AbstractID: 7116 Title: Cone-Beam CT with a Flat-Panel Imager: Dosimetric Considerations
Kilovoltage (kV) cone-beam computed tomography (CT) based upon flat-panel imager (FPI) technology
offers fully volumetric imaging of patient anatomy in an adaptable form. This imaging technology can be
readily adapted to conventional medical treatment devices such as an isocentric radiation therapy unit or an
isocentric C-arm for volumetric image-guided therapy. Investigations of cone-beam CT imaging
performance demonstrate soft-tissue contrast, high spatial resolution, and large field of view. Accurate
estimates of imaging dose are required if contrast-to-noise ratio (CNR) performance is to be quantitatively
evaluated and compared to that achieved with conventional CT systems. Measurements of dose in conebeam CT were performed on a well-controlled bench-top system that simulates the imaging geometry
employed on a medical linear accelerator equipped with kV cone-beam CT. Dose to air was measured in
cylindrical acrylic head and body phantoms (R=8 and 16 cm) at three radial positions (0, R/2, (R-1) cm)
using a calibrated Farmer-type chamber (ADCL, Capintec PR06C) for a range of x-ray beam qualities
(100-120 kVp) and cone-angles (2.8°-15.6°). For a cone-angle of 15.6°, the measured dose rate in the head
phantom was uniform (± 5%) at 0.01 cGy/mAs and for the body phantom varied from 0.005 cGy/mAs at
the center of the phantom to 0.009 cGy/mAs near the periphery. A typical cone-beam CT technique (300
projections at 120 kVp, 1 mAs/projection) results in an imaging dose of 1.5 - 3 cGy. Accurate estimates of
cone-beam CT imaging dose permit quantitative comparison of CNR performance with that reported for
conventional CT systems.