The rotate-plus-shift C-arm trajectory. Part I. Complete data with less than 180° rotation

Abstract

Purpose: In the last decade, C-arm-based cone-beam CT became a widely used modality for intraoperative imaging. Typically a C-arm CT scan is performed using a circular or elliptical trajectory around a region of interest. Therefore, an angular range of at least 180• plus fan angle must be covered to ensure a completely sampled data set. However, mobile C-arms designed with a focus on classical 2D applications like fluoroscopy may be limited to a mechanical rotation range of less than 180• to improve handling and usability. The method proposed in this paper allows for the acquisition of a fully sampled data set with a system limited to a mechanical rotation range of at least 180• minus fan angle using a new trajectory design. This enables CT like 3D imaging with a wide range of C-arm devices which are mainly designed for 2D imaging. Methods: The proposed trajectory extends the mechanical rotation range of the C-arm system with two additional linear shifts. Due to the divergent character of the fan-beam geometry, these two shifts lead to an additional angular range of half of the fan angle. Combining one shift at the beginning of the scan followed by a rotation and a second shift, the resulting rotate-plus-shift trajectory enables the acquisition of a completely sampled data set using only 180• minus fan angle of rotation. The shifts can be performed using, e.g., the two orthogonal positioning axes of a fully motorized C-arm system. The trajectory was evaluated in phantom and cadaver examinations using two prototype C-arm systems. Results: The proposed trajectory leads to reconstructions without limited angle artifacts. Compared to the limited angle reconstructions of 180• minus fan angle, image quality increased dramatically. Details in the rotate-plus-shift reconstructions were clearly depicted, whereas they are dominated by artifacts in the limited angle scan. Conclusions: The method proposed here employs 3D imaging using C-arms with less than 180• rotation range adding full 3D functionality to a C-arm device retaining both handling comfort and the usability of 2D imaging. This method has a clear potential for clinical use especially to meet the increasing demand for an intraoperative 3D imaging. C 2016 American Association of Physicists in Medicine. [http://dx

Current state and future applications of radiological image guidance for particle therapy

“…It is necessary to retract the hardware while not in use to avoid collision and not to limit the usable proton beam angles. Unless the nozzle‐mounted X ray source and imager can rotate 360° around the patient, which requires the longitudinal axis of the patient couch to point to the horizontal nozzle, a creative acquisition method needs to be adopted for obtaining large FOV images, such as 3D digital tomosynthesis imaging or repeated limited‐angle acquisitions with detector offsets …”

The rotate-plus-shift C-arm trajectory. Part II. Exact reconstruction from less than 180° rotation

“…However, reconstructions still require an appropriate redundancy weight for a standard filtered backprojection or specific iterative reconstruction algorithms. [7][8][9] Assuming the same angular sampling as in a conventional short scan, the RPS scan and the short scan can be performed with the same number of projections and thus the same dose. As projections in the shift are collimated, dose in RPS scans might be lower than in short scans and thus might be beneficial in terms of dose usage.…”

Technical Note: Intrinsic raw data‐based CT misalignment correction without redundant data

“…Similar to a circular scan with a limited angular range, a variety of trajectories without or with only a very limited amount of redundant data are in use. A popular example is the rotate‐plus‐shift trajectory used to overcome the limited angular range of C‐arm systems and to prevent a degradation of image quality by the resulting limited angle artifacts . Other examples of trajectories without or only limited redundant data include but are not limited to shifted detector scans with minimal overlap or scans originating from laminography and tomosynthesis units.…”