A FEASIBILITY STUDY FOR MEGAVOLTAGE CONE-BEAM CT USING A COMMERCIAL EPID

This study used a standard commercial electronic portal imaging device (EPID) area detector attached to an isocentric linear accelerator and the Feldkamp algorithm to produce cone beam tomographic reconstructio ns. The EPID has a active area of 32.5 x 32.5 cm(2), and can record 12 -bit images using two monitor units (MU), with a resolution of 2.1 x 2 .0 mm(2) FWHM. Since the EPID was not large enough to record the full patient projection at about 1.5 geometric magnification, it was necess ary to offset the detector to collect half-cone projections. Correctio ns are required to convert pixel values into units of exit dose and to realign the projections to overcome the +/-4 mm support arm sag. With a geometric magnification of 1.5 the sensitive volume is a cylinder o f radius 21 cm and length 17 cm. Unfortunately, the patient couch cont ains metal bed support rails that lie just outside this cylinder, and produce streak artefacts in the reconstruction. Using 90 views the sys tem delivers a central dose of 90 cGy, and has a density resolution of 4%.