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.