Beam Restriction

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Basic types of beam restrictors

Aperture diaphragms, cones, collimators

Describe aperture diaphragm?

Flat sheet of metal, usually lead, with a hole cut in center and attached to xray tube port

Simplest of all beam restriction devices

Aperture diaphragm

The size of the opening for an Aperture diaphragm depends on?

Desired field size, SID, distance from the aperture diaphragm to the focal spot

Advantages of aperture diaphragm

Simple design, low cost, ease of use

Disadvantage of aperture diaphragm?

Increase in penumbra- geometric unsharpness around the periphery of the image
- due to diaphragms close proximity to the tube port

Penumbra is reduced when

Beam restrictor is at an increased distance from the port

Reducing penumbra will improve?

the sharpness of the recoded image edges

Another problem with the aperture diaphragms is the increase in?

Off-focus (stem) radiation

Off-focus radiation originates within?

The xray tube

Cones and cylinders are essentially

Circular aperture diaphragms with metal extensions.

Describe a cone?

Has an extension that flares or diverges, with upper diameter smaller then the bottom flared end

Describe cyclinder?

Does not flare, has same diameter at the bottom of the extension as it has on the top. Possible extension sleeve which can expand or collapse to vary the degree of beam restriction

Advantages of cones and cylinder?

Inexpensive and simple
-Cylinder reduces penumbra and off-focus radiation

Cones & cylinders used most often in radiography of?

Skull, spine, gallbladder, & breast

Disadvantages of cones

Heavy, weight may cause tube to slightly angle, fixed field size

Most commonly used beam restrictor?

Collimator

Advantages of Collimator?

Provides a light source to outline exposure field and provides crosshair to identify the center of the beam

Collimator consist of?

Lead shutters at right angles to one another, which adjust to infinite number, attached to tube head

The purpose of a collimators shutters?

Regulate field size.
-Bottom shutter reduce penumbra along periphery, has 2 pairs of shutters for field length & width
-Upper shutters (closes to tube) help reduce amount of off-focus (stem) radiation reaching IR by absorbing radiation

Design of collimator (light field)?

Light field provided by mounting mirror in path of xray beam at angle of 45 degrees. Light source and xray source must be equidistant from each other

Name for automatic collimators?

Positive beam limitation (PBL) devices

What do PBL devices do?

When cassette is loaded, sensing device collimates lead shutter down to size of loaded cassette

Use of collimator results in some xray beam filtration due to?

Primary beam passing through the mirror in the collimator.
added filtration= 1 mm Al

Proper collimation is the responsibility of the?

Radiographer

Aperture Diaphragm Primarily used for?

Head & chest units

Disadvantages of cylinder?

Used only for small field sizes, fixed filled size

For the light field in the xray field to correspond accurately

Xray tube focal spot and light bulb must be exactly at the same distance from the center of the mirror

National Council on Radiation Protection (NRCP) guidelines state the collimators must be accurate within?

2% fo the SID

For PBL, the NCRP requires that the guidelines be accurate to within

3% of the SID for a singe side, and within 4% of the SID for all total sides

The NRCP guideline for manual collimation is to be within?

2% of the SID

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