[Mristudio-users] Slice thickness of DTI

[Santosh Yadav]

Dear Professor,

Thanks a lot for making a time and great supports.

With Best Regards

On Tue, Nov 27, 2012 at 7:37 PM, susumu mori <susumu at mri.jhu.edu> wrote:

> When you consider effects of imaging parameters on the final outcomes, you
> need to know variability and bias.
> If the outcomes have simple Gaussian noises, less scans (thus lower SNR)
> would lead to higher variability. In your case, suppose you draw an ROI on
> the internal capsule and measure Trace of 100 voxels in the ROI, your
> second time points would have larger standard deviations. However, you
> could argue that the average values are still comparable.
>
> FA is known to have bias; as SNR goes down (e.g. less signal averaging),
> FA goes up. Depending on tensor fitting algorithm, completely random noise
> seem to converge at around 0.8 - 1.0 or even higher. As SNR goes up, the
> measured FA values converge to the real FA value.
>
> If your image protocol have sufficient SNR with 1 signal averaging such
> that the measured FA values are already close to the real FA values, then
> you won't see much impact between 2 and 4 signal averaging. If your image
> protocol has sufficient SNR with 4 averaging but not with 2 averaging, you
> would see uniformly higher FA with 2 signal averaging. In this case, you
> can't combine the data. Practically, if you are using 12-orientation / 3T /
> 2.5mm protocol, you may find FA increase with 2 averaging compared to 4. If
> you are using protocols with less SNR (e.g. 2mm resolution or 1.5T), the
> effect would be worse.
>
>
> On Mon, Nov 26, 2012 at 3:56 PM, Santosh Yadav <
> santoshyadav20076 at gmail.com> wrote:
>
>>  Thanks a lot Sir,
>>
>>
>> I have another question regarding DTI data; I have longitudinal DTI data
>> of 3 time points. First time point have 4 averages second time point have 2
>> averages and third time point have 4 averages, can I combine these data
>> sets?
>>
>> So finally the question is what the effect of averaging of DTI data, is
>> it going to the change the DTI matrix such as FA and Trace?
>>
>>
>> With best Regards,
>>
>>
>>
>> On Mon, Nov 26, 2012 at 2:31 AM, susumu mori <susumu at mri.jhu.edu> wrote:
>>
>>> Hi Santosh,
>>>
>>> Practically, the slice thickness, which is the thinner the better, is
>>> determined by SNR.
>>> If you use 2.5 mm, you need about 50-55 slices to cover the entire brain.
>>> 2.2 mm needs 55-60 and 2mm needs more than 60. These are all without
>>> gaps.
>>>
>>> With 5-6 min scans, you can acquire about 40-50 DWIs for 2.5mm/50-slice
>>> 3D data. These numbers decrease as the thickness become thinner and thus
>>> more slices are required and takes longer time.
>>>
>>> 40-50DWI means about 4 repetitions of 12-orientations.
>>>
>>> If you use a 1.5T, you may need more than 60 DWIs with 2.5mm, which
>>> takes about 7-8min. With thinner slices (e.g. 2.2mm), you need longer scans
>>> to achieve good SNR.
>>>
>>> With a 3T, 30-40 DWIs would give decent SNR with 2.5mm. With 2-2.2mm
>>> slice, you want more than 50-60 DWIs (5-8min).
>>>
>>> These numbers are to achieve a good SNR, which depends on my subjective
>>> judgement. So different people could recommend different numbers, but they
>>> may not be that much different.
>>>
>>> As for image distortion, the most important factor is the size of the
>>> image matrix; the smaller the better (I assume that you already use
>>> parallel imaging, which reduces the matrix size into half).
>>>
>>> For example, 128x128points / 256x256mm and 96x96/192x192mm both give the
>>> same 2mm resolution, but 96x96 would give better (less) distortion and
>>> better SNR.
>>>
>>> On Mon, Nov 26, 2012 at 1:28 AM, Santosh Yadav <
>>> santoshyadav20076 at gmail.com> wrote:
>>>
>>>> Hi DTI experts,
>>>>
>>>> Thanks for great supports!
>>>>
>>>> I would like know the recommended slice thickness and gap between the
>>>> slice for DTI data acquisition for 12 directions. Is higher
>>>> Slice thickness( more that 3 mm) have partial volume effect?
>>>>
>>>> Second question: Is eye open during the DTI acquisition have any impact
>>>> on DTI distortion or image quality?
>>>>
>>>> With Best Regards,
>>>>
>>>> Santosh
>>>>
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>>>
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>>
>>
>> --
>> With Best Regards,
>>
>> Neuroscience and MR Research Program
>>
>> Department of Medicine, Queen's Medical Center
>>
>> University of Hawaii
>>
>> 1356 Lusitana Street, 7th Floor, Honolulu, HI 96813
>>
>> Phone No. 808-220-7152
>>
>> Fax No.  808-545-8970
>>
>>
>>
>>
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>
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-- 
With Best Regards,

Neuroscience and MR Research Program

Department of Medicine, Queen's Medical Center

University of Hawaii

1356 Lusitana Street, 7th Floor, Honolulu, HI 96813

Phone No. 808-220-7152

Fax No.  808-545-8970
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