2.3.1 Correction Factor Values
The mean and standard deviation of the correction factors for each axis of each scanner is summarized in Table 2.2. In addition, an average volumetric correction factor is provided to characterize the correction in voxel volume for each scanner arising from the linear axis correction factors. A correction factor > 1 indicates that distances in the uncorrected micro-CT images overestimated the true dimensions. There was no clear pattern to the magnitudes or directions (over or under estimation) of the errors along the
Table 2.2- Calculated average scanner correction factors for each axis. Scanner X-Axis Correction
Factor Y-Axis Correction Factor Z-Axis Correction Factor Volumetric Correction Factor eXplore Locus Ultra Isocenter Scanner One 0.9998 ± 0.00006 1.0009 ± 0.00012 1.0002 ± 0.00021 1.0008 ± 0.00034 eXplore Locus Ultra Offset Scanner One 1.0008 ± 0.00063 0.9968 ± 0.00018 0.9987 ± 0.00040 0.9962 ± 0.00063 eXplore Locus Ultra Isocenter Scanner Two 1.0033 ± 0.00013 0.9981 ± 0.00012 0.9978 ± 0.00014 1.0020 ± 0.00012 eXplore Locus Ultra Offset Scanner Two 1.0022 ± 0.00010 0.9999 ± 0.00009 0.9998 ± 0.00008 0.9993 ± 0.00012 eXplore SpecZT 1.0011 ± 0.00007 1.0008 ± 0.00002 0.9990 ± 0.00004 1.0009 ± 0.00043 eXplore CT 120 1.0027 ± 0.00006 1.0031 ± 0.00103 1.0005 ± 0.00014 1.0064 ± 0.00133 eXplore RS 0.9963 ± 0.00021 0.9965 ± 0.00069 0.9957 ± 0.00025 0.9886 ± 0.00092
three axes. A statistically significant difference was found between the correction factors in the X (p = 0.043 and p < 0.001), Y (p < 0.001 for both scanners) and Z (p < 0.001 for both scanners) axes at the two bore positions in both eXplore Locus Ultra scanners. These results suggest that the calibration phantom should ideally be placed as close as possible in the bore to the anticipated location of targets.
The results of Tukey tests comparing the correction factors for each scanner are summarized in Table 2.3. For three out of four scanner models, no significant difference was found between the X and Y correction factors for the scanners, indicating a single average correction factor can be used for the in-plane direction. The X and Y correction factors were significantly different for the two eXplore Locus Ultra scanners. However, for these two scanners the absolute difference was still < 1%. In 11 out of 14 cases, the X and Y correction factors were generally significantly different from the Z correction
Table 2.3-Results of Tukey test ( p <0.05) for differences in the mean correction factors for each pair of scanner axes. Scanner
X and Y X and Z Y and Z
eXplore Locus Ultra Isocenter
Scanner One Yes No Yes
eXplore Locus Ultra Offset Scanner
One Yes Yes No
eXplore Locus Ultra Isocentre
Scanner Two Yes Yes Yes
eXplore Locus Ultra Offset Scanner
Two Yes Yes Yes
eXplore SpecZT No Yes Yes
eXplore CT 120 No Yes Yes
factors, which suggest the out-of-plane correction factor is unique from the in-plane correction factors.
2.3.2 Geometric Correction to Calibration Phantom
The corrected and uncorrected bead separations within the calibration phantom are summarized in Table 2.4. Application of the correction factors significantly reduced the error in bead separations for in five out of seven image sets. In six out of the seven image sets, the corrected and uncorrected errors were found to be significantly different.
The mean difference in corrected and uncorrected errors ranged from ≤ 1 µm to 83 µm in
absolute terms or from 0.005% to 0.338% in relative terms.
Table 2.4-Uncorrected and corrected bead separation errors for the calibration phantom. Each error is described with a mean and standard deviation in both an absolute value of µm and as a percent of total bead separation. The p-value of the t-tests comparing the corrected and uncorrected errors of each scanner are provided.
Scanner Uncorrected Error (µm) Corrected Error (µm) Uncorrected Error (%) Corrected Error (%) p-Value eXplore Locus Ultra
Isocenter Scanner One 22 ± 2 22 ± 3 0.096 ± 0.013 0.101 ± 0.016 p = 0.03
eXplore Locus Ultra
Offset Scanner One 38 ± 3 31 ± 4 0.172 ± 0.014 0.148 ± 0.014 p = 0.002
eXplore Locus Ultra
Isocentre Scanner Two 19 ± 1 7 ± 1 0.078± 0.006 0.031± 0.007 p < 0.001
eXplore Locus Ultra
Offset Scanner Two 36 ± 1 35 ± 1 0.146± 0.003 0.150± 0.003 p = 0.16
eXplore SpecZT 18 ± 4 11 ± 3 0.080 ± 0.016 0.054 ± 0.014 p < 0.001
Explore CT 120 50 ± 7 7 ± 1 0.200 ± 0.027 0.007 ± 0.001 p = 0.001
2.3.3 Geometric Correction to Validation Phantom
The average error in bead separation within the validation phantom for all sets of images was calculated with and without application of the correction factors calculated from the calibration phantom. The corrected and uncorrected bead separations and the p- values for the two-tailed t-tests between the corrected and uncorrected errors are summarized in Table 2.5. In three scanners with minimal ( < 50 µm) geometric accuracy errors, application of the correction factors slightly increased the geometric errors of the images. However, this increase was not statistically significantly except for the offset position of the second eXplore Locus Ultra scanner. For the two scanners with > 60 µm uncorrected error, application of the correction factors significantly improved the geometric accuracy of the images.
Table 2.5-Uncorrected and corrected bead separation errors for the validation phantom. Each error is described with a mean and standard deviation in both an absolute value of µm and as a percent of total bead separation. The p-value of the t-tests comparing the corrected and uncorrected errors of each scanner are provided.
Scanner Uncorrected Error (µm) Corrected Error (µm) Uncorrected Error (%) Corrected Error (%) p-Value eXplore Locus Ultra
Isocenter Scanner One 37 ± 27 38 ± 26 0.100 ± 0.080 0.101 ± 0.074 p=0.46
eXplore Locus Ultra
Offset Scanner One 44 ± 36 54 ± 41 0.132 ±0.138 0.163 ±0.158 p=0.26
eXplore Locus Ultra
Isocentre Scanner Two 27 ± 15 29 ± 15 0.077 ± 0.068 0.084 ± 0.056 p=0.69
eXplore Locus Ultra
Offset Scanner Two 44 ± 22 80 ± 54 0.132 ± 0.117 0.215 ± 0.145 p= 0.01
eXplore SpecZT 15 ± 10 19 ± 13 0.047 ±0.041 0.049 ±0.033 p=0.35
eXplore CT 120 67 ± 27 22 ± 16 0.168 ± 0.037 0.052 ± 0.025 p<0.001
2.3.4 Comparison of Validation Phantom Errors
The eXplore CT 120, eXplore speCZT and eXplore RS all yielded images with a voxel size of approximately 50 µm. The uncorrected errors of all three scanners were significantly different, but, the corrected errors of the three scanners were not significantly different. This result indicates scanners will possess similar accuracies when corrected by the phantom. The uncorrected errors for each of the two eXplore Locus Ultra scanners were not significantly different from each other at both the
isocenter (p = 0.29) and offset (p = 0.94) positions. Similarly, the corrected isocenter (p
= 0.23) and offset (p = 0.21) positions of each of the two eXplore Locus Ultra scanners
were also not significantly different from each other.