Radiology

1. XRAY BEAM RESTRICTION
Islah Raoof
Dnb resident

2. ‣ These are devices to regulate the size
and shape of X-ray beam.
‣ Beam restricting devices include:
‣ APERTURE DIAPHRAGMS.
‣ CONES AND CYLINDERS.
‣ COLLIMATORS.
Xray beam restricting
devices

3. ‣Simplest xray beam restrictor.
‣ It is made of a thin sheet of lead with a
hole at its centre. Size and shape of xray
beam is determined by the size and
shape of the hole.
Aperture diaphragms

4. ADVANTAGE
▸Simple
▸Pb is soft so aperture
could be easily
altered into different
shape and size.
DISADVANTAGE
▸Produces a fairly
large penumbra
(unexposed area) at
the periphery of Xray
field
‣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.

5. Diagram of aperture
diaphragm

6. ‣ 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.
Cones and cylinders

7. ‣ The major disadvantage of these is that
a limited shapes and sizes are available
even a large assortment cannot fulfill
the demands and changing them is very
inconvenient.
Cones and cylinders

8. Diagram of cone and cylindrical
restrictors

9. ‣ Collimators are considered the best beam restrictor devices.
‣ It is having two advantages:-
1. It provides an infinite variety of rectangular
xray fields.
2. Provision for the light beam that shows the centre and
exact configuration of the xray field.
‣ It consists of two sets of SHUTTERS (S1 & S2) controlled
independently which move in opposite directions to remove
the PENUMBRA produced by each other.
‣ Each shutter has >= 4 lead plates.
Collimators

10. ‣ These lead plates move in independent pairs
which provides a no. of rectangularand square
fields.
‣ The second set of shutter totally cuts off the
penumbra of first shutter.
‣ 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 45
degree inside it.
Collimators

11. ‣ Xray field is illuminated by a light beam from a light
bulb in the collimator.
‣ The light beam is deflected by a mirror placed in the
path of Xray beam at an angle of 45 degree.
‣ Nowadays ( in DR) ,automatic collimators are used
which are known as POSITIVE BEAM LIMITING
DEVICES.
Collimators

12. Diagram of collimators

13. ‣ Distance of target of xraytube from cetre of
mirror = distance of the light bulb from
centre of mirror.
Collimators

14. ‣ Alignment of beams have to be checked
periodically
‣ MATERIAL REQUIRED:-
‣ Four L shaped wires.
‣ 14*17 inches film.
‣ Lead foil of R shape.
Testing xray beam and light
beam alignment

15. ‣ 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.
Procedure of test

16. Any problem in alignment will
appear as follows

17. ‣ Arrows indicate the area of the LIGHT BEAM
while exposed area indicates the area of
XRAY BEAM.
Procedure of test

18. Thank you