When you consider effects of imaging parameters on the final outcomes, you need to know variability and bias.<div>If the outcomes have simple Gaussian noises, less scans (thus lower SNR) would lead to higher variability. In your case, suppose you draw an ROI on the internal capsule and measure Trace of 100 voxels in the ROI, your second time points would have larger standard deviations. However, you could argue that the average values are still comparable.</div>
<div><br></div><div>FA is known to have bias; as SNR goes down (e.g. less signal averaging), FA goes up. Depending on tensor fitting algorithm, completely random noise seem to converge at around 0.8 - 1.0 or even higher. As SNR goes up, the measured FA values converge to the real FA value.</div>
<div><br></div><div>If your image protocol have sufficient SNR with 1 signal averaging such that the measured FA values are already close to the real FA values, then you won't see much impact between 2 and 4 signal averaging. If your image protocol has sufficient SNR with 4 averaging but not with 2 averaging, you would see uniformly higher FA with 2 signal averaging. In this case, you can't combine the data. Practically, if you are using 12-orientation / 3T / 2.5mm protocol, you may find FA increase with 2 averaging compared to 4. If you are using protocols with less SNR (e.g. 2mm resolution or 1.5T), the effect would be worse. <br>
<br><div class="gmail_quote">On Mon, Nov 26, 2012 at 3:56 PM, Santosh Yadav <span dir="ltr"><<a href="mailto:santoshyadav20076@gmail.com" target="_blank">santoshyadav20076@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<p class="MsoNormal"> Thanks a lot Sir,</p><p class="MsoNormal"><br></p>
<p class="MsoNormal">I have another question regarding DTI data; I have
longitudinal DTI data of 3 time points. First time point have 4 averages second
time point have 2 averages and third time point have 4 averages, can I combine
these data sets?</p>
<p class="MsoNormal">So finally the question is what the effect of averaging of
DTI data, is it going to the change the DTI matrix such as FA and Trace?</p><p class="MsoNormal"><br></p>
With best Regards,<div><div class="h5"><br><br><br><div class="gmail_quote">On Mon, Nov 26, 2012 at 2:31 AM, susumu mori <span dir="ltr"><<a href="mailto:susumu@mri.jhu.edu" target="_blank">susumu@mri.jhu.edu</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Hi Santosh,<div><br></div><div>Practically, the slice thickness, which is the thinner the better, is determined by SNR.</div><div>If you use 2.5 mm, you need about 50-55 slices to cover the entire brain.</div><div>2.2 mm needs 55-60 and 2mm needs more than 60. These are all without gaps.</div>
<div><br></div><div>With 5-6 min scans, you can acquire about 40-50 DWIs for 2.5mm/50-slice 3D data. These numbers decrease as the thickness become thinner and thus more slices are required and takes longer time.</div><div>
<br></div><div>40-50DWI means about 4 repetitions of 12-orientations.</div><div><br></div><div>If you use a 1.5T, you may need more than 60 DWIs with 2.5mm, which takes about 7-8min. With thinner slices (e.g. 2.2mm), you need longer scans to achieve good SNR.</div>
<div><br></div><div>With a 3T, 30-40 DWIs would give decent SNR with 2.5mm. With 2-2.2mm slice, you want more than 50-60 DWIs (5-8min).</div><div><br></div><div>These numbers are to achieve a good SNR, which depends on my subjective judgement. So different people could recommend different numbers, but they may not be that much different.</div>
<div><br></div><div>As for image distortion, the most important factor is the size of the image matrix; the smaller the better (I assume that you already use parallel imaging, which reduces the matrix size into half).</div>
<div><br></div><div>For example, 128x128points / 256x256mm and 96x96/192x192mm both give the same 2mm resolution, but 96x96 would give better (less) distortion and better SNR.<br><br><div class="gmail_quote"><div><div>
On Mon, Nov 26, 2012 at 1:28 AM, Santosh Yadav <span dir="ltr"><<a href="mailto:santoshyadav20076@gmail.com" target="_blank">santoshyadav20076@gmail.com</a>></span> wrote:<br>
</div></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div>Hi DTI experts,<div><br></div><div>Thanks for great supports!<br><div><br></div><div>
I would like know the recommended slice thickness and gap between the slice for DTI data acquisition for 12 directions. Is higher Slice thickness( more that 3 mm) have partial volume effect?</div>
<div><br></div><div>Second question: Is eye open during the DTI acquisition have any impact on DTI distortion or image quality?</div><div><br></div><div>With Best Regards,<br>
</div><div><br></div><div>Santosh</div></div>
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