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TH‐A‐18C‐11: An Investigation of KV CBCT Image Quality and Dose Reduction for Volume‐Of‐Interest Imaging Using Dynamic Collimation
Author(s) -
Parsons D,
Robar J
Publication year - 2014
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4889570
Subject(s) - isocenter , imaging phantom , image quality , optics , collimated light , aperture (computer memory) , physics , materials science , medical imaging , monte carlo method , contrast to noise ratio , nuclear medicine , computer science , mathematics , medicine , computer vision , artificial intelligence , acoustics , image (mathematics) , laser , statistics
Purpose: The focus of this work was to investigate the improvements in image quality and dose reduction for volume‐of‐interest (VOI) kV‐CBCT using dynamic collimation. Methods: A prototype iris aperture was used to track a VOI during a CBCT acquisition. The current aperture design is capable of one‐dimensional translation as a function of gantry angle and dynamic adjustment of the iris radius. The aperture occupies the location of the bow‐tie filter on a Varian OBI system. CBCT and planar image quality was investigated as a function of aperture radius, while maintaining the same dose to the VOI, for a 20 cm diameter cylindrical water phantom with a 9 mm diameter bone insert centered on isocenter. Corresponding scatter‐to‐primary ratios (SPR) were determined at the detector plane with Monte Carlo simulation using EGSnrc. Dose distributions for various anatomical sites were modeled using a dynamic BEAMnrc library and DOSXYZnrc. The resulting VOI dose distributions were compared to full‐field distributions. Results: SPR was reduced by a factor of 8.4 when decreasing iris diameter from 21.2 cm to 2.4 cm (at isocenter). Similarly, this change in iris diameter corresponds to a factor increase of approximately 1.4 and 1.5 in image contrast for CBCT and planar images, respectively, and similarly a factor decrease in image noise of approximately 1.7 and 1.5. This results in a measured gain in contrast‐to‐noise ratio of a factor of approximately 2.3 for both CBCT and planar images. Depending upon the anatomical site, dose was reduced to 10%–70% of the full field value along the central axis plane and down to 2% along the axial planes, while maintaining the same dose to the VOI compared to full‐field techniques. Conclusion: The presented VOI technique offers improved image quality for image‐guided radiotherapy while sparing the surrounding volume of unnecessary dose compared to full‐field techniques.