Dear MRTrix community,
Could you please clarify me on how the tractography is being performed in the presence of Edema? Is there any default correction that is being applied? I couldn’t find my answer in the documentation. So I’d appreciate any guidance on this matter.
There’s certainly not any mechanism that decides “this voxel contains edema; let’s perform tractography differently, and not tell the user about it”. That would be very much against our ethos.
This question necessitates answering in two parts:
How is deconvolution affected?
One must consider first how the DWI signal is changed by the presence of edema, and then correspondingly how the parameters of a fitted model may be changed by such. If performing multi-tissue deconvolution, then one would expect that hyperintensity in the b=0 data will be primarily attributed to the CSF-like compartment. But there could also be changes in T2, which, due to the way in which we deal with intensity normalisation, could influence the magnitude of the WM ODF, and that in turn could have an influence on tractography.
How is streamlines tractography affected?
Once you get to the point of performing tractography, it’s only the WM ODFs that are utilised; the rest of the decomposition is never touched. So the question is how may the WM ODFs differ in a way that would be consequential for tractography. This could be a reduction in overall amplitude, which if severe enough may result in completely forbidding streamlines from crossing due to failing to exceed the FOD amplitude criterion. Or it could be an increase in overall amplitude if the T2 of intra-cellular water is affected or the extra-cellular water becomes more restricted. Exactly what happens here depends on the nuances of how any given streamlines tractography algorithm operates.
Note that you should certainly not expect a correspondence between the magnitudes of WM ODFs in pathology and the density of streamlines. While the SIFT(2) methods attempt to establish correspondence between these two parameters, they (and related methods) do so on the premise of continuity of fibre density along each trajectory. If a localised pathology such as edema violates this premise (whether due to artifacts of the diffusion model or genuine biological discontinuity), this correspondence can’t be established fully, so these methods can’t wholly “correct” the recosntruction, and the metric of connectivity gets biased (just as happens for other metrics).
Here’s a relevant discussion if you feel like consuming some Cancer (thankfully there’s mutual understanding and a common goal now).
The behaviour of tractography may alternatively be affected due not to difference in the DWI signal, but to differences in explicit tissue segmentation as provided to ACT. If the edema is visible in the anatomical image used to derive that segmentation, then whatever tissue component(s) is/are allocated at that location will determine the anatomical priors that ACT applies to streamlines traversing that region. You may alternatively choose to manually allocate that region to the fifth volume in the ACT 5TT image, which will result in no anatomical priors being applied to streamlines as they traverse that region, and therefore whether or not they continue / terminate will be determined exclusively by the input DWI information.