[Mristudio-users] Relationship between acquisition and reconstructed voxel size.

[Dorian P.]

Dr. Mori, thank you for the exhaustive explanation.

I have two follow-up questions.

First, you wrote:
"Because field-of-view, which is determined by the brain size, is about 200
- 250 mm, DTI spatial resolution is limited to 2-2.5mm."
It appears that the number of acquired points depends on the FOV. Does this
mean that a smaller FOV have more SNR than a bigger FOV for the same voxel
resolution?

Second, I usually acquire with FOV=230mm. If smaller FOV gets more SNR
(depending on answer of 1st question), I can reduce FOV to match the
patients brain. Question is, will variable FOV be problematic in a
scientific publication?
I think that a fixed FOV keeps SNR standard between subjects, but brain
size affects DTI anyway. For example, within the same 2mm voxel size
chances are that a small brain will have more heterogenous fibers for each
voxel than a bigger brain. Therefore having more SNR for a smaller brain
may (kind of) balance the disadvantage. My plain thoughts anyway.


Thank you for your comments.

Dorian
TJU


2013/8/16 Susumu Mori <smoriw at gmail.com>

> MRI raw data is so-called time-domain data. This means, within about
> 10-100ms of time, signals are acquired and recorded. For example, if your
> image matrix is 128x128, there are 16,384 data points to acquire. For DTI,
> to freeze the motion effect, all 16,384 points are acquired at once (8,192
> points if you use a parallel imaging with factor = 2). Actually there are
> real and imaginary data points and therefore there are 16,384x2 points. By
> the way, because all 16,384 points are acquired at once, the data
> acquisition time becomes very long and there is not much signal left by the
> time all 16,384 points are read from the signal. Therefore, for DTI, there
> is no point to acquire 256x256 (=65,536) points because after about 20,000
> point-read, all the remaining points are reading just noise. This is why
> all DTI studies have been done using 96x96 or 128x128. Because
> field-of-view, which is determined by the brain size, is about 200 - 250
> mm, DTI spatial resolution is limited to 2-2.5mm.
>
> Now, when we do interpolation by the scanner, the scanner simply add "0"
> and extend the 128 points to 256 points. This is called zerofilling.
> After the fourier transform, the time-domain data is converted to the
> frequency-domain or image-domain data (the same thing with different
> names).
>
> If you have 128x128, after the fourier transformation, you get a
> 128x128-pixel image.
>
> Now, you have two options, do the interpolation by the scanner
> (time-domain interpolation), convert the 128x128 time-domain matrix to
> 256x256 time-domain matrix and FT it to the 256x256 image-domain matrix.
>
> Alternatively, you can FT first, get a 128x128 image-domain matrix and
> then digitally interpolate it to 256x256.
>
> A big question is, are they different? Signal processing people say,
> "sinc-interpolation of 128x128 image to 256x256 image is the same as
> time-domain zerofilling". However, things are not that easy because
> time-domain data has real and imaginary parts. If you have 10 physicists,
> I'm sure that you get two camps; one say they are the same and the other
> say, time-domain interpolation is better and you can never get the same
> quality after the time-domain data are converted to an image.
>
> There is a famous paper by a novel laureate, Dr. Ernst, proving the latter
> is the case, but the problem is, ordinary people like us can't understand
> the paper.
>
> Anyway, answering your question, you can always interpolate the data into
> higher resolution, but many people believe that it is just cosmetic,
> especially in the image-domain. We can argue that you are wasting the hard
> drive space. I always do twice time-domain zerofilling (128 becomes 256) by
> the scanner. In the imaging domain, we further digitally interpolate to
> 1x1x1mm because that is the voxel size of many atlases.
>
> I don't think there is a large impact on SNR by the interpolation.
>
>
> On Mon, Aug 12, 2013 at 12:40 PM, Dorian P. <alb.net at gmail.com> wrote:
>
>> Hi all,
>>
>> Is there any relationship between acquired and reconstructed voxelsize.
>> Is there any downside of reconstructing to much smaller voxels, for example
>> acquire at 3mm and reconstruct at 2mm or 1mm? Is SNR going to be the same?
>>
>>
>> Thank you.
>> Dorian
>> TJU
>>
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