Exposure Technique Factors

Chapter 4

Exposure Technique Factors


After completing this chapter, the reader will be able to perform the following:

1. Define all the key terms in this chapter.

2. State all the important relationships in this chapter.

3. Explain the relationship between milliamperage and exposure time with radiation production and image receptor (IR) exposure.

4. Calculate changes in milliamperage and exposure time to change or maintain exposure to the IR.

5. Compare the effect of changes in milliamperage (mA) and exposure time on digital and film-screen images.

6. Recognize how to correct exposure factors for a density error.

7. Explain how kilovoltage peak (kVp) affects radiation production and IR exposure.

8. Calculate changes in kVp to change or maintain exposure to the IR.

9. Compare the effect of changes in kVp on digital and film-screen images.

10. Recognize the factors that affect recorded detail and distortion.

11. Calculate changes in mAs for changes in source-to-image receptor distance.

12. Calculate the magnification factor, and determine image and object size.

13. Describe the use of grids and beam restriction and their effect on IR exposure and image quality.

14. Calculate changes in mAs when adding or removing a grid.

15. Recognize patient factors that may affect IR exposure.

16. Identify the exposure factors that can affect patient radiation exposure.

17. State exposure technique modifications for the following considerations: body habitus, pediatric patients, projections and positions, soft tissue, casts and splints, and pathologic conditions.

In Chapter 2, variables that affect both the quantity and the quality of the x-ray beam were presented. Milliamperage and time affect the quantity of radiation produced, and kilovoltage affects both the quantity and the quality. Chapter 3 emphasized that a good-quality radiographic image accurately represents the anatomic area of interest. The characteristics evaluated for image quality are density or brightness, contrast, recorded detail or spatial resolution, distortion, and noise. This chapter focuses on exposure techniques and the use of accessory devices and their effect on the radiation reaching the image receptor (IR) and the image produced. Radiographers have the responsibility of selecting the combination of exposure factors to produce a good-quality image. Knowledge of how these factors affect the image individually and in combination assists the radiographer to produce a radiographic image with the amount of information desired for a diagnosis.

Because various types of IRs respond differently to the radiation exiting the patient, these differences are noted throughout this chapter. Digital IRs separate acquisition from processing and image display; their response to changes in radiation exposure does not affect the amount of brightness displayed on the image. The level of brightness and contrast can be altered during computer processing and image display. However, the amount of exposure to the digital IR needs to be carefully selected, as with film-screen IRs, to produce a quality image with the least amount of exposure to the patient. Radiographic film acquires the latent image and needs to be chemically processed before the image can be displayed. Changes in the quantity and the quality of radiation exposure to a film-screen IR affect the amount of density and contrast visible on the processed radiograph. This chapter discusses all the primary and secondary factors and their effects on the radiation reaching the IR.

Primary Factors

The primary exposure technique factors the radiographer selects on the control panel are milliamperage (mA), time of exposure, and kilovoltage peak (kVp). Depending on the type of control panel, milliamperage and exposure time may be selected separately or combined as one factor, milliamperage/second (mAs). Regardless, it is important to understand how changing each separately or in combination affects the radiation reaching the IR and the radiographic image.

Milliamperage and Exposure Time

The quantity of radiation reaching the patient affects the amount of remnant radiation reaching the IR. The product of milliamperage and exposure time has a direct proportional relationship with the quantity of x-rays produced. Once the anatomic part is adequately penetrated, as the quantity of x-rays is increased, the exposure to the IR proportionally increases (Figure 4-1). Conversely, when the quantity of x-rays is decreased, the exposure to the IR decreases. Therefore, exposure to the IR can be increased or decreased by adjusting the amount of radiation (mAs).

Because the mAs is the product of milliamperage and exposure time, increasing milliamperage or time has the same effect on the radiation exposure.

As demonstrated in the Mathematical Application, mAs can be doubled by doubling the milliamperage or doubling the exposure time. A change in either milliamperage or exposure time proportionally changes the mAs. To maintain the same mAs, the radiographer must increase the milliamperage and proportionally decrease the exposure time.

It is important for the radiographer to determine the amount of mAs needed to produce a diagnostic image. This is not an easy task because there are so many variables that can affect the amount of mAs required. For example, single-phase generators produce less radiation for the same mAs compared with a high-frequency generator. A patient’s age, the general condition of the patient, and the presence of a pathologic condition also affect the amount of mAs required for the procedure. In addition, IRs respond differently for a given mAs. Digital IRs can detect a wide range of radiation intensities (wide dynamic range) exiting the patient and are not as dependent on the mAs as film-screen IRs. However, exposure errors can adversely affect the quality of the digital image. If the mAs is too low (low exposure to the digital IR), image brightness is adjusted during computer processing to achieve the desired level. Although the level of brightness has been adjusted, there may be increased quantum noise visible within the image (Figure 4-2). If the mAs selected is too high (high exposure to the digital IR), the brightness can also adjusted, but the patient has received more radiation than necessary.

Because the brightness of a digital image can be altered during image processing, information about the exposure to the IR is important. Manufacturers of each type of digital system specify the expected range of x-ray exposure sufficient to produce a quality image. A numeric value (exposure indicator) is displayed on the processed image to indicate the level of x-ray exposure received (incident exposure) to the digital IR. It is important for the radiographer to consider the indicated value because exposure errors, as stated previously, affect the quality of the digital image and the radiation dose to the patient. Exposure errors are not obvious by simply looking at the digital image because the digital data are normalized to provide images with diagnostic density or brightness. Most manufacturers of digital IRs suggest a range for the exposure indicator based on the radiographic procedure. If the exposure indicator value falls outside of this range, image quality or patient exposure or both could be compromised.

For film-screen IRs, the mAs controls the density produced in the image. There is a direct relationship between the amount of mAs and the amount of density produced when using film-screen IRs. When the mAs is increased, density is increased; when the mAs is decreased, density is decreased (Figure 4-3).

When a film image is too light (insufficient density), a greater increase in mAs may be needed to correct the density, or the mAs may need to be decreased to correct a film image that has excessive density. When using a film-screen IR, radiographers need to assess the level of density produced on the processed image and determine whether the density is sufficient to visualize the anatomic area of interest. The radiographer must decide how much of a change in mAs is needed to correct for the density error.

Generally, for repeat radiographs necessitated by density errors, the mAs is adjusted by a factor of 2; therefore, a minimum change involves doubling or halving the mAs. This change typically brings the film densities back to visualize the anatomic area of interest best. Radiographs that have sufficient but not optimal density usually are not repeated. If a radiograph must be repeated because of another error, such as positioning, the radiographer may also use the opportunity to make an adjustment in density to produce a radiograph of optimal quality. Making a visible change in radiographic density requires that the minimum amount of change in mAs be approximately 30% (depending on equipment, this may vary between 25% and 35%). Radiographic images generally are not repeated to make only a slight visible change. A radiographic image repeated because of insufficient or excessive density requires a change in mAs by a factor of at least 2.

To visualize the anatomic area of interest best, the mAs selected must produce a sufficient amount of radiation reaching the IR, regardless of type. An excessive or insufficient amount of mAs adversely affects image quality and patient radiation exposure.

Kilovoltage Peak

The kVp affects the exposure to the IR because it alters the amount and penetrating ability of the x-ray beam. The area of interest must be adequately penetrated before the mAs can be adjusted to produce a quality radiographic image. When adequate penetration is achieved, increasing the kVp further results in more radiation reaching the IR. In addition to affecting the amount of radiation exposure to the IR, the kVp also affects image contrast.

Kilovoltage Peak and Exposure to the Image Receptor

Because kVp affects the amount of radiation reaching the IR, its effect on the digital image is similar to the effect of mAs. Assuming that the anatomic part is adequately penetrated, too much radiation reaching the IR (within reason) produces a digital image with the appropriate level of brightness as a result of computer adjustment during image processing; however, the patient has been overexposed. Similarly, too little radiation reaching the IR (within reason) produces a digital image with the appropriate level of brightness, but the increased quantum noise decreases image quality. Excessive or insufficient radiation exposure to the digital IR, as a result of the mAs or kVp, should be reflected in the exposure indicator value.

The kVp has a greater effect on the image when using film-screen IRs. Increasing the kVp increases IR exposure and the density produced on a film image, and decreasing the kVp decreases IR exposure and the density produced on a film image (Figure 4-4).

For film-screen IRs, kVp has a direct relationship with density; however, the effect of the kVp on density is not equal throughout the range of kVp (low, middle, and high). A greater change in the kVp is needed when operating at a high kVp (>90) compared with operating at a low kVp (<70) (Figure 4-5).

Kilovoltage is not a factor typically manipulated to vary the amount of IR exposure because the kVp also affects contrast. However, it is sometimes necessary to manipulate the kVp to maintain the required exposure to the IR. For example, using portable or mobile x-ray equipment may limit choices of mAs settings, and the radiographer must adjust the kVp to maintain sufficient exposure to the IR.

Maintaining or adjusting exposure to the IR can be accomplished with kVp by using the 15% rule. The 15% rule states that changing the kVp by 15% has the same effect as doubling the mAs or reducing the mAs by 50%; for example, increasing the kVp from 82 to 94 (15%) produces the same exposure to the IR as increasing the mAs from 10 to 20.

Increasing the kVp by 15% increases the exposure to the IR, unless the mAs is decreased. Also, decreasing the kVp by 15% decreases the exposure to the IR, unless the mAs is increased. As mentioned earlier, the effects of changes in the kVp are not uniform throughout the range of kVp. When a low or high kVp is used, the amount of change in the kVp required to maintain the exposure to the IR may be greater than or less than 15%.

Kilovoltage Peak and Radiographic Contrast

Altering the penetrating power of the x-ray beam affects its absorption and transmission through the anatomic tissue being radiographed. Higher kVp increases the penetrating power of the x-ray beam and results in less absorption and more transmission in the anatomic tissues, which results in less variation in the x-ray intensities exiting the patient (lower subject contrast). As a result, images with lower contrast (more shades of gray) are produced (Figure 4-6). When a low kVp is used, the x-ray beam penetration is decreased, resulting in more absorption and less transmission, which results in greater variation in the x-ray intensities exiting the patient (higher subject contrast). A high-contrast (fewer shades of gray) radiographic image is produced (Figure 4-7).

In digital imaging, the kVp affects the variation in radiation intensities exiting the patient and image contrast; however, image brightness and contrast are primarily controlled during computer processing. When a kVp that is too low is selected, the brightness and contrast are adjusted, but quantum noise may be visible. Additionally, when a kVp that is too high is selected, the image brightness and contrast are adjusted, but patient exposure may be increased. Although image contrast can be adjusted when using a kVp that is too high, increased scatter radiation reaches the IR and may adversely affect image quality.

Changing the kVp affects its absorption and transmission as it interacts with anatomic tissue; however, using a higher kVp reduces the total number of interactions and increases the amount of x-rays transmitted. In these interactions, more Compton scattering than x-ray absorption occurs (photoelectric effect), and more scatter exits the patient. It is important to understand that in addition to kVp changing the subject contrast, it also affects the amount of scatter reaching the IR and therefore radiographic contrast.

The level of radiographic contrast desired—and therefore the kVp selected—depends on the type and composition of the anatomic tissue, the structures that must be visualized, and to some extent the type of IR. These factors make achieving a desired level of radiographic contrast more complex than achieving a desired level of exposure to the IR, especially for film-screen imaging. Radiographic film can be manufactured to display different levels of contrast. In addition to the type of film used, the kVp selected controls the level of contrast produced in the image.

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Mar 6, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Exposure Technique Factors

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