Scatter radiation, as described in Chapter 3, is primarily the result of the Compton interaction, in which the incoming x-ray photon loses energy and changes direction. Two major factors affect the amount and energy of scatter radiation exiting the patient: kilovoltage peak (kVp) and the volume of tissue irradiated. The volume of tissue depends on the thickness of the part and the x-ray beam field size. Increasing the volume of tissue irradiated results in increased scatter production. In addition, using a higher kVp increases x-ray transmission and reduces its overall absorption (photoelectric interactions); however, higher kVp increases the percentage of Compton interactions and the energy of scatter radiation exiting the patient. Using higher kVp or increasing the volume of tissue irradiated results in increased scatter radiation reaching the IR.

Important Relationship

kVp and Scatter

The amount and energy of scatter radiation exiting the patient depends, in part, on the kVp selected. Examinations using higher kVp produce a greater proportion of higher-energy scattered x-rays compared with examinations using low kVp.

Important Relationship

X-ray Beam Field Size, Thickness of the Part, and Scatter

The larger the x-ray beam field size, the greater the amount of scatter radiation produced. The thicker the part being imaged, the greater the amount of scatter radiation produced.

Important Relationship

Volume of Tissue Irradiated and Scatter

The volume of tissue irradiated is affected by both the part thickness and the x-ray beam field size. Therefore, the greater the volume of tissue irradiated, because of either or both factors, the greater the amount of scatter radiation produced.

Beam Restriction

It is the responsibility of the radiographer to limit the x-ray beam field size to the anatomic area of interest. Beam restriction serves two purposes: limiting patient exposure and reducing the amount of scatter radiation produced within the patient.

The unrestricted primary beam is cone- shaped and projects a round field on to the patient and IR (Figure 5-1). If not restricted in some way, the primary beam goes beyond the boundaries of the IR, resulting in unnecessary patient exposure. Any time the x-ray field extends beyond the anatomic area of interest, the patient is receiving unnecessary exposure. Limiting the x-ray beam field size is accomplished with a beam-restricting device. Located just below the x-ray tube housing, the beam-restricting device changes the shape and size of the primary beam.

FIGURE 5-1 The unrestricted primary beam is cone- shaped, projecting a circular field. A, Side view. B, View from above.

The terms beam restriction and collimation are used interchangeably; they refer to a decrease in the size of the projected radiation field. The term collimation is used more often than the term beam restriction because collimators are the most popular type of beam-restricting device. Increasing collimation means decreasing field size, and decreasing collimation means increasing field size.

Important Relationship

Beam Restriction and Patient Dose

As beam restriction or collimation increases, field size decreases, and patient dose decreases. As beam restriction or collimation decreases, field size increases, and patient dose increases.

Patient Protection Alert

Appropriate Beam Restriction

In performing a radiographic examination, the radiographer should be aware of the anatomic area of interest and limit the x-ray field size to just beyond this area. Collimating to the appropriate field size is a basic method for protecting the patient from unnecessary exposure.

Beam Restriction and Scatter Radiation

In addition to decreasing patient dose, beam-restricting devices reduce the amount of scatter radiation that is produced within the patient, reducing the amount of scatter the IR is exposed to, and thereby increasing the radiographic contrast. The relationship between collimation (field size) and quantity of scatter radiation is illustrated in Figure 5-2. As stated previously, collimation means decreasing the size of the projected field, so increasing collimation means decreasing field size, and decreasing collimation means increasing field size.

FIGURE 5-2 As field size increases, the relative quantity of scatter radiation increases.

Important Relationship

Collimation and Scatter Radiation

As collimation increases, the field size decreases, and the quantity of scatter radiation decreases; as collimation decreases, the field size increases, and the quantity of scatter radiation increases.

Collimation and Contrast

Because collimation decreases the x-ray beam field size, less scatter radiation is produced within the patient, and less scatter radiation reaches the IR. As described in Chapter 3, this affects the radiographic contrast.

Important Relationship

Collimation and Radiographic Contrast

As collimation increases, the quantity of scatter radiation decreases, and radiographic contrast increases; as collimation decreases, the quantity of scatter radiation increases, and radiographic contrast decreases.

Compensating for Collimation

Increasing the collimation decreases the volume of tissue irradiated, the amount of scatter radiation produced, the number of photons that strike the patient, and the number of x-ray photons reaching the IR to produce the latent image. As a result, the exposure technique factors may need to be increased when increasing collimation to maintain exposure to the IR.

Important Relationship

Collimation and Exposure to the Image Receptor

As collimation increases, exposure to the IR decreases; as collimation decreases, exposure to the IR increases.

It has been recommended that significant collimation requires an increase in 30% to 50% of the milliamperage/second (mAs) to compensate for the decrease in IR exposure.

Important relationships regarding the restriction of the primary beam are summarized in Table 5-1.

TABLE 5-1

Restricting the Primary Beam

Increased Factor	Result
Collimation	Patient dose decreases
	Scatter radiation decreases
	Radiographic contrast increases
	Exposure to image receptor decreases
Field size	Patient dose increases
	Scatter radiation increases
	Radiographic contrast decreases
	Exposure to image receptor increases

Types of Beam-Restricting Devices

Several types of beam-restricting devices, which differ in sophistication and utility, are available. All beam-restricting devices are made of metal or a combination of metals that readily absorb x-rays.

Aperture Diaphragms

The simplest type of beam-restricting device is the aperture diaphragm. An aperture diaphragm is a flat piece of lead (diaphragm) that has a hole (aperture) in it. Commercially made aperture diaphragms are available (Figure 5-3), or hospitals make their own for purposes specific to a radiographic unit. Aperture diaphragms are easy to use. They are placed directly below the x-ray tube window. An aperture diaphragm can be made by cutting rubberized lead into the size needed to create the diaphragm and cutting the center to create the shape and size of the aperture.

FIGURE 5-3 Commercially made aperture diaphragm.

Although the size and shape of the aperture can be changed, the aperture cannot be adjusted from the designed size, and therefore the projected field size is not adjustable. In addition, because of the aperture’s proximity to the radiation source (focal spot), a large area of unsharpness surrounds the radiographic image (Figure 5-4). Although aperture diaphragms are still used in some applications, their use is not as widespread as other types of beam-restricting devices.

FIGURE 5-4 Radiographic image unsharpness using an aperture diaphragm.

Cones and Cylinders

Cones and cylinders are shaped differently (Figure 5-5), but they have many of the same attributes. A cone or cylinder is essentially an aperture diaphragm that has an extended flange attached to it. The flange can vary in length and can be shaped as either a cone or a cylinder. The flange can also be made to telescope, increasing its total length (Figure 5-6). Similar to aperture diaphragms, cones and cylinders are easy to use. They slide onto the tube, directly below the window. Cones and cylinders limit unsharpness surrounding the radiographic image more than aperture diaphragms do, with cylinders accomplishing this task slightly better than cones (Figure 5-7). However, they are limited in terms of available sizes, and they are not interchangeable among tube housings. Cones have a particular disadvantage compared with cylinders. If the angle of the flange of the cone is greater than the angle of divergence of the primary beam, the base plate or aperture diaphragm of the cone is the only metal actually restricting the primary beam. Therefore, cylinders generally are more useful than cones. Cones and cylinders are almost always made to produce a circular projected field, and they can be used to advantage for particular radiographic procedures (Figure 5-8).

Collimators

The most sophisticated, useful, and accepted type of beam-restricting device is the collimator. Collimators are considered the best type of beam-restricting device available for radiography. Beam restriction accomplished with the use of a collimator is referred to as collimation. The terms collimation and beam restriction are used interchangeably.

A collimator has two or three sets of lead shutters (Figure 5-9). Located immediately below the tube window, the entrance shutters limit the x-ray beam much as the aperture diaphragm would. One or more sets of adjustable lead shutters are located 3 to 7 inches (8 to 18 cm) below the tube. These shutters consist of longitudinal and lateral leaves or blades, each with its own control. This design makes the collimator adjustable in terms of its ability to produce projected fields of varying sizes. The field shape produced by a collimator is always rectangular or square, unless an aperture diaphragm, cone, or cylinder is slid in below the collimator. Collimators are equipped with a white light source and a mirror to project a light field onto the patient. This light is intended to indicate accurately where the primary x-ray beam will be projected during exposure. In case of failure of this light, an x-ray field measurement guide (Figure 5-10) is present on the front of the collimator. This guide indicates the projected field size based on the adjusted size of the collimator opening at particular source-to-image receptor distances (SIDs). This guide helps ensure that the radiographer does not open the collimator to produce a field that is larger than the IR. Another problem that may occur is the lack of accuracy of the light field. The mirror that reflects the light down toward the patient or the light bulb itself could be slightly out of position, projecting a light field that inaccurately indicates where the primary beam will be projected. There is a means of testing the accuracy of this light field and the location of the center of projected beam (Box 5-1).

BOX 5-1 Quality Control Check: Collimator and Beam Alignment