basic description of the topic

1. BY:- NITISH RAJPUT
BSc. RADIODIAGNOSIS 2ND YEAR
BATCH-2k16

2. 1- SCATTERED RADIATION
2- BEAM LIMITING DEVICES
3- BEAM CENTERING DEVICES
4- RADIATION GRIDS
5- ASSESMENT OF GRID FUNCTION

3.  Xray’s belong to a group of radiation known as
the ELECTROMAGNETIC RADIATIONS.
 All the electromagnetic radiations have dual
characteristics.
 WAVE NATURE
 PARTICLE NATURE
 Wave nature can be understood by :- c=
 Particle nature can be understood by :- E=hv

6.  Scattering is basically of five types:-
 COHERENT
 COMPTON
 PHOTOELECTRIC
 PHOTODISINTEGRATION
 PAIR PRODUCTION
 (of these compton scattering occurs mostly in
diagnostic radiology)

7.  Basically, when x-rays are absorbed then they
cease to exist but when they are scattered then
they are deflected from there original path and
hence carry no useful information...
 At the end they only add up NOISE (unwanted
densities) to the film.
 The end result is that the radiographic image is
of a poor quality which makes it difficult to
diagnose it. Contrast of the film is also
degraded.

9. VARIOUS DEVICES TO REDUCE SCATTER:-
 Filters
 Aperture diaphragms
 Cones and cylinders
 Collimators
 Beam centering devices
 Radiographic Grids

10.  More scatter is created in bone than in soft
tissue.
 When the beam is restricted then less scatter
occurs and hence less of it reaches the film.
 Technical factors may have to be manipulated
to reduce the amount of scatter.

11.  Filtration is basically a process of shaping the
x-ray beam to increase the amount of useful
photons and decrease the low energy photons
so that we can get an image with better
contrast.
 This also decreases the patient radiation dose
and occupational hazards.

12.  When exposure is done then both high as well
as low energy x-ray photons are produced.
 When these x-ray photons interact with the
human body, only the high energy photons
penetrate while the low energy photons get
absorbed in the body, hence increasing the
patient radiation dose.
 Filtration basically absorbs the low energy
photons from the beam and hence increases the
image contrast.

14.  INHERENT FILTRATION(x-ray
tube and its housing).
 ADDED FILTRATION (sheets of
metal placed in the path of beam).
 THE PATIENT.

15.  The absorption of the low energy x-ray photons by
the x-ray tube components itself is known as the
inherent filtration.
 Glass housing, metal enclosure and the assembly
oil is responsible for inherent filtration.
 Inherent filtration of a tube is measured in
ALUMINIUM EQUIVALENT.
 It is basically the amount of aluminium required to
absorb the same radiation which the tube material
was absorbing.
 Inherent filtration is generally between 0.5-1.00mm
of aluminium equivalent.

16.  But disadvantage of inherent filtration is that it
causes a significant reduction in the image
contrast.
 At some places where unfiltered x-ray beam is
required, there BERYLLIUM filters are mostly
used.

18.  Added filtration is a result of any beam
absorber which is placed in the path of the xray
beam, this absorber absorbs the low or high
energy photons.
 Ideally an added filter should absorb all the
low energy photons and let the high energy
photons pass through it.
 But no such material exists.
 SOLUTION- we always use added filters
mostly in a group of Aluminium(13) +
Copper(29).

19.  They are arranged as the high at. No.(COPPER)
element faces the x-ray tube while the
other(ALUMINIUM) one faces the patient.
 Most of the filtration is done by copper.
 They can't be used separately.
 Patient radiation dose as well as the image
contrast is reduced.
 This combination of two layers of filters is also
known as COMPOUND FILTERS.
 It also increases the tube loading.

21.  Wedge filters are mostly used at places where the
body part to be radiographed varies greatly in
densities.(thick from one side and thin from
another side)
 Wedge filters are like the shape of a wedge , the
thin part is placed under thick body part while
thick part is placed under thin body part.
 Result is that ,beam attenuation by thick part is
more hence less radiation reaches the part and
beam attenuation by thin part is less hence more
radiation reaches thick part.
 Therefore a radiograph of uniform density is got.

23.  More will be the thickness of filters ,more will
be the attenuation of x-rays by it.
 In other words:-
 ATTENUATION 0<(directly proportional)
FILTER THICKNESS

24.  Filters with metals of high atomic number.
 Usually used with x-ray tubes of high capacity
and high speed intensifying screen.
 These filters basically use the K-EDGE
ABSORPTION of the element.
 K-EDGE is basically the

25.  Beam restricting devices include:-
 APERTURE DIAPHRAGMS.
 CONES AND CYLINDERS.
 COLLIMATORS.

26.  These are basically the simplest xray beam
restrictors.
 It is made of a sheet of lead which is having a hole
at its centre.
 Its advantage is its simplicity, it can be altered to
any shape or size because lead it is soft.
 Its disadvantage is that it produces a fairly large
PENUMBRA(unexposed area) shadow.
 Penumbra can be reduced by placing the
diaphragm far away from the x-ray tube.
 The centre of the xray field is exposed by the
circular area.

28.  These restrictors are the shape of cones and
cylinders.
 They reduce the UNEXPOSED AREA at edges
by reducing PENUMBRA.
 The flare of the cone is similar to the geometric
shape of the xray beam, but it is greater than
the flare of the xray beam.
 They are attached to the front of the x-ray tube
and may also be equipped with extensions to
give even better restriction.

29.  The major disadvantage of these is thtat a
limited shapes and sizes are available even a
large assortment cannot fulfill the demands
and changing them is very inconvenient.

31.  Collimators are considered the best beam
restrictor devices.
 It is having two advantages:-
 It provides an infinite variety of rectangular
xray fields.
 It also shows the centre point of the xray beam.
 It consists of two sets of SHUTTERS (S1 & S2)
which move in opposite directions to remove
the PENUMBRA produced by each other.

32.  One set of shutters can be moved without moving
the other shutters which provides a no. of
rectangular shapes.
 A plus sign mounted on the front of the collimator
(made of PLEXIGLASS) helps to identify the centre
of the xray beam.
 Collimators can also measure the film-source
distance with the help of mirrors placed at
45degree inside it.
 Nowadays ( in DR) ,automatic collimators are used
which are known as POSITIVE BEAM LIMITING
DEVICES.

34.  Alignment of beams have to be checked
periodically
 MATERIAL REQUIRED:-
 Four L shaped wires.
 14*17 inches film.
 Lead foil of R shape.

35.  Load the film in cassette and place it over the
xray table.
 Now open the collimator shutter to a
convenient size (10*10in.).
 Carefully place four L-shaped wires at the
corners of the light beam.
 Place the lead letter R at the bottom right
corner of the collimated area.
 NOW EXPOSE THE FILM.

37.  Arrows indicate the area of the LIGHT BEAM
while exposed area indicates the area of XRAY
BEAM.

38.  Grids were invented by DR. GUSTAVE
BUCKY in 1913.
 Grids are used to decrease the patient radiation
dose and the scattered radiation.
 Grids basically consists of LEAD
INTERSPACING (interspace is made up of
ORGANIC MATERIAL OR ALUMINIUM)
which is aligned with the geometry of xray
beam of a particular tube.

40.  Grid ratio is defined as the ratio of the height of
the lead strips to the distance between the lead
strips.
 Grid ratio is always expressed as e.g. 8:1(where
8 is the actual ratio and the second no. is
always one).
 In radiology ,grids of ratio between 4:1 to 16:1
are generally used.
 Usually the thickness of lead strips is about
o.o5mm.

41.  Basically there are two types of grid pattern :-
 1-LINEAR GRID
 2-CROSSED GRID
 LINEAR GRID is a grid in which the lead strips
are placed parallel to each other.
 Its advantage is that when using linear grid ,then
we can angle the tube also.
 CROSSED GRID is formed when two linear grids
are overlapped with each other. It cannot be used
with oblique techniques.

43.  A focused grid is a grid which is made up of
lead strips which are angled slightly so that
they focus in space.
 Distance b/w the FOCUS and the lead strips is
known as the FOCAL LENGTH of the grid.
 There focus is also of two types.:-
 1-convergent line focus.
 2-convergent point focus.
 Focal ranges of the grid are indicated on the
top of the grids y manufacturers.

45.  It is basically the no. of lead strips present per
inch of the grid.
 Its formula is :- 25.4
D+d
Where:- D=thickness of interspaces
d=thickness of lead strips
and 25.4 is the no. of mm in 1 inch

46.  There are three tests which are used evaluate
the grid performance.
 CONTRAST IMPROVEMENT FACTOR.
 PRIMARY TRANSMISSION.
 BUCKY FACTOR.

47.  Contrast improvement factor is the ratio of the
contrast with grid to without the grid (K).
 Its formula:- contrast with GRID
 contrast without GRID
 This is the ultimate test for grids performance
because it measures the ability of the grid to
improve its contrast.

48.  It is the percentage of the primary radiation
transmitted through the grid (ideally it should
be 100%).
 It requires a special equipment to measure the
primary transmission of the grid.

49.  Typical value of pt. is b/w 55% to 75%.
 Theoretical calculation > actual calculation.
 PRIMARY TRANSMISSION =intensitywitgrid*
intensity without grid

51.  It is the ratio of the incident radiation falling on
the grid to the transmitted radiation passing
through the grid.
 It is a practical determination because it
indicates how much we should increase or
decrease the factors when doing xrays with or
without grid.
 BUCKY FACTOR(B)=incident radiation
 transmitted radiation