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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Thanks very much Dr.Mori,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Venkat<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> mristudio-users-bounces@mristudio.org [mailto:mristudio-users-bounces@mristudio.org]
<b>On Behalf Of </b>susumu mori<br>
<b>Sent:</b> Thursday, February 14, 2013 5:09 PM<br>
<b>To:</b> DTI Studio, ROI Editor, DiffeoMap Questions/Support<br>
<b>Subject:</b> Re: [Mristudio-users] difference between ADC and MD<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Let me add little more history.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Suppose you do diffusion constant measurement using X, Y, and Z gradient individually. You get three results, diffusion along X (Dx), Y (Dy), and Z (Dz).<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">When you image an anisotropic sample like brain, Dx, Dy, Dz are not equal.<o:p></o:p></p>
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<p class="MsoNormal">Each of these is called "Apparent diffusion constant". They are "apparent" because depending on the gradient axis you use or the orientation of the sample, they vary.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Then people found that Dx+Dy+Dz is invariant, meaning it stays the same regardless of the sample orientation.<o:p></o:p></p>
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<p class="MsoNormal">Dx is rotation-variant because if you use X-gradient and measure the same sample (e.g. a chunk of meat) three times with different orientations, you get different Dx values.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">So, Dx+Dy+Dz is a convenient value because you don't have to worry about consistency of sample orientations. <o:p></o:p></p>
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<p class="MsoNormal">Dx+Dy+Dz is called "Trace". In DtiStudio, we use this "Trace".<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">(Dx+Dy+Dz)/3 is called "Mean diffusivity"<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Confusion arises when we start to use "Averaged diffusion constant", which also is (Dx+Dy+Dz)/3 and the same as "Mean diffusivity".<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">When we use the abbreviation, "ADC", it is not clear if it means "apparent diffusion constant" or "averaged diffusion constant".<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">So, you have to read the context carefully. MD is, in this sense, a better term.<o:p></o:p></p>
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<p class="MsoNormal">On Thu, Feb 14, 2013 at 2:46 PM, Abhijit Das <<a href="mailto:adas@kesslerfoundation.org" target="_blank">adas@kesslerfoundation.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">Dear Venkatesh and Hangyi,<br>
You clarified a big confusion for me today. <br>
Thanks,<br>
Abhijit<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-family:"Tahoma","sans-serif";color:black">From:</span></b><span style="font-family:"Tahoma","sans-serif";color:black">
<a href="mailto:mristudio-users-bounces@mristudio.org" target="_blank">mristudio-users-bounces@mristudio.org</a> [<a href="mailto:mristudio-users-bounces@mristudio.org" target="_blank">mristudio-users-bounces@mristudio.org</a>] on behalf of Hangyi Jiang [<a href="mailto:hjiang@jhmi.edu" target="_blank">hjiang@jhmi.edu</a>]<br>
<b>Sent:</b> Thursday, February 14, 2013 2:44 PM<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Tahoma","sans-serif";color:black"><br>
<b>To:</b> DTI Studio, ROI Editor, DiffeoMap Questions/Support<br>
<b>Subject:</b> Re: [Mristudio-users] difference between ADC and MD<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""><br>
some papers may use ADC for Averaging Diffusion Coefficient (<span style="color:#1F497D">synonymously with MD</span>). We don't use it in this way, it is so easy to get confused.<br>
<br>
I cannot say it is not right to use ADC as a synonymous term of MD (language changes all the time, :) ). but in the very beginning, ADC means apparent diffusion coefficient.<br>
<br>
best,<br>
<br>
hangyi<br>
<br>
<br>
<br>
<br>
<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><b><span style="font-family:"Tahoma","sans-serif";color:black">From:</span></b><span style="font-family:"Tahoma","sans-serif";color:black">
<a href="mailto:mristudio-users-bounces@mristudio.org" target="_blank">mristudio-users-bounces@mristudio.org</a> [<a href="mailto:mristudio-users-bounces@mristudio.org" target="_blank">mristudio-users-bounces@mristudio.org</a>] on behalf of Venkatesh Rajagopalan
[<a href="mailto:VRajagopalan@kesslerfoundation.org" target="_blank">VRajagopalan@kesslerfoundation.org</a>]<br>
<b>Sent:</b> Thursday, February 14, 2013 2:27 PM<br>
<b>To:</b> 'DTI Studio, ROI Editor, DiffeoMap Questions/Support'<br>
<b>Subject:</b> Re: [Mristudio-users] difference between ADC and MD</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">Hangyi,</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">Sorry again though. In some manuscripts ADC is used synonymously with MD, so this is wrong, right?.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">Thanks</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">Venkat</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">
<a href="mailto:mristudio-users-bounces@mristudio.org" target="_blank">mristudio-users-bounces@mristudio.org</a> [mailto:<a href="mailto:mristudio-users-bounces@mristudio.org" target="_blank">mristudio-users-bounces@mristudio.org</a>]
<b>On Behalf Of </b>Hangyi Jiang<br>
<b>Sent:</b> Thursday, February 14, 2013 2:12 PM<br>
<b>To:</b> DTI Studio, ROI Editor, DiffeoMap Questions/Support<br>
<b>Subject:</b> Re: [Mristudio-users] difference between ADC and MD</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">hi,<br>
<br>
apparent diffusion coefficient (ADC) is a measurement for a diffusion weighted image in a specific gradient. its definition is:<br>
<br>
ADC = ln(S/S0) / (-b)<br>
here, S is the gradient weighted image, S0 is the B0 image, b is the b value.<br>
<br>
so, if a data set has 30 gradient orientations, there will be 30 ADCs. for each gradient, there will be a ADC map for this particular DW-image.<br>
<br>
<br>
mean- diffusivity is defined as the mean value of eigen values (Lamda 1-3) of the tensor ellipsoid:<br>
Mean_Diffusivity = (Lamda_1 + Lamda_2 + Lamda_3) / 3;<br>
<br>
mathematically, this is equal to the average of the diagonal elements of the tensor matrix:<br>
Mean_Diffusivity = (Dxx + Dyy + Dzz) / 3;<br>
<br>
in noise-free situation (practically, it is impossible), mean-diffusivity is also equal to the mean value of ADCs of the DWI data set.<br>
e.g. given a data set with 30 orientations, then Mean_Diffusivity = (ADC1 + ADC2 + .. ADC30) / 30;<br>
<br>
BTW: the sum of eigen-values or the sum of the diagonal elements of the tensor matrix is called TRACE:<br>
TRACE = (Lamda_1 + Lamda_2 + Lamda_3) = (Dxx + Dyy + Dzz);<br>
<br>
so, Mean_Diffusivity = TRACE/3;<br>
<br>
<br>
best,<br>
<br>
hangyi<br>
<br>
<br>
</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">
<a href="mailto:mristudio-users-bounces@mristudio.org" target="_blank">mristudio-users-bounces@mristudio.org</a> [<a href="mailto:mristudio-users-bounces@mristudio.org" target="_blank">mristudio-users-bounces@mristudio.org</a>] on behalf of Venkatesh Rajagopalan
[<a href="mailto:VRajagopalan@kesslerfoundation.org" target="_blank">VRajagopalan@kesslerfoundation.org</a>]<br>
<b>Sent:</b> Thursday, February 14, 2013 1:40 PM<br>
<b>To:</b> 'DTI Studio, ROI Editor, DiffeoMap Questions/Support'<br>
<b>Subject:</b> [Mristudio-users] difference between ADC and MD</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Hi All,<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I very often come across apparent diffusion coefficient and mean diffusivity being measured in studies. I am wondering what is the difference between these two measures in terms
of the formula used.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Thanks<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Venkat<o:p></o:p></p>
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