[Rtk-users] Implementing a Motion-Compensated SART (MCSART) Reconstruction

[Andosca, Ryan]

Hi,

So, my experience with coding, especially in C, is extremely limited. I mostly work in MATLAB, and as such I've been primarily using the built-in console applications to perform reconstruction on CBCT datasets. The rtksart application works great and provides a beautiful reconstruction. My goal is to effectively do the same reconstruction, but tack on motion compensation to each iteration.

After delving into the algorithm implementation in rtksart as much as I am able, I wrote a MATLAB workflow that boils down to:

  *
Start with image of zeros
  *
Randomize gantry angle order
  *
Iterate over gantry angles:
     *
Load current projection forward and backward motion compensation DVFs
     *
Deform current image iteration to current gantry angle breathing state
     *
Forward project deformed image
     *
Subtract deformed projections from raw CBCT image data
     *
Back project the subtraction image
     *
NORMALIZE BACK PROJECTION (this is where I think my issue is)
     *
Deform normalized back projection back to the original image space
     *
Update current image: img = img + lambda * (normalized_back_projection / number_of_gantry_angles)
        *
Note: This step, as it also contains normalization, may also be a source of my issue...
     *
Set negatives to 0
     *
Repeat

I get an image that is much as I expect - a much sharper diaphragm and generally higher definition in soft tissue structures. So I must be generally on the right track. The issue is that the normalization is clearly off in some way. I have a bright circle artifact in the middle of the image and some striping throughout. Try as I might to understand it, I couldn't discern how normalization was done in the rtksart application. So, this is the method I used:

  *
Create an image of 1s of the size of the final (SART reconstructed) image.
  *
Forward project through this image of 1s
  *
Back project through the resulting forward projection
  *
In the normalization step in the MCSART process detailed above, I simply divide by this resulting back projection.

Knowing all of that, any idea what my issue might be leading to my artifacts? Ultimately, I know it would be more ideal if I just altered the rtksart application itself to include motion compensation, and then I'd just be using the built-in normalization that clearly works, but I find my abilities lacking when I attempt to do this... So instead I am just seeking to understand how that application implements normalization, so that I can use a similar normalization in my algorithm!

I greatly appreciate any help provided!

Best,
R. Andosca
GSR
UCLA Health





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