Hi Darren,<br><br>In a previous communication in the mailing list, I explained why we can not use regular k-space averaging for DTI and, therefore, we average data after magnitude calculation (we average images, not k-space data). If anybody remembers which thread it was, I would appreciate it.<br>
<br>Anyway, because we first have to calculate image before averaging, there is no point to ask scanners to do the averaging. One downside of the averaging-by-the-scanner is, if the images are mis-registered or there are corrupted images, we can not undo the averaging. <br>
<br>I believe that is why Siemens always save all images separately (without creating an "averaged image" even if multiple averaging is specified. <br><br>Please note the difference between "repeated scans (you submitted the same protocol twice)" and "averaged scans (you specified 2 signal averaging)" when you use Siemens. Either way, all images are saved individually and they are basically the same studies. However, in the former case, the "repeated images" are stored in different directories and Siemens console can not combine them for DTI calculation. With the latter case, the repeated images are stored in the same directory and Siemens console can combine them to calculate one DTI data.<br>
<br>Either way, when you use DtiStudio, you need to load all files at once and DtiStudio can combine them to calculate one DTI. <br><br>Now going back to your question;<br><br>1) You can specify the number of signal averaging in Siemens. I'm not sure why you thought you couldn't. However, if you meant, "Siemens saved all images separately without averaging", then you are correct.<br>
2) DtiStudio can read repeated scans and combine them for one DTI calculation. <br><br><div class="gmail_quote">On Sun, Feb 28, 2010 at 7:54 PM, Darren Gitelman <span dir="ltr"><<a href="mailto:d-gitelman@northwestern.edu">d-gitelman@northwestern.edu</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">Hello:<br>
<br>
Is there a way to combine data across several repeats of a DTI sequence?<br>
<br>
The reason I ask is that we have a Siemens Trio scanner but are<br>
working with another group that has a GE scanner. On the GE scanner<br>
the DTI sequence allows NEX > 1 (they are getting 4), which appears to<br>
improve their SNR considerably. The Siemens sequences do not allow<br>
this, so we are repeating the DTI sequence several times, and want to<br>
combine the data across runs. Can this be done within DTIstudio or<br>
some other program? Would it be best to do this at the level of the<br>
DTI images or the FA maps?<br>
<br>
Thanks,<br>
Darren<br>
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