[Mristudio-users] difference between ADC and MD

Fri Feb 15 09:53:26 EST 2013

Thanks very much Dr.Mori,

Venkat

From: mristudio-users-bounces at mristudio.org [mailto:mristudio-users-bounces at mristudio.org] On Behalf Of susumu mori
Sent: Thursday, February 14, 2013 5:09 PM
To: DTI Studio, ROI Editor, DiffeoMap Questions/Support
Subject: Re: [Mristudio-users] difference between ADC and MD

Let me add little more history.

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).

When you image an anisotropic sample like brain, Dx, Dy, Dz are not equal.
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.

Then people found that Dx+Dy+Dz is invariant, meaning it stays the same regardless of the sample orientation.
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.

So, Dx+Dy+Dz is a convenient value because you don't have to worry about consistency of sample orientations.
Dx+Dy+Dz is called "Trace". In DtiStudio, we use this "Trace".

(Dx+Dy+Dz)/3 is called "Mean diffusivity"

Confusion arises when we start to use "Averaged diffusion constant", which also is (Dx+Dy+Dz)/3 and the same as "Mean diffusivity".

When we use the abbreviation, "ADC", it is not clear if it means "apparent diffusion constant" or "averaged diffusion constant".

So, you have to read the context carefully. MD is, in this sense, a better term.
On Thu, Feb 14, 2013 at 2:46 PM, Abhijit Das <adas at kesslerfoundation.org<mailto:adas at kesslerfoundation.org>> wrote:
Dear Venkatesh and Hangyi,
You clarified a big confusion for me today.
Thanks,
Abhijit
________________________________
From: mristudio-users-bounces at mristudio.org<mailto:mristudio-users-bounces at mristudio.org> [mristudio-users-bounces at mristudio.org<mailto:mristudio-users-bounces at mristudio.org>] on behalf of Hangyi Jiang [hjiang at jhmi.edu<mailto:hjiang at jhmi.edu>]
Sent: Thursday, February 14, 2013 2:44 PM

To: DTI Studio, ROI Editor, DiffeoMap Questions/Support
Subject: Re: [Mristudio-users] difference between ADC and MD

some papers may use ADC for Averaging Diffusion Coefficient (synonymously with MD).   We don't use it in this way, it is so easy to get confused.

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.

best,

hangyi

________________________________
From: mristudio-users-bounces at mristudio.org<mailto:mristudio-users-bounces at mristudio.org> [mristudio-users-bounces at mristudio.org<mailto:mristudio-users-bounces at mristudio.org>] on behalf of Venkatesh Rajagopalan [VRajagopalan at kesslerfoundation.org<mailto:VRajagopalan at kesslerfoundation.org>]
Sent: Thursday, February 14, 2013 2:27 PM
To: 'DTI Studio, ROI Editor, DiffeoMap Questions/Support'
Subject: Re: [Mristudio-users] difference between ADC and MD
Hangyi,

Sorry again though. In some manuscripts ADC is used synonymously with MD, so this is wrong, right?.

Thanks
Venkat

From: mristudio-users-bounces at mristudio.org<mailto:mristudio-users-bounces at mristudio.org> [mailto:mristudio-users-bounces at mristudio.org<mailto:mristudio-users-bounces at mristudio.org>] On Behalf Of Hangyi Jiang
Sent: Thursday, February 14, 2013 2:12 PM
To: DTI Studio, ROI Editor, DiffeoMap Questions/Support
Subject: Re: [Mristudio-users] difference between ADC and MD

hi,

apparent diffusion coefficient (ADC) is a measurement for a diffusion weighted image in a specific gradient. its definition is:

ADC = ln(S/S0) / (-b)
here, S is the gradient weighted image, S0 is the B0 image, b is the b value.

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.

mean- diffusivity is defined as the mean value of eigen values (Lamda 1-3) of the tensor ellipsoid:
Mean_Diffusivity = (Lamda_1 + Lamda_2 + Lamda_3) / 3;

mathematically, this is equal to the average of the diagonal elements of the tensor matrix:
Mean_Diffusivity =  (Dxx + Dyy + Dzz) / 3;

in noise-free situation (practically, it is impossible), mean-diffusivity is also equal to the mean value of ADCs of the DWI data set.
e.g. given a data set with 30 orientations, then Mean_Diffusivity = (ADC1 + ADC2 + .. ADC30) / 30;

BTW: the sum of eigen-values or the sum of the diagonal elements of the tensor matrix is called TRACE:
TRACE =  (Lamda_1 + Lamda_2 + Lamda_3) = (Dxx + Dyy + Dzz);

so, Mean_Diffusivity = TRACE/3;

best,

hangyi

________________________________
From: mristudio-users-bounces at mristudio.org<mailto:mristudio-users-bounces at mristudio.org> [mristudio-users-bounces at mristudio.org<mailto:mristudio-users-bounces at mristudio.org>] on behalf of Venkatesh Rajagopalan [VRajagopalan at kesslerfoundation.org<mailto:VRajagopalan at kesslerfoundation.org>]
Sent: Thursday, February 14, 2013 1:40 PM
To: 'DTI Studio, ROI Editor, DiffeoMap Questions/Support'
Subject: [Mristudio-users] difference between ADC and MD
Hi All,

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.

Thanks
Venkat

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