Transcript Physics 6 - NYCC SP-01

Chapter 6
Control of Secondary
and
Scattered Radiation
CONTROL OF SECONDARY
AND SCATTERED RADIATION
 There
are various types of
auxiliary radiographic
equipment that can be used to
control the secondary and
scattered radiation.
 These include filters,
diaphragms, cones, and grids.
FILTERS
 are
thin sheets of material (copper,
aluminum) placed between the
tube and the patient, through which
the x-ray must pass before they
reach the film. They absorb some
of the soft/weak, undesirable
radiation, depending upon the
material they are made of.
FILTERS
 All
filters absorb radiation of all
wavelengths. They absorb, however,
relatively more of the soft rays (longer
wavelengths). The filter therefore
removes the rays that can cause skin
reactions or damage and which are
subject to greater scattering because of
their limited energies.
FILTERS

Part of the filter is permanently built into
the tube head - the inherent filter. this
includes the glass wall of the tube, the oil
surrounding the tube and the wall of the
tube head. These materials are thinned over
the area penetrated by the x-ray beam (the
x-ray window or port). The inherent
filtration amounts to approximately the
equivalent of 0.5 mm. of aluminum.
FILTERS

Directly in front of the x-ray window is a
channel for adding other filters. These are
made of aluminum for radiography (and of
aluminum, copper, tin, and even lead for
radiotherapy). For patient safety, you should
always maintain an additional 2.0 mm.
aluminum filter which, together with the
inherent filtration, gives a total filtration of
2.5 mm. of aluminum.
FILTERS

The introduction of 1 mm of
aluminum filtration reduces the
dosage to the patient by 60%, and
2mm. of aluminum reduces the
dosage to the skin by 80% at 50Kv
and 70% at 100 kV.
DIAPHRAGMS

These are no longer in use today.
Diaphragms are flat sheets of lead with
various size and shape apertures in their
centers. The size of the aperture used will
be determined by the size of the area being
examined and the film-anode distance. The
diaphragm is placed in a slot between the
patient and the tube and used as a mask,
very much like a custom made collimator.
CONES

Cones may be used in place of the diaphragm
and are placed in the same slot that the
diaphragm is used in. Cones are metal tubes
and there are three basic types. A flare cone is
one in which the aperture in the end facing the
patient is larger than the end attached to the
tube head. It allows for a larger field than the
cylinder cone. Remember that the beam exiting
through a cylinder cone may bounce off the
cylinder cone and thereby give weaker rays to
the patient
CONES

A cylindrical cone (or cylinder) is one in
which the aperture is the same throughout
its length. A cylindrical extension cone is
actually one cylinder within another slightly
larger cylinder. The inner cylinder is
attached to the tube head and the outer
cylinder can be extended to contact the skin
of the patient. This is used in spot-film work
exclusively.
CONES

Diaphragms and cones serve to limit the xray field to the part which is being
radiographed by cutting down the field size.
They also cut down on the quantity of soft
tissue exposed and thus reduce the amount
of scattered radiation and reducing film fog.
Since scattered radiation accounts in part
for the radiographic effect produced,
primary exposure must be increased (20%
in a 3 inch field).
GRIDS

are devices that are placed behind the
patient and in front of the film to minimize
the amount of secondary radiation from the
soft tissues reaching the film. The grid
appears to be a solid piece of metal but is
actually composed of lead strips alternating
with a radiolucent material (wood or
bakelite).
GRIDS
There
are two basic types
of grids:
Focused
Unfocused
GRIDS
A grid is unfocused when all the lead strips
are parallel to each other and perpendicular
to the grid surface.
 A grid is focused when all the lead strips
except those in the center are inclined at an
angle towards the center (they are actually
on radius of a circle whose center is the
focal spot of the tube when the correct
distance is used).

GRIDS
 Using
a parallel or unfocused
grids gives us the ability to
change distances. This may not
be as good a remover of scatter
as the focused, but it gives us
more freedom.
GRIDS

The focused grid can be used in only one
way - the side on which the lead strips are
closer must face the tube and the proper
distance for the grid should be used. If the
focused grid is used backwards or at an
improper distance, the radiation at the edges
of the film will be absorbed and the film
will be blank at these places.
Grid Radius
 is
the distance from the center of the grid
to a point where the projected planes of
the focused strips of the grid would meet.
 Grid radii of 30, 36, 40 and 48 inches are
in use. Although a 40 inch grid radii is
the most common used today, most
people don’t use focused grids at all.
Grid Radius

The focal spot of the tube should be at the
focus of the grid; then the divergent beam of
the primary rays is parallel to the lead strips
and a minimum of them will be absorbed by
the strips. In an unfocused grid the radius
would be infinity. The focused grid will be
marked “tube side” on the side that must
face the tube.
Grid Ratio
 is
the ratio of the height of the
lead strips to the width of the
space between them. Various
ratios include, but are not
limited to, 8:1, 10:1, 12:1,
16:1. The most common grid
ratios are 8:1 and 10:1.
Grid Efficiency

refers to the amount of maximum
absorption of secondary radiation as
compared with minimal absorption of
primary radiation. When the secondary
radiation is materially reduced without any
appreciable absorption of primary radiation,
the efficiency is high. Classically, the higher
the grid ratio, the more efficient the grid as
it decreases the amount of obliquely
scattered rays by four times.
Stationary Grids
 AKA “Wafer”
grids or Leisholm grids
 produce x-rays that have very fine white
lines on them. These are the area of the
film that are blocked from receiving any
x-ray by the lead strips.
 To eliminate these “grid lines” the moving
grid or Potter-Bucky diaphragm was
developed by Potter in 1913.
Potter-Bucky

The Potter-Bucky has all the characteristics
of the stationary grid with the addition of a
system to move the grid across the film
when the exposure is being made. When
this happens no part of the film is entirely
deprived of radiation, and there are no grid
lines of the finished film. The Bucky is
often connected into the timing circuit of
the x-ray machine, but it is a mechanical
device, depending upon the electricity only
for its release.
Potter-Bucky

It should be remembered when calculating
an exposure that the M.A.S. required when
using a Bucky or grid will be four times the
amount required without it, but the increase
in contrast and detail will make it worth
using. Never x-ray a part thicker than 10 or
11 centimeters without using a bucky or
grid due to the large amount of scattered
radiation that needs to be eliminated.
Grids
 Today,
stationary grids are being
used as the lead lines are made so
thin that they are invisible to the
naked eye. Stationary grids are
being produced at up to 140 lines
per inch and do an excellent job in
absorbing scatter.
THE INVERSE SQUARE LAW
 The
Inverse Square Law states
that the intensity of radiation
decreases in proportion to the
square of the distance.
 Remember that x-ray travels in
a straight line diverging from a
point.
THE INVERSE SQUARE LAW

When using 40” and 72” it is easy to see
that this is a basic doubling of the distance.
When shooting at 72” you are shooting an
area four times larger than that shot at 40”.
Therefore, you need to increase the MAS by
four times to compensate. When using four
times more MAS you should collimate
down four times more so that the patient
does not receive more radiation.