Dear Nacho, Torben, and SPMlist-
I'd like to add some points:)
>If you want to smooth anyway you should not care too much about cross talk
anyway
Smoothing will definitely attenuate "cross talk" effects but I feel it is
still better to avoid it as good as possible.
>If you are
> interested in high spatial resolution you would not accept gaps nor
> cross-talk, and I would go for the interleaved acquisition
...but depending on your TR and the interleaving procedure you may still end
up with some "cross talk" if you don't use any gaps.
>> we have a new eight channel
> > headcoil that can do 20 slices in less that a second... does it make
> > any sense with 1.5 T????
Watch out for your SNR when you acquire your images by some parallel
technique. In addition, some phase encodings won't work anymore.
Best regards-
Andreas
----- Original Message -----
From: "Torben E. Lund" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, September 24, 2003 2:38 PM
Subject: Re: interleaved adquisition for event related---> would you
recomend it to your friends???
> Dear Nacho and SPM'ers
>
> In short:
> The more you need the slicetiming (long TR) the more the assumptions for
> slicetiming are violated.
>
> The somewhat longer explanation:
> Slicetiming relies on the assumption that the acquired signal is
> band-limited and can therefore be interpolated based on our samples.
> While this is probably true for the BOLD signal (there is not much power
> in the BOLD response above 0.1 Hz) it is definitely not true for the
> coloured physiological noise (cardiac pulsation 1Hz, respiratory
> movement: 0.15Hz). These noise sources gets aliased when you do not
> sample them critically (i.e. TR shorter than 500ms for the fundamental
> cardiac frequency). This means that slicetiming will go wrong when
> trying to estimate what the signal in a given slice would have been had
> it been acquired at the same time as the reference slice. And the more
> under sampled the cardiac noise is the more incorrect slice-timing will
get.
> One reason for using interleaved acquisition is to avoid cross-talk
> between slices, and moreover some people has also found the interleaved
> acquisition to be more temporally stable. The drawback is that if you
> want to smooth afterwards you would like the BOLD signal in adjacent
> voxels (slices) to be the same, and not delayed by TR/2. Again the
> longer the TR the bigger the problem. If you want to smooth anyway you
> should not care too much about cross talk anyway, and then you should
> just use a flexible basis set and ascending acquisition. If you are
> interested in high spatial resolution you would not accept gaps nor
> cross-talk, and I would go for the interleaved acquisition and again a
> flexible basis set. Another approach would be to use a slice specific
> designmatrix like in Worsley's fmristat, but this still doesn't solve
> the issue of smoothing like with like.
> In SPM99 the smoothing issue could be solved by using smoothed contrast
> images for the second level analysis, but with SPM2 (which uses ReML
> estimation) this approach is not strictly correct any longer.
>
> Hope this helps
>
> Torben Lund, Danish Research Centre for MR
>
>
>
> Ignacio Vallines wrote:
>
> > Hi SPMers,
> >
> > we are using a Siemens Sonata 1.5T running Syngo... could anyone point
> > me out what the advantages/drawbacks of using interleaved aquisition
> > are???? of course for fine event related designs where timming is
> > crucial.... how reliable is the time correction algorithm in SPM2???..
> > what about the influence of short TRs??? we have a new eight channel
> > headcoil that can do 20 slices in less that a second... does it make
> > any sense with 1.5 T???? I have heard arguments and explanations for
> > and against it... what is the feeling from an SPM users perspective????
> >
> > I wold really appreciatte tips on this!
> >
> > thanks!
> >
> > Nacho
> >
>
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