X-ray scattering in single- and dual-source CT

For medical imaging applications, such as cardiac imaging, dual-source computed tomography (CT) improves the temporal resolution by the simultaneous use of two cone beams, which acquire twice as many projections as single-source CT does within the same time interval. Besides this advantage, a drawback of such a system is additional x-ray scatter originating from the extra (cross-illuminating) cone beam. In this work, a comparison with single-source CT images is performed under same-dose conditions for two different thorax phantoms, and for different cone beam angles corresponding to a coverage of 20, 40, 80, and 160 mm on the rotation axis (z coverage). As a general result, the HU-magnitude of scatter-induced streak and cupping artifacts scale almost proportional to the illuminated volume. In dual-source CT, cross scatter induces a further factor of almost 2 in the scaling of artifacts in comparison to single-source CT. For all examined systems, the scatter-induced noise reduces the contrast-to-noise ratio (CNR). In the case of an ideal scatter correction, the CNR is reduced even more, but contrast and CNR can be restored by an additional x-ray dose. With a 32-slice single-source CT (z overage of 20 mm) taken as a reference, a corresponding dual-source CT requires 7% more dose to maintain the same CNR. A CT system with a z coverage of 40, 80, and 160 mm requires 8%, 23%, and 54% more dose in a single-source configuration, respectively, and 20%, 47%, and 102% more dose in a dual-source configuration, respectively. In conclusion, a dual-source CT is comparable to a single-source CT with twice the z coverage concerning image degradation by scatter.