Transcript Sensitometry

Sensitometry
By Prof. Stelmark
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 B+F. 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.
The characteristic curve of a fast image receptor is positioned to the left—closer to
the y-axis—of that of a slow image receptor. Radiographic image receptors are
identified as fast or slow according to their sensitivity to x-ray exposure.
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.
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.
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.
As development time or temperature increases, changes occur in the shape and
relative position of the characteristic curve.
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.
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.