[Mristudio-users] 1.5 Tesla versus 3 Tesla

[Ardalan Vossoughi]

Thanks for your significantly helpful comment;
Sincerely
AV

On Sat, Mar 2, 2013 at 6:00 AM, susumu mori <susumu at mri.jhu.edu> wrote:

> We all know that T1 and T2 relaxation times are sensitive to the magnetic
> field strength (B0).
> However, diffusion constant is not related to B0. So, theoretically,
> diffusion-based measurements should be independent of B0.
> However, B0 may have independent effects. Here are some thoughts;
>
> 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.
> 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.
> 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.
>
> On Fri, Mar 1, 2013 at 11:07 PM, Ardalan Vossoughi <
> clearmindpmdoc at hotmail.com> wrote:
>
>>  Dear Dr Mori, MRI-studio professors and researchers
>> I would like to thank you in advance for this great learning opportunity;
>> I have a question that is more theoretical and need your opinion on this
>> if possible: there
>>  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
>> 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?
>>
>> Sincerely
>> AV
>> Sent from Windows Mail
>>
>>
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>
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