Hi,

I’m processing 3T and 7T diffusion data. The results for the 3T data look alright but 7T is a mess (fibre orientation is off). When I tried to figured out what’s going on with my data I came across this post and after plotting mmy bvec distribution with the script @ThijsDhollander provided (many thanks for that! ) my 7T set came out like this:

b1000

b2500

Data is acquired on a siemens scanner so for 7T the shells are acquired separately. I can tell that this distribution is far from optimal and will probably explain the wrong fibre orientations.

Also, I checked the bvec distribution for my 3T data and from what I can tell, this one is not quite good as well:

My guess is that because the shells at 3T were acquired all in once and so there are more directions, the fibre orientation for 3T is quite reasonable. Would this make sense?

The question I’m left with now is how to interpret the outcome of `dirstat`

. Below is the output for my 3T data set. I know the `lmax`

should be around 1 and so the fit for the 8 order SH for b2500 is a bit too high but what about the energy? What are reasonable number to expect there? And how ‘wrong’ is the distribution of my bvecs when looking at the `dirstat`

output?

```
bvecs.txt [ 97 volumes ]
b = 0 [ 8 directions ]
Bipolar electrostatic repulsion model:
nearest-neighbour angles: mean = 180, range [ 180 - 180 ]
energy: total = inf, mean = inf, range [ 1.79769e+308 - inf ]
Unipolar electrostatic repulsion model:
nearest-neighbour angles: mean = 180, range [ 180 - 180 ]
energy: total = inf, mean = inf, range [ 1.79769e+308 - inf ]
Spherical Harmonic fit:
condition numbers for lmax = 2 -> 2: nan
b = 1000 [ 27 directions ]
Bipolar electrostatic repulsion model:
nearest-neighbour angles: mean = 24.3208, range [ 21.6325 - 28.4583 ]
energy: total = 616.206, mean = 45.6449, range [ 44.1563 - 46.7251 ]
Unipolar electrostatic repulsion model:
nearest-neighbour angles: mean = 26.5889, range [ 21.6325 - 41.1454 ]
energy: total = 328.571, mean = 24.3386, range [ 18.7545 - 29.4113 ]
Spherical Harmonic fit:
condition numbers for lmax = 2 -> 4: 1.16352 1.44367
b = 2500 [ 62 directions ]
Bipolar electrostatic repulsion model:
nearest-neighbour angles: mean = 15.7685, range [ 12.8085 - 19.3174 ]
energy: total = 3459.65, mean = 111.601, range [ 109.681 - 114.217 ]
Unipolar electrostatic repulsion model:
nearest-neighbour angles: mean = 17.5848, range [ 13.9219 - 24.9838 ]
energy: total = 1709.1, mean = 55.1321, range [ 50.2074 - 61.723 ]
Spherical Harmonic fit:
condition numbers for lmax = 2 -> 8: 1.06112 1.18295 1.35523 2.15758
```

Many thanks for your help!

Lotte