Angular resolution of the look up tables

This tutorial illustrates how to change the resolution of the look-up tables (a.k.a. LUT) that is internally used by AMICO to discretize the possible orientations of the fiber populations in each voxel.

Change the resolution

In order to change the resolution of the LUT internally used by AMICO, we can specify the desired number of directions using the parameter ndirs of the generate_kernels() method.

For example, if we want to instruct AMICO to use only 500 directions (over the half-sphere) to discretize the possible orientations of the fiber populations in each voxel, we sh. The modified code should be like this:

import amico

amico.setup()
ae = amico.Evaluation()

# Load data and set model

ae.generate_kernels(ndirs=500, regenerate=True)

The value of ndirs can take one of the following values: 1, 500, 1000, 1500, 2000, 2500, 3000, 3500, 4000, 4500, 5000, 5500, 6000, 6500, 7000, 7500, 8000, 8500, 9000, 9500, 10000, 32761. Please note that when the value of ndirs is not specified, AMICO will use 500 directions by default.

Differences between the two reconstructions

Memory consumption

The following table shows the differences in memory of the LUT.

ndirs value	memory usage
500	50 Megabytes
1000	100 Megabytes
1500	150 Megabytes
2000	200 Megabytes
2500	250 Megabytes
3000	300 Megabytes
3500	350 Megabytes
4000	400 Megabytes
4500	450 Megabytes
5000	500 Megabytes
5500	550 Megabytes
6000	600 Megabytes
6500	650 Megabytes
7000	700 Megabytes
7500	750 Megabytes
8000	800 Megabytes
8500	850 Megabytes
9000	900 Megabytes
9500	950 Megabytes
10000	1 Gigabytes
32761	3.2 Gigabytes

Quality of the fit

A change in the resolution affects the fitting process. The following figure illustrates the differences in the fitting between ndirs=500 and ndirs=32761.