diff --git a/etomo/operators/radon/radon.py b/etomo/operators/radon/radon.py
index e42c34d..931c5ab 100644
--- a/etomo/operators/radon/radon.py
+++ b/etomo/operators/radon/radon.py
@@ -184,6 +184,8 @@ def __init__(self, angles, img_size, nb_slices=None, n_channels=1,
             angles acquired (in degrees)
         img_size: int
             size of the images (only square images)
+        nb_slices: int, optional
+            Number of slices. If None, assumes equal to img_size
         n_channels: int, default 1
             Number of images for multichannel reconstructions
         normalized: bool, default True