Analysis of vertical and horizontal circular C-arm trajectories

Abstract

C-arm angiography systems offer great flexibility in the acquisition of trajectories for computed tomography. Theoretically, these systems are able to scan patients while standing in an upright position. This would allow novel insights into structural changes of the human anatomy while weight bearing. However, this would require a scan on a horizontal trajectory parallel to the ground floor which is currently not supported by standard C-arm CT acquisition protocols.In this paper, we compared the standard vertical and the new horizontal scanning trajectories by analysis of the source positions and source to detector distances during the scan. We employed a C-arm calibration phantom to compute the exact scan geometry. Based on the analysis of the projection matrices, we computed the source position in 3D and the source to detector distance for each projection. We then used the calibrated scan geometries to reconstruct the calibration phantom. Based on this reconstruction in comparison to the ideal phantom geometry we also evaluated the geometric reconstruction error.As expected, both the vertical and the horizontal scan trajectories exhibit a significant C-arm "wobble". But in both kinds of trajectories, the reproducibility over several scans was comparable. We were able to reconstruct the calibration phantom with satisfactory geometric reconstruction accuracy. With a reconstruction error of 0.2 mm, we conclude that horizontal C-arm scans are possible and show properties similar to those of vertical C-arm scans.The remaining challenge is compensation for the involuntary movement of the standing subject during a weight-bearing acquisition. We investigated this using an optical tracking system and found that the average movement at the knee while standing upright for 5 seconds is between 0.42 mm and 0.54 mm, and goes up to as much as 12 mm when the subject is holding a 60º squat. This involuntary motion is much larger than the reconstruction accuracy. Hence, we expect artifacts in reconstructions to be significant for upright positions, and overwhelming in squat positions if no motion correction is applied.

Fiducial marker‐based correction for involuntary motion in weight‐bearing C‐arm CT scanning of knees. Part I. Numerical model‐based optimization

“…In evaluation of human subjects, the feasibility for CT reconstruction was demonstrated using a rotational angiogram in standing position, i.e., weight-bearing position. 1 Novel dedicated conebeam CT (CBCT) systems for orthopedics have also been developed to acquire weight-bearing imaging of the lower extremities. 2,3 CBCT systems with a flat panel provide superior spatial resolution with wide volumetric beam coverage along the axial direction compared to conventional CT, which allows high-resolution 3D images of entire knee in a single sweep without table translating per rotation.…”

Fiducial marker-based correction for involuntary motion in weight-bearing C-arm CT scanning of knees. II. Experiment

“…34 However, both trajectories showed comparable reproducibility over multiple scans acquired within one day and across one week. 34 As long as the trajectory is reproducible, we can geometrically calibrate the system using the projection matrix approach.…”

“…34 However, both trajectories showed comparable reproducibility over multiple scans acquired within one day and across one week. 34 As long as the trajectory is reproducible, we can geometrically calibrate the system using the projection matrix approach. The amount of wobble of the C-arm gantry should not impact the performance of the proposed subject-motion correction methods since the projection matrices are properly incorporated into the governing equations, Eqs.…”

Dynamic detector offsets for field of view extension in C‐arm computed tomography with application to weight‐bearing imaging

“…Here, the focus is on imaging the body in an upright position, as this is also the position in which the surgery is performed. 9 Maier et al 10 showed that C-arm CT devices allow horizontal trajectories. This renders medical examination of the weight-bearing knees possible with already available devices (Fig.…”

“…1). 10,15 Currently, these trajectories and their reconstruction methods are designed for circular FOVs, but many anatomical structures may be better described by a noncircular boundary, e.g., an ellipse.…”