Hi,
This is also certainly an overestimate of the echo spacing as it doesn't
account for any of the time spent doing excitation and, more
importantly,
waiting between excitation and the start of the readout, since to get
the
required TE usually means there is some time spent without the readout
in operation.
If you can find out how long the total read-out takes then you can
divide
this by the number of k-space lines acquired in this time. The
alternative
is to try and work it out from the bandwidth and the number of read-out
points, but be careful that the bandwidth is equal to 1/(time between
read-out samples) and not an oversampled bandwidth.
All the best,
Mark
On 5 Aug 2010, at 14:45, Michael Roche wrote:
> Hi,
>
> Thanks for the quick reply. Well the sequence I am using is
> definitely a single shot EPI sequence. The acquired matrix is 80x63
> and there is an acceleration factor making the echo train length
> equal 47. I have tried the formula ES = TR/Number of Slices/Number
> of Phase Encoding Steps, did you mean ES = TR/(Number of
> Slices*Number of Phase encoding steps)? Using the version of the
> formula I have assumed I calculated an ES of 1.81ms. Which is a
> value which makes sense to me. Would I be correct in assuming this
> value is the ES?
>
> TR = 849ms
> Number of Slices = 10
> echo train length = 47
>
> Regards
>
> Michael
>
> On Thu, Aug 5, 2010 at 1:23 PM, Auer, Tibor M.D. Ph.D.
> <[log in to unmask]> wrote:
> Hello,
>
> Echo spacing is the time btw. the phase encoding steps (i.e. btw.
> the K-space lines). You case seems to be a bit more complicated b/c
> it seems to be a multi-echo sequence, but not an EPI.
>
> In EPI:
> - Number of Phase Encoding Steps = Echo Train Length
> - 1 excitation pulse/slice
> - TR = time btw. volumes
> The basic equation is:
> ES = TR/Number of Slices/Number of Phase Encoding Steps
>
> So I suppose, your case is:
> - TR = time btw. excitation pulses
> - Echo Train Length = Number of Phase Encoding Steps btw. two
> excitation pulses
> The equation may be:
> ES = Min TR/Echo Train Length
>
> Does that sounds correct?
>
> Auer, Tibor M.D. Ph.D.
> Biomedizinische NMR Forschungs GmbH
> am Max-Planck Institut für Biophysikalische Chemie
> Am Fassberg 11
> 37077 Göttingen
> Germany
> Phone/Work: +49-(0)551-201-1725
> Phone/Home: +49-(0)551-387-0076
> Mobile: +49-(0)176-8012-7921
> Mail: [log in to unmask]
>
>
>
> 2010.08.05. 13:59 keltezéssel, Michael Roche írta:
> Hello,
> My query is in relation to calculating the echo spacing (ES) for use
> in distortion correction of DWI images using FUGUE. I am scanning
> with a 3T Philips Achieva which does not specify the ES and neither
> does the DICOM header file. I have asked the local Physicist and he
> is unsure on how to calculate the ES. The following are my
> acquisition parameters:
> Scanning Sequence: SE
> Slice thickness: 3mm
> TE: 76.56ms
> Number of Phase Encoding Steps: 80
> Echo Train Length: 47
> Pixel Bandwidth: 3295.2Hz
> Min TR: 849ms
> Calculating the ES has been causing me major problems so any help on
> this topic would be greatly appreciated. If anyone could outline the
> steps involved in calculating the ES that would be great. Feel free
> to suggest any parameters i may need to calculate the ES.
> Thanks
> Michael
>
>
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