diff --git a/spyrit/core/prep.py b/spyrit/core/prep.py
index 070a96fd..77d9904f 100644
--- a/spyrit/core/prep.py
+++ b/spyrit/core/prep.py
@@ -302,8 +302,8 @@ def sigma(self, x: torch.tensor) -> torch.tensor:
             :attr:`x`: batch of images in the Hadamard domain
             
         Shape:
-            - Input: :math:`(B,2*M)` where :math:`B` is the batch dimension
-            - Output: :math:`(B, M)`
+            - Input: :math:`(*,2*M)` :math:`*` indicates one or more dimensions
+            - Output: :math:`(*, M)`
             
         Example:
             >>> x = torch.rand([10,2*400], dtype=torch.float)
@@ -312,7 +312,7 @@ def sigma(self, x: torch.tensor) -> torch.tensor:
             torch.Size([10, 400])
             
         """
-        x = x[:,self.even_index] + x[:,self.odd_index]
+        x = x[...,self.even_index] + x[...,self.odd_index]
         x = 4*x/(self.alpha**2) # Cov is in [-1,1] so *4
         return x