Enabling tomography with low-cost C-arm systems

Abstract

In scenarios where the use of a Computed Tomography (CT) is difficult, such as during surgery or in the ICU, the use of a C-arm system to generate tomographic information could contribute with interesting additional clinical information. Recent days are seeing the development of the so-called cone-beam CT (CBCT) based on advanced motorized isocentric C-arm systems. To be able to make use of more basic C-arm systems, apart from the geometric non-idealities common to any CBCT, we need to address other difficulties. First, the trajectory of the source-detector pair may differ from a circular path and the system may suffer mechanical strains that modify the relative positions of the source and detector for different projection angles. Second, and more importantly, the exact position of the source and detector elements may not be repeatable for consecutive rotations due to low mechanical precision, thus preventing an accurate geometrical calibration of the system. Finally, the limitation of the angular span and the difficulty of obtaining a high number of projections pose a great challenge to the image reconstruction. In this work, we present a novel method to adapt a standard C-arm, originally designed for planar imaging, to be used as a tomograph. The key parts of the new acquisition protocol are (1) a geometrical calibration method to compensate mechanical inaccuracies that prevent an accurate repetition of source-detector position between acquisitions, and (2) an advanced image reconstruction method able to deal with limited angle data, sparse projections and non-circular trajectories. Both methods exploit surface information from the patient, which can be obtained using a 3D surface scanner. The proposed method was evaluated with two real C-arm systems, based on an image intensifier and a flat panel detector respectively, showing the feasibility of the proposal.

Fig 1. Workflow of the acquisition/reconstruction process.

Fig 2. Scheme depicting the coordinate system and the geometrical misalignments in the detector panel.

Fig 3. System geometric calibration tool showing the ellipses for one projection of the calibration phantom.

Fig 4. Phantom projection for the case with no detector panel inclinations (top-left), for roll angle φ = 60 degrees (bottom-left) and for pitch angle θ = 60 degrees (right).

Fig 5. Workflow of the adaptive refinement of geometrical calibration algorithm.

Yellow and blue lines show the contour of the projected mask and the projection data respectively.

Fig 6. Design of the calibration phantom (left) and real phantom (right).

Fig 7. Central axial slice of the CT volume corresponding to the hand (left) and the foot (right) phantoms with the ROI used for the calculation of LiVA overimposed in green.

Fig 8

Left: Electronic board consisting of a cupper layer on a plastic plate used to evaluate distortions in the image intensifier. Right: Phantom projection acquired with the C-arm; red lines follow the obtained pattern and yellow lines show the ideal directions and shapes of the phantom.

Fig 9. SIREMOBIL geometry (left), non-isocentric vertical rotation movement showing the dimensions of detector and FOV (center), and horizontal rotation movement (right).

Fig 10. Setting for calibration phantom (left) and clinical phantom (right) acquisitions, showing the angular position recording system.

Fig 11. Axial and coronal views of the reconstruction of the PBU-60 phantom.

A and B correspond to FDK before and after adaptive calibration respectively. C is the reconstruction with SCoLD and D is the CT volume acquired on the helical scanner.

Fig 12. Setting with the in-house C-arm for the clinical phantom acquisition.

Fig 13

Top: Horizontal offset (left) and skew (right) for two calibrations. Bottom: Coronal views of the reconstructed image using FDK before (A, C) and after correcting the calibration parameters (B, D).

Fig 14. Axial and coronal views of the CT volume acquired on the helical scanner and of the reconstruction of the limited data obtained on the C-arm with an angular span of 120 degrees and 25 and 49 projections with FDK and SCoLD.

Fig 15. Axial and coronal views of the CT volume acquired on the helical scanner (left) and of the reconstruction of the limited data with 25 projections obtained on the C-arm with FDK (center), and with SCoLD using the acquired mask (right).