Misalignment Compensation for Ultra-High-Resolution and Fast CBCT Acquisitions

The acquisition time of cone-beam CT (CBCT) systems is limited by different technical constraints. One important factor is the mechanical stability of the system components, especially when using C-arm or robotic systems. This leads to the fact that today's acquisition protocols are performed at a system speed, where geometrical reproducibility can be guaranteed. However, from an application point of view faster acquisition times are useful since the time for breath-holding or being restraint in a static position has direct impact on patient comfort and image quality. Moreover, for certain applications, like imaging of extremities, a higher resolution might offer additional diagnostic value. In this work, we show that it is possible to intentionally exceed the conventional acquisition limits by accepting geometrical inaccuracies. To compensate deviations from the assumed scanning trajectory, a marker-free auto-focus method based on the gray-level histogram entropy was developed and evaluated. First experiments on a modified twin-robotic X-ray system (Multitom Rax, Siemens Healthcare GmbH, Erlangen, Germany) show that the acquisition time could be reduced from 14 s down to 9 s, while maintaining the same high-level image quality. In addition to that, by using optimized acquisition protocols, ultra-high-resolution imaging techniques become accessible.