Hi Scott,
In multiple spin echoes, errors in the flip angle for the refocusing pulses
can lead to reduced signal on odd echoes. These are compensated for to some
extent on even echoes.
Even if you get perfect 180 pulses at the centre of your image slices, the
slice profile won't be perfectly square, and you will have reduced flip
angle further from the centre of the slice.
Do all your even echoes have higher signal than expected?
Also, remember the T2 for CSF is quite long.
Dave
On 5/5/08 10:05, "Scott Kolbe" <[log in to unmask]> wrote:
> Dear FSL list
> this is a question not directly related to FSL but I am a bit lost for
> an alternative place to ask for advice. I have acquired multiecho T2
> images to study relaxation times in a patient group. i found that for
> the first echo image (13ms) the CSF signal is actually darker than in
> the second echo image (26ms) by about 10%. i have acquired 8 echos and
> for the final echo image (132ms), the CSF signal is only 3% darker than
> for the second echo.
> My question is, does anyone have any idea why the first echo would be
> darker than the second. i had presumed that the earlier the echo is
> collected, the more signal there is. is that true in practice?
>
> thanks in advance
> Scott
>
> ========================
> Scott Kolbe
> Postgraduate Student
> Neuroimaging Group
> Howard Florey Institute &
> Centre for Neuroscience
> University of Melbourne
> VIC, Australia, 3010.
>
> ph: +61 3 8344 1887
> email: [log in to unmask]
> website: http://www.neuroimaging.org.au/index.php?id=383
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