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A Novel Technology for Volume-Of-Interest KV Cone Beam CT

J Robar

J Robar1,2*, (1) Dalhousie University, (2) Nova Scotia Cancer Centre, Halifax, Nova Scotia

TH-C-BRA-2 Thursday 10:30:00 AM - 12:30:00 PM Room: Ballroom A

Purpose: In this work we describe the development of a new technology allowing volume-of-interest (VOI) kV CBCT. A device for dynamic kV beam collimation is described and initial imaging and dosimetric results are given.
Methods: We have manufactured a prototype of an iris aperture capable of tracking a chosen VOI during kV CBCT imaging. The aperture and housing is compact and occupies an area comparable to the bow-tie filter when mounted to the source of an on-board imaging system. The system allows two-dimensional translation of the aperture as a function of gantry angle and dynamic adjustment of iris diameter. Iris leaves are non-overlapping and therefore can be made sufficiently thick to attenuate higher energy kV beams used. The device was tested for functionality on a Varian OBI system. Dose reduction in the VOI was measured and compared to full-field imaging, for planar and CBCT imaging protocols, as a function of iris dimension. Initial projection and CBCT images of a head phantom were acquired.
Results: The system provides a versatile tool for dynamic collimation of the kV beam during CBCT. While dose reduction in VOI CBCT is achieved primarily by blocking of non-VOI anatomy, the dose inside the imaged VOI is also reduced substantially, by 5% to 60% for 20 cm and 2 cm iris diameters, respectively. The magnitude of dose reduction increases slightly with increased tube potential, between 65 and 125 kVp. Projection images demonstrate that the iris produces effective collimation with minimal leakage. Reconstructed CBCT images are affected by truncation artefact, but this may be remedied by data filling techniques or other (e.g. pi-line) reconstruction algorithms.
Conclusions: The technology presented is a novel approach to kV CBCT, localizing imaging dose to a chosen VOI while, compared to full-field CBCT, reducing dose both inside and outside of the VOI.

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