X4D HowTo Optimize Xray DRR Settings
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To make it easier to find the best set of parameter values for a particular trial, the image optimization widget performs a simulated annealing optimization on the 15 parameters. To use it, follow these steps:
- Choose an x-ray frame in which the bone you're tracking is fairly easy to see. It is recommended that you select a single bone to optimize the image settings.
- Manually move the bone into the correct pose as best you can.
- In the Object Tracking widget, perform a quick optimization (about 500 iterations with ranges of 1 mm and 1 deg) to fine-tune the pose. If this optimization makes the pose worse, go back to the manually adjusted pose.
- Set the tracking optimization ranges to larger values (e.g., 3 mm and 3 deg) and keep the number of iterations at about 500.
- In the Image Optimization widget, set the max iterations value. A value between 200 and 500 is recommended. This is the number of tracking optimizations that will be performed on the current bone in the current frame. Each of these optimizations will use a different set of image processing parameters and will run for the number of iterations specified in the Object Tracking widget.
- Set the ranges of the 15 image processing parameters to values appropriate for the current frame. You may want to experiment with their values in the Xray/DRR Settings widget to get a sense of how they affect the images. For parameters that have a large effect on the images (e.g., X-ray Min Edge), set their ranges smaller so that the optimizer focuses more on the optimal region.
- Press Optimize to start the optimization. For each iteration, a simulated annealing algorithm will randomly choose values for the 15 parameters within the ranges you specified, then perform a tracking optimization starting from the bone pose you specified at the start.
- When the optimization is finished, the image processing parameters will be set to their optimal values. The optimal values are the values that caused the tracking optimization to lock-in on a pose closest to the initial pose. You can then try tracking the bone in other frames to see if the parameter values work on the rest of the trial.
Note: Turning off the display of the DRRs and CT bones will increase the speed of the optimization.
Note: For any given x-ray frame and bone, there is no guarantee that there exists a set of image processing parameters such that the correct bone pose is the global minimum bone pose. This is because of the inherent differences between the x-ray and DRR images and the quality and extent of the CT data. There is also no guarantee that the optimal parameters for one frame will work on any other frame. However, image optimization is still often useful for finding a neighborhood of parameter values that improve the performance of the tracking optimization.