DIR (Deformable image registration) is a registration technique which generates a displacement vector field (DVF) to move each pixel of a moving image to the corresponding pixel position of the target image, and deforms the moving image to match the target image. This article focuses the algorithm and fundamental matters of DIR by introducing intensity-based, contour-based algorithms, and hybrid algorithm. The intensity-based algorithm calculates the DVF by optimizing the index that measures the degree of coincidence of the intensity of two images. On the other hand, the contour-based algorithm calculates DVF by applying a physical model that moves the contour points on the moving image to the corresponding contour points of the target image. Recently, hybrid methods using both image intensity information and contour information can also be used. Factors influencing accuracy of DIR include the algorithm, parameters of the algorithm, the combination of image modality, and user's skill. In order to judge whether the result of DIR is clinically acceptable or not, it is necessary to evaluate the accuracy of DIR comprehensively by visual and quantitative evaluation for each case.

Current status of deformable image registration (DIR) and auto-segmentation for various medical imaging types (e.g., CT, MR, and CBCT) is reported. First, we introduce the advantage/disadvantage of DIR between (1) CT and CT, (2) CT and CBCT, (3) MR and MR, and (4) CT and MR. Next, we explain an atlas-based segmentation. Our explanation about DIR and auto-segmentation will help for understanding DIR technique.

Deformable image registration (DIR) can be used for accurate dose mapping between multiple radiotherapy image set. Dose accumulation based on DIR is playing an important role in advanced radiation therapy, such as 4 dimensional radiation therapy and adaptive Radiotherapy. The accuracy of dose mapping depends on the accuracy of the deformation vector fields arising from DIR and on the local dose gradient in the irradiated geometry. Therefore, in clinical use, patient-specific verification should be performed. In this article, challenges and points to notice on DIR based dose accumulation are overviewed and discussed briefly.

One of the characteristics of deformable image registration (DIR) is to detect anatomical coincidence points between two different images. The advantages of using DIR with radiotherapy are as follows. (1) to support the determination of the target using images of modality and series different from the treatment plan image, and (2) to evaluate the dose administered to the target and risk organs by summing the multiple dose distributions. Although DIR has been widely used in the field of external irradiation, application in the field of brachytherapy has been widespread in recent years. Also, the use of DIR is tried for quantitative assessment of anatomical changes over time during the treatment period and determination of the treatment effect after completion of treatment.

In this article, we describe the outline of the technique and the evidence of the current status about the application of DIR in clinical practice.