Hi Danilo
There are several noise contributors to noise in fMRI (low-frequency
drift, cardiac pulsation and respiration), and therefore it is
difficult to create a realistic artificial data. Phantoms are great
tools to investigate scanner drift, I normally use a gel-phantom to
avoid convection effects, and preferably you would use a phantom with
some internal structures, since the drift is normally pronounced near
the edges. Ideally the phantom would contain three different
compartments with T1 and T2 close to those of WM GM and CSF.
The cardiac and respiratory induced noise could be modelled as aliased
oscillations with original frequencies near 0.1 and 1Hz plus some
higher harmonics. So those you can create e.g. a 1Hz oscillation and
sample it at your TR. Alternatively you can use the function bellow to
calculate the aliased frequency, fa, for your specific frequency, f, at
a sampling rate fs=1/TR. If you want to be sophisticated you can even
let the frequency change slightly over time e.g. to model higher pulse
during stimulation.
Best,
Torben
Torben E. Lund
Danish Research Centre for MR
Copenhagen University Hospital
Kettegaard Allé 30
2650 Hvidovre
Denmark
email: [log in to unmask]
webpage: http://www.drcmr.dk
%-----------------------------------------------------------------------
------------------------------------------------------------------------
----------------
function fa=f_alias(f,fs)
% syntax: fa=f_alias(f,fs)
% This function calculates the aliased frequency, fa, when given a
frequency, f, and a sampling frequency, fs.
fn=fs/2;
n=floor(f/fn);
fd=f-n*fn;
if n/2-floor(n/2)==0 %n is even
fa=fd;
else
fa=fn-fd;
end
%-----------------------------------------------------------------------
------------------------------------------------------------------------
----------------
On 14 Dec 2005, at 12:52, danilo dongiovanni wrote:
> Hi all,
>
> is there any available phantom time series dataset ?
>
> Or some matlab tool to simulate fmri noise accurately?
>
> Or some set of basic rules to follow when trying to build phantom time
> series?
>
>
> thanks for any help
>
> danilo
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