X-ray or gamma ray systems or devices – Accessory – Testing or calibration
Reexamination Certificate
2000-12-22
2002-12-24
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Accessory
Testing or calibration
C378S038000, C378S168000
Reexamination Certificate
active
06497511
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods and devices for imaging in digital dental radioscopy, and especially to methods and devices making use of sensor arrays whose image elements are smaller than would be necessary for the desired local resolution.
2. Description of Prior Art
Dental X-ray diagnostic units in which images are produced digitally making use of CCD sensors (CCD =charge coupled device) have been known for some time. The sensors used normally comprise the CCD sensor itself and a scintillation layer which is applied directly to the CCD sensor and which converts the incident X-radiation into visible light which will then generate an electric charge in the underlying CCD sensor. The image information is then read from the CCD sensor by applying suitable clock signals, whereupon it is preprocessed, digitized and finally transmitted to a computer system, e.g. a personal computer, for display and storage. In comparison with a conventional film technique, this method is primarily advantageous insofar as images are obtained much faster.
An example of such X-ray diagnostic units is described in DE-A-19615178. The diagnostic unit described in this publication additionally comprises a correction means for compensating fluctuations of the electric signals of the individual elements of the CCD array caused by a dark current of these elements, by different conversion efficiencies of these elements and by inhomogeneities of the scintillation layer.
The size of the image elements of the CCD sensor, which are also referred to as pixels, is normally adapted to the desired local resolution. Furthermore, image elements which are smaller than the desired local resolution can be used, image information of neighbouring image elements being then already combined on the sensor. This course of action is referred to as binning. An effective image element whose size corresponds again to the desired local resolution is obtained in this way. The resolution demanded in the field of digital dental radioscopy is typically a resolution of from 50 &mgr;m to 60 &mgr;m. A higher resolution will normally not make sense, since the scintillator required for converting the X-radiation into visible light will normally not permit a higher resolution. It is true that a higher resolution can be achieved by particularly thin scintillation layers, but this is disadvantageous insofar as only a very small percentage of the X-radiation can be detected by means of these very thin scintillation layers and, due to a correspondingly lower signal-to-noise ratio, this will lead to a deterioration of the image quality.
The necessary total area of such a sensor for dental radioscopy amounts to several square centimetres, since, if the total area of the sensor is smaller than that, it will be difficult to show a whole tooth on a single X-ray image. The size of the sensors is normally adapted to the size of conventional tooth films, which usually starts with a size of 2×3 cm
2
.
Known dental radioscopy techniques have a plurality of disadvantages. On the one hand, the image quality of digital dental X-ray photographs is usually not yet satisfactory, since the image is very noisy. One reason for this is that only a small number of X-ray quanta per image element contributes to the production of the image, since the patient should only be exposed to a small dose for medical reasons; this leads to quantum noise. In addition to this quantum noise, a second source of noise exists, since only part of the X-radiation falling on the X-ray sensor is absorbed by the scintillation layer and converted into visible light, whereas part of the X-radiation passes through the scintillation layer and can be absorbed in the CCD sensor itself, the CCD sensor consisting preferably of silicon. Such an absorption of X-ray quanta in the silicon of the CCD sensor will be referred to as “direct hit” in the following. This effect is undesired, since X-ray quanta converted in the scintillation layer generate only a small charge in the CCD sensor, typically a charge in the range of a few hundred electrons. Direct hits, however, generate in comparison therewith a very high charge, typically a charge in the range of ten thousands of electrons. Hence, a direct hit will typically produce a charge which is 50 times as high as an X-ray quantum converted in the scintillator. Direct hits will therefore contribute to image noise to a very high extent.
Another problem of known sensors is to be seen in that, only in the most advantageous cases will it be possible to produce sensors of the above-mentioned geometrical size which do not have any fault whatsoever. Frequently, individual image elements or several neighbouring image elements, so-called clusters, or even complete lines or columns of the sensor will fail to contribute. In this connection defects occur in the case of which the defective image element or pixel generates a large image signal without exposure as well as defects in the case of which the defective image element, though exposed, does not generate any image signal. Depending on the number and kind of such defects, a distinction is made between various sensor qualities which are suitable for different cases of use. When the number of defects is excessively large, i.e. when the quality of the sensor is too poor, the sensor cannot be employed for the intended use, i.e. for dental radioscopy, since, due to the defective image elements, the dentist may perhaps fail to notice important image information. This, however, has the consequence that a substantial percentage of the sensors produced cannot be used, and this leads to a considerable increase in the price of the usable sensors.
U.S. Pat. No. 5,617,461 teaches that, making use of at least one calibration image, a defect image is produced. An image recorded from an object is then corrected making use of the defect image, this correction being carried out by means of linear interpolation.
U.S. Pat. No. 5465284 shows how a plurality of CCD sensors, which have been combined to form an image element, can be processed. The local resolution of the image is reduced in comparison with the resolution of the sensor in this way, whereas the signal-to-noise ratio is improved.
SUMMARY OF THE INVENTION
It is the object of the present invention to provide methods and devices for imaging in digital dental radioscopy, which permit the use of sensor arrays that are less expensive to produce while maintaining or improving the quality of the image produced.
According to a first aspect of the present invention, this object is achieved by a method for imaging in digital dental radioscopy making use of a sensor array, the individual image elements of which are smaller than a desired local resolution, so that a plurality of image elements forms a respective effective image element, comprising the steps of:
a) detecting first reference signals which are generated by the image elements of the sensor array when said sensor array is not exposed to X-radiation;
b) detecting second reference signals which are generated by the image elements of the sensor array when said sensor array is exposed to X-radiation;
c) determining defective image elements on the basis of the detected first and second reference signals; and
d) detecting third electric signals which are generated by the image elements of the sensor array when said sensor array is exposed to an X-radiation representative of an image of an object; and
e) producing the image by forming a respective output signal for the respective effective image elements using exclusively those third electric signals which are generated by image elements of the effective image element that have been determined as being non-defective.
Thus, according to the first aspect, the present invention provides a method for imaging in digital dental radioscopy making use of a sensor array in which a plurality of image elements forms a respective effective image element in the case of which defective image elements of the sensor ar
Hanke Randolf
Kostka Günther
Schmitt Peter
Barber Therese
Dunn Drew A.
Fraunhofer-Gesellschaft zur Foerderung der angewandten Forschung
Patton & Boggs LLP
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