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Hi Todd, I am no longer on site so I must wait until the morning to get the wfs for the sequence off the scanner. The equation sounds like it could work. I'll check it in the morning and see what type of result I get. What is the significance of the number 434.215 and how did you arrive at this number? Have you managed to find a way to get the correct scaling factors for the b0 map? What type of fieldmap are you able to obtain from your scanner? Michael On Thu, Aug 5, 2010 at 4:35 PM, Todd L. Richards <[log in to unmask]>wrote: > For Philips Acheiva, the best equation for echo spacing is > > echo spacing =(1000*wfs)/(434.215 *(etl+1)) > > where wfs is the Philips value of water fat shift in pixels > > etl is the echo train length or Philips calls this the epi factor > > > > The problem we have had even though we can get the echo train length is to > get the correct scaling factors > > for the b0 map. > > Todd > > > > > > > ------------------------------ > > *From:* FSL - FMRIB's Software Library [mailto:[log in to unmask]] *On > Behalf Of *Auer, Tibor M.D. Ph.D. > *Sent:* Thursday, August 05, 2010 8:15 AM > > *To:* [log in to unmask] > *Subject:* Re: [FSL] Echo Spacing (FUGUE) > > > > I still believe that the key is the echo train length. > If it is really a single-shot EPI, than ETL could mean time to acquire a > full echo train (i.e. slice). > In this case your ES = ETL/PEsteps: 47ms/80 = 0.5875ms > > 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. 16:50 keltezéssel, Michael Roche írta: > > I now see that an echo spacing of 1.8 would be way too high. I will try to > look at the sequence in the scanner and see if I can measure the time > between the middle of each of the readout blocks. But in case this is not > possible could anybody confirm if there is any mathematical way to > calculate ES? It is the only parameter that is holding me back from my > processing. > > > > Thanks > > > > Michael > > > > On Thu, Aug 5, 2010 at 3:28 PM, kochunov <[log in to unmask]> wrote: > > > > I think the echo space for this sequence should be somewhere between > 0.5-0.9ms range. The easiest way to measure it would be by looking at the > sequence in simulator and just measuring the time between the middle of each > of the readout blocks. If I’m not mistaken this can be done on a console of > a phillips scanner. > > pk > ------------------------------ > > *From:* FSL - FMRIB's Software Library [mailto:[log in to unmask]] *On > Behalf Of *Matt Glasser > *Sent:* Thursday, August 05, 2010 9:09 AM > > > *To:* [log in to unmask] > *Subject:* Re: [FSL] Echo Spacing (FUGUE) > > > > Why isn’t echo spacing simply echo train length (I am assuming you gave it > in ms) divided by number of phase encoding steps? That would give 0.5875ms, > which is in the range of the echo spacings I usually get from the Siemens > scanner. 1.81ms seems high to me for an echo spacing on an EPI diffusion > sequence. The echo spacing is used as a scaling factor for applying the > field map correction, and with a TE of ~77ms you should have less distortion > than one would have with a higher TE e.g. 90-100ms that gives an echo > spacing of around 0.7ms. > > > > Peace, > > > Matt. > > > ------------------------------ > > *From:* FSL - FMRIB's Software Library [mailto:[log in to unmask]] *On > Behalf Of *kochunov > *Sent:* Thursday, August 05, 2010 9:07 AM > *To:* [log in to unmask] > *Subject:* Re: [FSL] Echo Spacing (FUGUE) > > > > Hi, > > Given that your bandwidth per pixel is over 3Khz e.g. It just takes 0.3 ms > to read a line in a k-space, I’d say your echo spacing should be smaller > than 1.8. > > Cheers > > pk > ------------------------------ > > *From:* FSL - FMRIB's Software Library [mailto:[log in to unmask]] *On > Behalf Of *Michael Roche > *Sent:* Thursday, August 05, 2010 8:46 AM > *To:* [log in to unmask] > *Subject:* Re: [FSL] Echo Spacing (FUGUE) > > > > 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 > > > > > > >