A Comparative Evaluation of 3 Different Free-Form Deformable Image Registration and Contour Propagation Methods for Head and Neck MRI: The Case of Parotid Changes During Radiotherapy

Abstract

Purpose: To validate and compare the deformable image registration and parotid contour propagation process for head and neck magnetic resonance imaging in patients treated with radiotherapy using 3 different approaches-the commercial MIM, the open-source Elastix software, and an optimized version of it.

Materials and methods: Twelve patients with head and neck cancer previously treated with radiotherapy were considered. Deformable image registration and parotid contour propagation were evaluated by considering the magnetic resonance images acquired before and after the end of the treatment. Deformable image registration, based on free-form deformation method, and contour propagation available on MIM were compared to Elastix. Two different contour propagation approaches were implemented for Elastix software, a conventional one (DIR_Trx) and an optimized homemade version, based on mesh deformation (DIR_Mesh). The accuracy of these 3 approaches was estimated by comparing propagated to manual contours in terms of average symmetric distance, maximum symmetric distance, Dice similarity coefficient, sensitivity, and inclusiveness.

Results: A good agreement was generally found between the manual contours and the propagated ones, without differences among the 3 methods; in few critical cases with complex deformations, DIR_Mesh proved to be more accurate, having the lowest values of average symmetric distance and maximum symmetric distance and the highest value of Dice similarity coefficient, although nonsignificant. The average propagation errors with respect to the reference contours are lower than the voxel diagonal (2 mm), and Dice similarity coefficient is around 0.8 for all 3 methods.

Conclusion: The 3 free-form deformation approaches were not significantly different in terms of deformable image registration accuracy and can be safely adopted for the registration and parotid contour propagation during radiotherapy on magnetic resonance imaging. More optimized approaches (as DIR_Mesh) could be preferable for critical deformations.

Keywords: Elastix; MIM; accuracy evaluation; contour propagation; head and neck MRI; image registration.

Figure 1.

Schematic description of the DIR_Mesh contour propagation algorithm. From the delineated contour on MRI_1, a mesh was generated (A); the estimated vector field was applied to it (B) in order to deform the vertices of the mesh (C); the obtained deformed mesh (D) was finally cut on the correspondent slice on MRI_2.

Figure 2.

Examples of contour propagation using MIM (left), DIR_Trx (center), and DIR_Mesh (right) methods. Manual reference contours are also reported in red.

Figure 3.

Average symmetric distance (ASD), maximum symmetric distance (MSD), and Dice similarity coefficient (DSC) indices calculated for each patient and each parotid with the 3 different DIR methods (MIM, DIR_Trx, DIR_Mesh). LP indicates left parotid; RP, right parotid.

Figure 4.

Modified receiver operating characteristic (ROC) curve for the 3 methods.

Figure 5.

Example of a failed contour propagation, especially for MIM (green line). A, Magnetic resonance imaging (MRI) acquired before radiation therapy (RT) with manual parotid contours. B, Magnetic resonance imaging acquired after RT with manual parotid contours. C, Magnetic resonance imaging acquired after RT with manual and deformed contours.