Four-Dimensional MVCT Imaging and Spatial Translation of Helical Tomotherapy Dose Distributions Using Sinogram Modification

Helical tomotherapy systems have the unique attribute of using sinograms as a machine input and a machine output. These sinograms have well understood properties that may allow for operational changes that simplify and/or improve radiation treatment.

In the first part of this work, the detector array output sinogram of an MVCT imaging study is used to produce images of an object at different locations along a pattern of motion and the multi-leaf collimator (MLC) control sinogram is used to translate dose distributions in space. A temporal re-binning algorithm was written that divided the MVCT sinogram projections into positional bins that were subsequently used to produce new sinograms for each bin. A CT resolution phantom was placed upon a motion phantom with a periodic range of motion, and a multiple gantry rotation MVCT scan was performed upon the phantom when stationary and in-motion. The detector sinogram was separated into several bins, before a sinogram was compiled from each bin. Images were produced from the sinograms using a filtered-back projection image reconstruction program. The resultant re-binned images of the in-motion phantom were compared to the reconstructed images of the in-motion and stationary phantom. Improvements were seen in the resolution of the CT phantom in the re-binned images as compared to the in-motion phantom without re-binning. In some cases, the CT phantom resolution in the images was seen to closely approach the resolution in the image of the stationary phantom. In the second part of this work, a multileaf collimator control sinogram of a controlled treatment plan was modified so as to translate the treatment in the lateral and vertical directions. A combination of ion chamber measurements and radiograph analysis was used to verify the position, dose and shape of the translated treatment. The translated treatments resulted in closely matching the original treatment with approximately 3-mm accuracy.