Effects of scanning orbit wobbling and detector tilting on cone-beam tomography

Cone-beam computed tomography (CBCT) provides a real volume imaging modality, which can reconstruct a digital volume with an isotropic grid resolution. However, in practical CT applications, because of the errors associated with machine manufacturing and system integration, the circular scanning orbit is not a perfect circle, and the detector plane may not be perpendicular to the isoray of the cone-beam projection. As a result, the cone-beam projection images acquired under the wobbled scanning orbit and tilted detector will deteriorate the cone-beam volume reconstruction. Specifically, the non-circular scanning orbit will cause severe spatial blurring and geometrical distortion, especially for the regions away from the midplane; the detector tilting will cause the asymmetrical distortion and spatial blurring. These spatial blurring and shape distortion effects can be characterized through the use of three-dimensional (3D) point spread function (PSF) measurement. In general, the orbit wobbling boosts the spatial variance of the cone-beam volume imaging in terms of blurring. The detector tilting causes asymmetry of 3D PSF because of the fact that: one detector side locates away from the source further than the other side due to the obliqueness. By computer simulations with bead phantom scanning and reconstruction, we demonstrate the effects of scanning-orbit wobbling and detector tilting on cone-beam computed tomography, under a variety of parameter settings. The findings in the simulation study are useful for constructing a cone-beam tomographic system.