[Mristudio-users] The b matrix

[darshan pai]

Thanx for the description

Darshan


--- On Mon, 7/21/08, susumu <susumu at mri.jhu.edu> wrote:

> From: susumu <susumu at mri.jhu.edu>
> Subject: Re: [Mristudio-users] The b matrix
> To: "'DTI Studio, ROI Editor, Landmarker Questions/Support'" <mristudio-users at mristudio.org>
> Date: Monday, July 21, 2008, 7:10 PM
> Hi Darshan,
> 
> If we assume one tensor (one fiber population) in each
> pixel, b-value of 700
> - 1,500 should provide similar results. 
> 
> If b-value is small, the image intensity is high, echo time
> shorter, less
> motion sensitive, and less eddy current. The downside is,
> you have less
> signal attenuation and determination of the slope (=D =
> proportional to
> Log(S/S0)) becomes unstable. For example, if there is only
> 5-10% expected
> signal loss while there is +/- 5% signal fluctuation by
> SNR, you have a
> problem.
> 
> When b is higher than 1,000, you expect more than 50% of
> signal attenuation
> in average, which is enough to determine D, but the signal
> goes closer to
> the noise floor and have other practical issues such as
> longer TE (thus
> lower SNR), more motion sensitive, and eddy current. 
> 
> So too small b and too high b are not good from the SNR
> point of view.
> Empirically, the optimum b should be somewhere around 700 -
> 1,500.
> 
> Here, the b - SNR relationship is the center of our
> interest when we use the
> simple tensor model.
> 
> There are some simulation-based papers that tired to
> theoretically determine
> the optimum b, but it is not always straightforward because
> the impacts of
> TE/motion/eddy currents are difficult to model. Danny
> Alexander has the most
> comprehensive simulation paper in this regard. Our internal
> data with actual
> human data didn't show much difference between b = 750
> to 1,500. Start to
> see problems with b-values lower than 500 and more than
> 2,000 .
> 
> If you are interested in more than one tensor in each
> pixel, there is a
> totally different factor. First of all, the decay is not
> mono-exponential.
> An interesting feature of exponential decay is, if you want
> to differentiate
> bi-exponential and mono-exponential, you have to look at
> high-b range
> because they behave identical in low-b range. In other
> words, everything
> looks single-tensor if low b-values are used. This
> simulation can be seen in
> Larry Frank's HARDI paper, showing two-crossing-fiber
> pixel looks like a
> single pancake tensor, not "X" with a low
> b-value. The higher the b-value,
> the higher the power to delineate the "X" shape
> accurately. The smaller the
> crossing angle, the more difficult to resolve with smaller
> b. This is why
> the original Van Wedeen paper used b = 30,000.
> 
> It is still under investigation by many researchers to
> figure out how much b
> is needed. Some say 3,000 is enough, but it should be a
> function of crossing
> angle, SNR, and the number of b-orientations. I don't
> know the answer.
> 
> I hope this helps.
> 
> Susumu
> 
> 
> 
> 
> -----Original Message-----
> From: mristudio-users-bounces at mristudio.org
> [mailto:mristudio-users-bounces at mristudio.org] On Behalf Of
> darshan pai
> Sent: Friday, July 18, 2008 4:46 PM
> To: mristudio-users at mristudio.org
> Subject: [Mristudio-users] The b matrix
> 
> Hello all,
> 
> I just had a question regarding the Weighting matrix . I
> have always seen
> that the b value is usually set to be 1000 . Is there a
> specific reason to
> do that . I have seen literature which suggest that having
> b value to 1000
> is appropriate to at least resolve two fiber populations.
> Which means that
> increasing the value should be advantageous for multiple
> orientations. Is
> there an SNR issue to take care of at higher b values ?
> 
> Maybe the question is too basic and maybe a little too
> trivial . Just
> curious to know
> 
> Regards
> Darshan
> 
> 
> 
>       
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