AbstractID: 7682 Title: Low-dose megavoltage cone-beam CT with an amorphous silicon electronic portal
imaging device
We examine the quality of reconstructions with a megavoltage cone-beam system using low doses of 10 to
70 cGy. Such low doses are required if full field megavoltage CT is to be used regularly for patient position
verification. We use a 6 MV linac beam and an amorphous silicon electronic portal imaging device (EPID)
to acquire projection images for reconstruction. Low doses are achieved using a linac-EPID feedback
system that permits automatic image acquisition using less than 0.5 MU per projection image. With this
system the linac dose rate output varies by approximately 6%. We correct for this variability using readings
from a diode secured under an open leaf of the multi-leaf collimator. We perform reconstructions of a
water-filled phantom with contrast cylinders of varying densities with respect to water, as well as an
anthropomorphic thorax phantom. We use 100 projection images and the full 22x29 cm EPID image size.
With 35 MU total (~28 cGy at 15 cm depth), all of the contrast cylinders with relative densities greater than
5% are distinguishable from the background water. Lowering the total ‘dose’ to 12 MU (~10 cGy) still
permits reconstruction of the 8% and 10% contrast cylinders. In a 12 MU scan of the thorax phantom,
bones and soft tissue are distinguishable, although the image quality is improved as the dose is increased.
These data suggest that low dose cone-beam CT is possible with this EPID and may be useful as a 3D
patient localization system.