Scans with dynamically collimated beam and with non-coaxial field of view. Local tomography.



Dynamic collimation


Typically x-ray beam is collimated (restricted) such way that there are no direct rays missing the detector. However sometimes a required field of view is relatively small and in most cases is not coaxial with the axis of rotation. A requirement of illumination only field of view causes a necessity of moving collimator blades

Dynamically collimated views

Dynamically collimated views

Dynamic collimation

Dynamic collimation

Angular dependent projection cropping


Dynamic (angular dependent) collimation results into partially illuminated views where exposed area gets different positions from projections to projection. A direct utilization of such data causes major artifacts. To avoid this it is needed to involve information about exposed area position within every view. So the software might take it into account by internal cutting off (cropping) all non-relevant pixels. There are two .bin files in Cobra for describing this processing mode.

Local tomography


Dynamic collimation typically leaves a substantial part of the object outside field of view. This situation is well known in CT as object protrusion. Cobra incorporates a special algorithm Dynamic slice scale calibration to mitigate related artifacts

 
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Describing a reconstruction task

Within the COBRA reconstruction package, all input, output and intermediate data are kept on the hard disk. The user defines and controls the reconstruction task through a parameter file, a text file with extension .xxm. This file contains tags and assigned values, and describes the scanner hardware setup, input and output data characteristics and representation, and the chosen reconstruction method. Some of the reconstruction options are only accessible through SDK API’s. These are related to synchronization and control of in-line reconstruction (recon concurrent with acquisition), and the setup of COBRA’s working folders.
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Defining position dependent cropping

CropLeft.bin

CropLeft.bin

The width (pixels) of the strip (left side) which should be excluded from preprocessing (this strip is assumed to be shadowed by a collimator blade) CropLeft.bin is a binary file, no header, one 4-byte floating point per position Defaults to the value assigned in xxm parameter file if CropLeft.bin is omitted.
CropRight.bin

CropRight.bin

The width (pixels) of the strip (right side) which should be excluded from preprocessing (this strip is assumed to be shadowed by a collimator blade) CropLeft.bin is a binary file, no header, one 4-byte floating point per position Defaults to the value assigned in xxm parameter file if CropRight.bin is omitted.
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