diff --git a/src/pyronn_torch/conebeam.py b/src/pyronn_torch/conebeam.py
index c9409fb2b5e6bff73e8b07694f33ecf375142009..846ef6306b0c71372dfaa890a3dc0a8954cf0b14 100644
--- a/src/pyronn_torch/conebeam.py
+++ b/src/pyronn_torch/conebeam.py
@@ -85,6 +85,7 @@ class _ForwardProjection(torch.autograd.Function):
             state.projection_multiplier, volume_grad, *state.volume_origin,
             *state.volume_spacing)
 
+        self.state = state
         if return_none:
             return volume_grad, None
         else:
@@ -92,8 +93,8 @@ class _ForwardProjection(torch.autograd.Function):
 
 
 class _BackwardProjection(torch.autograd.Function):
-    backward = _ForwardProjection.forward
-    forward = _ForwardProjection.backward
+    backward = staticmethod(_ForwardProjection.forward)
+    forward = staticmethod(_ForwardProjection.backward)
 
 
 class ConeBeamProjector: