diff --git a/voluseg/_steps/step4.py b/voluseg/_steps/step4.py
index 75110a9..d7c4555 100755
--- a/voluseg/_steps/step4.py
+++ b/voluseg/_steps/step4.py
@@ -77,7 +77,7 @@ def detect_cells(parameters):
 
             # get number of voxels in each cell
             n_blocks, block_valids, xyz0, xyz1 = \
-                define_blocks(lx, ly, lz, p.n_cells_block, n_voxels_cell, volume_mask)
+                define_blocks(lx, ly, lz, p.n_cells_block, n_voxels_cell, volume_mask, volume_peak)
 
             # save number and indices of blocks
             with h5py.File(fullname_volmean+hdf, 'r+') as file_handle:
diff --git a/voluseg/_steps/step4a.py b/voluseg/_steps/step4a.py
index ffdd96f..4aad1d0 100755
--- a/voluseg/_steps/step4a.py
+++ b/voluseg/_steps/step4a.py
@@ -1,4 +1,4 @@
-def define_blocks(lx, ly, lz, n_cells_block, n_voxels_cell, volume_mask):
+def define_blocks(lx, ly, lz, n_cells_block, n_voxels_cell, volume_mask, volume_peak):
     '''get coordinates of individual blocks'''
 
     import os
@@ -54,6 +54,8 @@ def define_blocks(lx, ly, lz, n_cells_block, n_voxels_cell, volume_mask):
     # get indices of masked blocks
     block_valids = np.ones(n_blocks, dtype=bool)
     for i in range(n_blocks):
-        block_valids[i] = np.any(volume_mask[x0[i]:x1[i], y0[i]:y1[i], z0[i]:z1[i]])
+        mask_i = volume_mask[x0[i]:x1[i], y0[i]:y1[i], z0[i]:z1[i]]
+        peak_i = volume_peak[x0[i]:x1[i], y0[i]:y1[i], z0[i]:z1[i]]
+        block_valids[i] = np.any(np.logical_and(mask_i, peak_i))
 
     return n_blocks, block_valids, xyz0, xyz1