Lecture 4 Beam restricting Devices, their
Use and Maintenance
• Topic 1 Importance of beam restricting
devices
• Topic 2 Types of beam restricting
devices
• Topic 3 Quality assurance tests of beam
restricting devices
• Topic 4 Compression of area of interest
Topic 1 Importance of beam- restricting
devices
Topic 1 Importance of beam restricting
devices
• These devices restrict the primary x-ray beam to the area
of interest.
• The larger the area covered by primary x-ray beam, the
greater the scattered radiation produced.
• Scattered radiation must be minimized. They increase
radiation dose and add to film darkening thus reducing
visualization of details.
Topic 1 Importance of beam- restricting
devices
Beam was not
restricted to the
chest – the area of
interest. Entire
abdomen and part
of face of baby
was exposed
unnecessarily.
Topic 1 Importance of beam- restricting
devices
Unrestricted beam-considerable scatter
Restricted beam – less scattered radiation
Topic 2 Types of beam-restricting
devices
a. Attached to x-ray tube housing
 Aperture diaphragms - lead sheets with
circular, square, or rectangular openings
 Cones – detachable metal tubes of different
lengths, shapes, and sizes of opening
 Variable aperture collimators – adjustable
lead plates or shutters , contain "cross hairs",
a light source, and a mirror to project the
light
Beam- restricting devices
Removable
metal cone
Variable aperture collimator
Variable aperture, multiple shutter
collimator
Diaphragm closer to window reduces off-focus
x rays.
Multiple
shutters
Smaller sized field reduces size of
primary beam.
Multiple
shutters
Topic 2 Types of beam - restricting
devices
b. Not attached to x ray tube housing
Lead blockers – used to divide a
cassette when more than one
exposure is made on a single film;
reduces unnecessary exposure of
film thus improving image quality
Use of lead blockers
• Place blocker on couch for lateral projections of thoracic,
lumbar, sacrum or
coccyx spine.
• Position blocker next to patient's back to absorb radiation
that will not pass through spine.
• Defective lead rubber aprons could be cut
into a range of sizes to serve as blockers.
Topic 3 Quality assurance tests of beam
limiting devices
• Collimation or limiting the x-ray beam to the area of
interest requires use of beam restricting devices. It is
good practice to collimate such that the resulting image
has collimated edges on all four sides of film
Topic 3 Quality assurance tests of beamrestricting devices
• Bring tube to rest on table top to check
that all four edges of square light beam
diaphragm touch the table top when
using vertical beam. Do several times
during the day.
• Do the same test with cone inserted in
front of the x ray tube window.
Caution: Cones sometimes become bent when
dropped or bumped.
Topic 3 Quality Assurance tests of beam
restricting devices
• Do collimator-beam alignment check at least every six
months.
• Do check of alignment of centre of x ray
beam at least every six months.
Collimator-beam alignment test
• Using 100 cm focus-film distance (FFD), set
collimator at 20 cm x 20 cm field size on surface
of loaded cassette.
• Place metal coins or paper clips on top of
cassette as shown in next slide.
• Expose the film at 60 kVp and 4 – 8 mAs.
• Process the film.
• Check whether the distance between outer edges
of image and of paper clips/coins is
within +/- 2% of FFD.
Line diagram showing metal coins and
field size
Coin to identify right
upper corner.
Edge of light.
Cassette.
Collimator-beam alignment test result
Check of alignment of centre of
x-ray beam
• Place unexposed loaded cassette in centre of bucky tray
and centre tube to cassette.
• Move tube to 100 cm FFD to bucky tray. Reduce
longitudinal collimators to a thin slit (e.g. 0.5 cm). Close
lateral collimators. Expose using 60 kVp and 4 – 8 mAs.
• Do not remove cassette.
Check of alignment of centre of x ray
beam (cont)
• Close longitudinal collimators and open lateral
collimators to a thin slit. Expose film again.
Process film.
• Bend film in half and check that exposed "cross"
is in film centre.
• Acceptable deviation: 1 cm on either side of
centre.
• Can also be done for non-bucky radiography.
Just move tube so 100 cm FFD is to cassette
top.
Centre of beam alignment test result
Topic 3 Summary
Beam restricting devices
• may be attached or not attached to tube housing
• require periodic checks
• are necessary for radiation protection and contribute to
good image quality
• minimize primary radiation
• minimize scatter radiation
Topic 4 Compression of area of interest
• Reducing patient thickness by compression minimizes
production of scattered radiation, thus reducing radiation
dose.
• Device used : compression band.
Topic 4 Compression of area of interest
Compression band
over abdomen
reduces its volume.
This reduces
scatter radiation.
Lecture 4 Summary
• Beam-restricting devices are important in
radiation protection.
• These devices should be checked regularly.
• Regular performance of quality assurance tests is in
accordance with ALARA.