Parallel reconstruction using CSDΒΆ

This example shows how to use parallelism (multiprocessing) using peaks_from_model in order to speedup the signal reconstruction process. For this example will we use the same initial steps as we used in Reconstruction with Constrained Spherical Deconvolution.

Import modules, fetch and read data, apply the mask and calculate the response function.

import multiprocessing

from dipy.data import fetch_stanford_hardi, read_stanford_hardi

fetch_stanford_hardi()
img, gtab = read_stanford_hardi()
data = img.get_data()

from dipy.segment.mask import median_otsu

maskdata, mask = median_otsu(data, 3, 1, False,
                             vol_idx=range(10, 50), dilate=2)

from dipy.reconst.csdeconv import auto_response

response, ratio = auto_response(gtab, maskdata, roi_radius=10, fa_thr=0.7)

data = maskdata[:, :, 33:37]
mask = mask[:, :, 33:37]

Now we are ready to import the CSD model and fit the datasets.

from dipy.reconst.csdeconv import ConstrainedSphericalDeconvModel

csd_model = ConstrainedSphericalDeconvModel(gtab, response)

from dipy.data import get_sphere

sphere = get_sphere('symmetric724')

Compute the CSD-based ODFs using peaks_from_model. This function has a parameter called parallel which allows for the voxels to be processed in parallel. If nbr_processes is None it will figure out automatically the number of CPUs available in your system. Alternatively, you can set nbr_processes manually. Here, we show an example where we compare the duration of execution with or without parallelism.

import time
from dipy.direction import peaks_from_model

start_time = time.time()
csd_peaks_parallel = peaks_from_model(model=csd_model,
                                      data=data,
                                      sphere=sphere,
                                      relative_peak_threshold=.5,
                                      min_separation_angle=25,
                                      mask=mask,
                                      return_sh=True,
                                      return_odf=False,
                                      normalize_peaks=True,
                                      npeaks=5,
                                      parallel=True,
                                      nbr_processes=None)

time_parallel = time.time() - start_time
print("peaks_from_model using " + str(multiprocessing.cpu_count())
      + " process ran in :" + str(time_parallel) + " seconds")

peaks_from_model using 8 processes ran in 114.425682068 seconds

start_time = time.time()
csd_peaks = peaks_from_model(model=csd_model,
                             data=data,
                             sphere=sphere,
                             relative_peak_threshold=.5,
                             min_separation_angle=25,
                             mask=mask,
                             return_sh=True,
                             return_odf=False,
                             normalize_peaks=True,
                             npeaks=5,
                             parallel=False,
                             nbr_processes=None)

time_single = time.time() - start_time
print("peaks_from_model ran in :" + str(time_single) + " seconds")

peaks_from_model ran in 242.772505999 seconds

print("Speedup factor : " + str(time_single / time_parallel))

Speedup factor : 2.12166099088

In Windows if you get a runtime error about frozen executable please start your script by adding your code above in a main function and use:

if __name__ == '__main__':
    import multiprocessing
    multiprocessing.freeze_support()
    main()

Example source code

You can download the full source code of this example. This same script is also included in the dipy source distribution under the doc/examples/ directory.