An investigation of kV CBCT image quality and dose reduction for volume-of-interest imaging using dynamic collimation.
OBJECTIVE
The focus of this work was to investigate the improvements in image quality and dose reduction for volume-of-interest (VOI) kilovoltage-cone beam CT (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 1D 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 On-Board Imager system. CBCT and planar image quality were 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 sizes VOI 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 to 2.4 cm (at isocenter). Depending upon VOI location and size, dose was reduced to 16%-90% of the full-field value along the central axis plane and down to 4% along the axis of rotation, while maintaining the same dose to the VOI compared to full-field techniques. When maintaining constant dose to the VOI, this change in iris diameter corresponds to a factor increase of approximately 1.6 in image contrast and a factor decrease in image noise of approximately 1.2. This results in a measured gain in contrast-to-noise ratio by a factor of approximately 2.0.


CONCLUSIONS
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.
Department of Physics and Atmospheric Science, Dalhousie University, 5820 University Avenue, Halifax, Nova Scotia B3H 1V7, Canada.
David
D
Parsons
Department of Radiation Oncology and Department of Physics and Atmospheric Science, Dalhousie University, 5820 University Avenue, Halifax, Nova Scotia B3H 1V7, Canada.
James L
JL
Robar
D000818:Animals; D054893:Cone-Beam Computed Tomography / Q000295:instrumentation / Q000379:methods*; D006801:Humans; D009010:Monte Carlo Method; D010315:Particle Accelerators; D011786:Quality Control; D011829:Radiation Dosage; D011856:Radiographic Image Enhancement; D012371:Robotics; D012984:Software; D013552:Swine