[Mristudio-users] Is better to do 3 DTI series or just one well done?

Fri Dec 11 21:14:01 EST 2009

Do you mean you are doing 33-orientation x 3 signal averaging for 5 mm
slice? That is equivalent to 99 DWIs. With such as thick slice and the 99
DWI equivalent, you should have enough SNR even with 1.5T.

3 NEX and 3 series are equivalent in terms of SNR. The only difference is
whether the scanner average the data for you or you get them saved
separately. Usually, if you do 3 separate series, MR scanner can not combine
them and does not offer an FA map based on the all 3 series. So, for
clinical scans, you have no choice but using 3 NEX.

If you are using Siemens, 3NEX and 3 series are completely identical because
they actually save the 3 NEX scans separately and use all of them for tensor
calculation. For example, if you do 3NEX of 33-orientaiton, you can find 99
Mosaic files. They don't actually average 3 scans. So it's the same as 3
series.

I'm not sure how GE works. I believe if you do 3NEX of 33-orientaiton, you
may get only 33 images, in which the 3 scans are already averaged. The
disadvantage of this approach is, if the subject moved or one of the images
are corrupted, you can not correct them once 3 images are averaged. These
issues can be corrected to some degree if you are using off-line programs
like DtiStudio or FSL. However, if you rely on FA map generation by the
scanner, I guess you'd better use 3NEX.

On Fri, Dec 11, 2009 at 11:05 AM, Lucas Lessa <lucas_lessa at yahoo.com.br>wrote:

> Once again thank you Dr Mori.
> and what do you think about increase the SNR in GE 1.5T scanners?
> We only do one serie with 33 directions, but we have to use slices with 5mm
> thickness, and NEX 3. Is it better to do 3 series like in Phillips or push
> the parameters?
>
>
> --- Em *qui, 10/12/09, susumu mori <susumu at mri.jhu.edu>* escreveu:
>
>
> De: susumu mori <susumu at mri.jhu.edu>
> Assunto: Re: [Mristudio-users] Is better to do 3 DTI series or just one
> well done?
> Para: "DTI Studio, ROI Editor, Landmarker Questions/Support" <
> mristudio-users at mristudio.org>
> Data: Quinta-feira, 10 de Dezembro de 2009, 16:59
>
> I wouldn't push the resolution that much. I even hesitate to use 128x128,
> unless we can use SENSE factor of 4 so that the echotrain is 128/4 =32
>
> On Thu, Dec 10, 2009 at 1:33 PM, Lucas Lessa <lucas_lessa at yahoo.com.br<http://mc/compose?to=lucas_lessa@yahoo.com.br>
> > wrote:
>
>> Dr. Susumu Mori you helped me a lot.
>> I'm using 3 repetions of a 33 dir DTI in a Philips 1.5T (the voxel is
>> about 2.2mm.) when I want to do an exam for a research, for the real life I
>> use just one repetion.
>> Indeed the SNR is much better with 3 repetions, but just in DTI Studio,
>> because Philip's software don't handle the 3 series like one.
>> What do you think in increase my resolution to 256x256 in this scanner?
>> Will the exam take so long? Can I use just 2 repetitions?
>> Thank you.
>>
>> Lucas Lessa.
>>
>>
>>
>> --- Em *qui, 10/12/09, susumu mori <susumu at mri.jhu.edu<http://mc/compose?to=susumu@mri.jhu.edu>
>> >* escreveu:
>>
>>
>> De: susumu mori <susumu at mri.jhu.edu<http://mc/compose?to=susumu@mri.jhu.edu>
>> >
>> Assunto: Re: [Mristudio-users] Is better to do 3 DTI series or just one
>> well done?
>> Para: "DTI Studio, ROI Editor, Landmarker Questions/Support" <
>> mristudio-users at mristudio.org<http://mc/compose?to=mristudio-users@mristudio.org>
>> >
>> Data: Quinta-feira, 10 de Dezembro de 2009, 15:22
>>
>>
>> If you are using a 1.5 T scanner, I think 2.5 mm isotropic resolution is a
>> good compromise between scanning time and SNR. With a 30-orientation scheme,
>> you may want to do at least two repetition (total 60 DWIs) to get a decent
>> SNR. If you want to increase the resolution, you need more dwis and a longer
>> scanning time.
>>
>> What is important is the total number of images, in this case 60. So, if
>> you are using a 15-orientation scheme, you need 4 repetitions. A
>> 30-orientation scheme should take about 4-5min in Siemens and Philips. GE
>> may take 7-8 min in some versions because of gradient overheating precaution
>> (TR can't be too short). So, two repetitions (total 60 dwis) should take
>> 8-10 min in Siemens/Philips. This is certainly a long scan, especially for
>> clinical. If time is important, you can do one repetition (total 30 dwis),
>> scarifying the SNR. In this case, you definitely don't want to increase the
>> resolution, which would make the SNR worse.
>>
>> For 3T, 30 dwis can give you a decent SNR with 2.5mm resolution. Or if you
>> stick with a 60 dwis protocol, you can use 2.2 mm resolution. I have seen
>> nice 2.0 mm resolution with the latest head coils. For 2.2 mm, you need to
>> use 112x112 matrix. Please note that larger matrix leads to more distortion.
>> Because 3T has larger distortion than 1.5 T, using a larger matrix in 3T is
>> not a good idea in general.
>>
>> Hope it helps.
>>
>> Susumu
>>
>> On Thu, Dec 10, 2009 at 9:47 AM, Lucas Lessa <lucas_lessa at yahoo.com.br<http://br.mc1102.mail.yahoo.com/mc/compose?to=lucas_lessa@yahoo.com.br>
>> > wrote:
>>
>>>   Good morning,
>>>
>>> As I said in aonther post, I'm in the middle of a DTI study that,
>>> obsviously, uses DTI studio to process the data.
>>> The other researcher brought (a think from John Hopkins) a model that
>>> uses 3 DTI series, just to increase the ratio between sign/noise.
>>> The exams are taking so long and I would like to increase the resolution
>>> from 112x110 to 256x256, and maybe, 512x512.
>>> Is this a good idea? Or it's better to continue making the exams with the
>>> 3 DTI series?
>>> And wich is the bennefit of doing 3 DTI series (the vendor software only
>>> uses one at a time)?
>>> One more time thank you.
>>>
>>> Lucas Lessa
>>> São Paulo/Brazil
>>>
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