[MURG] MRI resolution

[Eugen Leitl]

On Sun, Nov 28, 2004 at 10:27:07AM -0500, Eric Zilli wrote:

> a 7T magnet like the one I mentioned above is very powerful and quite
> rare.. most work is done with 1.5T or 3T magnets, but you asked for
> the limit.. one problem may be that as magnets get more powerful, they
> also get less safe.. I didn't even know humans could be used in a 7T
> magnet until I found that study.. I thought it was considered too
> unhealthy

As a start, just inserting people into a high-Tesla magnet will give them
seizures. Water droplets and frogs will levitate in a very strong field 
with a strong gradient.

You could levitate a person, but you would probably kill her in the process.
I.e. right now the raw Teslas (but not homogenous ones!) are no longer the 
limit, but how much you can stress the patient. You already have to exclude
certain regions in pulse sequence space because they're inducing voltage
gradients, and tissue heating in general (a problem with very obese patients).
 
> B. someone else may have a better estimate, but I'd say if you can
> image dendritic spines you're pretty close.. the smallest spine necks

If you see that structure, you're not seeing the properties of that
structure, which might taking samples at literally submolecular resolution.
I doubt you can tell the function (type) from shape and the context. Sure you
could label, but how many different types of labels can you have in a
specimen? And how you pull that trick in vivo/postmortem?

I think the basic approach "do not harm" suggests best structural
preservation possible, yet not fixate nor label for fix of introducing
artifacts. I do think that processing the (presectioned for parallel
processivity) bulk through surface abrasion is the way to go. Thin slicing
and 2D imaging in one go might be an option.

> can be as narrow as 40-50 nm, so we'd want a volume on that scale.. if
> we use the 4T structural voxel size as standard, it'd mean we need an
> improvement of about 18750 times to get to that scale (0.9375 mm / 50
> nm)
> remember that we can't use fmri at this scale: fmri is based on the
> hemodynamic response, so it's really only meaningful on the scale of
> blood vessels, much, much larger than this size
> also keep in mind that mri improvements alone won't do.. to use this
> route of noninvasive uploading probably also requires a high
> resolution mr spectroscopy system because I think there's still a need
> to extract information about receptor densities and such

You can just use the math of how many nuclei are there in the specimen, how
high your field, and how absorbance are you getting in a given voxel size
(shaping field gradients about a noggin-sized, alive, noisy object and
environment -- there are novel noise-supressing tricks, but they only lead
you that far, and your HF sensitivity is about as good as it gets).
 
> C. this one I can't answer, I don't know enough to predict.. if
> there's a moore's law sort of thing happening that cuts the volume of

Physical laws are fixed, and do not allow any sort of progress. If your
technology is pushing the limits (MRI today almost does) any further step is
harder than previous, and produces diminishing, asymptoting improvements.

> a voxel down by 8 each year (i.e. each dimension halved), it'd take
> about 15 years to get there.. but I very much doubt things are going
> that quickly
> 
> keep in mind any of my answers could be wrong :)
> 
> On Sun, 28 Nov 2004 15:47:22 +0100, Giu1i0 Pri5c0 <pgptag at gmail.com> wrote:
> > A) What is the current resolution limit that can be achieved by MRI on
> > an object the size of a human brain (not on an extra-thin slice)?
> > B) What is a realistic theoretical estimate of the MRI resolution
> > required for mind backup applications?
> > C) When do you guys think B) can be feasible?
> > _______________________________________________
> > MURG mailing list
> > MURG at minduploading.org
> > http://minduploading.org/mailman/listinfo/murg
> > 
> 
> 
> -- 
> Eric Zilli
> Hasselmo Lab - Computational Neurophysiology
> Center for Memory and Brain
> Boston University
> 2 Cummington St.
> Boston MA, 02215
> digfarenough at gmail.com -- www.digfarenough.com
> _______________________________________________
> MURG mailing list
> MURG at minduploading.org
> http://minduploading.org/mailman/listinfo/murg
-- 
Eugen* Leitl <a href="http://leitl.org">leitl</a>
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