Beam Restriction

Scattered x-rays―caused by high kVp, field size, and patient or part thickness―create a density on the film that is not related to patient anatomy.  Beam limiting devices control the production of scatter by limiting the size of the x-ray beam and thus reducing the amount of tissue irradiated.  The disadvantage is that it increases patient dose because we are decreasing the amount of scatter produced that would contribute to the density of the image.  Beam limiting devices, such as aperture diaphragms, cones, and collimators, are attached directly to the tube housing.

Aperture Diaphragms

Aperture diaphragms are made from a flat sheet of metal―lead or lead-lined―with a hole cut in the center and attached to the x-ray tube port.  The opening can be any shape, although they are usually round or rectangle.  Aperture diaphragms are used with dedicated units (chest and head) where the SID does not change and the field size is fixed.  A machine that uses aperture diaphragms most likely does not have a light source.  The disadvantages of aperture diaphragms are an increase in focal spot blur and off-focus radiation (compared to collimators and cylinders) because the diaphragm is so close to the tube port.

Cones

There are two basic types of cones, cylinder type and flare type.  A cone is an extended metal structure that restricts the useful beam to the required size; some of them can telescope out.  A cylinder type cone extends out straight and a flare type cone matches the divergence of the beam.  Cones are used mostly in dentistry.  Common uses in radiography include sinus exams and L5-S1 spot projections.  Cones are heavy and can change the angle of the tube, causing a misalignment with the image receptor.  When using a cone, collimate if you can, for better image quality. 

Collimators

Collimators are the most common and most efficient type of beam limiting device.  Collimators are a variable instead of a fixed aperture because of its adjustable lead shutters.  It is the radiographer's responsibility to make sure that the exposure field does not exceed the size of the image receptor.  X-ray equipment with manual collimation has a scale―on the collimator housing―that indicates field size is used as a back up when the collimator light fails.  Most x-ray equipment today has automatic collimation or positive beam limitation (PBL).  When an image receptor is placed in the bucky, the collimators automatically adjust the field size according to the size of the image receptor.  PBL provides 4-sided collimation just smaller than the image receptor. 

The light field is reflected by a mirror and it corresponds to the size of the beam field.  The light field and beam field must be accurate within 2% (tested annually by physicist).  A double set of adjustable shutters working in opposite pairs that are about 3mm of lead thickness and positioned at 90° angles to each other.  The near (1st stage) shutters located just below glass window are used to control off-focus radiation and they are not adjustable by the technologist.  The far (2nd stage) shutters is what the technologist manipulates to control the field size and thus reducing focal spot blur.

The tube housing has positioning lines that cross each other to show the center of the beam.  Some tube housings have the locations of the photo sensors using when phototiming (AEC) an exposure.  The collimator tube housing has an equivalent of about 1mm of Al filtration due to the beam passing through the mirror.  The minimum collimator to skin distance is 15 cm. 

Contrast

Because of the use of beam restricting devices, image contrast is increased.  This occurs as a result of a reduction in scattered radiation. 

Density

A reduction in  scattered radiation, as a result of beam restricting devices, causes a decrease in density.  This occurs because the scatter radiation that can be used to create a density on the radiograph is removed from the primary beam.

Detail

Beam restricting devices increase the visibility of detail around the edges of the image as a result of a decrease in focal spot blur and off-focus radiation. 

Patient Dose

Initially, beam restricting devices decrease patient dose; however due to the fact that mAs needs to be increased to maintain density, patient dose is increased.

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