In medical imaging, controlled use of radiation is paramount to obtaining necessary diagnostic information while ensuring safety. X-ray beams, which are generated from a focal point, naturally spread out or diverge as they travel toward the patient. To precisely manage this energy, a fundamental process called collimation is employed in every radiographic procedure.
Defining Beam Restriction
Collimation is the process of physically limiting the size and shape of the X-ray beam to include only the specific anatomical region of interest. The goal is to match the radiation field as closely as possible to the size of the image receptor or the exact area being examined. By restricting the beam, medical professionals ensure that X-ray energy is directed solely where it is needed for the image. Limiting the field size prevents unnecessary exposure to healthy tissues and structures surrounding the target area. The restriction is achieved by using specialized components, typically made of lead, which absorb the X-rays that would otherwise travel outside the designated path.
Equipment Used to Control the Beam
A variety of devices, known as beam-restricting devices, are used to achieve accurate collimation. The simplest form is the aperture diaphragm, which is essentially a fixed lead plate with a central opening that is mounted near the X-ray tube. While simple and inexpensive, these fixed-size devices must be manually exchanged if a different field size is required. Cones and cylinders are modifications of the aperture diaphragm, often used in dental radiography, that further restrict the beam to a circular or rectangular shape.
The most common and sophisticated device in general radiography is the variable-aperture collimator, housed in a box-like structure beneath the X-ray tube. This device uses two sets of adjustable lead shutters or blades that can be independently moved to create a rectangular field of various sizes. These movable shutters allow the technologist to precisely adjust the X-ray field without needing to change any physical components. Many modern collimators include a light field system, which uses a small bulb and mirror to project a visible light outline onto the patient’s skin, showing the exact area the X-ray beam will cover.
Dual Purpose: Safety and Clarity
The practice of collimation serves two distinct, but equally significant, purposes in medical imaging: enhancing patient safety and improving diagnostic image clarity. By restricting the size of the X-ray beam, a smaller volume of tissue is exposed to radiation, which directly reduces the total absorbed dose for the patient.
The second major benefit of collimation is the reduction of scatter radiation, which is X-ray energy that has interacted with the patient’s body and changed direction. When a larger area is irradiated, more tissue is available to produce this secondary radiation. Scatter radiation travels toward the image receptor, creating a generalized fogging effect that degrades the image, much like light leaks onto a photograph. Reducing the field size limits the production of this scatter, which in turn increases the image contrast and detail.