X-ray Production/ dental courses

X-ray Production
The following slides identify atomic
structure, the forces at work inside the
atom, types of electromagnetic radiation
(including x-rays), x-ray characteristics,
components of an x-ray machine and x-
ray tube, how x-rays are formed and
ways to modify the x-ray beam.
INDIAN DENTAL ACADEMY
Leader in continuing Dental
Education
www.indiandentalacademy.com

In navigating through the slides, you should click
on the left mouse button when you see the
mouse holding an x-ray tubehead or you are
done reading a slide. Hitting “Enter” or “Page
Down” will also work. To go back to the previous
slide, hit “backspace” or “page up”.

An atom is composed of electrons (with a negative
charge), protons (with a positive charge) and
neutrons (no charge). The protons and neutrons
are found in the nucleus of the atom and the
electrons rotate (orbit) around the nucleus. The
number of electrons equals the number of protons
in an atom so that the atom has no net charge
(electrically neutral). Different materials (for
example, gold and lead) will have different
numbers of protons/electrons in their atoms.
However, all the atoms in a given material will have
the same number of electrons and protons. (See
diagram next slide)
Atomic Structure

Atom
This atom has 7 protons and 7 neutrons in the nucleus.
There are 7 electrons orbiting around the nucleus.
protons
neutrons
electrons

The electrons are maintained in their orbits
around the nucleus by two opposing forces.
The first of these, known as electrostatic force,
is the attraction between the negative electrons
and the positive protons. This attraction causes
the electrons to be pulled toward the protons in
the nucleus. In order to keep the electrons from
dropping into the nucleus, the other force,
known as centrifugal force, pulls the electrons
away. The balance between these two forces
keeps the electrons in orbit.
(See next three slides)

Electrostatic force is the attraction between the
positive protons and negative electrons. Electrons in
the orbit closest to the nucleus (the K-shell) will have
a greater electrostatic force than will electrons in
orbits further from the nucleus. Another term often
used is binding energy; this basically represents the
amount of energy required to overcome the
electrostatic force to remove an electron from its
orbit. For our purposes, electrostatic force and
binding energy are the same. The higher the atomic
number of an atom (more protons), the higher the
electrostatic force will be for all electrons in that
atom.

Centrifugal force pulls the electrons
away from the nucleus

The balance between electrostatic force and
centrifugal force keeps the electrons in orbit
around the nucleus
EF CF

Electromagnetic Radiation
An x-ray is one type of electromagnetic
radiation. Electromagnetic radiation
represents the movement of energy through
space as a combination of electric and
magnetic fields. All types of electromagnetic
radiation, which also includes radiowaves, tv
waves, visible light, microwaves and gamma
rays, travel at the speed of light (186,000 miles
per second). They travel through space in
wave form.

D
W
W
The waves of electromagnetic radiation have two
basic properties: wavelength and frequency. The
wavelength (W) is the distance from the crest of one
wave to the crest of the next wave. The frequency (F)
is the number of waves in a given distance (D). If the
distance between waves decreases (W becomes
shorter), the frequency will increase. The top wave
above has a shorter wavelength and a higher
frequency than the wave below it.
F = 3
F = 2

radio
waves
tv
waves
visible
light
x-rays gamma
rays
cosmic
rays
Which of the above examples of electromagnetic
radiation has the shortest wavelength?
Which of the above has the lowest frequency?
Cosmic rays
Radio waves

The energy of a wave of electromagnetic
radiation represents the ability to penetrate
an object. The higher the energy, the more
easily the wave will pass through the object.
The shorter the wavelength, the greater the
energy will be and the higher the frequency,
the greater the energy will be.
X-ray Energy

A
B
C
Which of the above x-rays has the highest energy?
A: It has the shortest wavelength, highest frequencywww.indiandentalacademy.com

X-ray Characteristics
• X-rays are high energy waves, with very short
wavelengths, and travel at the speed of light.
• X-rays have no mass (weight) and no charge
(neutral). You cannot see x-rays; they are
invisible.
• X-rays travel in straight lines; they can not
curve around a corner.
• An x-ray beam cannot be focused to a point; the
x-ray beam diverges (spreads out) as it travels
toward and through the patient. This is similar
to a flashlight beam.

• X-rays are differentially absorbed by the
materials they pass through. More dense
materials (like an amalgam restoration) will
absorb more x-rays than less dense material (like
skin tissue). This characteristic allows us to see
images on an x-ray film.
• X-rays will cause certain materials to fluoresce
(give off light). We use this property with
intensifying screens used in extraoral radiography.
• X-rays can be harmful to living tissue. Because of
this, you must keep the number of films taken to
the minimum number needed to make a proper
diagnosis.
X-ray Characteristics (continued)

X-ray Equipment
X-ray equipment has three basic components:
(1) the x-ray tubehead, which produces the x-
rays, (2) support arms, which allow you to
move the tubehead around the patient’s head
and (3) the control panel, which allows you to
alter the duration of the x-ray beam (exposure
time) and, on some x-ray machines, the
intensity (energy) of the x-ray beam.
1
3
2

PID
(cone)
X-ray
Tubehead
degrees
The x-ray tubehead is attached to the support arms
so that it can rotate up and down (vertically;measured
in degrees) and sideways (horizontally) to facilitate
proper alignment of the x-ray beam. The PID (Position
Indicating Device) is attached to the x-ray tubehead
where the x-ray beam exits and it identifies the
location of the x-ray beam. Some people refer to the
PID as a “cone”; the PID’s on very old x-ray machines
used to be coneshaped.

The control panel, like the one above left, allows you
to change exposure time but nothing else. Some
machines, like the one above right, have controls for
changing the mA and kVp settings in addition to
exposure time. The individual controls will be
discussed more later.
exposure time kVp control
mA control

X-ray Tube
X-rays are produced in the x-ray tube, which is
located in the x-ray tubehead. X-rays are
generated when electrons from the filament
cross the tube and interact with the target. The
two main components of the x-ray tube are the
cathode and the anode.

(tungsten)
Cathode
Focusing
cup
Filament
The cathode is composed of a tungsten filament
which is centered in a focusing cup. Electrons are
produced by the filament (see next slide) and are
focused on the target of the anode where the x-rays
are produced. The focusing cup has a negative
charge, like the electrons, and this helps direct the
electrons to the target (“focuses” them; electrons
can be focused, x-rays cannot).
side view
(cross-section)
front view
(facing target)

Thermionic Emission
x-section
of
filament
hot
filament
When you depress the exposure button, electricity
flows through the filament in the cathode, causing it to
get hot. The hot filament then releases electrons which
surround the filament (thermionic emission). The hotter
the filament gets, the greater the number of electrons
that are released. (Click to depress exposure button
and heat filament).
electrons

Anode
Copper stem
Target
The anode in the x-ray tube is composed of a
tungsten target embedded in a copper stem. When
electrons from the filament enter the target and
generate x-rays, a lot of heat is produced. The
copper helps to take some of the heat away from
the target so that it doesn’t get too hot.
side view front view
Target

X-ray Tube Components
1
2
4
3
5
8
6
7
9
1. focusing cup 6. copper stem
2. filament 7. leaded glass
3. electron stream 8. x-rays
4. vacuum 9. beryllium window
5. target
(for description, see next slide)www.indiandentalacademy.com

1. Focusing cup: focuses electrons on target
2. Filament: releases electrons when heated
3. Electron stream: electrons cross from filament to
target during length of exposure
4. Vacuum: no air or gases inside x-ray tube that might
interact with electrons crossing tube
5. Target: x-rays produced when electrons strike target
6. Copper stem: helps remove heat from target
7. Leaded glass: Keeps x-rays from exiting tube in
wrong direction
8. X-rays produced in target are emitted in all
directions
9. Beryllium window: this non-leaded glass allows
x-rays to pass through. The PID would be
located directly in line with this window.
X-ray Tube Components (continued)

Target
Beryllium Window
Focusing cup
(filament located inside)
Photo of an X-ray Tube
Leaded glass

X-ray Machine
Components
Control Panel
X-ray
Tubehead
110, 220 line Timer
Exposure switch
mA selector
kVp selector
Autotransformer
Step-down transformer
Step-up transformer
X-ray Tube
Wires
Oilwww.indiandentalacademy.com

The x-ray machine is plugged into a 110-volt outlet
(most machines) or a 220-volt outlet (some extraoral
machines). The current flowing from these outlets is
60-cycle alternating current. Each cycle is
composed of a positive and negative phase. X-rays
are only produced during the positive phase; the
target needs to be positive to attract the negative
electrons from the filament. During the positive
portion of the cycle, the voltage starts out at zero
and climbs to the maximum voltage before
dropping back down to zero and entering the
negative phase. Each complete cycle lasts 1/60 of a
second; there are 60 cycles per second.
(See next slide)
X-ray Machine Voltage

+ 110, 220
- 110, 220
positive
negative
target positive;
electrons flow
target negative;
no electron flow
target positive;
electrons flow
0
voltage starts at zero and reaches
a maximum of 110 or 220 before
going back to zero

Direct Current (Constant Potential)
60-cycle Alternating Current
Many machines now convert the alternating current
into a direct current (constant potential). Instead of
cycles going from zero to the maximum, both
positive and negative, the voltage stays at the
maximum positive value, creating more effective x-
ray production. This allows for shorter exposure
times.

Timer
The timer controls the length of the exposure. The
black numbers above represent impulses. The red
numbers are seconds.

Number of Impulses
60
= seconds
1/60 sec.With alternating current, there are 60
complete cycles each second; each
cycle represents an impulse and is 1/60
of a second. To change impulses into
seconds, divide the number of impulses
by 60. To convert seconds to impulses,
multiply by 60.
Number of seconds X 60 = impulses
60 impulses/60 = 1 second
30 impulses/60 = 0.5 (1/2) second
15 impulses/60 = 0.25 (1/4) second
0.75 (3/4) second X 60 = 45 impulses
0.1 (1/10) second X 60 = 6 impulseswww.indiandentalacademy.com

There are two electrical circuits operating during an
x-ray exposure. The first of these is the low-voltage
circuit that controls the heating of the filament.
When the exposure button is depressed, this low
voltage circuit operates for ½ second or less to heat
up the filament. There are no x-rays produced during
this time. As you continue to depress the exposure
button, the high-voltage circuit is activated. This
circuit controls the flow of electrons across the x-ray
tube; during the positive portion of the alternating
current cycle, the negative electrons are pulled
across the x-ray tube to the positive target. X-rays
are produced until the exposure time ends. The
length of time the high-voltage circuit is operating
represents the exposure time. (See next slide).

X-ray Exposure
2. Activate low-voltage circuit to heat filament3. Activate high-voltage circuit to pull electrons across tube4. Electrons cross tube, strike target and produce x-rays1. Depress exposure button5. X-ray production stops when exposure time ends.
Release exposure button

Exposure Button
The timer determines the length of the exposure,
not how long you hold down the exposure
button; you cannot overexpose by holding the
exposure button down for an extended period.
However, you can underexpose by releasing the
exposure button too soon; the exposure
terminates as soon as you release the button.www.indiandentalacademy.com

mA setting
milliAmpere (mA) selector
The mA (milliAmpere) setting determines the
amount of current that will flow through the
filament in the cathode. This filament is very
thin; it doesn’t take much current (voltage) to
make it very hot. The higher the mA setting, the
higher the filament temperature and the greater
the number of electrons that are produced.

Step-Down Transformer
If the voltage flowing through the filament is too
high, the filament will burn up. In order to reduce
the voltage, the current flows through a step-
down transformer before reaching the filament.
The voltage reaching the step-down transformer
is determined by the mA setting. The step-down
transformer reduces the incoming voltage to
about 10 volts, which results in a current of 4-5
amps flowing through the filament.

Step-Down Transformer
Primary
Secondary
110 volts
or less
currentflow
10 volts
currentflow
The current enters the step-down transformer on the
primary (input) side and exits on the secondary (output)
side. The fewer turns in the coil on the secondary side, the
lower the output voltage will be. The primary coil below
would have 110 turns, the secondary coil would have 10.
(Each loop of the coil is a “turn”; the number of turns in the
diagram below has been reduced for easier viewing).

kiloVolt peak (kVp) control
kVp readout
kVp control knob
The kVp control regulates the voltage across the x-
ray tube. (A kilovolt represents 1000 volts; 70 kV
equals 70,000 volts. A 70 kVp setting means the peak,
or maximum voltage, is 70,000 volts). The higher the
voltage, the faster the electrons will travel from the
filament to the target. The kVp control knob regulates
the autotransformer (see next slide).www.indiandentalacademy.com

Autotransformer
The autotransformer determines how much
voltage will go to the step-up transformer.
Basically, a transformer is a series of wire coils.
In the autotransformer, the more turns of the coil
that are selected (using the kVp control knob),
the higher the voltage across the x-ray tube will
be. This is similar to the function of a rheostat.
The following slide shows how this works. The
incoming line voltage will be 110 volts. The
exiting voltage will be 65 volts if the kVp control
is set at 65. The exiting voltage will be 80 volts if
the kVp setting is 80.

110 V
65 volts
currentflow
Autotransformer: the initial setting is 65; 65 volts
leave the autotransformer.
80 volts
to step-up transformer
kVp
selector
Autotransformer: if the setting is changed to 80,
80 volts leave the autotransformer.

The voltage coming from the autotransformer next
passes through the step-up transformer, where it is
dramatically increased. The ultimate voltage coming
from the step-up transformer is roughly a thousand
times more than the entering voltage. For example, if
you set the kVp control knob to 65, 65 volts will exit
the autotransformer. This 65 volts is increased to
65,000 volts by the step-up transformer. (The “k” in
kVp stands for one thousand; 65 kV is 65,000 volts).
The side of the step-up transformer where the voltage
enters (primary side) has far fewer turns in the coil
than the exit (secondary) side.
Step-Up Transformer

Step-Up Transformer
Primary
Secondary
65-90 volts
currentflow
65,000 to
90,000 volts
currentflow
The current enters the step-down transformer on the
primary (input) side and exits on the secondary (output)
side. The more turns in the coil on the secondary side, the
higher the output voltage will be. The secondary coil in the
step-up transformer has 1000 times as many turns as the
primary coil. (Again, the number of turns has been reduced
for easier viewing).

65,000 to
90,000 volts
kVp
filament
110 volts
10 volts
The relationship of the various x-ray machine components
are shown in the diagram below. They form the high-voltage
and low-voltage circuits. For a more detailed review of the
components, see next slide.

exposure
button
oil
filter
The x-ray machine is plugged into the electrical
outlet (110 volts usually).
The length of the exposure is selected with the timer.When the exposure button is depressed, the current can
flow into the x-ray tubehead. This activates the low-
voltage circuit which heats the filament; this lasts for ½
second (Click to depress exposure button).
While keeping the exposure button depressed, the
high-voltage circuit is activated to pull the
electrons from the filament to the target, producing
x-rays. (Click to produce x-rays).
filament
The x-rays pass through the filter and collimator
before exiting through the PID. (Click for next slide)

The tubehead is filled with oil which surrounds the
transformers, x-ray tube and electrical wires. The primary
function of the oil is to insulate the electrical
components. It also helps to cool the anode and, as we
will discuss later, it helps in filtration of the x-ray beam.
The barrier material prevents the oil from leaking out of
the tubehead but still allows most x-rays to pass through.
oil
barrier
material
Step-up
Trans
Step-down
Trans

X-ray Production
There are two types of x-rays produced in the
target of the x-ray tube. The majority are
called Bremmstrahlung radiation and the
others are called Characteristic radiation.

Bremmstrahlung x-rays are produced when high-
speed electrons from the filament are slowed
down as they pass close to, or strike, the nuclei of
the target atoms. The closer the electrons are to
the nucleus, the more they will be slowed down.
The higher the speed of the electrons crossing the
target, the higher the average energy of the x-rays
produced. The electrons may interact with several
target atoms before losing all of their energy.
Bremsstrahlung Radiation
(Also known as braking radiation or general
radiation)

Bremsstrahlung X-ray Production
+High-speed
electron from
filament enters
tungsten atom
Electron slowed
down by positive
charge of
nucelus; energy
released in form
of x-ray
Electron continues on in
different direction to interact
with other atoms until all of its
energy is lostwww.indiandentalacademy.com

Bremsstrahlung X-ray Production
Maximum energy
High-speed electron
from filament enters
tungsten atom and
strikes target, losing
all its energy and
disappearing
The x-ray produced has energy
equal to the energy of the
high-speed electron; this is the
maximum energy possible
+

Characteristic Radiation
Characteristic x-rays are produced when a high-
speed electron from the filament collides with an
electron in one of the orbits of a target atom; the
electron is knocked out of its orbit, creating a
void (open space). This space is immediately
filled by an electron from an outer orbit. When the
electron drops into the open space, energy is
released in the form of a characteristic x-ray. The
energy of the high-speed electron must be higher
than the binding energy of the target electron
with which it interacts in order to eject the target
electron. Both electrons leave the atom.

Characteristic x-rays have energies “characteristic” of
the target material. The energy will equal the difference
between the binding energies of the target electrons
involved. For example, if a K-shell electron is ejected
and an L-shell electron drops into the space, the energy
of the x-ray will be equal to the difference in binding
energies between the K- and L-shells. The binding
energies are different for each type of material; it is
dependent on the number of protons in the nucleus
(the atomic number).
Characteristic Radiation (continued)
K-shell
M-shell
L-shell

Characteristic X-ray Production
L
K
M
High-speed electron
with at least 70 keV
of energy (must be
more than the
binding energy of k-
shell Tungsten atom)
strikes electron in
the K shell, knocking
it out of its orbit
Ejected electron
leaves atom
Recoil electron
(with very little
energy) exits
atom
vacancy
X-ray with 59
keV of energy
produced. 70
(binding
energy of K-
shell electron)
minus 11
(binding
energy of L-
shell electron)
= 59.
Electron in L-shell
drops down to fill
vacancy in K-shell

X-ray Spectrum
An x-ray beam will have a wide range of x-ray energies;
this is called an x-ray spectrum. The average energy of
the beam will be approximately 1/3 of the maximum
energy. The maximum energy is determined by the kVp
setting. If the kVp is 90, the maximum energy is 90 keV
(90,000 electron volts); the average energy will be 30. As
shown below, characteristic x-rays contribute a very
small number of x-rays to the spectrum.
X-ray energy (keV)
characteristic
x-rays
(59 & 67 keV)
#ofx-rays
Bremmstrahlung
x-rays

X-ray Spectrum (continued)
The x-ray spectrum results from three
factors:
(1) the varying distances between the
high-speed electrons and the
nucleus of the target atoms
(2) multiple electron interactions. The
high-speed electrons keep going
until all energy is lost.
(3) varying voltage. With an alternating
current, the speed of the electrons
will change as the voltage changes.
The higher the voltage, the faster
the electrons will travel. This is not
a factor when the newer constant
potential x-ray units are used.www.indiandentalacademy.com

X-ray production is a very inefficient process. Only
1% of the interactions between the high-speed
electrons and the target atoms result in x-rays. 99
% of the interactions result in heat production. The
excess heat is controlled by the high melting point
of the tungsten target, the conductive properties of
the copper sleeve and the cooling from the oil
surrounding the x-ray tube.
heat

X-ray Beam Modifiers
The following slides identify the various ways
of changing the energy of the x-ray beam and
the number of x-rays produced during an x-
ray exposure.

Exposure Factors
The energy of the x-ray beam and the number
of x-rays are primarily regulated by the kVp
control, the mA setting and the exposure time.
One, two or all three of these exposure factors
may need to be adjusted, depending on the
size of the patient’s head, the likelihood of
patient movement due to tremors or the
inability to hold still, etc.. If the exposure
factors are not set properly for the current
patient, the resultant film may be too light or
too dark (see next slide).

Exposure factors too high
(too dark)
Correct exposure factors
Exposure factors too low
(too light)

kVp (kilovolt peak)
The kVp primarily controls the energy or
penetrating quality of the x-ray beam. The higher
the kVp, the higher the maximum energy and the
higher the average energy of the beam. A higher
kVp allows the x-ray beam to pass through more
dense tissue in a larger individual, resulting in a
more acceptable radiographic image. In addition
to increasing penetrating ability, a higher kVp will
also result in the production of more x-rays.
Because of this, an increase in kVp will allow for a
decrease in exposure time, which may be helpful
in children or in adults with uncontrolled head
movement.

kVp (kiloVolt peak)
X-ray Energy (keV)
NumberofX-rays
70 90
90 kVp
70 kVp
In switching from 70 kVp to 90 kVp, the average
energy increases (dotted lines below), the maximum
energy increases (from 70 keV to 90 keV) and the
number of x-rays increases. (Click to change from
70 kVp to 90 kVp).

mA (milliampere)
The mA setting determines the heating of the filament.
The hotter the filament, the more electrons that are
emitted; the more electrons crossing the x-ray tube, the
greater the number of x-rays that result. There is no
change in the average energy or maximum energy of the
x-ray beam. Doubling the mA setting results in twice as
many x-rays. (Click to change from 5 mA to 10 mA).
NumberofX-rays
X-ray Energy
10 mA (twice as many x-rays)
5 mA
maximum energy
average energy
(no change)
(no change)

NumberofX-rays
X-ray Energy
10 impulses
(twice as many x-rays)
5 impulses
maximum energy
average energy
Exposure Time
An increase in exposure time will result in an
increase in the number of x-rays. Doubling the
exposure time doubles the number of x-rays
produced. Exposure time has no effect on the
average or maximum energy of the x-ray beam.
(Click to change exposure time from 5 impulses
to 10 impulses).
(no change)
(no change)

mAs or mAi
mAs = milliamperes (mA) x seconds (s)
mAi = milliamperes (mA) x impulses (i)
All x-ray machines have an mA setting (may be fixed or
variable) and an exposure time setting (always variable) for
each radiograph taken. The product of the mA setting times
the exposure time equals mAs or mAi, depending on whether
the exposure time is in seconds or impulses. As long as the
mAs remains constant for a given patient size, the x-ray
output will remain the same. For example, if the mA setting is
5 and the exposure time is 30 impulses, the mAi would be 150
(5 times 30). If we change the mA setting to 10 and decrease
the exposure time to 15, the mAi is still 150 (10 times 15).
There will be no change in the number of x-rays. If an x-ray
machine has variable mA settings, increasing the mA will
allow for a decrease in exposure time; this will be
advantageous in most cases.www.indiandentalacademy.com

1. Recommended kVp, mA, exposure time (e.t.)
2. Increase mA; no change in kVp, e.t.
3. Decrease e.t.; no change in kVp, mA
4. Increase kVp; no change in mA, e.t.
5. Double mA, halve e.t.; no change in kVp
A CB
B
A
C
A
B
overexposed correct exposure underexposed
In the following situations, would you expect the x-ray
film to be (A), overexposed, (B) correctly exposed or
(C) underexposed? (No change in patient size).

Filtration
Low-energy x-rays do not contribute to the formation
of an x-ray image; all they do is expose the body to
radiation. Therefore, we need to get rid of them. The
process of removing these low-energy x-rays from
the x-ray beam is known as filtration. Filtration
increases the average energy (quality) of the x-ray
beam.
There are two components to x-ray filtration. The first
of these, called inherent filtration, results from the
materials present in the x-ray machine that the x-rays
have to pass through. These include the beryllium
window of the x-ray tube, the oil in the tubehead and
the barrier material that keeps the oil from leaking
out of the tubehead. This removes very weak x-rays.

Filtration (continued)
The second component is the addition of aluminum
disks placed in the path of the x-ray beam (added
filtration). These disks remove the x-rays that had
enough energy to get through the inherent filtration but
are still not energetic enough to contribute to image
formation.
Disks of varying thicknesses, when combined with the
inherent filtration, produce the total filtration for the x-
ray machine. Federal regulations require that an x-ray
machine capable of operating at 70 kVp or higher must
have total filtration of 2.5 mm aluminum equivalent. (The
inherent filtration is “equivalent” to a certain thickness
of aluminum). X-ray machines operating below 70 kVp
need to have a total filtration of 1.5 mm aluminum
equivalent. www.indiandentalacademy.com

Filtration
Inherent
beryllium window
of x-ray tube
Added
Aluminum filter (s)
Total
Oil/Metal barrier
filter
PID
collimator
barrier
material
beryllium
window
oil

filter
PID
The filter is usually
located in the end of
the PID which attaches
to the tubehead.

primary x-ray
scattered x-ray
Collimation
Collimation is used to restrict the area of the head that the
x-rays will contact. We want to cover the entire film with
the x-ray beam, but don’t want to overexpose the patient.
Also, when x-rays from the tubehead interact with the
tissues of the face, scatter radiation is produced (see
below). This scatter radiation creates additional exposure
of the patient and also decreases the quality of the x-ray
image. (Scatter will be discussed in greater detail in the
section on biological effects of x-rays).

Collimation
The collimator, located in the end of the PID where
it attaches to the tubehead, is a lead disk with a
hole in the middle (basically a lead washer). The
size of the hole determines the ultimate size of the
x-ray beam. The shape of the hole will determine
the shape of the x-ray beam.
You are looking up through the
PID at the collimator (red arrows),
which is a circular lead washer
with a circular cutout in the
middle. This will produce a round
x-ray beam. The light gray area in
the center is the aluminum filter.

The shape of the opening
in the collimator
determines the shape of
the x-ray beam. The size
of the opening
determines the size of the
beam at the end of the
PID. PID’s come in
varying lengths; longer
PID’s have a smaller
opening in the collimator.
round
rectangular
Collimation

The x-ray beam continues
to spread out as you get
further from the x-ray
source (target). More
surface is exposed on the
exit side of the patient
than the entrance side. By
collimating the beam, less
overall surface is exposed
and as a result, less
scatter radiation is
produced. Both of these
things reduce patient
exposure. 2.75 inches (7
cm) is the maximum
diameter of a circular
beam or the maximum
length of the long side of a
rectangular beam at the
end of the PID.
collimated
beamcollimator
target
(x-ray source)
Collimation

If you switch from a 7 cm
round PID to a 6 cm
round PID, the patient
receives 25% less
radiation because the
area covered by the
beam is reduced by 25%.
Rectangular collimation
(dotted line at left)
results in the patient
receiving 55 % less
radiation when compared
to what they would
receive with a 7 cm
round PID.
6 cm round
film
(4.5 cm long)
entrance
entrance
exit
exit
6 cm
7 cm
area covered at skin surface (6 cm round PID)
area covered as beam exits (6 cm round PID)
area covered at skin surface (7 cm round PID)
area covered as beam exits (7 cm round PID)
Collimation

Quality Quantity
(primarily)kVp
mA
Time
Filtration
no change
no change
Collimation does not change the energy or number of
x-rays in the x-ray beam that reach the film; it just
limits the size and shape of the beam.
The quality, or average energy, of the x-ray beam is
increased with an increase in kVp or an increase in
filtration. The quantity, or number of x-rays, is
increased with an increase in kVp, mA setting and
kVp setting.

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