[Mristudio-users] Regarding gradient table

[susumu mori]

The generic table is only a generic table.
In old days when the gradient information is not stored in DICOM files, the
generic tables are the only information we had.
Usually, the generic table is what the pulse programs for DTI were using
inside the scanner. The pulse programs have only one internal generic
table. Then you specify patient position (head first / foot first), patient
orientation (face up / face down), image orientation (cor/sag/ax), oblique
information, etc. Based on these study-specific parameters, the pulse
program dynamically generate RF and gradient waveforms. During this dynamic
generation of waveforms, the generic table is translated into actual time
sequences of the physical X, Y, Z gradients. If you record what actual X,
Y, Z gradient do and compared to x, y, z components in the generic gradient
table, you may find X, Y, Z and x, y, z are not the same; they could be
permutated,  the +/- sign of X and x may be flipped, or X, Y, Z is rotated
with respect to x, y, z.

Therefore, in old days, we have to figure out the "correct" table from the
generic table. For example, x, y, z of a old generic gradient table was Y,
X, -Z in Philips. In DICOM files, you can find gradient information after a
study-specific gradient table is generated. Therefore, X, Y, Z permutation
and +/- sign are supposed to be correct.

As I mentioned in a previous email, the oblique information is still tricky
because you may still have to rotate the gradient table in the DICOM based
on the oblique angle.


On Fri, Apr 12, 2013 at 7:48 PM, Santosh Yadav
<santoshyadav20076 at gmail.com>wrote:

> Thanks Dr. Susumu.
>
> So for taking the exact correct gradient table, for example I should
> extract gradient table from DICOM files using DTI studio having the option
> called (Get gradient from DICOM file header) or gradient file from the
> scanner. Since i have the discrepancy between gradient file obtained from
> scanner provided by manufacturer and extracted from DICOM file header using
> DTIstudio. Which one is more accurate?
>
> With best Regards,
>
> Santosh
>
>
>
>
>
>
> On Fri, Apr 12, 2013 at 4:10 PM, susumu mori <susumu at mri.jhu.edu> wrote:
>
>> Hi Santosh,
>>
>> If the gradient table is wrong, all calculation would be wrong.
>> There is a mistake that could be corrected after the calculation but it
>> is only when the +/- sign of entire column (say, y-component column) of the
>> table is wrong. In this case, I think FA/MD are not affected but the vector
>> signs are flipped. So you can flip the sign of the y-components before
>> tracking. This rule is, however, no longer valid when you do gradient table
>> rotation for oblique-imaging or you rotate the image for image registration.
>>
>>
>>
>> On Fri, Apr 12, 2013 at 6:28 PM, Santosh Yadav <
>> santoshyadav20076 at gmail.com> wrote:
>>
>>> Hi Dr. Susumu,
>>>
>>> I have one question regarding gradient table in calculation of DTI
>>> metric and fiber tractography.
>>>
>>> Is gradient table have only effect on fiber tractography and doesn’t
>>> affect ADC and FA values, Is this correct?
>>>
>>> With best Regards,
>>>
>>> Santosh
>>>
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>
>
> --
> With Best Regards,
>
> Neuroscience and MR Research Program
>
> Department of Medicine, Queen's Medical Center
>
> University of Hawaii
>
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>
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