Comparison of Variable and Fixed Focal Length Cone Beam CT in Diagnostic Imaging

摘要

We use a task-based study to objectively evaluate the effect of variable versus fixed focal length in determining the position of a lesion in helical cone-beam computed tomography (HCBCT). This method will be used to assess whether variable focal length CBCT scans provide a measurable improvement in estimating lesion position relative to fixed focal length CBCT in diagnostic applications. In this simulation study a 1 cm diameter spherical lesion is placed at four different positions within a three-dimensional Shepp-Logan head phantom. The axial plane is taken to point along the z-axis, which is also the central axis of the helix. The lesion is placed at the center of the Shepp-Logan phantom, at positions displaced ±5 cm in x, and at a position displaced 5 cm in y. Four different scans of pitch length 10 cm are then performed using 128 views over 360° with a 100×300 pixel (20 cm×60 cm) detector. Two scans have a fixed focal length of 50 cm between the X-ray source and the center of rotation (COR), varying only in the starting angle of the source (0° and 90°). We call this the circular configuration. The other two scans have a variable focal length following the curvature of the head phantom and ranging from 37.5 cm to 50 cm. We call this the elliptical configuration. The detector rotates with the source but maintains a constant distance of 30 cm from the COR. A likelihood gridding technique is used to assess bias and variance in the position estimates determined from each scan configuration. We find that the biases are small relative to the variances, and have no apparent preferred direction. Of the 24 circular to elliptical comparisons made, we find that in 14 cases the elliptical scan has a smaller variance that is statistically significant(p ≤ 0.05). By contrast, we find no statistically significant cases in which the circular scan gives a smaller variance compared to the elliptical scan. We conclude that using a variable focal length adapted to the contours of the head phantom provides more precise results, but caution that; this is a limited pilot study and many more factors will be accounted for in future work.