SANDI

SANDI - Soma And Neurite Density Imaging

With this tutorial you will learn how to fit the SANDI model to a sample dataset.

[!IMPOERTANT]

• Default setting we present here should be used only on data with at least 5 b shells. Although we strongly discourage it, to fit the model to data with less b shells, one or more model parameters must be fixed (e.g., for data with only 3 shells, 2 of the 5 model parameters must be fixed to a reasonable value).
• Interpretation of the compartments of this model as meaningful biological structures holds under specific assumptions that can be valid in specific experimental regimes (see Palombo, Marco, et al. 2020). In particular, for in vivo studies, diffusion time <~25 ms and some data at b>3000s/mm^2. Although we strongly discourage it, the model can be used to analyze also data outside this experimental regime, but then the biological interpretation of the model parameters should be taken vary carefully.

Prepare the dataset

Download the sample dataset available from the BrainHack repository:

• BrainHack repository

Note

These data were kindly provided by Dr. Andrada Ianus and Dr. Noam Shemesh were acquired in the Preclinical MRI Lab at Champalimaud Foundation.

After unzipping, your directory structure should look like this:

SANDI_Data_BrainHack_Shared/
├── bvals.bval
├── bvecs.bvec
├── MouseData_Slice_Mask.nii
└── MouseData_Slice.nii

Preprocess the data

Usually, DWI images need some preprocessing (e.g., eddy current correction, head movement correction, and skull stripping). You need to perform these pre-preprocessing steps before fitting the model. Assuming this pre-processing has already been done for this sample dataset, we skip those steps here.

Run AMICO

Initialization

Move into the SANDI_Data_BrainHack_Shared directory and run a Python interpreter:

cd SANDI_Data_BrainHack_Shared
python

In the Python shell, import the AMICO library and setup/initialize the framework:

import amico
amico.setup()

-> Precomputing rotation matrices:
   [ DONE ]

Note

This step will precompute all the necessary rotation matrices and store them in ~/.dipy. This initialization step is necessary only the first time you use AMICO.

Now you can instantiate an Evaluation object and start the analysis:

ae = amico.Evaluation()

To fit the SANDI model you also need to perform a directional average on your data. You can do it by setting the doDirectionalAverage flag to True with the set_config() method:

ae.set_config('doDirectionalAverage', True)

You can generate the scheme file from the bvals/bvecs files and values of delta and small_delta of your acquisition with the sandi2scheme() method:

delta = 0.02 # Time between pulses [s]
small_delta = 0.0055 # Pulses duration in [s]
TE = 0.03 # Echo time if different from delta+small_delta [s] (optional)
amico.util.sandi2scheme('bvals.bval', 'bvecs.bvec', delta, small_delta, TE_data=TE, schemeFilename='SANDI_scheme.txt', bStep=100)

-> Rounding b-values to nearest multiple of 100.0

-> Writing scheme file to [ SANDI_scheme.txt ]
'SANDI_scheme.txt'

Note

TE is not mandatory and if not provided it will be computed internally as the sum of delta and small_delta. Moreover, in case delta and small_delta (and echo time) have different values for different directions and/or shell the corresponding files can be provided in the same format as bvals.bval.

Load the data

Load your data with the load_data() method:

ae.load_data(dwi_filename='MouseData_Slice.nii', scheme_filename='SANDI_scheme.txt', mask_filename='MouseData_Slice_Mask.nii', b0_thr=10)

-> Loading data:
	* DWI signal
		- dim    = 118 x 100 x 2 x 336
		- pixdim = 0.120 x 0.120 x 0.400
	* Acquisition scheme
		- 336 samples, 8 shells
		- 16 @ b=0 , 40 @ b=1000.0 , 40 @ b=4000.0 , 40 @ b=7000.0 , 40 @ b=10000.0 , 40 @ b=2500.0 , 40 @ b=5500.0 , 40 @ b=8500.0 , 40 @ b=12500.0 
	* Binary mask
		- dim    = 118 x 100 x 2
		- pixdim = 0.120 x 0.120 x 0.400
		- voxels = 8107
   [ 0.1 seconds ]

-> Preprocessing:
	* Normalizing to b0... [ min=0.00,  mean=0.47, max=3.96 ]
	* Keeping all b0 volume(s)
	* Performing the directional average on the signal of each shell... 
		- dim    = 118 x 100 x 2 x 9
		- pixdim = 0.120 x 0.120 x 0.400
	* Acquisition scheme
		- 9 samples, 8 shells
		- 1 @ b=0 , 1 @ b=1000.0 , 1 @ b=2500.0 , 1 @ b=4000.0 , 1 @ b=5500.0 , 1 @ b=7000.0 , 1 @ b=8500.0 , 1 @ b=10000.0 , 1 @ b=12500.0 
   [ 0.1 seconds ]

Compute the response functions

Set the SANDI model with the set_model() method and generate the response functions with the generate_kernels() method:

ae.set_model('SANDI')
d_is = 3e-3 # Intra-soma diffusivity [mm^2/s]
Rs = np.array([1.55555556e-06, 3.44444444e-06, 4.44444444e-06, 5.33333333e-06, 6.00000000e-06, 6.55555556e-06, 8.11111111e-06, 9.55555556e-06, 1.16666667e-05]) # Radii of the soma [meters]
d_in = np.array([0.00091667, 0.00169444, 0.003]) # Intra-neurite diffusivitie(s) [mm^2/s]
d_isos = np.array([0.00036111, 0.00163889, 0.003]) # Extra-cellular isotropic mean diffusivitie(s) [mm^2/s]
ae.model.set(d_is, Rs, d_in, d_isos)
ae.generate_kernels(regenerate=True, ndirs=1)

-> Creating LUT for "SANDI" model:
   [ 0.8 seconds ]

Important

• The dictionary provided as default inside the code was optimized for the specific acquisition of the data in SANDI_Data_BrainHack_Shared. If you need to change them you can modify model-specific variables and then set the model with the set() method.
• You need to compute the reponse functions only once per study; in fact, scheme files with same b-values but different number/distribution of samples on each shell will result in the same precomputed kernels (which are actually computed at higher angular resolution). The method generate_kernels() does not recompute the kernels if they already exist, unless the flag regenerate is set, e.g. generate_kernels(regenerate=True).

Load the precomputed kernels (at higher resolution) and adapt them to the actual scheme (distribution of points on each shell) of the current subject with the load_kernels() method:

ae.load_kernels()

-> Resampling LUT for subject ".":
   [ 0.0 seconds ]

Fit the model to the data

Fit the model to the data with the fit() method:

lambda1 = 0 # L1 regularization term (MUST be varied according to data)
lambda2 = 5e-3 # L2 regularization term (MUST be varied according to data)
ae.set_solver(lambda1=lambda1, lambda2=lambda2)
ae.fit()

-> Fitting 'SANDI' model to 8107 voxels (using 32 threads):
   [ 00h 00m 00s ]

Note

Now you need to set the regularisation parameters according to your data and if you need to apply L1 or L2 regularization or a combination of both.

Save the results

Finally, save the results as NIfTI images with the save_results() method:

ae.save_results(save_dir_avg=True)

-> Saving output to "AMICO/SANDI/*":
	- configuration  [OK]
	- fit_fsoma.nii.gz  [OK]
	- fit_fneurite.nii.gz  [OK]
	- fit_fextra.nii.gz  [OK]
	- fit_Rsoma.nii.gz  [OK]
	- fit_Din.nii.gz  [OK]
	- fit_De.nii.gz  [OK]
	- dir_avg_signal.nii.gz  [OK]
	- dir_avg.scheme  [OK]
   [ DONE ]

Visualize the results

🎉Congratulations! You have successfully fitted the SANDI model to your data.🎉 You will find the estimated parameters in the SANDI_Data_BrainHack_Shared/AMICO/SANDI directory:

SANDI_Data_BrainHack_Shared/AMICO/SANDI/
├── config.pickle
├── dir_avg.scheme
├── dir_avg_signal.nii.gz
├── fit_De.nii.gz
├── fit_Din.nii.gz
├── fit_fextra.nii.gz
├── fit_fneurite.nii.gz
├── fit_fsoma.nii.gz
└── fit_Rsoma.nii.gz

Open them with your favorite viewer.