A Registration Algorithm for Dose Correction in Radiation Therapy Validation of Deformable Registration Using Phantoms V. Zambrano1,2,3, D. Fabri3, M. Stock2,4, W. Birkfellner3,4, D. Georg2,4 1EBG MedAustron GmbH, Wr. Neustadt, Austria 2Department of Radiotherapy, Medical University of Vienna, Austria 3Centre of Medical Physics and Biomedical Engineering, Medical University of Vienna,Austria 4Department of Radiation Oncology & Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology Medical University of Vienna, Austria Valentina Zambrano – 2 nd March, 2012 ICTR-PHE 2012 – Génève, Switzerland
Contents - The Algorithm - A First Phantom Study - A Second Phantom Study - Discussion and Conclusion
The Algorithm
(a) (b) An example of target (a) and source (b) images. The images (source and target) are divided into featurelets (mega- voxels) which are then rigidly registered as sub-images.
The Deformation field (pixels vector field) can be extracted, visualized or applied to other images. An example of deformation field.
A First Phantom Study
25 cm 25 cm 15 cm The inside of the first The first phantom. phantom. The first phantom scanning.
The 24 fiducial markers displacement can be evaluated for the two deformation configurations. The smaller the marker displacement, the better the two images (source and deformed) match. The fiducial markers on the 3D-Slicer.
Truth [mm] Rigid [mm] B-spline [mm] Featurelets [mm] t-Student t-Student First Second First Second First Second First Second 10vx - 20vx 10vx - 30vx p-value p-value 2.72 2.08 Mean 4.57 3.84 2.78 1.86 2.43 1.84 1.40 1.43 0.009 0.05 SD 1.25 1.41 1.58 1.12 1.46 1.13 1.15 0.91 max 7.83 6.90 7.15 5.00 6.16 5.08 5.62 4.30 min 2.50 1.45 1.04 0.71 0.47 0.21 0.00 0.48 The two-sided paired t-Student test gave significant results at 95% C.L. as well as the p- value did for the both phantom's configuration.
A Second Phantom Study
polystyrene = prostate bag = colon balloon = bladder The bottom inside of the The second phantom. second phantom. The second phantom's filling The second phantom scanning. system.
The overall VOI (Volume Overlapping Index, oVOI) percentage was calculated for the three structures in the four registration modalities. CT - CT CT - CBCT oVOI % oVOI % 100 100 90 90 80 80 70 70 Balloon Balloon 60 60 Polystyrene Polystyrene 50 50 Bag Bag 40 40 30 30 20 20 10 10 0 0 RR NC MI IPLAN RR NC MI IPLAN
Rigid Featurelets NC Featurelets MI iPlan t-Student t-Student CT-CT CT-CBCT CT-CT CT-CBCT CT-CT CT-CBCT CT-CT CT-CBCT p-value p-value 1.28 3.27 oVOI% 57.48 65.07 77.36 58.93 62.97 69.72 75.39 57.90 0.26 0.01 SD 3.97 6.95 3.04 5.96 5.74 4.88 1.30 6.23 A two-sided paired t-Student test at 95% C.L. and the p-values were calculated. The featurelets algorithm showed an improvement when compared to the iPlan software, although the result was statistically significant only for the inter-modality approach when the MI method was used.
Discussion and Conclusion
Take home message THE IDEA BEHIND IMAGE REGISTRATION IN CLINICAL ROUTINE Registering images is useful to evaluate the anatomical (hence dosimetric) changes which may occur in the patient during the course of the treatment and therefore decide when to re-plan... Note: do always perform a rigid registration first!
Conclusion The featurelets deformable registration algorithm provided promising results in both phantom studies showing comparable, if not better, results against commercially available algorithms for deformable registration.
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