| Determining the bounding box or Region of Interest (ROI) for registration |
For the initial RIR, be careful not to include/clip high‐contrast structures that move relative to the target soft tissue structures within a rectangular ROI as these will bias the registration, for example pubis when registering prostate. Individual ROIs should be defined appropriately for each registration application, based on the clinical goal of the registration. If bounding boxes are used for DIR, the box should include enough contrast and, if possible, should encompass entire organs that may deform, to avoid discontinuity at borders. If a good result cannot be obtained for the full registration ROI, try using sequentially smaller regions to progressively tune the result. Watch out for discontinuity between regions.
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| Initial RIR is critical for effective DIR |
Ensure the RIR is accounting for systematic variation between images (provides a global/coarse fit in the region of interest), so that the DIR can focus on deformation alone. In images with large variations, the RIR should be optimised to provide the strongest registration at areas of greatest clinical importance. Potentially, multiple registrations are needed to focus on separate areas across the image.
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| Contrast within the ROI |
Regions of low contrast provide little intensity variation ‘features’ for algorithms to compute the deformation and thus may give incorrect or non‐physical results when using DIR. This is of importance when deforming PET or dose images according to the registration between two CT images. Use thresholds and window/level settings to improve contrast where possible.
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| Understand the limitations of RIR and DIR |
Image registration is a mathematical tool, with limited or no biological information involved in the process. There are limitations in compensating for large changes in pose, expansions and contractions, and differential movement of tissues with varying biomechanical properties and attachment. Recognise when RIR/DIR is appropriate, and consider viewing images side by side if neither RIR/DIR provide accuracy required. Communicate and document the accuracy or uncertainty level which represents a recommendation for end use; include residual errors or uncertainties for downstream processes. Limitations may be due to software, the images used, operator experience or the task itself.
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| Iterative deformation can improve accuracy |
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