X-ray Beam Composition and Collimation

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X-ray Beam Composition and Collimation

• By Professor Stelmark

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Leakage radiation is radiation generated in the
x-ray tube that does not exit from the collimator
opening but rather penetrates through the
protective tube housing and to some degree
through the sides of the collimator
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Protective housing covers the x-ray tube and
provides the following three functions It
reduces leakage radiation to less than 100 mR/hr
at 1 m.
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X-ray tubes are designed so that projectile
electrons from the cathode interact with the
target only at the focal spot. However, some of
the electrons bounce off the focal spot and then
land on other areas of the target, causing x-rays
to be produced from outside of the focal
spot. These x-rays are called off-focus
radiation. This is not unlike squirting a water
pistol at a concrete pavement Some of the water
splashes off the pavement and lands in a larger
area.
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Extreme effect of off-focus radiation. You can
see the image of the nose from the off-focus
radiation
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Off focus radiation is undesirable because it
extends the size of the focal spot. The
additional x-ray beam area increases skin dose
modestly but unnecessarily. Off focus radiation
can significantly reduce image contrast. Finally,
off focus radiation can image patient tissue
that was intended to be excluded by the
variable-aperture collimators. Examples of such
undesirable images are the ears in a skull
examination, the soft tissue beyond the cervical
spine, and the lung beyond the borders of the
thoracic spine.
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X-rays are carriers of manmade, electromagnetic
energy. If x-rays enter a material such as human
tissue, they may interact with the atoms of the
biologic material in the patient or pass through
without interaction. If they interact,
electromagnetic energy is transferred from the
x-rays to the atoms of the patient's biologic
material. This transference of electromagnetic
energy to the atoms of the material is called
absorption
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Primary radiation emerges from the x-ray tube
target and consists of x-ray photons of various
energies. It is produced when the positively
charged target is bombarded with a stream of
high-speed electrons and these electrons interact
with the atoms of the target.
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Although all photons in a diagnostic x-ray beam
do not have the same energy, the most energetic
photons in the beam can have no more energy than
the electrons that bombard the target. The energy
of the electrons inside the x-ray tube is
expressed in terms of the electrical voltage
applied across the tube. In diagnostic radiology,
this is expressed in thousands of volts, or
kilovolts (kV). Because the voltage across the
tube fluctuates, it is usually expressed in
kilovolt peak (kVp)
The emerging x-ray photon beam is collectively
referred to as primary radiation (primary signal
DR)
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• When an x-ray beam passes through a patient, it
  goes through a process called attenuation.
  Attenuation is simply the reduction in the number
  of primary photons in the x-ray beam through
• Absorption (a total loss of radiation energy)
• Scatter (a change in direction of travel that may
  also involve a partial loss of radiation energy)
  as the beam passes through the patient in its
  path.

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Some primary x-ray photons will also traverse the
patient without interacting. This may be called
direct transmission. These non-interacting x-ray
photons reach the radiographic image receptor
(e.g., phosphor plate, digital radiography
receptor or radiographic film). Other primary
photons can undergo interactions and as a result
may be scattered or deflected. Such photons may
still traverse the patient and strike the image
receptor. This process is called indirect
transmission. The optimal x-ray image is formed
when only direct transmission x-ray photons reach
the image receptor. In clinical situations,
scattered photons do reach the image receptor and
degrade image quality.
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Before the four photons produced by the x-ray
source enter human tissue, they are referred to
as primary photons. Only two photons emerge from
the tissue and strike the radiographic image
receptor below it. They are referred to as exit
or image-formation photons. The two that do not
strike the image receptor are attenuated.
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Photon 4 seems to disappear. It has transferred
all its energy to the atoms of the patient and
has therefore been eliminated. Because a photon
has no mass, it ceases to exist when it gives up
its energy. Attenuation, then, refers to both
absorption and scatter processes that prevent
photons from reaching a predefined destination.
Figure shows that the path of photon 2 was bent,
but not so much that the photon missed its
target. Because photon 2 reaches the image
receptor, it is part of the exit, or
image-formation, radiation, but the bending of
its path represents what is called small-angle
scatter. Scattered photons in this category have
essentially the same energy as the incident
photons. Small-angle scatter degrades the
appearance of a completed radiographic image by
blurring the sharp outlines of dense structures
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Differential absorption is the difference between
the x-ray photons that are absorbed
photoelectrically versus those that penetrate the
body.
A radiographic image is created by passing an
x-ray beam through the patient and interacting
with an image receptor, such as a film-screen or
digital system. The variations in absorption and
transmission of the exiting x-ray beam
structurally represent the anatomic area of
interest.
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When the attenuated x-ray beam leaves the
patient, the remaining x-ray beam, referred to as
exit radiation or remnant radiation, is composed
of both transmitted and scattered radiation. The
varying amounts of transmitted and absorbed
radiation (differential absorption) create an
image that structurally represents the anatomic
area of interest. Scatter exit radiation that
reaches the image receptor does not provide any
diagnostic information about the anatomic area.
Scatter radiation creates unwanted exposure on
the image called fog.
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Components of Exit or Remnant Radiation

• Off focus radiation
• Scattered radiation
• Image forming radiation
• No-image forming radiation
• Directly transmitted radiation

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Secondary Radiation

• Attenuated radiation (scattered photons)

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Because many billions of such scatter events
occur, radiographic density of the image also
increases overall, interfering with the
radiologist's ability to distinguish different
structures in the image. This undesirable,
additional density is called radiographic fog.
Reducing the amount of tissue irradiated reduces
the amount of fog produced by small-angle
scatter. Therefore, adequately collimating the
x-ray beam is one way to reduce fog.
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Scatter Control
Factors Affecting the Amount of Scatter
Radiation The greater the volume of tissue
irradiated because of part thickness or x-ray
beam field size, the greater the amount of
scatter radiation produced. The higher the kVp
used, the greater the energy of scattered x-rays
exiting the patient.
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Controlling the amount of scatter radiation
produced in the patient and ultimately reaching
the image receptor (IR) is essential in creating
a diagnostic-quality image. Scatter radiation is
detrimental to radiographic quality because it
adds unwanted exposure (fog) to the image without
adding any patient information. Digital IRs are
more sensitive to lower energy levels of
radiation such as scatter, which results in
increased fog in the image. Additionally,
scatter radiation decreases radiographic contrast
for both film-screen and digital images.
Increased scatter radiation either produced
within the patient or higher-energy scatter
exiting the patient affects the exposure to the
patient and anyone within close proximity.
Therefore the radiographer must act to minimize
the amount of scatter radiation produced and
reaching the IR.
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Beam Restriction It is up to each radiographer
to limit the x-ray beam field size to the
anatomic area of interest. Beam restriction
serves two purposes limiting patient exposure
and reducing the amount of scatter radiation
produced within the patient
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Collimation and Scatter Radiation As collimation
increases, the field size decreases and the
quantity of scatter radiation decreases as
collimation decreases, the field size increases
and the quantity of scatter radiation increases.
Collimators The most sophisticated, useful, and
accepted beam-restricting device is the
collimator. Collimators are considered the best
beam-restricting device.
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A collimator has two or three sets of lead
shutters. Located immediately below the tube
window, the entrance shutters limit the x-ray
beam much as the aperture diaphragm does. One or
more sets of adjustable lead shutters are located
3 to 7 inches (8 - 18 cm) below the tube. These
shutters consist of longitudinal and lateral
leaves or blades, each with its own control. This
makes the collimator adjustable in that it can
produce projected fields of varying sizes. The
field shape produced by a collimator is always
rectangular or square.
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Automatic Collimators An automatic collimator,
also called a positive beam-limiting (PBL)
device, automatically limits the size and shape
of the primary beam to the size and shape of the
IR. For a number of years, automatic collimators
were required by U.S. federal law on all new
radiographic installations. This law has since
been rescinded, and automatic collimators are no
longer a requirement on any radiographic
equipment. However, they are still widely used.
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Automatic collimators mechanically adjust the
primary beam size and shape to that of the IR
when the IR is placed in the Bucky tray, just
below the tabletop. Automatic collimation makes
it difficult for the radiographer to increase the
size of the primary beam to a field larger than
the IR, which would result in increasing the
patient's radiation exposure. PBL devices were
seen as a way of protecting patients from
overexposure to radiation. However, it should be
noted that automatic collimators have an override
mechanism that allows the radiographer to
disengage this feature.
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 Restricting the Primary Beam
Increased Factor Result
Collimation Patient dose decreases. Scatter radiation decreases. Radiographic contrast increases. Film-screen Radiographic density decreases. Digital Quantum noise increases.
Field Size Patient dose increases. Scatter radiation increases. Radiographic contrast decreases. Film-screen Radiographic density increases. Digital Quantum noise decreases.
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Collimation and Intensity
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Collimation and Intensity
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Collimation and Intensity
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Collimation and Intensity
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