unit. We have discussed
the tube design and the power supply, now we need to know how
to control the energy and intensity of the radiation being generated.
There are three principle controls to a standard x-ray system,
which are the ma control, the kV control, and a timer. The first
two are the most important in terms of the radiation characteristics.
We will briefly describe the timer control. The controls for the
system are usually housed in a panel.
Current
Control
The ma control on
an x-ray system commonly includes some type of a panel meter or
digital display (millimeter) which is a rheostat connected to
the circuit that allows adjustment in tube current. Adjusting
the current being applied to the filament results in variations
in the radiation intensity. Remember that the filament provides
the electrons for interaction with the target. When the tube current
is varied, the number of electrons being supplied to the anode
(target) varies.
Voltage Control
The kV control on
a x-ray system is similar to the ma control in that it includes
some type of metered display (kilovolt meter) and a rheostat in
the circuit. The voltage being supplied to the anode is referred
to as the tube voltage, and is primarily measured in kilovolts.
Variations in the tube voltage affects the energy of the radiation;
penetrating power varies with the voltage. Increasing the tube
voltage increases the speed of the electrons interacting with
the target. Remember from our previous discussions that the energy
of radiation is a function of the wavelength. Increasing the energy
results in a shorter wavelength x-ray photon with greater penetrating
power.
Time Control
The third control
feature of an x-ray system is the timer. The timer is no different
then one you set when baking cookies. The timer is much like that
of a stop watch. It may be an analog or digital display of some
sort. The function of the timer is simply to control the duration
of the exposure, in other words, how much time the tube is generating
radiation. It is, however, connected to the circuits of the system.
When the time has elapsed, the system shuts down and no more radiation
will be produced until the system is reset.
Review:
The three
main parts to an x-ray generator setup are an x-ray tube, a
high voltage power supply, and a control unit.
The X-ray
generator provides three things that are required to produce
X-rays, and they are a source of electrons, a means of acceleration,
and a target for interaction.
