diff --git a/diffdrr/_modidx.py b/diffdrr/_modidx.py
index a4c667afa..9b34a313e 100644
--- a/diffdrr/_modidx.py
+++ b/diffdrr/_modidx.py
@@ -29,6 +29,7 @@
                              'diffdrr.drr.DRR.affine_inverse': ('api/drr.html#drr.affine_inverse', 'diffdrr/drr.py'),
                              'diffdrr.drr.DRR.forward': ('api/drr.html#drr.forward', 'diffdrr/drr.py'),
                              'diffdrr.drr.DRR.inverse_projection': ('api/drr.html#drr.inverse_projection', 'diffdrr/drr.py'),
+                             'diffdrr.drr.DRR.n_patches': ('api/drr.html#drr.n_patches', 'diffdrr/drr.py'),
                              'diffdrr.drr.DRR.perspective_projection': ('api/drr.html#drr.perspective_projection', 'diffdrr/drr.py'),
                              'diffdrr.drr.DRR.render': ('api/drr.html#drr.render', 'diffdrr/drr.py'),
                              'diffdrr.drr.DRR.rescale_detector_': ('api/drr.html#drr.rescale_detector_', 'diffdrr/drr.py'),
diff --git a/diffdrr/drr.py b/diffdrr/drr.py
index 8cc29eb9d..c02778d83 100644
--- a/diffdrr/drr.py
+++ b/diffdrr/drr.py
@@ -98,8 +98,6 @@ def __init__(
             )
         self.reshape = reshape
         self.patch_size = patch_size
-        if self.patch_size is not None:
-            self.n_patches = (height * width) // (self.patch_size**2)
 
     def reshape_transform(self, img, batch_size):
         if self.reshape:
@@ -122,6 +120,10 @@ def affine(self):
     def affine_inverse(self):
         return RigidTransform(self._affine_inverse)
 
+    @property
+    def n_patches(self):
+        return (self.detector.height * self.detector.width) // (self.patch_size**2)
+
 # %% ../notebooks/api/00_drr.ipynb 8
 def reshape_subsampled_drr(img: torch.Tensor, detector: Detector, batch_size: int):
     n_points = detector.height * detector.width
diff --git a/notebooks/api/00_drr.ipynb b/notebooks/api/00_drr.ipynb
index 7273d1b35..95da3da1e 100644
--- a/notebooks/api/00_drr.ipynb
+++ b/notebooks/api/00_drr.ipynb
@@ -193,8 +193,6 @@
     "            )\n",
     "        self.reshape = reshape\n",
     "        self.patch_size = patch_size\n",
-    "        if self.patch_size is not None:\n",
-    "            self.n_patches = (height * width) // (self.patch_size**2)\n",
     "\n",
     "    def reshape_transform(self, img, batch_size):\n",
     "        if self.reshape:\n",
@@ -215,7 +213,11 @@
     "\n",
     "    @property\n",
     "    def affine_inverse(self):\n",
-    "        return RigidTransform(self._affine_inverse)"
+    "        return RigidTransform(self._affine_inverse)\n",
+    "\n",
+    "    @property\n",
+    "    def n_patches(self):\n",
+    "        return (self.detector.height * self.detector.width) // (self.patch_size**2)"
    ]
   },
   {