Television – Responsive to nonvisible energy
Patent
1994-11-07
1996-05-21
Greening, Wendy R.
Television
Responsive to nonvisible energy
348243, 378 988, 378 982, H04N 314
Patent
active
055194370
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to obtain on a display a curve of luminous flux corresponding to a photographic film when using an image sensor formed by a CCD member (Charge Coupled Device) instead of x-ray film for dental x-raying and more exactly to a method for compensation of dark current in the use of a CCD member for dental x-raying due to the CCD member having a strongly temperature and integration time dependent dark signal.
2. Description of the Related Art
In dental x-ray examinations small pieces of photographic film encapsuled in what could be called a cover to protect the film from light have for a long time been used, whereby said cover was being brought into the oral cavity. This film particularly adapted for exposure by x-ray radiation is in darkness developed after exposure and removal of the cover in a normal way, whereby an image is obtained where e.g. the jawbone and teeth will appear lighter in the film than for example soft tissues due to the difference in material density.
Today according to prior art the photographic film is replaced by electronic image sensors, usually in form of an image sensor of a CCD type (Charge Coupled Device). Such image sensing members are also frequently to be found in e.g. video technique but are then meant to especially operate within the wavelength range of visual light. The technique implies that immediately after the exposure with x-ray radiation a corresponding image is obtained for example on some type of display and thereby one avoids all of the developing process simultaneously as the radiation dose in most cases essentially is decreased due to that the image sensor itself may sense the necessary radiation dose to obtain the sufficient exposure. Such a system for dental x-raying named SEN-A-RAY is marketed for example by Regam Medical Systems AB, Sundsvall. When these images produced by the previously used dental x-ray film were evaluated, this was done by permitting light from behind to fall through the obtained image. The darkening of the film may be expressed by the optical density D as a function of the light intensity I.sub.1 reaching the eye relative to the light intensity I.sub.0 falling in.
In a dental film, D is basically proportional to the exposure dose. If one thereby, according to FIG. 2, draws a graph of the light intensity as a function of the optical density D, a nonlinear curve having the largest slope at high material densities of the object within the interval marked by a in FIG. 2 is obtained, i.e. the areas in the exposed film corresponding to enamel and dentine. Thus it desirable that the intensity curve within this density range be as steep as possible to be able to discriminate small changes in e.g. the enamel of a tooth.
In the case of a CCD sensor an intensity value is obtained, which is proportional to the exposure, and primarily the film and the sensor in this respect have equal qualities. Upon the presentation of the signal from such a CCD sensor, a visualization is generally obtained where the relation between intensity of light on the display and D will be a linear function, as is exemplified in FIG. 3. But simultaneously as a high resolution of gray levels within the area comprising e.g. the enamel of a tooth is desired, also still a resolution is desired at high optical densities corresponding to portions more opaque to the x-ray radiation, to e.g. clearly be able to indicate an eventual root infection and the like. This is why the problem is not simply solved only by making the linear function steep within a limited density interval. Accordingly an image having the linear intensity imaging does not to the user appear quite similar compared to film images and may sometimes result in certain interpretation difficulties. This is why it is desirable to be able to present image data also on a video monitor having the logarithmic or non-linear curve of FIG. 2.
By utilization of different technical methods it is of course possible by hardware or software to transform
REFERENCES:
patent: 4467351 (1984-08-01), Wang
patent: 4628357 (1986-12-01), Fenster
patent: 4652918 (1987-03-01), Liu et al.
patent: 4901336 (1990-02-01), Nishiki
Greening Wendy R.
Regam Medical AB
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