[Mristudio-users] ADC values decrease with increasing b-values

[susumu]

Hi Gustav,

Your observation is correct. When we determine ADC, we perform
mono-exponential fitting, but the signal decay is hardly mono-exponential.
When the intensity as a function of b-value is plotted in a log scale, it
should be a straight line if it is mono-exponential, but reality is, it
curves. That means, if we do 2-point measurements like 0 and 500, 0 and
1000, or 0 and 1,500, the determined slope gets less and less steep. I
attached our internal data acquired by Ming-Chung Chou. You can see very
strong dependency of ADC on b. This could be due to hitting a noise floor
but even with high SNR, the non mon-exponential behavior can be observed. As
a matter of fact, if we do simulation with different SNR, ADC values are
more stable than FA. 

This is somewhat the opposite of FA (second slide). Again this is
Ming-Chung's data. As you can see from the graph, FA is rather stable in
750-1500 range, but as we know, FA values is sensitive to SNR when SNR is
low.

Hope this will help.

Susumu

________________________________________
From: mristudio-users-bounces at mristudio.org
[mailto:mristudio-users-bounces at mristudio.org] On Behalf Of Gustav Andreisek
Sent: Friday, April 04, 2008 12:09 AM
To: Mristudio-users at mristudio.org
Subject: [Mristudio-users] ADC values decrease with increasing b-values

Dear All,

We imaged a cohort of volunteers at different b-values and determined the
ADC values. We found decreasing ADC with increasing b-values. This would be
in accordance to the literature, as long as we can assume that DTIstudio
does not automatically uses a correction factor for varying SNR at different
diffusion weightings (Dietrich O, Heiland S, Sartor K. Noise correction for
the exact determination of apparent diffusion coefficients at low SNR. Magn
Reson Med 2001; 45:448-453.).
Is that true?

Best regards and greetings from Canada,
Gustav

Gustav Andreisek, M.D.
Postdoctoral Research Fellow
Department of Medical Imaging, 5th Floor
Mount Sinai Hospital and the University Health Network
University of Toronto
600 University Ave
Toronto, ON, M5G 1X5
Canada  
Tel:     +1 416 586 4800 ext 2935
Fax:    +1 416 586 8695
E-mail: gustav at andreisek.de

-------------- next part --------------
A non-text attachment was scrubbed...
Name: TraceADC.ppt
Type: application/vnd.ms-powerpoint
Size: 163840 bytes
Desc: not available
Url : http://lists.mristudio.org/pipermail/mristudio-users/attachments/20080404/2bbe4e2c/attachment-0001.ppt