Lecture 4 Beam restricting Devices, their

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15 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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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.

Smaller sized field reduces size of
primary beam.

Multiple
shutters

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 beam
-

restricting 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.