4. Plotting brain images

In this section, we detail the general tools to visualize neuroimaging volumes with nilearn.

Nilearn comes with plotting function to display brain maps coming from Nifti-like images, in the nilearn.plotting module.

Code examples

Nilearn has a whole section of the example gallery on plotting.

A small tour of the plotting functions can be found in the example Plotting tools in nilearn.

Finally, note that, as always in the nilearn documentation, clicking on a figure will take you to the code that generates it.

4.1. Different plotting functions

Nilearn has a set of plotting functions to plot brain volumes that are fined tuned to specific applications. Amongst other things, they use different heuristics to find cutting coordinates.

   
plot_anat plot_anat
Plotting an anatomical image
plot_epi plot_epi
Plotting an EPI, or T2* image
plot_glass_brain plot_glass_brain
Glass brain visualization. By default plots maximum intensity projection of the absolute values. To plot positive and negative values set plot_abs parameter to False.
plot_stat_map plot_stat_map
Plotting a statistical map, like a T-map, a Z-map, or an ICA, with an optional background
plot_roi plot_roi
Plotting ROIs, or a mask, with an optional background
plot_connectome plot_connectome
Plotting a connectome
plot_prob_atlas plot_prob_atlas
Plotting 4D probabilistic atlas maps
plot_img plot_img
General-purpose function, with no specific presets

Warning

Opening too many figures without closing

Each call to a plotting function creates a new figure by default. When used in non-interactive settings, such as a script or a program, these are not displayed, but still accumulate and eventually lead to slowing the execution and running out of memory.

To avoid this, you must close the plot as follow:

>>> from nilearn import plotting
>>> display = plotting.plot_stat_map(img)     
>>> display.close()     

4.2. Different display modes

   
plot_ortho display_mode=’ortho’, cut_coords=[36, -27, 60]
Ortho slicer: 3 cuts along the x, y, z directions
plot_z_many display_mode=’z’, cut_coords=5
Cutting in the z direction, specifying the number of cuts
plot_x display_mode=’x’, cut_coords=[-36, 36]
Cutting in the x direction, specifying the exact cuts
plot_y_small display_mode=’y’, cut_coords=1
Cutting in the y direction, with only 1 cut, that is automatically positionned
plot_z_small display_mode=’z’, cut_coords=1, colorbar=False
Cutting in the z direction, with only 1 cut, that is automatically positionned
plot_xz display_mode=’xz’, cut_coords=[36, 60]
Cutting in the x and z direction, with cuts manually positionned
plot_yx display_mode=’yx’, cut_coords=[-27, 36]
Cutting in the y and x direction, with cuts manually positionned
plot_yz display_mode=’yz’, cut_coords=[-27, 60]
Cutting in the y and z direction, with cuts manually positionned
plot_lzr Glass brain display_mode=’lzr’
Glass brain and Connectome provide additional display modes due to the possibility of doing hemispheric projections. Check out: ‘l’, ‘r’, ‘lr’, ‘lzr’, ‘lyr’, ‘lzry’, ‘lyrz’.
plot_lyrz Glass brain display_mode=’lyrz’
Glass brain and Connectome provide additional display modes due to the possibility of doing hemispheric projections. Check out: ‘l’, ‘r’, ‘lr’, ‘lzr’, ‘lyr’, ‘lzry’, ‘lyrz’.

4.3. Adding overlays, edges, contours, contour fillings and markers

To add overlays, contours, or edges, use the return value of the plotting functions. Indeed, these return a display object, such as the nilearn.plotting.displays.OrthoSlicer. This object represents the plot, and has methods to add overlays, contours or edge maps:

display = plotting.plot_epi(...)
   
plot_edges display.add_edges(img)
Add a plot of the edges of img, where edges are extracted using a Canny edge-detection routine. This is typically useful to check registration. Note that img should have some visible sharp edges. Typically an EPI img does not, but a T1 does.
plot_contours display.add_contours(img, levels=[.5], colors=’r’)
Add a plot of the contours of img, where contours are computed for constant values, specified in ‘levels’. This is typically useful to outline a mask, or ROI on top of another map.
Example: Plot Haxby masks
plot_fill display.add_contours(img, filled=True, alpha=0.7, levels=[0.5], colors=’b’)
Add a plot of img with contours filled with colors
plot_overlay display.add_overlay(img, cmap=plotting.cm.purple_green, threshold=3)
Add a new overlay on the existing figure
Example: Visualizing a probablistic atlas: the default mode in the MSDL atlas
plot_markers display.add_markers(coords, marker_color=’y’, marker_size=100)
Add seed based MNI coordinates as spheres on top of statistical image or EPI image. This is useful for seed based regions specific interpretation of brain images.
Example: Producing single subject maps of seed-to-voxel correlation

4.4. Displaying or saving to an image file

To display the figure when running a script, you need to call nilearn.plotting.show: (this is just an alias to matplotlib.pyplot.show):

>>> from nilearn import plotting
>>> plotting.show() 

The simplest way to output an image file from the plotting functions is to specify the output_file argument:

>>> from nilearn import plotting
>>> plotting.plot_stat_map(img, output_file='pretty_brain.png')     

In this case, the display is closed automatically and the plotting function returns None.


The display object returned by the plotting function has a savefig method that can be used to save the plot to an image file:

>>> from nilearn import plotting
>>> display = plotting.plot_stat_map(img)     
>>> display.savefig('pretty_brain.png')     
# Don't forget to close the display
>>> display.close()     

4.5. Surface plotting

Plotting functions required to plot surface data or statistical maps on a brain surface.

New in version 0.3.

NOTE: These functions works for only with matplotlib higher than 1.3.1.

   
plot_surf_roi plot_surf_roi
Plotting surface atlases on a brain surface
Example: Loading and plotting of a cortical surface atlas
plot_surf_stat_map plot_surf_stat_map
Plotting statistical maps onto a brain surface
Example: Seed-based connectivity on the surface