C-arm fluoroscopy is modelled as a perspective projection, the parameters of which are estimated through a calibration procedure. It has been universally accepted that precise intra-procedural calibration is a prerequisite for accurate quantitative C-arm fluoroscopy guidance. Calibration, however, significantly adds to system complexity, which is a major impediment to clinical practice. We challenge the status quo by questioning the assumption that precise intra-procedural calibration is really necessary. We derived theoretical bounds for the sensitivity of 3D measurements to mis-calibration. Experimental results corroborated the theory in that mis-calibration in the focal spot by as much as 50
still allows for tracking with an accuracy of 0.5
in translation and 0.65
in rotation, and such mis-calibration does not impose any additional error on the reconstruction of small objects.