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This project assesses the four dimensional computed tomography (4DCT) capabilities of the Philips Brilliance Big Bore CT scanner (Philips Medical Systems, Cleveland, OH). A mechanical phantom imparts clinically relevant motions to acrylic spheres of various diameters. The size, shape, and position of these spheres, as measured with 4DCT, are compared to their true size, shape, and position. An evaluation of image quality is also performed. Maximum discrepancies between physical and imaged volumes, for all sphere sizes and motion ranges, did not exceed 2.6 mm (mean = 1.2 mm, standard deviation = 0.4 mm). For approximately tissue equivalent density objects, mean CT# in 4DCT images differed from those in standard clinical thoracic images by only a few Hounsfield units. Measured geometric precision along with the accuracy of mean CT#s observed in 4DCT phase images indicate that 4DCT is an appropriate imaging technique for treatment planning. / Medical Physics