X-ray or gamma ray systems or devices – Accessory – Testing or calibration
Reexamination Certificate
1999-11-12
2001-10-02
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Accessory
Testing or calibration
C378S098800, C382S272000, C382S275000
Reexamination Certificate
active
06296387
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for correcting image defects from a matrix-type X or &ggr;-ray detector, and in particular, for correcting defective or missing pixels in this image, by assigning to each image pixel a confidence rating between 0 and 1.
The invention is implemented in all fields using digital images produced through X or &ggr;-rays, and more particularly, in the medical field and the dental field, in X or &ggr;-ray digital radiographic imaging.
STATE OF THE ART
In the field of X or &ggr;-ray digital radiographic imaging, the system used is generally a standard image acquisition system. Such system can comprise an X or &ggr; radiation source (e.g. a bidimensional X tube or X detector, for X-rays), that provides, at the output, an image representative of the amount of X or &ggr;-photons transmitted through the radiographed object that is placed between the source and the detector. In some instances, the object itself is a source of &ggr;-rays.
However, the images thus acquired may contain a number of defects, most of which are related to the detector. These defects come from one or more pixels, called defective pixels, which behave absurdly and consequently provide a value that is not related to the amount of X or &ggr;-photons received in this region of the detector. Such defective pixels can also be due to a lack of measurement, i.e. that these defective pixels form an image area where the detector could not take a measurement.
For instance, the solid X-ray detector, described in “Recent developments on a CdTe based X-ray detector for digital radiography”, F. GLASSER et al., SPIE conference on Medical Imaging, Feb. 22/28, 1997, and designed for digital dental radiography, is composed of a CdTe semiconductor with four CMOS circuits arranged in a 2×2 matrix. The connection between the semiconductor and the scanning circuits is performed by means of indium beads. Such detectors have two types of defects:
random defects related in particular to the semiconductor and the connections between the semiconductor and the scanning circuit; and
cross-shaped missing lines and columns at the junction between the four scanning circuits.
The defects of this detector appear, in particular in
FIGS. 3A and 3C
, which show the image of a known object and the image of a tooth to be examined, respectively.
Obviously, these defective pixels are detrimental to the visual image quality and consequently to its usage. Therefore, such pixels must be detected and their values corrected.
When the defective pixels are isolated, correcting the image is simple; it is done by interpolation with regard to neighboring pixels of the defective pixel considered.
In other cases, these defective pixels can be grouped and form whole image areas, which may be rather big and of any shape. These areas can form, e.g. a line or a column, or else a set of lines or columns of defective pixels, or even a cluster of any shape.
There are already methods for correcting a set of defective pixels. Such methods generally use a conventional technique consisting in a binary determination of defective pixels based on thresholding a criterion. The choice of this criterion is difficult; indeed, if the threshold is chosen too low, defective pixels may not be detected; if it is too high, the areas of defective pixels will be determined as being larger than they really are, which makes a proper correction of the defective pixels more difficult.
The difficulty of choosing the criterion therefore implies a difficult implementation of the method. In addition, the results obtained with this method are not very reliable as they depend on the criterion chosen.
DESCRIPTION OF THE INVENTION
It is precisely the object of the invention to remedy the disadvantages of the method described before. For this purpose, it provides a method for correcting X or &ggr; image defects, based on constructing a confidence map grouping the confidence ratings (being continuous, between 0 and 1) of all the detector pixels.
More precisely, the invention relates to a method for correcting the image defects (related to defective pixels or pixels not measured by the detector), from a matrix-type X or &ggr;-ray detector, consisting in calibrating the detector, then correcting the defective pixels of the image to be corrected. The method is characterized in that the calibration step comprises the following operations:
a) acquiring a black image;
b) acquiring at least one image of a known object;
c) determining at least one offset and mean gain image;
d) acquiring at least one image of the object to be measured with uniform attenuation when the detector is of the X-ray type and uniform emission when it is of the &ggr;-ray type;
e) determining a first average and a first standard deviation for the whole image of the object to be measured;
f) producing a first confidence map depending on the gray levels of the image pixels as well as the standard deviation and the average determined at step e);
and in that the correction step consists in acquiring an image of the object to be measured and correcting the image of the object on the basis of the confidence map and the last object image acquired.
According to an alternative embodiment of the invention, the calibration step comprises, after the operation f) of producing a first confidence map, the following operations:
g) in the image of the known object, determining a gray level gradient (
18
);
h) determining (
19
-
22
) a second average and a second standard deviation, then a second confidence map (C
2
); and
i) determining (
24
) a third confidence map (C
3
) by multiplying, pixel-wise, the first and second confidence maps.
Advantageously, the first and second confidence maps are composed of the set of confidence ratings assigned to each image pixel, the confidence rating of each pixel being determined depending on the pixel's gray level, as well as the standard deviation and the average of the whole image of the object to be measured.
According to a preferred embodiment of the invention, step b) of acquiring the image of a known object is carried out with a uniform attenuation.
According to an alternative embodiment of the invention, where the image defect are the result of the lack of measurement in at least one image area, the calibration step, after operation f) or i) consists in:
magnifying (
241
) the image of the object to be measured;
adding (
242
), at the location of the image area not measured, pixels with zero confidence ratings; and
determining (
243
) a fourth confidence map, on the basis of the first and third confidence maps, taking into account the zero confidence ratings;
and in that the correction step consists in performing another image acquisition of the object to be measured (
244
), applying the magnification to it and correcting the object image on the basis of the fourth confidence map and the last object image acquired.
REFERENCES:
patent: 5617461 (1997-04-01), Schreiner
patent: 5832055 (1998-11-01), Dewaele
patent: 5881182 (1999-03-01), Fiete et al.
patent: 5886353 (1999-03-01), Spivey et al.
patent: 0 775 414 (1997-05-01), None
patent: WO 95/01045 (1995-01-01), None
H. Knutsson, et al., IEEE Computer Society Conference on Computer Vision a Pattern Recognition, pp. 515-523, “Normalized and Differential Convolution. Methods for Interpolation and Filtering of Incomplete and Uncertain Data,” Jun. 15-18, 1993.
F. Glasser, et al., SPIE, Medical Imaging, vol. 3032, pp. 513, “Recent Developments on a CdTe Based X-Ray Detector for Digital Radiography,” Feb. 22/28, 1997.
Commissariat A l'Energie Atomique
Ho Allen C
Kim Robert H.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
LandOfFree
Method for correcting image defects from a matrix-type X or... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for correcting image defects from a matrix-type X or..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for correcting image defects from a matrix-type X or... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2552063