[Camino-users] How to derive the noise variance

[Ed Gronenschild]

Hi,

I'm sorry that I did not correctly formulated my question.
There are a few things I want to clarify in relation to
using RESTORE:

1.
The man page on estimatesnr tells that the noise standard
deviation can be estimated as

sigma = sqrt( 2.0 / ( 4.0 - pi ) ) * stddev( bgnd signal).

In my case: stddev( bgnd signal) = 1.10, so that would
result in sigma = 1.68

Is this the value I should pass to RESTORE?

2.
I've downloaded the DIG ActiveAX data set as illustrated
in the DTI tutorial. I've derived the sttdev of the
bgnd signal and found a value of 107. According to
the formula above this would result in a sigma of 162.
However, in the tutorial you are using sigma = 200.

3.
In addition, it's confusing that the man pages on RESTORE
are telling that one should supply <noise std> as argument
whereas the tutorial on DTI says that it is the noise
VARIANCE (see chapter 9.3).

My first experience with RESTORE is that the results are
dependent on the supplied value of the noise. So I would
appreciate if you could advice me on a valid method
to derive a reliable estimate of the noise.

Cheers,
Ed


On 2 Nov 2011, at 23:04, Philip A Cook wrote:

> Hi,
>
> I believe you pass the noise sigma directly. You can use the Camino  
> command estimatesnr to assist in this computation
>
> On Oct 31, 2011, at 11:28 AM, Ed Gronenschild wrote:
>
>> Hi,
>>
>> It;s not entirely clear to me how to derive the noise variance
>> in the DTI data, the value I have to supply with the RESTORE
>> command.
>> Suppose I define contours in the background of the B0 volume
>> and I derive a standard deviation of 1.68. What should I
>> specify in that case?
>>
>> Cheers,
>> Ed
>>
>> _______________________________________________
>> Camino-users mailing list
>> Camino-users at www.nitrc.org
>> http://www.nitrc.org/mailman/listinfo/camino-users
>