We all know that T1 and T2 relaxation times are sensitive to the magnetic field strength (B0).<div>However, diffusion constant is not related to B0. So, theoretically, diffusion-based measurements should be independent of B0.</div>
<div>However, B0 may have independent effects. Here are some thoughts;</div><div><br></div><div>1) Effect of noise: From simulation studies, we know that anisotropy measures such as FA contains bias (FA increases) as SNR goes down. In general, any measurements have more variability as SNR goes down. Suppose our measurements is 10 +/- 2 with high SNR, we expect 10+/-4 with lower SNR; variability goes up as SNR goes down, while the averages stay the same. Diffusion constant measurements are like this. To measure anisotropy, you measure multiple diffusion constants along multiple orientations, from which FA is calculated. Suppose FA is 0.5 +/- 0.1 with high SNR, as SNR goes down, FA becomes like 0.7 +/- 0.2; both the average value and variability go up. This shift of average (from 0.5 to 0.7) is "bias". If we do two scans; one with 3T and the other with 1.5T with the same resolution and scan time, 1.5T would give lower SNR and therefore it's possible that you observe higher FA.</div>
<div>2) Effect of resolution: In general, as resolution goes down, one pixel contains more axons with different orientations. Therefore you expect lower FA. You often use higher resolution with 3T because of the SNR gain. Then you can't compare FA values between 1.5T and 3T.</div>
<div>3) DTI is based on EPI data acquisition, in which all 128 echoes are acquired after one excitation. If your field of view is 256mm, you get 2mm pixel resolution. To increase the resolution to, say 1mm, you need to increase the number of echoes, say 256 echoes. You can do it and get 1mm resolution. However, your image would not look like high-resolution image because after 128 echoes, there is not much signal left and remaining 128 echoes are just measuring noise. The resolution of EPI data is thus deceptive. As B0 goes up, signal decay during the long 128 echoes increases. This means, with 1.5 T, we have enough signal left after 128 echoes and you may want to try 144 echoes to increase the resolution. With 7T, you have to wonder if any signal left after 128 echoes and there is not much point to sample 144 echoes. This is why DTI with 7T is difficult. We get much larger signal in the beginning of the echo train but it decays so fast during the echo train. <br>
<br><div class="gmail_quote">On Fri, Mar 1, 2013 at 11:07 PM, Ardalan Vossoughi <span dir="ltr"><<a href="mailto:clearmindpmdoc@hotmail.com" target="_blank">clearmindpmdoc@hotmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div><div style="font-family:Calibri,'Segoe UI',Meiryo,'Microsoft YaHei UI','Microsoft JhengHei UI','Malgun Gothic','Khmer UI','Nirmala UI',Tunga,'Lao UI',Ebrima,sans-serif;font-size:16px">
<div>Dear Dr Mori, MRI-studio professors and researchers<br>I would like to thank you in advance for this great learning opportunity;<br>I have a question that is more theoretical and need your opinion on this if possible: there<br>
are some papers indicating that there could be significant differences in DWI findings based on the scanner Tesla power; and apparently 3 Tesla is more sensitive than 1.5 Tesla; I would like to know<br>a. Is it a general rule: can we also conclude that higher Tesla scanners would possibly help more or not necessarily? what are the implications of such differences in terms of possible DTI analysis (if any at all) ;Is there any limitations or considerations that should be taken for lower versus higher Tesla imaging while analyzing the data?</div>
<div><div> </div><div>Sincerely</div><div>AV</div><div>Sent from Windows Mail</div><div> </div></div></div></div><br>_______________________________________________<br>
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