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I received this inquiry from a member. I am going to post the response here so that it can help others with the same question: QUESTION: "I am printing out a spine model.... Why are there so many defects in the rendering? I can't print this out on a 3d printer, half of the vertebrae are hollow. I get these from a 3d CT and on a computer monitor, the vertebrae are whole. Just take a look at the thumbnails and you'll know what I'm talking about. I don't have the expertise or time to fill all of the defects. Is there a paid service somewhere that could do this for me? I'm just surprised the STL file wouldn't look like the 3d CT since they use the same dicom imagery?" ANSWER: If you are creating bony models and are finding that the bones have holes or other large defects in them (see above), this is probably an issue with the Threshold value used during the conversion. Threshold is the number of Hounsfield units to use to create the surface of the model. Anything above the threshold value is considered bone and is included. Anything below is not considered bone and is excluded. Normal cortical bone is very dense, greater than 300 Hounsfield units, so the default threshold of 150 is more than enough to catch it. The inside of the bone (medullary, or marrow cavity) is filled with fatty bone marrow and is a much lower Hounsfield value. If the patient has osteoporosis or very thin cortical bones they may not register as bone if the default threshold of 150 is used. You can decrease this to a lower threshold value (maybe 100 or so) and you will be more likely to capture this thin, deossified bone. If you go too low though (60 or so) you will start to capture non-bony structures like muscle. Another thing that may help get the highest quality models is using premium operations such as Very Detailed Bone and Ultra quality level. These operations are time-consuming however. To save on time, you can run your scan through democratiz3D using free operations such as Detailed Bone and medium or high quality until you find the threshold you like. Once you find the threshold value you like, you can run you scan through a final time using the highest quality (and slowest) operation settings, such as Very Detailed Bone and Ultra quality. Hope this helps! Dr. Mike
I've released two tutorials on using Blender to get 3D bone models ready for 3D printing. Those of you looking for a freeware solution to preparing models for 3D printing may find this helpful. 3D Printing of Bones from CT Scans: A Tutorial on Quickly Correcting Extensive Mesh Errors using Blender and MeshMixer Preparing CT scans for 3D printing. Cleaning and repairing STL file mesh from bones using Blender, an advanced tutorial