On 2022-04-07 20:30:32, user Robert Turner wrote:
I'm delighted that your <br />
group is pursuing the very important goal of characterizing cortical <br />
micro-architecture in vivo. The paper you sent is quite well-written, <br />
yet I find it puzzling in several areas. I hope<br />
you will find these comments helpful as you prepare it further for full<br />
publication, and in your next work.
1) References in the Introduction.
I was surprised to see no mention of the work of:
a) Turner, Oros-Peusquens et al (2008), who were able to reliably image <br />
the stria of Gennari in several volunteers, with nearly isotropic voxels<br />
(0.4x0.4x0.5) mm3 at 3T, using an IR-TSE sequence. While you mentioned <br />
the slightly earlier work of Barbier, it<br />
is obvious that isotropic voxels with 0.5 mm resolution or smaller are <br />
essential for characterization of microstructure, and it was the Turner <br />
paper that pioneered this breakthrough.
b) Trampel, Ott and Turner (2011), who actually used ultra-high <br />
resolution (0.5 mm isotropic) 7T structural MRI to address an important <br />
neuroscientific and clinical question--the extent to which congenital <br />
blindness alters the structure of the visual cortex.<br />
This paper, on its own, contradicts the current paper's statement: <br />
"However, in vivo mesoscopic MRI has not advanced beyond the <br />
proof-of-concept stage, and has not been incorporated into the toolkit <br />
of practicing neuroscientists." Indeed there are other recent<br />
papers which use structural 7T MRI at high resolution to discover new <br />
knowledge about brain organization.
c) Bazin, Dinse et al (2014) and Waehnert, Dinse et al (2016) which <br />
present a complete suite of image processing tools that can "optimally <br />
process mesoscopic imaging data." These tools, largely created by <br />
Pierre-Louis Bazin and including equivolume cortical<br />
layering, have already been used widely in some groundbreaking studies,<br />
for example Gau et al, Elife 2020. If your group found good reason not <br />
to use these tools, for instance preferring to segment your images <br />
manually, it would help the community to explain<br />
why.
It might therefore be a good idea--more courteous and accurate--to tone <br />
down some of the claims of novelty made in this Introduction section.
2) Remarks about layer structure in primary visual cortex.
You comment (end of Section 3) that "It can be seen that there are lower<br />
T2* values around the middle of the cortical thickness. This structure <br />
is likely the stria of Gennari...". Of course it is the Stria of <br />
Gennari. Many researchers have now compared myelin-stained<br />
cadaver brain sections with ex vivo and in vivo MRI scans of primary <br />
visual cortex. The increased myelin density and higher iron content in <br />
the Stria of Gennari significantly reduce T1 and T2*, and this is <br />
unquestionably the source of the line in an MRI scan<br />
with sufficient spatial resolution, whether qT1, qT2*, phase imaging or<br />
simply T1w. No further explanation is needed! The Stueber et al paper <br />
of 2014 (not cited here) is quite conclusive.
A further comment. The Stria of Gennari in your T1 maps (Figure 6 of <br />
your paper) shows up quite poorly. Indeed, there is hardly a decrease in<br />
T1 to be seen in the middle of the cortex, especially in the average <br />
along the cortical plane.
It has become a familiar observation from published papers (such as Duyn<br />
et al) that the Stria shows up more clearly and sharply in T2*-weighted<br />
images than it does in MP2RAGE T1 maps with the same nominal spatial <br />
resolution. It was clear to me several years<br />
ago that this effect was probably due to the MRI sequence, rather than<br />
any anatomical subtlety. Iron-stained sections of primary visual cortex<br />
show a thickness of the Stria very similar to that seen in myelin <br />
stained sections, with a similar increased myelin<br />
and iron density towards the white matter. The image sharpness issue <br />
seems to be the point spread function of the MP2RAGE sequence. Because <br />
each of the long train of echoes is actually recorded at a different <br />
inversion time, the point spread is convolved with<br />
the recovery curve of the longitudinal magnetization, and is thus <br />
broadened. It was to circumvent this problem that I devised the <br />
Multi-Shot Multi-Slice Inversion Recovery EPI sequence, now implemented <br />
brilliantly by Rosa Sanchez-Panchuelo and colleagues at<br />
Nottingham. Here each of the EPI acquisition windows is short in <br />
duration compared with T1, and the broadening is much reduced. You can <br />
see this in Figure 8 of our 2021 paper (attached), where our MS-IR-EPI <br />
T1 maps show a much sharper Stria. Fabrizio Fasano<br />
and I demonstrated this at last year's ISMRM using a gel phantom with a<br />
sharp boundary between two gels with different T1s.
I would recommend this sequence to anyone interested in quantitative mapping of cortical myelin at high resolution.
3) In Section 3.4.2 your paper states: "T1 contrast is also known to be <br />
related to myelination and therefore used for delineating areal borders <br />
(Cohen-Adad et al., 2012; Deistung et al., 2013; Dick et al., 2012; <br />
Haast et al., 2016; Marques et al., 2017). While<br />
there have been efforts to acquire mesoscopic resolution T1 and <br />
T1-weighted images in the past (Federau & Gallichan, 2016; Lüsebrink<br />
et al., 2021; Lüsebrink et al., 2017), there has not yet been a <br />
quantitative T1 dataset together with T2* at mesoscopic resolution<br />
in the living human brain."
Yes, indeed, T1 contrast is related to myelination. There are several <br />
papers (e.g. Leuze 2017, Morawski 2018) showing that if CLARITY is used <br />
to clear the myelin from cadaver brain, T1 contrast completely <br />
disappears. Could you put this first line a little more<br />
strongly, perhaps?
The first paper to argue that quantitative T1 maps should be the most <br />
accurate in vivo guide for cortical parcellation was actually Geyer et <br />
al (2011, attached), describing results shown at the OHBM meeting in <br />
2010, which then in fact inspired David van Essen<br />
and Matt Glasser to attempt their own parcellation of the cortex, <br />
partly based on the arbitrary ratio of the image intensities of <br />
T1-weighted MP-RAGE images and TSE images (also mostly inversely T1 <br />
weighted, but inaccurately denoted 'T2-weighted' images).<br />
In the attached Geyer paper you can find a reasonably complete summary <br />
of previous related work.
4) At the beginning of Section 4.2 you state: "However, as argued within<br />
(Wallace et al.,2016), mesoscopic in vivo imaging may still be <br />
insufficient to capture the subtle changes in myeloarchitecture aside <br />
from primary visual cortex where the stria of Gennari<br />
is extremely thick." Perhaps you have missed the papers by Skeide <br />
(2018) showing hypermyelination of left auditory cortex in dyslexic <br />
individuals, and Kuehn (2017) showing subtle variations in myelination <br />
of BA3b relating to somatosensory fields.
5) One last comment--have you tried denoising your images? With luck, <br />
you might gain a factor of 3 in SNR, and they would look more <br />
convincing.