Sensitometry

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Sensitometry

• By Prof. Stelmark

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The study of the relationship between the
intensity of exposure of the film and the
blackness after processing is called
sensitometry. Knowledge of the sensitometric
aspects of radiographic film is essential for
maintaining adequate quality control.
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Characteristic Curve The two principal
measurements involved in sensitometry are the
exposure to the film and the percentage of light
transmitted through the processed film. Such
measurements are used to describe the
relationship between OD and radiation exposure.
This relationship is called a characteristic
curve, or sometimes the H D curve after Hurter
and Driffield, who first described this
relationship.
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Two pieces of apparatus are needed to construct a
characteristic curve an optical step wedge,
sometimes called a sensitometer, and a
densitometer, a device that measures OD. An
aluminum step wedge, or penetrometer, can also be
used as an alternative to the sensitometer.
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Radiographic film is sensitive over a wide range
of exposures. Film-screen, for example, responds
to radiation intensities from less than 1 to
greater than 1000 mR. Consequently, the exposure
values for a characteristic curve are presented
in logarithmic fashion. Furthermore, it is not
the absolute exposure that is of interest but
rather the change in OD over each exposure
interval. Therefore, log relative exposure (LRE)
is used as the scale along the x-axis.
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The LRE scale usually is presented in increments
of 0.3 because the log of 2, doubling the
exposure, is 0.3. Doubling the exposure can be
achieved by doubling the mAs.
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The useful range of OD is approximately 0.25 to
2.5. Most radiographs, however, show image
patterns in the range of 0.5 to 1.25 OD.
Attention to this part of the characteristic
curve is essential. However, very low OD may be
too light to contain an image, whereas very high
OD requires a hot light to view the image.
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ODs of unexposed film are due to base density and
fog density. Base density is the OD that is
inherent in the base of the film. It is due to
the composition of the base and the tint added to
the base to make the radiograph more pleasing to
the eye. Base density has a value of
approximately 0.1.
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Film contrast is related to the slope of the
straight-line portion of the characteristic curve.
The characteristic curve of an image receptor
allows one to judge at a glance the relative
degree of contrast. If the slope or steepness of
the straight-line portion of the characteristic
curve had a value of 1, then it would be angled
at 45 degrees. An increase of 1 unit along the
LRE axis would result in an increase of 1 unit
along the OD axis. The contrast would be 1.
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An image receptor that has a contrast of 1 has
very low contrast. Image receptors with a
contrast higher than 1 amplify the subject
contrast during x-ray examination. An image
receptor with a contrast of 3, for instance,
would show large OD differences over a small
range of x-ray exposure.
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Speed The ability of an image receptor to
respond to a low x-ray exposure is a measure of
its sensitivity or, more commonly, its speed. An
exposure of less than 1 mR can be detected with a
film-screen combination, whereas several mR are
necessary to produce a measurable exposure with
direct-exposure film.
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Speed Point The speed of radiographic film
typically is determined by locating the point on
a sensitometric curve that corresponds to the
optical density of 1.0 plus BF. This point is
called the speed point. This optical density
point is used because it is within the
straight-line portion of the sensitometric curve.
The speed point serves as a standard method of
indicating film speed.
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The characteristic curve of a fast image receptor
is positioned to the leftcloser to the y-axisof
that of a slow image receptor. Radiographic image
receptors are identified as fast or slow
according to their sensitivity to x-ray exposure.
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Usually, identification of a given image receptor
as so many times faster than another is
sufficient for the radiologic technologist. If A
were twice as fast as B, image receptor A would
require only half the mAs required by B to
produce a given OD. Moreover, the image on image
receptor A might be of poor quality because of
increased radiographic noise.
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Latitude An additional image receptor feature
easily obtained from the characteristic curve is
latitude. Latitude refers to the range of
exposures over which the image receptor responds
with ODs in the diagnostically useful range.
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Latitude also can be thought of as the margin of
error in technical factors. With wider latitude,
mAs can vary more and still produce a diagnostic
image. Image receptor B responds to a much wider
range of exposures than A and is said to have a
wider latitude than A.
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As development time or temperature increases,
changes occur in the shape and relative position
of the characteristic curve.
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Digital Imaging The response of a digital image
receptor to the intensity of radiation exposure
is different when compared with that of
radiographic film. The digital image receptor is
more responsive to the wide range of x-ray
intensities exiting the anatomic part. In
addition, a digital imaging system can retain
significantly more information than radiographic
film. The information received from the digital
image receptor and processed in the computer
represents the dynamic range capabilities of the
digital system. Dynamic range refers to the range
of exposure intensities an image receptor can
accurately detect. The greater the number of
x-ray photon intensities recorded and available
to create an image, the wider the dynamic range
of the imaging system. Digital imaging systems
have the ability to visually display a wider
range of densities than film radiography.
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As evidenced by the sensitometric curve for film,
x-ray intensities must fall within a smaller
range to display radiographic densities that can
be visible. The linear response of a digital
image receptor results in a greater range of
densities available for display within the
digital image. The digital image can display a
shade of gray that represents low x-ray
intensity, as well as medium and high x-ray
intensities.