<aside>
An X-ray collimator is a beam-limiting device attached to the tube housing that defines the size and shape of the X-ray field to correspond to the area of interest on the detector.
</aside>
The x-ray tube collimator assembly is attached to the housing at the tube port, typically on a collar that allows it to be rotated. A light source, positioned at a virtual focal spot location, illuminates the field area from an angled mirror positioned in the x-ray beam path to indicate the x-ray field for patient positioning. The lead collimator blades define the light field and the x-ray field, and annual quality control tests verify their congruence within 2% of the source to image distance on any collimated edge.
Bushberg, J. T., Seibert, J. A. (2022). The Essential Physics of Medical Imaging Study Guide. United States: Wolters Kluwer Health.
Collimator Assembly – usually a light-beam diaphragm (LBD) consisting of:
| Entrance shutters (fixed) | Absorb off-focus radiation close to the focal spot. |
|---|---|
| Adjustable lead shutters | |
| (two pairs at right angles) | Define rectangular/square fields. |
| Light source & mirror system | Projects a visible light field on the patient, coinciding with the X-ray field. |
| Scale/indicators | Provide field size readouts. |
| Added filtration slot | Often incorporated for aluminum or copper filters. |
| Fixed aperture diaphragm | Simple metal plate with a hole; used in early systems, now obsolete. |
|---|---|
| Cones & Cylinders | Fixed beam limitation for specific studies (e.g., dental, skull). |
| Variable-aperture collimators (LBD) | Modern standard; motorized versions in fluoroscopy and CT. |
| Reduces patient dose | By irradiating only the area of interest, minimizing exposure to adjacent tissues. |
|---|---|
| Improves image quality | Less scatter radiation reaching the detector → increased contrast. |
| Radiation protection | Complies with ALARA (As Low As Reasonably Achievable) principle. |
| Positioning aid | Light field ensures accurate centering. |