What must the radiographer do to correct a radiograph that had the proper amount of exposure but demonstrated excessive quantum noise?

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.

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.