Subject specific scaling based from medical images

Hello,

I had a couple of questions regarding incorporating subject specific geometries from the CT images into the MoCap models and was wondering if anyone could help me.

  1. I followed the tutorial where the femur from the standard anybody femur (source) is scaled to the femur from the CT scan (target). Now, I am looking to do the same thing, but for the tibia and spine. In order to do so, I would need the source geometry of the tibia and the spine. Where would I be able to locate the source files for those segments?

I tried exporting the geometry from the Anybody as a stl file itself. However, the exported stl seem to be very coarse. For instance, when I exported the femur from the Anybody, the exported stl file is almost 4 times smaller in size compared to the source femur stl from the tutorial.

  1. My another question is how are the scaled segments placed and connected to other segments in the Anybody model? Are their placement and connection to other segments solely based on their position in space or are there any other anatomical features that need to be identified as well?

Any assistance would be helpful.

Thanks,
Anup

Hello Anup,

  1. Load a standing model with #define BM_SCALING SCALING_NONE. Double-clilck on the right bone to find the surface object in the model tree. Right-click on it and use "Export Surface" class-operation with "local frame" option with scaling factor = 1.

The coarse geometry is generally not a problem unless you want to do contact analysis. For this purpose we register the CT surface to the morphed AnyBody surface (which should be very similar).

  1. Scaled segments are assembled through predefined joints and it assumes good scaling of the joint nodes. For linear scaling it is typically okay, and it is a little more complex for non-linear cases, where bone axes change. In those cases people implement different methods to reconstruct joint nodes either through fitting analytical surfaces (e.g. hip joint centre as a centre of the sphere, check ref. manual for AnySurfEllipsoidFit), or through landmarks that define joint axes (e.g. condyles of the femur), or a combination of those.

Kind regards,
Pavel