Chapters 7 and 8 discuss the interactions of x-rays with matter. Scatter radiation 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 of scatter radiation produced and exiting the patient: the volume of tissue irradiated and the kilovoltage peak (kVp) . The volume of tissue depends on the thickness of the part as well as the x-ray beam field size. Increasing the volume of

Make the Physics Connection 11-1

Chapter 7

Compton scattering is one of the most prevalent interactions between x-ray photons and the human body in general diagnostic imaging and is responsible for most of the scatter that fogs the image. The probability of Compton scattering does not depend on the atomic number of atoms involved. Compton scattering may occur in both soft tissue and bone. The probability of Compton scattering is related to the energy of the photon. As x-ray photon energy increases, the probability of that photon penetrating a given tissue without interaction increases. However, with this increase in photon energy, the likelihood of Compton interactions relative to photoelectric interactions also increases.

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, a higher kVp increases the energy of scatter radiation exiting the patient.

Chapter 9 introduces the characteristics of a quality image and explains that scatter radiation provides no useful information about the anatomic area of interest. Controlling the amount of scatter radiation produced in the patient and ultimately reaching the image receptor (IR)

Critical Concept 11-1

Factors Affecting the Amount of Scatter Radiation

The greater the volume of tissue irradiated because of part thickness or x-ray beam field size, the greater the amount of scatter radiation produced. The higher the kVp used, the greater the energy of scattered x-rays exiting the patient.

is essential in creating a diagnostic-quality image. Scatter radiation is detrimental to radiographic quality because it adds unwanted exposure (fog) to the image without adding any patient information. Digital IRs are more sensitive to lower energy levels of radiation such as scatter, which results in increased fog in the image. Additionally, scatter radiation decreases radiographic contrast for both film-screen and digital images. Increased scatter radiation either produced within the patient or higher-energy scatter exiting the patient affects the exposure to the patient and anyone within close proximity. Therefore the radiographer must act to minimize the amount of scatter radiation produced and reaching the IR.

Beam-restricting devices and radiographic grids are tools the radiographer can use to limit the amount of scatter radiation that affects the radiographic image and exposure to the patient or personnel. Beam-restricting devices decrease the x-ray beam field size and the amount of tissue irradiated, thereby reducing the amount of scatter radiation produced. Radiographic grids are used to improve radiographic image quality by absorbing scatter radiation that exits the patient, reducing the amount of scatter reaching the IR. It should be noted that grids do nothing to prevent scatter production; they merely reduce the amount of scatter reaching the IR.

Beam Restriction

It is up to each 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 the patient and IR (Figure 11-1). If not restricted in some way, the primary beam goes beyond the boundaries of the anatomic area of interest and IR size, resulting in unnecessary patient exposure. Any time the x-ray field extends beyond the anatomic area of interest, the patient receives 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. 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 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.

FIGURE 11-1 **The Unrestricted Primary Beam.**
The unrestricted primary beam is cone shaped, projecting a circular field. A, Side view. B, View from above.

Critical Concept 11-2

Beam Restriction and Patient Dose

Collimating to the appropriate field size is a basic method for protecting the patient from unnecessary exposure. 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.

Beam Restriction and Scatter Radiation

In addition to decreasing patient dose, beam-restricting devices also reduce the amount of scatter radiation 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 11-2. 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 11-2 **X-ray Field Size and Scatter Radiation.**
As the field size increases, the relative quantity of scatter radiation increases.

Critical Concept 11-3

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. Therefore less scatter radiation reaches the IR. As described in Chapter 9, this affects the radiographic contrast.

Critical Concept 11-4

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

An increase in collimation also affects the number of x-ray photons reaching the IR to produce the latent image. Increasing collimation decreases the number of photons that strike the patient and decreases the amount of scatter radiation produced. Therefore exposure factors should be increased when increasing collimation. For example, when collimating significantly (changing from an 11 × 14-inch field size to a small, 4-inch-diameter cone), the radiographer must increase exposure to compensate for the decrease in the number of x-ray photons that otherwise occurs. The kVp is typically not increased because it increases the proportion of scatter produced in the patient and results in decreased image contrast. To maintain exposure to the IR, the mAs (milliamperes/second) should be altered.

Important relationships regarding the restriction of the primary beam are summarized in Table 11-1.

TABLE 11-1

Restricting the Primary Beam

Increased Factor	Result
Collimation	Patient dose decreases. Scatter radiation decreases. Radiographic contrast increases. Film-screen: Radiographic density decreases. Digital: Quantum noise increases.
Field Size	Patient dose increases. Scatter radiation increases. Radiographic contrast decreases. Film-screen: Radiographic density increases. Digital: Quantum noise decreases.

Types of Beam-Restricting Devices

Several types of beam-restricting devices are available; they differ in sophistication and utility. 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 11-3), as are those that are “homemade” (hospital-made) 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 to the size needed to create the diaphragm and cutting the center to create the shape and size of the aperture.

FIGURE 11-3 **Aperture Diaphragm.**
A commercially made aperture diaphragm.

Although the aperture’s size and shape can be changed, the aperture cannot be adjusted from the designed size. 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 11-4). Although aperture diaphragms are still used in some applications, their use is not as widespread as that of other types of beam-restricting devices.

FIGURE 11-4 **Image Unsharpness and Aperture Diaphragm.**
Radiographic image unsharpness using an aperture diaphragm.

Cones and Cylinders

Cones and cylinders are shaped differently (Figure 11-5), 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, thereby increasing its total length (Figure 11-6). Like 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 11-7). However, they are limited in terms of the sizes that are available, and they are not necessarily 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 11-8).

FIGURE 11-5 **Cylinders and Cones.**
A, A cylinder. B, A cone.

Collimators

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

A collimator has two or three sets of lead shutters (Figure 11-9). Located immediately below the tube window, the entrance shutters limit the x-ray beam much as the aperture diaphragm does. One or more sets of adjustable lead shutters are located 3 to 7 inches (8 – 18 cm) below the tube. These shutters consist of longitudinal and lateral leaves or blades, each with its own control. This makes the collimator adjustable in that it can 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 placed 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 accurately indicate 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 11-10) is present on the front of the collimator. It indicates the projected field size based on the adjusted size of the collimator opening at particular source-to-image receptor distances (SIDs). This 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 11-1).

BOX 11-1 Quality Control Check: Collimator and Beam Alignment