Definitions and measures of hypoattenuation height: (A) measurements used in the classification and description of HAT on the atrial surface of the watchman FLX. In an approximated coronal cross section, a line (solid blue) is placed parallel to the bottom of the screw hub cove. Another line (dotted blue) is then placed at the level of the most proximal part of the frame. HAT present in the screw hub cove (red) between the blue lines is considered subfabric. The thickness of superficial HAT is then calculated by subtracting the height of the cove (small yellow arrow) from the total thickness of the HAT (large red arrow). (B) Uncompressed WATCHMAN with fabric (grey). (C) 11% compressed WATCHMAN resulting in an increased space between the frame and fabric (black arrow).