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Dr. Mike

Holes in bone models with democratiz3D

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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?"

L spine defects.jpg

 

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

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I'd like to elaborate on this topic a bit, as I recently had another member inquire about this issue. The member was creating a model from a CT scan of the clavicles. As you can see, there are holes in the medial (midline) ends of both clavicles. What is causing this? Is it a problem with democratiz3D? How can it be fixed?

clavicle.jpg


The issue lies with the patient's anatomy and the quality of the original CT scan. In the human body there are areas where bones are naturally very thin. Sometimes, the bone surface (cortex) can be  paper thin. Also, some patients who have conditions like osteoporosis may have very little calcium in their bones. Issues like this make it very hard for the CT scanner to detect the bone wall, as you can see from the image below which shows the area on the left clavicle that has a hole in the final model (red arrow). The problem isn't with democratiz3D, but with the quality of the CT scan or with the patient having thin bones (how dare they!).  democratiz3D is actually creating the model exactly as it appears on the CT, its just that the CT has holes we don't want!

 

clavicle CT.PNG

 

So, what can be done? If you encounter this problem you have two options. 1) Manually fix the holes in the model with a mesh editor like Meshmixer, or 2) decrease the threshold value in democratiz3D and re-process the scan. Decreasing the threshold tells the system to capture more voxels in your model, potentially capturing more thin or osteoporotic bone. But, be careful. If you reduce the threshold too much (less than 100), you run the risk of starting to capture muscle, organs, and vessels in your bone model. If you are not sure what threshold to use, you can experiment by running your scan through democratiz3D using different thresholds. To save time, I suggest you do this on low or medium quality setting. When you find a threshold that works, you can generate your final model using a higher (and more time consuming) quality setting, like High or Ultra.

 

If you are familiar with mesh editing software, that is probably the fastest way to correct this problem. Just delete the edge of the hole, fill it in with a new face, and run a quick smooth operation on the area. It's a 1 minute fix if you know the keyboard shortcuts.

I hope this tip helps.

Dr. Mike

 

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