Primary Exposure Factors IV
By Professor Stelmark
Film-Screen Characteristics
The visibility of the anatomic structures and the accuracy of their structural
lines recorded determine the overall quality of the radiographic image. For
film-screen, visibility of the recorded detail refers to the photographic
properties of the image. Visibility of the recorded detail is achieved by the
proper balance of radiographic density and radiographic contrast.
Radiographic Density
Radiographic film that has been exposed to radiant energy and chemically
processed is composed of minute deposits of black metallic silver visualized as
density. The varying densities on the processed film represent the attenuation
properties of the anatomic part imaged.
Radiographic density is the amount of overall blackness produced on the
processed image. A radiograph must have sufficient density to visualize the
anatomic structures of interest. The radiographer must evaluate the overall
density on the image to determine whether it is sufficient to visualize the
anatomic area of interest. He or she then decides whether the radiograph is
diagnostic or unacceptable.
If a radiograph is deemed unacceptable, the radiographer must determine
what factors contributed to the density error. Knowledge about the factors
that affect the density on a radiographic image is critical to developing
effective problem-solving skills
Optimal
Insufficient Density
Excessive Density
Exposure Intensity and Optical Density
Increasing the exposure intensity to the film-screen image receptor increases
the optical density, whereas decreasing the exposure intensity to the filmscreen image receptor decreases the optical density.
Radiographic Contrast
Producing a radiographic image with diagnostic densities is important to
visualize the anatomic area of interest. To differentiate among the anatomic
tissues, there must be density differences.
Density differences are a result of the tissues’ differential absorption of
the x-ray photons. Differential absorption is the difference between the xray photons that are absorbed versus those that penetrate the body. It is
called differential because different body structures absorb x-ray photons
to different extents. Anatomic structures such as bone are denser and
absorb more x-ray photons than structures filled with air such as the
lungs.
Radiographic contrast is the degree of difference or ratio between adjacent
densities. The ability to distinguish between densities enables differences in
anatomic tissues to be visualized. An image that has a diagnostic density but
no differences in densities appears as a homogeneous object.
When the absorption characteristics of an object differ, the image presents with
varying densities.
Radiographic images are typically described by their scale of contrast or the
range of densities visible. A radiograph with few densities but great differences
among them is said to have high contrast. This is also described as shortscale contrast . A radiograph with a large number of densities but little
differences among them is said to have low contrast. This is also described as
long-scale contrast.
High Contrast
Low Contrast
Radiographic contrast is the combined result of multiple factors associated with the
anatomic structure, quality of the radiation, and the capabilities of the film. Subject contrast
refers to the absorption characteristics of the anatomic tissue radiographed along with the
quality of the x-ray beam. Differences in tissue thickness, density, and effective atomic
number contribute to subject contrast.
Low Contrast
The radiographer must evaluate the composition of the anatomic structure
to be radiographed and determine the factors to manipulate to produce the
desired level of radiographic contrast. Achieving the desired level of
contrast that best visualizes the anatomic area of interest maximizes the
amount of information visible for a diagnosis.
High Contrast
Low Contrast
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.
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.
Changes in mAs have a direct effect on density
mAs is the main controlling factor of 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. When the radiograph is
deemed unacceptable it 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. If the radiograph necessitates an adjustment greater than a
factor of 2, the radiographer should multiply or divide the mAs by 4.
¼ of required mAs
4 x required mAs
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 ensity 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.
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.
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 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.
kVp is the main controlling factor of 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.
Low kVp
High Contrast
High kVp
Low Contrast
Increasing the kVp increases IR exposure and the density produced on a
film image.
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
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.
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.