Lesson 1, Topic 1
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Primary X-Ray Beam and Scatter Radiation

April 11, 2024

Primary X-Ray Beam and Scatter Radiation

Learning Objective: Differentiate between the primary x-ray beam and scatter radiation.

Before we discuss the radiographic equipment, the following sections will discuss the primary x-ray beam and scatter radiation .

FIGURE 37.6  X-ray tube inside protective housing. From Johnston J, Fauber TL: Essentials of radiographic physics and imaging, ed 3, St. Louis, 2020, Mosby.

Primary X-ray Beam

Learning Objective: Define primary x-ray beam.

The source of x-rays is the x-ray tube. X-rays are formed within a very small area inside the x-ray tube. Once the x-rays are created, they are directed to exit the tube and diverge as they travel toward the patient. A lead-lined tube housing surrounds the x-ray tube. X-rays that are created exit the housing through an opening called the tube port or window (FIGURE 37.6). The radiation that leaves the tube is known as primary radiation . The squared area of the x-ray beam that strikes the patient and x-ray table is the radiation field . The center of the x-ray beam and perpendicular to the long axis of the x-ray tube is the central ray   (CR) . The CR is vital in positioning the patient because this point is used to align the x-ray tube to the body part to be imaged. Because the CR is essential, the limited operator knows exactly where to place the beam so that a light shines on the patient’s body (FIGURE 37.7). The light field has crosshairs to identify the exact center of the x-ray beam, which is the central ray.
During a radiographic exposure, x-rays from the tube are directed through the patient to the IR (FIGURE 37.8). As x-rays pass through the patient, some of them are absorbed by the patient, and others are not. Some anatomy, such as bone, will absorb more radiation than other anatomy, such as muscle. Absorption is based on the density (mass) of the structure. This results in varying intensity in the x-ray beam as it exits the opposite side of the patient. The radiation, known as remnant radiation, then passes through to the IR. The IR now contains the latent image, which is the image until it is processed. The image remains stored in the IR until it is processed. Processing will convert the latent image into a visible image.

Tissue Densities on X-Rays

• Bones: Are dense and thus absorb more x-rays. Bones appear white on the image.
• Fluid and soft tissue: Blood and muscles absorb fewer x-rays than bones and appear darker on the image than bones.
• Fat: Appears darker than fluid and soft tissues.
• Air: Absorbs fewer x-rays and thus appears the darkest on images. Air is most visible in the lungs on chest x-rays.

Scatter Radiation

Learning Objective: Define scatter radiation.

When the x-ray beam strikes the patient or the IR, part of its energy is absorbed in the patient or IR. Attenuation is the absorption of the x-ray beam. Attenuated x-rays can be absorbed within the body or scattered outside the body. The patient is the primary source of scatter radiation. Scatter radiation is radiation from the primary beam that is randomly scattered within or outside of the body. Scatter radiation has less energy than primary radiation, but it is not easily controlled. It travels from the absorbing matter in all directions, causing unwanted exposure to the IR or anyone in the room. This is why radiation safety is essential. Radiation safety will be discussed briefly later in this chapter. The unwanted image exposure caused by scatter radiation is called fog . See TABLE 37.2 for a summary of primary, remnant, and scatter radiation.