[Rtk-users] Support for translate-rotate geometries with small FOV detectors

[Sean Phillip Hood]

Hi,

I am using RTK to reconstruct spectral CBCT images from projections obtained with a photon counting detector (PCD) mounted on an Elekta linac.

The main challenge is the size of the PCD (at isocentre the FOV is approximately 80 mm x 10 mm). To address this we repeat CBCT acquisitions with the detector offset by multiples of its width for each gantry rotation.

Unfortunately, the sampling of gantry angles varies between acquisitions, and so the projections cannot be simply stitched together. Furthermore, obtaining projections by scanning across the FOV at a series of gantry angles is far too slow.

Therefore, I wanted to see if there was a solution using RTK. The first thing I have tried is to feed the command line applications all of the sorted PCD projections from separate acquisitions as well as a concatenated geometry file. The concatenation involves extracting the geometry information from each Elekta XML file and appending it to a single geometry file. The translation is added to the projection offset in the x-direction.

For RTK v2.5.0, this results in the following exception thrown by rtkDisplacedDetectorImageFilter when using rtkadmmtotalvariation:

Description: ITK ERROR: Cannot account for detector displacement larger than 50% of panel size. Corner inf=59.661 and corner sup=682.68

A similar error is returned by rtkfdk. It seems that projection offsets larger than 50% of the detector cannot be used. Would it be possible to add support for this, or for translate-rotate geometries in general?

I only ask because I have also tried this method with an older version of RTK (I do not know the exact version since I was provided with pre-compiled executables). These errors did not occur and I was able to at least produce an output from rtkadmmtotalvariation, albeit one that had clear geometry issues (I suspect there are differences in definitions that I'm not accounting for between RTKThreeDCircularGeometry versions 2 and 3, the former being used by the older version of RTK).

Please let me know if I can provide any more info, and I would be grateful for any assistance. Thanks!

Kind regards,
Sean Hood
School of Physics, Faculty of Engineering and Information Sciences
University of Wollongong, Australia
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