Beam limitation and filtration pt 2 ( this is the limitation portion)

Understand with this image the first shutter is the green area and the blue area is the second shutter.

Off focus radiation  or stem radiation

x-rays emitted from parts of the tube other
than the focal spot and at various angles from the x-ray tube
window.

A lead-lined metal diagnostic-type protective tube housing protects patients and imaging
personnel from off-focus, or leakage, radiation by restricting x-ray emission to the area of the primary
(useful) beam

This is everything together

Is this image good collimation or bad

GOOD

Is this image good collimation or bad

BAD

X-Ray Beam Limitation Devices

The primary x-ray beam must be collimated so that it is no larger than the size of the image receptor (IR) being used for the examination.

Light-localizing variable-aperture rectangular collimator

Used to manually or automatically adjust the size and shape of the x-ray beam; currently the predominant beam-limiting device.

Collimator assembly

The device used in modern radiology to change and customize the x-ray field size to fit the anatomy being imaged.

Importance of collimation

Reduces patient dose by limiting tissue exposure and decreases scatter radiation, improving image quality.

Collimation effects

Less scatter means lower dose to both the patient and technologist, especially important in thick anatomy like the lateral spine.

Benefits of collimation

Reduces patient dose, reduces scatter radiation, improves image contrast and overall image quality.

Collimator construction

Box-shaped device containing a series of adjustable components that control field size and beam shape.

Collimator components

Includes two sets of shutters (upper and lower), a light source to illuminate the field, and a mirror to reflect the light toward the patient.

First set of shutters

Located near the tube window; eliminates off-focus radiation that exits the tube at an angle.

Second set of shutters

Located below the light and mirror; reduces patient skin exposure by absorbing stray electrons produced by photon interactions.

Skin sparing

Minimizes skin exposure to electrons produced when photons interact with the lower collimator shutters.

Collimator safety standard

There must be at least 15 cm between the patient’s skin and the lowest set of collimator shutters to prevent electron exposure.

Field size

The x-ray field should never be larger than the image receptor, as that would unnecessarily expose more tissue.

Collimator light

The light must be bright enough to clearly outline the exposure field for accurate alignment.

Illuminated field standard

The illuminated light field must be within 2% of the actual x-ray field at the indicated SID.

Positive Beam Limitation (PBL)

Automatic collimation that restricts the beam to the size of the image receptor; no longer required since 1994 but still used in some systems.

PBL accuracy standard

PBL must be within 2% of the actual exposed field (2% of SID).

Cones

A fixed circular beam-limiting device that provides one field size; slides over the x-ray tube.

Use of cones

Commonly used for facial bones and sinus imaging.

Disadvantages of cones

Bulky and limited in flexibility; largely replaced by variable collimators.

Beam alignment

The radiographer must ensure the beam is properly aligned and collimated so it does not exceed the size of the IR.