Thread by @hipposubfields

It's Wednesday...so you know what that means! It's time for #SubfieldWednesday! 🥳📢

(1/n)

Two weeks ago we posted about whether you can visualize subfields on in vivo MRI. Here is a link if you missed it!

https://twitter.com/hipposubfields/status/1303720623898857474?s=20

#SubfieldWednesday (2/n)

You might recall that the hippocampus is a layered structure (just like the neocortex!) and that some of the layers have different contrast properties due to different amounts of myelination and cell densities

#SubfieldWednesday (3/n)

The most distinctive contrast feature within the hippocampus itself appears as a "dark band" on T2-weighted MRI & reflects several layers including the:

Stratum radiatum
Stratum laconusom
Stratum moleculare

And these layers are often abbreviated "SRLM"

#SubfieldWednesday (4/n)

...FYI the dark band also includes the Hippocampal sulcus!...

And the dark band can be used to separate the pyramidal cell layers of cornu ammonis layers from the granule cell layer of the dentate gyrus (DG)

#SubfieldWednesday (5/n)

So, what kind of MRI and image type do you need to visualize the subfields?

#SubfieldWednesday (6/n)

The SRLM is a very thin layer-- @RobindeFlores @lemwisse et al reported that the dark band is on average ~0.5mm thick in non-demented older adults. That means that you need a very small (<1mm) voxel size in the coronal plane to visualize the dark band!

#SubfieldWednesday (7/n)

Also, it is important to know that the SRLM is affected by Alzheimer's disease! So it will be even thinner in dementia patient populations!

#SubfieldWednesday (8/n)

Ok, now finally to talk MRI scanner strength (3T, 7T), image type (T1-weighted, T2-weighted) and voxel size!

#SubfieldWednesday (9/n)

Some examples from our 2015 NeuroImage paper (Yushkevich et al.), same subject scanned in with different sequences. Both are from a 3 tesla scanner.

#SubfieldWednesday (10/n)

^ Left is a standard 1mm isotropic T1-weighted image. Right is high resolution T2-weighted image (0.4x0.4mm in the coronal plane). Note that the SRLM much more clearly visible only on the right image with the higher in-plane (coronal) resolution.

#SubfieldWednesday (11/n)

Ok, what about 7T? Well, as you might imagine, you the contrast between layers, including the dark band, is even better!

#SubfieldWednesday (12/n)

A few more scan types to mention!

As mentioned last time, at 3T @mallarchkrvrty1 and colleagues have used ultra high resolution (0.3mm isotropic voxels!) to study hippocampal subfields + white matter structures (alveus, fimbria and fornix)

#SubfieldWednesday (13/n)

and at 7T (.75mm iso voxels) @neuroak and @jordandekraker have derived surface maps from in vivo MRI & post-mortem histology to characterize the organization of the subfields along the three different axes (transverse, longitudinal, and radial)!

#SubfieldWednesday (14/n)

Ok, that's it for this week! We hope you enjoyed this week's #SubfieldWednesday! See you next week!

(end)

I forgot to mention one other image sequence and protocol that is a good resource for learning about hippocampal subfields!

#SubfieldWednesday (end+1)

Dalton and colleagues have used 3T T2-weighted MRI (0.52 isotropic voxels) to visualize the hippocampal subfields!

#SubfieldWednesday (end+2)

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