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qazit t1_jed8uu5 wrote

If an MRI voxel is 1 cubic mm, that just means that the MRI can’t resolve anything smaller than that. If a significant proportion of that 1 cubic mm is made up of white matter, no matter the thickness of the white matter, you’ll see evidence of the white matter in that voxel.

An analogy would be a pixelated image. If you look at a pixelated image of somebody wearing a shirt with some symbol on it that has a lot of thin lines, but is a distinctly different color than the rest of the shirt, you will still see a contribution from that colored symbol in a pixelated image of the shirt.

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Zealousideal-Alarm37 OP t1_jegwhtc wrote

DTI, which has a lower resolution than other forms of MRI, supposedly maps the paths axons take in white matter and show the physical connectivity of the brain.

Are these Voxels overlapping to any extent to artificially make higher resolutions? Or is there something I'm missing.

There's a fine line between mapping where white matter is and mapping the individual connections and directions within. I'm having trouble wrapping my head around that, I know that DTI can read for diffusion resistance and thus directionality, but that would seem to require a higher resolution given the complexity of the white matter tracts.

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Brain_Hawk t1_jedfu27 wrote

First, the average thickness of the cortex is more like 2.5 mm

Second, we aren't explicitly measuring ig each voxel is cortex or non-cortex with a binary 1-0 mask. The way that cortical thickness is measured is by looking at the signal intensity to each voxel, and an algorithm estimates the barrier between gray matter and white matter, or gray matter and the dura matter on the outside of the brain. It does so by considering the intensity of this voxel relative to deeper voxels that are clearly white matter, or voxels that appear more likely to be gray matter.

So let's pretend a voxel that a cortical voxel would have an intensity value of one. Let's say that a voxel of white matter has an intensity value of 0.5

If you encountered a voxel that was roughly 0.75, you could infer that roughly half of that voxel was in the cortex.

Free surfer runs the algorithm across the entire image and attempts to estimate the inner and outer barrier using this approach. I simplified it of course, it's not really saying 0.75 and half the voxel, the modeling is more complicated than that and frankly I don't understand it more than that.

But, the long story short is that by looking at signal intensity the different voxels we can build an estimate a model of where the ribbon of the cortex is relative to the edges of that voxel.

Interestingly these estimates are very reliable across MRI scans. If I scan you today and scan you next week, most cortical thickness measurements will be very stable

Then once you have the outer ribbon, representing the outside of the cortex, and the inner ribbon representing the inside, you can place a perpendicular vertex in that section and estimate the thickness of the cortex

The process isn't perfect, and errors are not uncommon. Particularly in lower quality MRI scans.

I hope that help explains it

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Zealousideal-Alarm37 OP t1_jegvrv1 wrote

My issue with this explanation is that techniques like diffusion tensor imaging (a form of dMRI) can map paths taken in the white matter (ie the actually axons of neurons, and the myelin on this axons that make white matter white in the first place). Axons are very thin iirc, and while the cortex is thicker than the resolution of the MRI, how can it map things smaller than that resolution?

Do Voxels overlap?

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Brain_Hawk t1_jeh46nf wrote

Dti does not measure axons.

It makes a model of white matter tracts based on the characteristics os the direction of diffusion. Like so many things MRI, it's a model, and an estimate of how large bundles of axons are organized and traverse through the WM.

If you wanna learn more I bet you can find some good videos on YouTube. :)

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