Hello

I would like to confirm which sequence the 257 gradient table refers to. We have the mgh diffusion sequences, i guess it is the 257 half sphere sequence mode. Do you keep the direction scheme on single?

As a matter of interest, why do you limit the gradient amplitude?

Thanks for your help

Thanks Ruoepng

 

I must confess I am assuming you are
from MGH and use the MGH diffusion pulse sequences on a TRIO. Within
this sequence on the sequence / special card you can select the
diffusion gradient encoding scheme in the sequence mode box. There
are a variety of dsi schemes available, but DTK requires one with 257
directions. There is only 1 available called 257 half sphere.
Unfortunately, the gradient table for 257 half sphere does not appear
to match the one provided in DTK. An excerpt from the start of the
257 ½ sphere is below

 

0 0 0 0

250 0 0.229039 0.973417

250 0 0.973748 -0.227629

250 0.999887 -0.00833239 0.0124986

500 0 0.548489 -0.836158

500 0 0.848689 0.528893

 

while the 257 DTK table goes like

 

 0.000000,  0.000000,  0.200000
          0.000000,  0.200000,  0.000000
          0.200000,  0.000000,  0.000000
          0.000000,  0.200000, -0.200000
          0.000000,  0.200000,  0.200000
          0.200000, -0.200000,  0.000000
          0.200000,  0.000000, -0.200000
          0.200000,  0.000000,  0.200000
          0.200000,  0.200000,  0.000000
          0.200000, -0.200000, -0.200000
          0.200000, -0.200000,  0.200000
          0.200000,  0.200000, -0.200000 

I did assume that the MGH DSI sequences would match the MGH software.

 

Any suggestions for a pulse sequence
that matches the 257 DTK table? Alternatively, is there a way of
using an alternate gradient table for DSI in DTK

 

The description of the 257 ½ sphere
notes that the gradients for the pulse sequence are limited to 36.1
mT/m. As you are from MGH, I was wondering if you knew why this
restriction had been implemented. It was just for my interest. Thanks
for your help

 

Richard

With my last post, the gradient table
came from DSI studio, after you import the data you can save the
table. DSI studio reads the gradients and B-vals from the dicom
headers as it reads the dicoms in. I probably did have the scans
oblique. Today I repeated with direct axial slices and retrieved the
gradient table using dicon2nii. It automatically produces a text file
***.bvec as it converts dicom to nifti format. An exert from the
start of the file is below

 

0 0 0

-0.99993062019348 0 0

-0.7041670680046 -0.71081352233886
-0.71007132530212

-0.7041670680046 -0.57787042856216
-0.57760721445083

-0.57787042856216 -0.57760721445083 0

-0.00103517004754 -0.99998259544372 0

 

which still don't match .

 

Richard