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SID independent Grid structure correction for Flat Detector systems Disclosure Number: IPCOM000206911D
Publication Date: 2011-May-13
Document File: 3 page(s) / 72K

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SID independent Grid structure correction for Flat Detector systems

1: Description:

In Flat Detector systems gain correction is commonly used to correct for all kinds of pixel-to- pixel sensitivity differences. These sensitivity differences are caused by the X-ray beam (non homogene), local and global in-homogeneity of the Cesium Iodide layer on top of the array, image element-to-element sensitivity differences in the (TFT) array and differences in the components of the electronics (amplifiers, AtoD converters etc.). The aforementioned sensitivity differences are corrected by multiplying each pixel of the image with a correction factor for that specific pixel. The gain correction factor for each pixel is calculated after an acquisition of a number of "homogene" images, averaging and calculation during a calibration routine (during manufacturing or a service action). Besides this gain correction, offset - , non linearity - and defect correction are also applied. Compared to conventional detection methods (such as IITV's) where - due to various reasons - very limited corrections are possible, the FD images, after the corrections, are of a very high image quality.

The anti scatter grid is also responsible for introducing structure in the image and would be correctable in the same way as discussed before (i.e. by mounting the grid in the system before the calibration) however since in most practical systems a variable SID (source image distance) is used AND because of the fact that the grid is positioned at a certain distance from the detector array (4 to 6 cm) magnification differences are responsible for the fact that the grid is imaged onto the array in various magnifications. The image quality would than only be optimal at the SID where the calibration was performed. See figure 1


Figure 1: Image quality degradation due to varying SID

By extending the calibration routine to various SID values between SIDmax and SIDmin , extra correction factors per pixel as a function of SID can be obtained. By interpolation, the correction factor for every SID can be calculated. This SID correction factor should be multiplied with the nominal gain correction factor. The SID information is necessary for selecting the SID correction factor. This SID information is available in X-ray systems. The resulting gain correction factor should be used to correct the image. The result is an IQ which is not dependent of a change in SID. See f...