This study compared patient dose and image quality of a mobile O-arm™ cone beam imaging system in the 3D scan acquisition mode to those of a 64 slice Computed Tomography (CT) imaging system. The investigation included patient dose, scattered radiation, and image quality measurements. The patient dose was measured using a 0.6cc Farmer ion chamber and 30cm long Computed Tomography (CT) head and body polymethylmethacrylate (PMMA) phantoms. The results show that under identical radiographic techniques (kVp, mAs, etc.) and with the same scan length, the O-arm™ in 3D scan acquisition mode delivers approximately half the radiation dose of a 64 slice CT scanner. Scattered radiation was measured at several locations around the O-arm™, at 1 m, 2 m and 3 m distances in 3D CT scan acquisition mode with a RadCal 10 × 5-180 pancake ion chamber using a 30 cm long CT body phantom as the source of scatter. Similar measurements were made in a 64 slice CT scanner. The data demonstrate that scattered radiation from the O-arm™ to personnel involved in a clinical procedure is comparable to that from a 64 slice CT scanner. Image quality was compared by exposing a CATPHAN phantom to comparable doses in both the O-arm™ and the CT scanner. The resultant images were then evaluated for modulation transfer function (MTF), high-contrast spatial resolution, and low contrast sensitivity for clinical application purpose. The O-arm™ shows comparable high contrast to the CT (7 lp/cm vs. 8 lp/cm). The low contrast in the O-arm™ is not visible due to fixed pattern noise. For image guided surgery applications where the location of a structure is emphasized over a survey of all image details, the O-arm™ has some advantages due to wide radiation beam coverage and lower patient dose. The image quality of the O-arm™ needs significant improvement for other clinical applications where high image quality is desired.