fix: exclude NaN scores from FROC counts

Author: znanan

monai/metrics/froc.py

@@ -149,8 +149,8 @@ def compute_froc_curve_data(
     # smallest score) while avoiding a full scan of both arrays at every threshold.
     all_probs = sorted(set(list(fp_probs) + list(tp_probs)))
     thresholds = all_probs[1:]
-    sorted_fp_probs = np.sort(fp_probs)
-    sorted_tp_probs = np.sort(tp_probs)
+    sorted_fp_probs = np.sort(fp_probs[~np.isnan(fp_probs)])
+    sorted_tp_probs = np.sort(tp_probs[~np.isnan(tp_probs)])
     total_fps = len(sorted_fp_probs) - np.searchsorted(sorted_fp_probs, thresholds, side="left")
     total_tps = len(sorted_tp_probs) - np.searchsorted(sorted_tp_probs, thresholds, side="left")
     total_fps = np.append(total_fps, 0)

tests/metrics/test_compute_froc.py

@@ -199,6 +199,12 @@ def test_matches_reference(self, _, fp_probs, tp_probs):
             np.testing.assert_array_equal(actual_array, expected_array)
             self.assertEqual(actual_array.dtype, expected_array.dtype)
 
+    def test_nan_fp_probs_are_not_counted(self):
+        fp_probs = np.asarray([np.nan])
+        tp_probs = np.asarray([0.5, 0.8])
+        fps_per_image, _ = compute_froc_curve_data(fp_probs, tp_probs, num_targets=2, num_images=1)
+        np.testing.assert_array_equal(fps_per_image, np.zeros_like(fps_per_image))
+
 
 if __name__ == "__main__":
     unittest.main()