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Non-Circular Scanning Trajectories with Varian Developer Mode

A Davis

A Davis*, E Pearson , X Pan , C Pelizzari , Univ Chicago, Chicago, IL


WE-G-BRF-7 Wednesday 4:30PM - 6:00PM Room: Ballroom F

Purpose: Cone-beam CT (CBCT) in image-guide radiation therapy (IGRT) typically
acquires scan data via the circular trajectory of the linear
accelerator’s (linac) gantry rotation. Though this lends itself to
analytic reconstruction algorithms like FDK, iterative reconstruction
algorithms allow for a broader range of scanning trajectories. We
implemented a non-circular scanning trajectory with Varian’s TrueBeam
Developer Mode and performed some preliminary reconstructions to
verify the geometry.

Methods: We used TrueBeam Developer Mode to program a new scanning trajectory
that increases the field of view (FOV) along the gantry rotation axis
without moving the patient. This trajectory consisted of moving the
gantry in a circle, then translating the source and detector along the
axial direction before acquiring another circular scan 19 cm away from
the first. The linear portion of the trajectory includes an additional
4.5 cm above and below the axial planes of the source's circular
rotation. We scanned a calibration phantom consisting of a lucite tube
with a spiral pattern of CT spots and used the maximum-likelihood
algorithm to iteratively reconstruct the CBCT volume.

Results: With the TrueBeam trajectory definition, we acquired projection data
of the calibration phantom using the previously described trajectory.
We obtained a scan of the treatment couch for log normalization by
scanning with the same trajectory but without the phantom present.
Using the nominal geometric parameters reported in the projection
headers with our iterative reconstruction algorithm, we obtained a
correct reconstruction of the calibration phantom.

Conclusion: The ability to implement new scanning trajectories with the TrueBeam
Developer Mode enables us access to a new parameter space for imaging
with CBCT for IGRT. Previous simulations and simple dual circle scans
have shown iterative reconstruction with non-circular trajectories can
increase the axial FOV with CBCT. Use of Developer Mode allows
experimentally testing these and other new scanning trajectories.

Funding Support, Disclosures, and Conflict of Interest: Support was provided in part by the University of Chicago Research Computing Center, Varian Medical Systems, and NIH Grants 1RO1CA120540, T32EB002103, S10 RR021039 and P30 CA14599. The contents of this work are solely the responsibility of the authors and do not necessarily represent the official views of the supporting organizations.

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