Scatter Radiation and Its Control



Scatter Radiation and Its Control




Scatter radiation, introduced in Chapter 2, is produced as a result of the attenuation of the x-ray beam by matter. This chapter explores the production of scatter radiation and the factors that influence its formation. In addition, this chapter covers methods used to minimize the fog that this radiation causes on radiographs.



Radiation Interactions with Matter


When x-rays completely penetrate the body, there is no interaction with matter, and no scatter or scattered radiation is formed as a result. When x-rays are absorbed in the body, however, their energy is “scattered,” or converted into new scatter x-rays. Three types of interactions occur when radiation is absorbed by matter: coherent scattering, Compton effect, and photoelectric effect.


The result of either coherent scattering or the Compton effect is termed scatter radiation or simply scatter. Radiation produced by the photoelectric effect is correctly referred to as secondary radiation. Since more than one type of interaction takes place during radiography and the resulting radiation is so similar, the terms are often used interchangeably. When referring to both scatter and secondary radiation, this text uses the term scatter radiation.


The interactions that produce scatter radiation in radiography occur primarily within the patient. Some scattering also occurs as a result of interactions between the x-ray beam and the tabletop and image receptor (IR), and any other matter that happens to be within the radiation field.



Coherent Scattering


Coherent scattering is also known as Thompson scatter. This type of interaction takes place at relatively low energy levels (below 10 keV). Fig. 9-1 shows the path of the x-ray photon during this interaction. Because coherent scattering occurs in the very low energy ranges, and outside the usual range for diagnostic imaging, this interaction has no significance to our daily work. It is mentioned here only to demonstrate that at very low kilovolts peak (kVp) levels there is an interaction in the body.




Compton Effect


The Compton effect occurs at energy levels throughout the diagnostic x-ray range of 40 to 125 kVp. The incoming x-ray photon interacts with an outer orbital electron of an atom, removing it from the atom (ionization), and then proceeds in a different direction. The majority of the photon’s energy is converted into a new photon of scatter radiation (Fig. 9-2). This new photon has less energy than the incoming primary beam photon and therefore a longer wavelength. It also travels in a new direction. Compton scatter travels in all directions. If it is directed back toward the x-ray tube, it is termed backscatter. Most of the photons that are scattered will scatter in a more forward direction. As the kVp is increased, Compton interactions are increased.




Photoelectric Effect


The photoelectric effect is similar to that which forms characteristic radiation in the x-ray tube (see Chapter 5). In this case, however, the incoming energy is an x-ray photon interacting with an atom in the body rather than an electron interacting with the tungsten anode.


In a photoelectric interaction, the incoming photon from the primary beam collides with an inner orbital electron of an atom. The photon is totally absorbed in the process and creates an absorbed dose in the patient. The electron’s departure leaves a “hole” in the orbit, which is filled by an electron from an outer shell. The difference in binding energy between the two shells is emitted as a new x-ray photon (Fig. 9-3). This photon is referred to as a characteristic photon and is considered secondary radiation because it is radiation actually produced in the body. The photon will have a new direction. Its energy will be less than that of the primary photon. Photoelectric interactions are less prevalent in the diagnostic energy range than Compton interactions. The likelihood of a photoelectric interaction is determined by both the kVp level and the electron-binding energy of the atom in which the interaction occurs.



Because no part of the energy of the incoming photon exits the atom, photoelectric interactions are sometimes referred to as true absorption. In this text, references to scatter also apply to secondary radiation formed by the photoelectric effect. As kVp is increased, photoelectric effect is decreased. Note, this is the opposite of the Compton effect. In the diagnostic range of kVp used (50 to 100 kVp) the majority of radiation interactions with the body are Compton interactions.



Radiographic Effect of Scatter Radiation


The production of scatter radiation during an exposure results in fog on the radiograph. Fog is unwanted exposure to the image. It does not strike the IR in a pattern that represents the subject, and it contributes nothing of value to the image. This fog produces an overall increase in radiographic density. The result is also a reduction in radiographic contrast, as stated in Chapter 7. Although increased density in the darker areas of the image is scarcely noticeable, areas that would otherwise be bright or white will instead be gray because of the fog. The intermediate gray tones will appear more similar to each other, which makes it difficult to distinguish recorded detail within those portions of the image that have similar tissue densities. In other words, scatter radiation creates fog that reduces both contrast and the visibility of detail (Fig. 9-4).




Factors Affecting Quantity of Scatter Radiation Fog


Four primary factors directly affect the quantity of scatter radiation fog on the radiograph (Box 9-1): volume of tissue, kVp, density of the matter, and field size.




Volume of Tissue


The primary scatter consideration is the volume of tissue irradiated. The thicker or larger the body part is, the greater are the scatter and the fog. When there is a greater quantity of tissue in the path of the x-ray beam, there will be greater absorption of the x-ray beam and more interactions that produce scatter radiation. The volume of tissue irradiated is determined by the thickness of the subject and the size of the radiation field. When the subject is more than 10 to 12 cm in thickness, the amount of fog becomes objectionable unless the field size is very small.


Because a thicker subject requires a greater quantity of exposure, there will be more primary x-ray photons and more interactions. This is another reason why there will be more scatter radiation when the thickness of the subject is increased.


Mar 7, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Scatter Radiation and Its Control
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