optimal CT imaging for printing of long bones

[sam17franklin]

I have a couple questions about obtaining optimal CT data for subsequent 3DP of long bones.  First, I was told at a 3D medical printing conference that for such applications I would ideally want to use the "raw data" from the CT rather than using the reformatted sequences. I don't exactly understand what the radiologist meant when he told me this. I have always just downloaded the sequences from the PACS after the CT is done and then proceeded to go through the processes of generating the STL files and cleaning them up prior to printing.  What exactly are the "raw data" and how do I acquire those rather than using the sequences sent to the PACS.  

 

Second, the same radiologist told me that I would ideally want to use the CT sequence performed using a soft tissue (rather than bone) window level. This is counter intuitive to me considering that I want to print bones. Can you please tell me if this is true and why?

 

Lastly, from reading one of the tutorials here on embodi3D (Choosing the Best Medical Imaging Scan to Create a 3D Printed Medical Model) it appears that using a sharp kernel will provide the greatest detail on the surface of the bone. However, the tutorial then goes on to say that "However, for 3D printing smoother reconstruction kernels are generally best."  This latter statement confuses me. It would seem to me that for printing of bones I would want the sharp kernel. Can someone please clarify.  Thank you!

[Dr. Mike]

sam17franklin, thanks for the post. 

Which conference did you attend? The reason I ask is that I attend and often present at most of them, so I am curious. RNSA? Mayo 3D Printing? 

Raw data from a CT scan is useless from a 3D printing standpoint. It is the raw data that is acquired from the x ray tube spinning around the patient in a helix. FROM the raw data axial, sagittal, and coronal image series can be generated by computing the attenuations at given rotation angles and converting that into a matrix that can represent a "slice" in the stack of images. From the raw data you can generate 5mm slices, or 2.5 mm slices, or 1.25 mm slices, or 0.75 mm slices, although with diminishing image quality the thinner you go. Raw data is used to generate the standard slices from a scan, and then it is typically deleted from the scanner to save storage space since it consumes a lot of it. I think what the radiologist meant was that if you have access to the raw data, you can reconstruct a scan with very thin slice intervals (1.25 mm of less) for 3D printing that may not have been generated as the default slice thickness for that type of study.
 

I agree with the radiologist's advice on soft tissue reconstruction. Soft tissue is a kind of "soft kernel" reconstruction as described in my tutorial about choosing the right scan for 3D printing. Bone reconstructions are grainy as they are a type of "hard kernel" or "sharp kernel". They are designed to make it easy to see fractures, and thus have a lot of edge enhancement applied to make edges appear unnaturally noticeable. Look at the picture at the bottom left. Notice that right outside the bone is unusually dark? That darkness isn't really there. It is a artifact of the algorithm and is designed to make the bone-tissue interface have higher contrast. This makes it easier to see fractures but isn't good for 3D printing because the pixels are noisier, i.e. they look speckled like a photograph in low light. Any bright pixels confuse the software because it thinks there are thousands of bright, pixels-sized speck of bone everywhere, but really it is just noise. Each bright pixel can therefore be converted into a speck of bone in your model. Bone and lung reconstruction algorithms have this type of edge enhancement. Soft tissue reconstruction algorithms are best for 3D printing because they don't have the speckling and thus don't generate a "sea of sand" when converting to a model. 



Hope this helps.
 

[sam17franklin]

Thanks Dr. Mike for clarifying.  I have just one more question, I understand that using the sharp kernel for bone imaging and printing will result in a sea of bone speckles or noise around the bone. But, I find this is relatively easy to get rid of in programs like Meshmixer. If I'm willing to take that extra step to get rid of those extra and erroneous speckles of bone will using the sharp kernel preserve more ideal and accurate topography of the bone itself? I'm ultimately looking to create cutting guides that will need to closely match the surface topography of the bone so that they will fit on the bones in only one location and at one orientation. Consequently I'm worried about unnecessary 'smoothing' of the bone that will make those guides less 'specific' to a particular spot on the bone.  Or, will the sharp kernel actually result in an inaccurate and unnatural bone contour that then works against my goal of creating cutting guides? 

 

Thanks for all your help!

[Dr. Mike]

With a sharp or bone reconstruction kernel you are still going to have unnatural edge enhancement. These algorithms are designed to provide maximum conspicuity of fractures, not accuracy of the bony surface. The soft tissue algorithm will provide a truer approximation of the real bone surface.

[sam17franklin]

Perfect, that's what I needed to know.  Thank you!