[Mristudio-users] Calculating SNR of DTI data

[susumu mori]

As SC mentioned, it is not easy to measure SNR for DTI using parallel
imaging. First of all, SNR has spatial dependency; higher at the edge
(closer to the coils) and lower in the center.

As for your point #2, SNR is important for cross-institution calibration
because FA has SNR dependence for low SNR regions; the lower the SNR, the
higher the FA. Therefore, you have to make sure that you have a protocol
with SNR high enough so that small difference in SNR across institutes won't
affect the DTI results that much. Namely, if SNR is high enough, the FA
values are stable against SNR difference.

We published empirical measurement results of the SNR-FA relationship in;

Farrell, JAD, Landman, BA, Jones, CK, Smith, SA, Prince, JL, van Zijl, PCM,
*Mori, S. *"Effect of SNR on the accuracy and reproducibility of DTI-derived
fractional anisotropy, mean diffusivity, and principal eigenvector
measurements at 1.5T", J. Magn. Reson. Imag., 26, 756-767 (2007)

Currently we are trying to see if this kind of SNR (scan time) - FA
relationship can be used for cross-insitution calibration. This type of
measurement may not be enough for calibration, but certainly important.

On Tue, Jun 29, 2010 at 10:23 AM, [Seongjin] <choisj70 at gmail.com> wrote:

> My opinions to (3) and (4)
>
> (3) In parallel imaging, SNR decreases as acceleration factors increases (R
> in Philips system).
> But increased acceleration factors improves geometric distortions (i.e.
> better shape).
> You need to find trade off point with your data. In Philips 3T system, I
> observed significant noise in the images using acceleration factor larger
> than 3 (R > 3).
>
> (4) When you do DTI using parallel imaging, normal SNR methods will not
> work. You need to have signal image and noise only image. Get the mean
> signal and noise standard deviation from an ROI then calculate the SNR. You
> can get noise-only image in RF-off mode.
>
> -Seongjin
>
>
> On Mon, Jun 28, 2010 at 7:37 PM, Juranek, Jenifer <
> Jenifer.Juranek at uth.tmc.edu> wrote:
>
>>  While the issue of SNR effects on DTI metrics is discussed in the
>> published literature, I have found relatively little information regarding
>> calculating SNR as a screening procedure for image quality. A few authors
>> have reported the SNR range (or cut-off) of their datasets - but not many.
>> I'm curious about the following questions...any responses/opinions would be
>> very helpful.
>>
>>
>>
>> 1) Is calculation of SNR an effective screening tool for image quality in
>> DTI research?
>>
>> 2) Is calculation of SNR helpful for determining the feasibility of
>> pooling 3T DTI data from different scanner platforms (e.g. Siemens and
>> Philips)
>>
>> 3) To what extent do parallel imaging and acceleration factors affect
>> measures of SNR?
>>
>> 4) If calculation of SNR is a "good thing to do and report", then what is
>> the best approach?
>>
>>
>>
>> In particular, datatype for analysis (e.g. b=0, trace, DW) and location
>> "inside the brain" seem to be important things to consider. In terms of
>> datatype, I have seen folks use single non-averaged b=0 for each subject,
>> and a mathematical relation between sequential b=0 images, and some have
>> averaged their b=0 data on the scanner.
>>
>>
>>
>> Many thanks for any feedback,
>>
>> Jenifer
>>
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>
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