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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.
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