Мoreover, the time required to image a volume element or voxel with a certain statistical confidence increases drastically as the size of the voxel decreases. An object of smaller cross-section will absorb fewer photons and therefore requires longer exposure time to assure acceptable counting statistics. Consequently, increasing spatial resolutions require larger incident photon intensity or longer integration times. Because tube X-ray sources emit only a small fraction of their dissipated power as X-rays, obtaining high spatial resolution with these types of sources is often obtained at the cost of counting statistics and the ability to distinguish more subtle low-contrast features in an object. Synchrotron-based radiation on the other hand is well suited for high-resolution imaging because of the extremely high photon flux available. However, because it is difficult to produce energies above approximately 50 keV with synchrotron radiation sources, maximum sample size is generally limited to a few centimeters to assure that the beam can penetrate the sample, whereas larger samples can be examined in conventional systems that generally use higher energies. As a common rule, one can expect a spatial resolution on the order of 200–500 μm for medical CT systems, between 50 and 100 μm for industrial systems (no dose restrictions) designed to examine small samples (Kinney and Nichols, 1992), and from 50 μm down to approximately 1 μm for synchrotron based CT systems.
