GQI-ISO and restricted diffusion imaging

[Michele Ferrandino]

Hello,

I am working on DTI and GQI analysis. I know better DTI than GQI.

My DTI acquisition is a single-shell with B=1000 and 64 directions (+ 1 B0)

For DTI, I am also analyzing the Free-Water components. It has values between 0 (no free-water) and 1 (totally free-water, for instance in the CSF).

I am also running similar analysis by using the ISO maps.

I read on the DSI Studio website:

The isotropic value of the ODF [9], termed "ISO", is the minimum distribution value of an ODF. It represent background isotropic diffusion contributed from CSF or non-directional restricted diffusion (e.g. diffusion within or between cells). In multi-shell scheme, ISO is more correlated with restricted non-directional diffusion because of the higher b-value weighting, whereas in grid scheme, ISO has more weighting from free water diffusion.

1) My acquisition scheme is a single shell. Therefore, this means that I cannot analyze correctly the ISO maps?

2) Another thing: I have already run the GQI pre-processing for some my subjects. For instance, I have the same subject scanned in two different time-points (after 24 months). In the first case, the average ISO value inside the white matter is around 1.34, in the second case is 1.87. In my opinion I have a too big difference. And additionally, do we have a kind of range for ISO index?

3) Last question: I have also created the RDI and nRDI. If I understood well, the RDI is connected to the restricted diffusion, while nRDI is related to the free water diffusion, correct? Because, I analyzed these two values inside the CSF (at 08L) and I had:

RDI = 1.52

nRDI  = 0.025

How these two values are connected?

Sorry for my questions,

Micky

[Fang-Cheng Yeh]

On Mon, Sep 16, 2019 at 6:35 PM Michele Ferrandino
<mferra...@gmail.com> wrote:
>
> Hello,
> I am working on DTI and GQI analysis. I know better DTI than GQI.
> My DTI acquisition is a single-shell with B=1000 and 64 directions (+ 1 B0)
> For DTI, I am also analyzing the Free-Water components. It has values between 0 (no free-water) and 1 (totally free-water, for instance in the CSF).
> I am also running similar analysis by using the ISO maps.
>
> I read on the DSI Studio website:
> The isotropic value of the ODF [9], termed "ISO", is the minimum distribution value of an ODF. It represent background isotropic diffusion contributed from CSF or non-directional restricted diffusion (e.g. diffusion within or between cells). In multi-shell scheme, ISO is more correlated with restricted non-directional diffusion because of the higher b-value weighting, whereas in grid scheme, ISO has more weighting from free water diffusion.
>
> 1) My acquisition scheme is a single shell. Therefore, this means that I cannot analyze correctly the ISO maps?

It should still work for mapping free water, but the value may include
tissue edema.

Multiple b-value with RDI will allow for differentiating different
level of restricted/free water.

>
> 2) Another thing: I have already run the GQI pre-processing for some my subjects. For instance, I have the same subject scanned in two different time-points (after 24 months). In the first case, the average ISO value inside the white matter is around 1.34, in the second case is 1.87. In my opinion I have a too big difference. And additionally, do we have a kind of range for ISO index?

ISO is sensitive to signal gain, and it could be that the scanner
receiver gain is very different between the scans.

A more robust way to test the difference is "differential
tractography": https://www.sciencedirect.com/science/article/abs/pii/S1053811919307220

>
> 3) Last question: I have also created the RDI and nRDI. If I understood well, the RDI is connected to the restricted diffusion, while nRDI is related to the free water diffusion, correct? Because, I analyzed these two values inside the CSF (at 08L) and I had:
> RDI = 1.52
> nRDI = 0.025
>

RDI and nRDI would need multiple b-values to be fully functional. I am
sorry that the results may not be reliable in your data.

> How these two values are connected?

Total amount of water = restricted water (RDI) + non-restricted water (nRDI)

Hope this helps.
Frank

>
> Sorry for my questions,
> Micky
>

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[Michele Ferrandino]

Thanks so much Frank,

only last question:

you wrote "ISO is sensitive to signal gain, and it could be that the scanner

receiver gain is very different between the scans."

Is there any method to have a 'normalized' ISO (like nQA) to use for groups or subjects comparison?

Micky

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[Ali Kazemi]

Dear Frank. Could you explain more about these parameters? In a study, I have examined the development of arcuate fasciculus tract through time with QSDR. If I have realized correctly, rdi is related to within axon space. Hence, its developmental behaviour should be similar to fa. Respectively, nrdi is related to between axon space; and, iso is related to csf. Am I correct?

[Fang-Cheng Yeh]

I have a summary here: https://dsi-studio.labsolver.org/doc/how_to_interpret_dmri.html

Hope it helps,

Frank

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[Ali Kazemi]

I really appreciate your quick response. Thank you for the crystal-clear tabular explanation. Just one more question. It is mentioned that increase in rdi illustrates edema. Does it mean that its decrease shows that axons are getting more compact?

[Fang-Cheng Yeh]

Sorry for my late reply.

On Mon, Sep 19, 2022 at 10:37 AM Ali Kazemi <alikaz...@gmail.com> wrote:

I really appreciate your quick response. Thank you for the crystal-clear tabular explanation. Just one more question. It is mentioned that increase in rdi illustrates edema.

Should be "nrdi"

RDI quantifies restricted diffusion, whereas NRDI quantifies non-restricted diffusion, which is more specific to edema. 

Best,

Frank

 

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