|
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 60° 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.
|
