diff --git a/docs/source/changelog.rst b/docs/source/changelog.rst
index b418ce3a..4d2aceb0 100644
--- a/docs/source/changelog.rst
+++ b/docs/source/changelog.rst
@@ -10,6 +10,16 @@ Legend
 * :bdg-primary:`Refactor` denotes a code refactor; usually this means an efficiency boost or code cleanup.
 * :bdg-danger:`Change` denotes a change that may break existing code.
 
+v 3.31.0
+--------
+
+ACR MRI
+^^^^^^^
+
+* :bdg-warning:`Fixed` The positive and negative diagonal geometric distortion calculations (``results_data().geometric_distortion_module.profiles["negative diagonal"]['width (mm)']``) for the ACR Large MRI phantom were not scaled correctly.
+  Since the pixel distance is diagonal the physical spacing between pixels is actually :math:`\sqrt{2}` times the pixel spacing.
+  This is now fixed. Prior values can be scaled by :math:`\sqrt{2}` to get the correct values.
+
 v 3.30.0
 --------
 
diff --git a/pylinac/acr.py b/pylinac/acr.py
index 8439a5a2..a3732c42 100644
--- a/pylinac/acr.py
+++ b/pylinac/acr.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import io
+import math
 import textwrap
 import warnings
 import webbrowser
@@ -1079,7 +1080,6 @@ def _setup_rois(self) -> None:
         ys = xs + b
         coords = ndimage.map_coordinates(bin_image, [ys, xs], order=1, mode="mirror")
         f_data = fill_middle_zeros(coords, cutoff_px=px_to_cut_off)
-        # pixels are now diagonal and thus spacing between pixels is now the hypotenuse
         prof = FWXMProfile(values=f_data)
         line = Line(
             Point(
@@ -1091,8 +1091,11 @@ def _setup_rois(self) -> None:
                 ys[int(round(prof.field_edge_idx(side="right")))],
             ),
         )
+        # pixels are now diagonal and thus spacing between pixels is now the hypotenuse
+        # We don't have to fix the line above because that's in pixels. The issue is the
+        # geometric distance and thus we only have to correct here.
         self.profiles["negative diagonal"] = {
-            "width (mm)": prof.field_width_px * self.mm_per_pixel,
+            "width (mm)": prof.field_width_px * self.mm_per_pixel * math.sqrt(2),
             "line": line,
         }
         # calculate positive diagonal
@@ -1114,7 +1117,7 @@ def _setup_rois(self) -> None:
             ),
         )
         self.profiles["positive diagonal"] = {
-            "width (mm)": prof.field_width_px * self.mm_per_pixel,
+            "width (mm)": prof.field_width_px * self.mm_per_pixel * math.sqrt(2),
             "line": line,
         }
 
diff --git a/tests_basic/test_acr.py b/tests_basic/test_acr.py
index 0629f932..39c19496 100644
--- a/tests_basic/test_acr.py
+++ b/tests_basic/test_acr.py
@@ -300,6 +300,36 @@ def test_load_from_list_of_streams(self):
             paths = [io.BytesIO(open(p, "rb").read()) for p in paths]
             ACRMRILarge(paths)
 
+    def test_geometric_distortion_profile_lengths(self):
+        path = get_file_from_cloud_test_repo([*TEST_DIR_MR, "GE 3T.zip"])
+        mri = ACRMRILarge.from_zip(path)
+        mri.analyze()
+        data = mri.results_data()
+        self.assertAlmostEqual(
+            data.geometric_distortion_module.profiles["negative diagonal"][
+                "width (mm)"
+            ],
+            190.63,
+            delta=0.05,
+        )
+        self.assertAlmostEqual(
+            data.geometric_distortion_module.profiles["positive diagonal"][
+                "width (mm)"
+            ],
+            190.59,
+            delta=0.05,
+        )
+        self.assertAlmostEqual(
+            data.geometric_distortion_module.profiles["vertical"]["width (mm)"],
+            190.44,
+            delta=0.05,
+        )
+        self.assertAlmostEqual(
+            data.geometric_distortion_module.profiles["horizontal"]["width (mm)"],
+            190.44,
+            delta=0.05,
+        )
+
 
 class TestACRMRIResultData(TestCase, ResultsDataBase):
     def construct_analyzed_instance(self):