[Mristudio-users] Adjusting Gradients for Oblique Angles

[Meredith Reid]

I think the last row of your rotation matrix should be the cross product of
the two rows from ImageOrientationPatient, resulting in:

    1.0000         0         0
         0    0.8846   -0.4664
         0    0.4664   0.8846

Meredith


On Mon, Dec 13, 2010 at 5:03 PM, Jeff Sadino <jsadino.queens at gmail.com>wrote:

> Hello,
>
> Thank you Venkat and Hangyi for you insights.  I tried both methods, but I
> was not able to replicate the values that DTIStudio gave me.  The values
> that DTIStudio gave me, after clicking the "rotate gradients if applicable"
> button and for the first direction, is:
> 0.861, -0.489, 0.142
>
> Here is my original gradient table:
>          0         0         0
>     0.8608   -0.3657    0.3538
>     0.8643    0.3611    0.3502
>      0.8608    0.3657   -0.3538
>     0.8644   -0.3608   -0.3502
>     0.3582   -0.3523   -0.8646
>     0.3571   -0.8658   -0.3504
>     0.3554   -0.8606    0.3648
>     0.3566   -0.3652    0.8599
>     0.3579    0.3521    0.8648
>     0.3571    0.8658    0.3504
>     0.3557    0.8609   -0.3638
>     0.3569    0.3655   -0.8597
>
> And here is my ImageOrientationPatient info:
>     1.0000         0         0
>          0    0.8846   -0.4664
>
> If I do it according to Venkat, it becomes:
>      1.0000         0         0
>          0    0.8846   -0.4664
>          0         0    1.0000
>
> If I multiply my gradient by this, for the first direction, I get:
>     0.8608   -0.3235    0.5244
>
> If I do it according to Hangyi, the rotation matrix becomes:
>     1.0000         0         0
>          0    0.8846   -0.4664
>          0    0.8846   -0.4664
>
> If I multiply my gradient by this, for the first direction, I get:
>     0.8608   -0.0106    0.0056
>
> The only thing that I can think of is there is probably a step in there
> that normalizes the gradient to one.  i.e.: sqrt(grad_x^2 + grad_y^2 +
> grad_z^2) = 1.
>
> Can anybody help point me to the next step?    Thank you very much!!
> Jeff Sadino
>
> On Fri, Dec 10, 2010 at 4:07 AM, Hangyi Jiang <hjiang at jhmi.edu> wrote:
>
>> hello,
>>
>> the 3rd row should be the outer product of the given two vectors,   not
>> [0,0,1]
>>
>> regards
>>
>> hangyi
>>
>>
>>
>>
>>
>>
>>
>> ________________________________
>> From: mristudio-users-bounces at mristudio.org [
>> mristudio-users-bounces at mristudio.org] On Behalf Of Rajagopalan,
>> Venkateswaran [rajagov2 at ccf.org]
>> Sent: Friday, December 10, 2010 9:01 AM
>> To: DTI Studio, ROI Editor, DiffeoMap Questions/Support
>> Subject: Re: [Mristudio-users] Adjusting Gradients for Oblique Angles
>>
>> Dear Jeff Sadino,
>>
>> This is what I did for manual oblique angle correction
>>
>> multiply your gradient matrix (I don't know how many directions you have)
>> if you have 12 directional data then you will have 3*13 (one b=0 values too)
>> with the "Image Orientation patient" matrix
>>
>> [ 1 0 0 0 .8846 -.4664] rewritten as (these are unit vectors with z
>> direction being 0 0 1 not mentioned in the Image Image orientation patient
>> from Dicom header
>>
>> [ 1    0     0
>>  0  0.8846  -0.4664
>>  0   0      1]
>>
>> 3*3
>>
>> Multiply the ablove Image orientation patient with your gradient matrix.
>>
>> Thanks
>>
>> Venkat
>>
>> ________________________________
>> From: mristudio-users-bounces at mristudio.org on behalf of Jeff Sadino
>> Sent: Thu 12/9/2010 6:09 PM
>> To: DTI Studio, ROI Editor, DiffeoMap Questions/Support
>> Subject: [Mristudio-users] Adjusting Gradients for Oblique Angles
>>
>> Hello,
>>
>> Thank you for the DTI training session in Turtle Bay last week.  It was
>> very helpful for us!
>>
>> I am wondering how to manually adjust the gradients for an oblique angle.
>>  I am using Siemens 3T Trio Tim VB17.
>>
>> My default gradient (for one direction) is:
>> 0.861, -0.366, 0.354
>>
>> After clicking "Rotate gradients if applicable" in DTIStudio 3.0.2
>> (March2010), the gradient is:
>> 0.861, -0.489, 0.142
>>
>> I couldn't find exactly which tag in the DICOM header contains the
>> rotation information.  The closest I could find is
>> "ImageOrientationPatient".  The value for this is:
>> [ 1 0 0 0 .8846 -.4664]
>>
>> How do I go from the default gradient to the rotated gradient?  Also, can
>> I use this same method on the VB13 and VA25 software?
>>
>> Thank you very much for your help!
>> Jeff Sadino
>>
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>
>
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