In order to estimate the reliability and accuracy of the image registration algorithms, a series of pelvic phantom images with known shifts, rotations and magnification changes was taken. The results detected by the algorithms were then compared with the known values. Both the intra-user and inter-user variation in image registration were studied. As the images were obtained by digitizing films, errors in alignment between the images existed due to movement of the film in the cassette, and on the scanner. Therefore before alignment took place these errors were corrected by digitally transforming the images such that the bottom field edge was horizontal and the field centre was in the same position in all images.
Table 2; shows the results of the intra-user variation for the alignment of image pairs using six pairs of fiducial points on pelvic phantom images. Each image pair was aligned on five separate occasions, and the shifts have been converted into millimetres using pixel size and image magnification. The shifts produced by the affine transformation are in the xy plane as opposed to shifts along and perpendicular to the line of rotation that the simulator/treatment machines use. The figures given here have been converted into machine geometry. Table 3 shows the average inter-user variation for the alignment of image pairs, again using six pairs of fiducial points.
| Image pair | Rotation (°) | Magnification (%) | Long (mm) | Lateral (mm) |
| 1-2 | 2.0 | 1.0000 | 10 | 0 |
| 1-3 | -1.1 | 1.0000 | 10 | 5 |
| 1-4 | -0.6 | 1.0714 | -8 | 5 |
| 1-5 | -1.6 | 1.0357 | -8 | -10 |
| 1-6 | -0.9 | 1.0357 | -4 | -7 |
| 2-3 | -3.1 | 1.0000 | 0 | 5 |
| 2-4 | -2.6 | 1.0714 | -18 | 5 |
| 2-5 | -3.6 | 1.0357 | -18 | -10 |
| 2-6 | -2.9 | 1.0357 | -14 | -7 |
| 3-4 | 0.5 | 1.0714 | -18 | 0 |
| 3-5 | -0.5 | 1.0357 | -18 | -15 |
| 3-6 | 0.2 | 1.0357 | -14 | -12 |
| 4-5 | -1.0 | 0.9615 | 0 | -15 |
| 4-6 | -0.4 | 0.9615 | 4 | -12 |
| 5-6 | 0.7 | 1.0000 | 4 | 3 |
Table 1: Differences between images as set on machine.
| Image pair | Rotation (°) | Magnification (%) | Long (mm) | Lateral (mm) |
| 1-2 | -0.14 ± 0.12 | 0.09 ± 0.40 | 1.2 ± 0.14 | 0.6 ± 0.31 |
| 1-3 | -0.01 ± 0.13 | -0.14 ± 0.18 | 0.9 ± 0.18 | -0.4 ± 0.19 |
| 1-4 | 0.28 ± 0.10 | 0.34 ± 0.20 | 0.1 ± 0.19 | 0.6 ± 0.13 |
| 1-5 | 0.09 ± 0.09 | 0.30 ± 0.14 | -0.6 ± 0.16 | 1.1 ± 0.18 |
| 1-6 | 0.01 ± 0.10 | -0.11 ± 0.16 | 0.3 ± 0.23 | 0.2 ± 0.15 |
| 2-3 | 0.05 ± 0.09 | -0.01 ± 0.18 | 0.5 ± 0.13 | 0.3 ± 0.14 |
| 2-4 | 0.32 ± 0.14 | 0.63 ± 0.33 | -0.9 ± 0.28 | 1.4 ± 0.13 |
| 2-5 | 0.20 ± 0.20 | 0.28 ± 0.09 | -1.1 ± 0.12 | 2.0 ± 0.11 |
| 2-6 | 0.11 ± 0.09 | 0.13 ± 0.19 | -0.3 ± 0.14 | 0.8 ± 0.12 |
| 3-4 | 0.25 ± 0.06 | 0.28 ± 0.10 | -0.5 ± 0.25 | 0.7 ± 0.28 |
| 3-5 | -0.03 ± 0.09 | 0.00 ± 0.28 | -1.4 ± 0.23 | 1.5 ± 0.18 |
| 3-6 | 0.09 ± 0.17 | -0.05 ± 0.25 | -0.4 ± 0.22 | 0.3 ± 0.11 |
| 4-5 | -0.06 ± 0.11 | 0.27 ± 0.18 | -0.0 ± 0.22 | 0.8 ± 0.17 |
| 4-6 | -0.11 ± 0.22 | 0.05 ± 0.25 | 0.8 ± 0.32 | -0.1 ± 0.15 |
| 5-6 | -0.02 ± 0.22 | -0.09 ± 0.25 | 0.9 ± 0.21 | -0.7 ± 0.30 |
| Average | 0.07 ± 0.19 | 0.13 ± 0.31 | -0.0± 0.80 | 0.6 ± 0.71 |
Table 2: Average errors and standard deviations for intra-user registration
| Image pair | Rotation (°) | Magnification (%) | Long (mm) | Lateral (mm) |
| 1-2 | -0.10 ± 0.06 | -0.06 ± 0.42 | 1.1 ± 0.22 | 0.6 ± 0.45 |
| 1-3 | 0.06 ± 0.08 | -0.33 ± 0.18 | 0.9 ± 0.33 | -0.4 ± 0.12 |
| 1-4 | 0.28 ± 0.18 | 0.07 ± 0.40 | 0.1 ± 0.25 | 0.7 ± 0.28 |
| 1-5 | 0.17 ± 0.07 | 0.12 ± 0.20 | -0.6± 0.22 | 0.8 ± 0.32 |
| 1-6 | 0.12 ± 0.08 | -0.23 ± 0.11 | 0.4 ± 0.38 | -0.9± 1.82 |
| 2-3 | 0.33 ± 0.54 | 0.11 ± 0.41 | 0.2 ± 0.78 | -0.0 ± 0.41 |
| 2-4 | 0.26 ± 0.24 | 0.19 ± 0.25 | -0.9 ± 0.21 | 1.1 ± 0.40 |
| 2-5 | 0.39 ± 0.09 | 0.21 ± 0.24 | -1.0 ± 0.40 | 1.7 ± 0.49 |
| 2-6 | 0.02 ± 0.10 | -0.05 ± 0.46 | -0.5 ± 0.17 | 0.8 ± 0.14 |
| 3-4 | 0.11 ± 0.07 | 0.35 ± 0.32 | -0.7 ± 0.27 | 0.9 ± 0.21 |
| 3-5 | 0.01 ± 0.12 | -0.07 ± 0.26 | -1.7 ± 0.22 | 1.3 ± 0.48 |
| 3-6 | 0.16 ± 0.17 | -0.23 ± 0.21 | -0.3 ± 0.39 | 0.4 ± 0.06 |
| 4-5 | 0.02 ± 0.22 | 0.42 ± 0.43 | -0.0 ± 0.43 | 0.3 ± 0.35 |
| 4-6 | -0.01 ± 0.23 | 0.44 ± 0.12 | 0.5 ± 0.11 | -0.1 ± 0.26 |
| 5-6 | 0.06 ± 0.17 | -0.35 ± 0.11 | 0.9 ± 0.18 | -0.6 ± 0.21 |
| Average | 0.13 ± 0.24 | 0.04 ± 0.39 | -0.1 ± 0.86 | 0.4 ± 0.91 |
Table 3: Average errors and standard deviations for inter-user registration