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Table 2 Change in syndesmophyte volume (CT) and mSASSS (radiography)
CT
Radiography
1,095 (
±
1,278) mm
3
Mean (
±
std) at baseline
4.2 (
±
5.6)
Baseline to year 1
Number of patients with change > 0
24 (73 %)
4 (12 %)
87.0 (
±
186) mm
3
Mean (
±
std) change
0.24 (
±
0.97)
Baseline to year 2
Number of patients with change > 0
26 (79 %)
4 (12 %)
366) mm
3
Mean (
±
std) change
201 (
±
0.30 (
±
1.4)
only 4 (12 %) had a mSASSS increase at year 1 and 2 (Table
2
). From baseline to
year 1, 18 patients (55 %) had an increase larger than 3 %, the 95 % limit of
agreement derived from Bland-Altman analysis in the reliability study. From
baseline to year 2, 23 patients (70 %) had an increase larger than 3 %. Additionally,
two patients in whom the algorithm detected no syndesmophytes in all 4 IDSs at
baseline developed new syndesmophytes at year 1, and three patients did so at year
2. For these patients, the rate of change cannot be computed because their baseline
was 0.
Figure
17
shows the cumulative probability plots for computed volume changes
and mSASSS changes. The curves for computed volumes show the progressivity of
the disease. The curves for year 1 and 2 are clearly distinguishable and syndes-
mophyte volume changes are larger for year 2 than for year 1. By contrast, for
mSASSS the two curves are nearly identical and both mostly located at zero.
5 Discussion and Future Challenges
The algorithm is still new and has so far been validated on a relatively small
numbers of patients. More extensive work is needed to establish the method. The
method still requires an operator to place a seed to initiate the segmentation.
Automation of this task should be explored. The algorithm also requires high
resolution especially in the z direction (slice thickness of 1.5 mm and spacing
between slices of 0.7 mm). Additional work is needed to adapt the method to more
common lower resolution scans. It is probable that lower resolution will entail
lower precision in volume measurements.
Registration makes the choice of the ridgeline (baseline, year 1 or year 2)
unimportant. In our work, we chose the baseline ridgeline as the reference. Aver-
aging ridgelines may be advantageous, since an average is generally more robust to
errors. Although registration will ensure that the same errors are made for the scans
to be compared and will therefore not impact the computed syndesmophyte volume
differences, it is always bene
c to start with the most accurate ridgeline. Many
methods can be proposed to de
ne the average of 2 or more curves. In our case
