[Mristudio-users] Adjusting Gradients for Oblique Angles

[Jeff Sadino]

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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