Soft Tissue Visualization Using a Highly Efficient Megavoltage Cone ...

The corrected (Offset, Gain, Dead pixel) and pre-processed (resolution/dose adaptive filtering) projection
images were normalized using the pixel data in the open area of the image.
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Volumetric reconstruction is accomplished with using an extension of Feldkamp filtered back projection in
cone beam geometry 5 . The reconstruction time for generating 256 3 CT data was ??
2.6 Phantoms for Image Quality Analysis
The followings are the list of phantoms that were used and all were positioned at isocenter. The source to
imager distance (SID) was 140cm for all phantom images.
• A QC-3V phantom 3 was used to determine the spatial resolution and contrast to noise ratio (CNR)
of 2D projection images.
• A ball phantom with a diameter of 16cm consisting of water-equivalent plastic was used for
contrast/spatial resolution experiments. The 1cm thick slice in the middle of the ball contains
several tissue equivalent inserts that are 3 cm in diameter (Figure-2). There are also some inserts
with smaller diameters (1, 0.5, and 0.25 cm). There are air filled holes with diameters of 1, 1.5, 2,
and 3mm within the middle slice of phantom for spatial resolution analysis.
• A Quality Assurance System for Advanced Radiotherapy (QUASAR) body phantom 4 that
contains Electron Density (ED) rods of lung inhale, polyethylene, water equivalent, trabecular
bone, and dense bone with EDs of 0.190, 0.945, 1.002, 1.117, and 1.512; respectively. The ED of
acrylic is 1.137.
• An anthropomorphic head phantom with implanted gold seeds (1mm in diameter and 3mm long)