Introduction

In Chapter 6, variables that affect both the quantity and quality of the x-ray beam are presented. Milliamperage and time affect the quantity of radiation produced and kilovoltage affects both the quantity and quality. Chapter 9 emphasizes 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 radiographic 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 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 diagnostic interpretation.

Because various types of IRs respond differently to the radiation exiting the patient (remnant), these differences are noted throughout this chapter. However, regardless of the type of IR, the patient should be exposed to the least amount of radiation necessary to produce a diagnostic-quality radiograph. This chapter discusses all of 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, 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.

Make the Physics Connection 10-1

Chapter 6

Because the milliamperage/second (mAs) controls the number of electrons boiled off of the filament and available to produce x-rays, it is considered the primary factor controlling quantity. All other factors remaining constant, an increase in milliamperage increases the amplitude of both the continuous and discrete portions of the spectrum.

Once the anatomic part is adequately penetrated, as the quantity of x-rays is increased, the exposure to the IR proportionally increases (Figure 10-1). 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).

Critical Concept 10-1

mAs and Quantity of Radiation

As the mAs is increased, the quantity of radiation reaching the IR is increased. As the mAs is decreased, the amount of radiation reaching the IR is decreased.

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

Math Application 10-1

Adjusting Milliamperage or Exposure Time

100mA×0.1s=10mAs

To increase the mAs to 20, you could use:

100mA×0.2s=20mAs

200mA×0.1s=20mAs

As demonstrated in Math Application 10-1, 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.

Critical Concept 10-2

Milliamperage and Exposure Time

Milliamperage and exposure time have an inverse relationship when maintaining the same mAs.

Math Application 10-2

Adjusting Milliamperage and Exposure Time to Maintain mAs

100mA×100ms(0.1s)=10mAs

To maintain the mAs, use:

50mA×200ms(0.2s)=10mAs

200mA×50ms(0.05s)=10mAs

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 when compared with a high-frequency generator.

Make the Physics Connection 10-2

Chapter 6

High-frequency units produce x-rays much more efficiently than single-phase units. A high-frequency unit produces more x-rays using the same amount of electricity.

A patient’s age, condition, and the presence of a pathologic condition also affect the amount of mAs required for the procedure. Additionally, for a given mAs, IRs respond differently. 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. For example, when the mAs is increased, density is increased; when the mAs is decreased, density is decreased (Figure 10-2).

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. This relationship between radiation exposure intensity and density is discussed in more detail in Chapter 9. The film characteristic, speed, and chemical processing determine the amount of optical density produced on the image for a given mAs.

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. When the radiograph is deemed unacceptable, this means the optical densities lie outside the film’s sensitometric curve’s straight-line portion, and may need to be repeated. The radiographer must decide how much of a change in mAs is needed to correct for the density error.

In general, 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 typically brings the optical densities back within the straight-line portion of the film’s sensitometric curve to best visualize the anatomic area of interest. As mentioned previously, it may take more than doubling the mAs to correct for a density error. If the radiograph necessitates an adjustment greater than a factor of 2, the radiographer should multiply or divide the mAs by 4 (Figure 10-3).

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.

Critical Concept 10-3

mAs and Film-Screen Density

The mAs has a direct effect on the amount of radiographic density produced when using a film-screen IR. The minimum change needed to correct for a density error is determined by multiplying or dividing the mAs by 2. When a greater change in mAs is needed, the radiographer should multiply or divide by 4, 8, and so on.

Digital IRs can detect a wider range of radiation intensities (wider dynamic range) exiting the patient and therefore 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. If the mAs selected is too high (high exposure to the digital IR), the brightness can also be adjusted, but the patient has received more radiation than necessary.

Critical Concept 10-4

mAs and Digital Image Brightness

The mAs does not have a direct effect on image brightness when using digital IRs. During computer processing, image brightness is maintained when the mAs is too low or too high. A lower-than-required mAs produces an image with increased quantum noise and a higher-than-needed mAs exposes the patient to unnecessary radiation.

The radiographer should be diligent in monitoring exposure indicator values to ensure that quality images are obtained with the lowest possible radiation dose to the patient.

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

Kilovoltage

The kVp affects the exposure to the IR because it alters the amount and penetrating ability of the x-ray beam.

Make the Physics Connection 10-3

Chapter 6

When the kVp is increased at the control panel, a larger potential difference occurs in the x-ray tube, giving more electrons the kinetic energy to produce x-rays and increasing the kinetic energy overall. The result is more photons (quantity) and higher energy photons (quality).

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, further increasing the kVp results in more radiation reaching the IR. Unlike mAs, the kVp affects the amount of radiation exposure to the IR and radiographic contrast.

Critical Concept 10-5

kVp and the Radiographic Image

Increasing or decreasing the kVp changes the amount of radiation exposure to the IR and the contrast produced within the image.

However, 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 10-4).

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 (greater than 90) compared with operating at a low kVp (less than 70) (Figure 10-5).

Because kVp affects the amount of radiation reaching the IR, its effect on the digital image is similar to the effect of mAs. 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 noise decreases image quality.

Kilovoltage is not a factor typically manipulated to vary the amount of IR exposure in film-screen imaging 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 therefore 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.

Critical Concept 10-6

kVp and the 15% Rule

A 15% increase in kVp has the same effect as doubling the mAs. A 15% decrease in kVp has the same effect as decreasing the mAs by half.

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 low or high kVps are used, the amount of change in the kVp required to maintain the exposure to the IR may be greater or less than 15%.

Math Application 10-3

Using the 15% Rule

To increase exposure to the IR, multiply the kVp by 1.15 (original kVp + 15%).

80kVp×1.15=92kVp

To decrease exposure to the IR, multiply the kVp by 0.85 (original kVp − 15%).

80kVp×0.85=68kVp

To maintain exposure to the IR, when increasing the kVp by 15% (kVp × 1.15), divide the original mAs by 2.

80kVp×1.15=92kVp and mAs/2

When decreasing the kVp by 15% (kVp × 0.85), multiply the mAs by 2.

80kVp×0.85and mAs×2

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 (remnant). As a result, images with lower contrast are produced (Figure 10-6). 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 (remnant). This produces a high-contrast radiographic image (Figure 10-7).

Critical Concept 10-7

kVp and Radiographic Contrast

A high kVp results in less absorption and more transmission in the anatomic tissues, which results in less variation in the x-ray intensities exiting the patient (remnant), producing a low-contrast (long-scale) image. A low kVp results in more absorption and less transmission in the anatomic tissues, but with more variation in the x-ray intensities exiting the patient, resulting in a high-contrast (short-scale) image.

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.

Make the Physics Connection 10-4

Chapter 7

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. Scatter radiation remains a concern at higher kVp and provides no useful information and always decreases the radiographic contrast.

Critical Concept 10-8

Kilovoltage, Scatter Radiation, and Radiographic Contrast

At higher kVp, a greater proportion of Compton scattering occurs compared with x-ray absorption (photoelectric effect), which decreases radiographic contrast. Decreasing the kVp decreases the proportion of Compton scattering and increases 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 diagnostician’s preference. These factors make achieving a desired level of radiographic contrast more complex than achieving a desired level of radiographic density, especially for film-screen imaging.

For most anatomic regions, an accepted range of kVp provides an appropriate level of radiographic contrast. As long as the kVp selected is sufficient to penetrate the anatomic part, the kVp can be further manipulated to alter the radiographic contrast.

Radiographs generally are not repeated because of contrast errors. More often, the radiographer evaluates the level of contrast achieved to improve the contrast for additional radiographs or similar circumstances that arise with a different patient. If a repeat radiograph is necessary and kVp is to be adjusted to either increase or decrease the level of contrast, the 15% rule provides an acceptable method of adjustment. In addition, whenever a 15% change is made in the kVp to maintain the exposure to the IR, the radiographer must adjust the mAs by a factor of 2. Remember that a 15% change in kVp does not produce the same effect across the entire range of kVp used in radiography. A greater increase is needed for high kVp (90 and above) than for low kVp (below 70).

Adequate penetration of the anatomic area of interest is equally important when using digital IRs, and therefore kVp selection is important in producing a quality image. Assuming that the anatomic part is adequately penetrated, changing the kVp does not affect the digital image in the same way as a film-screen image. The kVp affects the contrast in a digital image; 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.

Critical Concept 10-9

Kilovoltage and Digital Image Quality

Assuming that the anatomic part is adequately penetrated, changing the kVp does not affect the digital image the same as a film-screen image. Image brightness and contrast are primarily controlled during computer processing.

The selection of kVp alters its absorption and transmission through the anatomic part regardless of the type of IR used and therefore must be selected wisely. Exposure techniques using higher kVp with lower mAs exposure techniques are recommended in digital imaging because contrast is primarily controlled during computer processing. Higher kVp and lower mAs values are not recommended as a general rule during film-screen imaging because of the contrast required to best visualize the anatomic structures.

Secondary Factors

Many secondary factors affect the radiation reaching the IR and image quality. It is important for the radiographer to understand their effects individually and in combination.

Focal Spot Size

On the control panel the radiographer can select whether to use a small or large focal spot size. The physical dimensions of the focal spot on the anode target in x-ray tubes used in standard radiographic applications usually range from 0.5 to 1.2 mm. Small focal spot sizes are usually 0.5 or 0.6 mm, and large focal spot sizes are usually 1 or 1.2 mm in size. Focal spot size is determined by the filament size. When the radiographer selects a particular focal spot size, he or she is actually selecting a filament size that is energized during x-ray production. Focal spot size is an important consideration for the radiographer because the focal spot size affects recorded detail.

Make the Physics Connection 10-5

Chapter 5

The actual focal spot is the area being bombarded by the filament electrons. The size of the electron stream depends on the size of the filament. The smaller this stream, the greater the heat generated in a small area; therefore it is desirable to have a larger actual focal spot area. The effective focal spot is the origin of the x-ray beam and is the area as seen from the patient’s perspective. The smaller this area of origin, the sharper the image. It is desirable to keep this as small as practical to improve image quality (Figure 10-8).

Critical Concepts 10-10

Focal Spot Size and Recorded Detail

As focal spot size increases, unsharpness increases and recorded detail decreases; as focal spot size decreases, unsharpness decreases and recorded detail increases.

In general, the smallest focal spot size available should be used for every exposure. Unfortunately, exposure is limited with a small focal spot size. When a small focal spot is used, the heat created during the x-ray exposure is concentrated in a smaller area and could cause tube damage. The radiographer must weigh the importance of improved recorded detail for a particular examination or anatomic part against the amount of radiation exposure used. Modern radiographic x-ray generators are equipped with safety circuits that prevent an exposure from being made if that exposure exceeds the tube loading capacity for the focal spot size selected. Repeated exposures made just under the limit over a long period can still jeopardize the life of the x-ray tube.

Source-to-Image Receptor Distance

The distance between the source of the radiation and the IR, source-to-image receptor distance (SID)

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