Tag | (0046,0204) |
---|---|

Type | Conditionally Required (1C) |

Keyword | PupilCentroidYCoordinate |

Value Multiplicity | 1 |

Value Representation | Single (FL) |

The vertical coordinate of the centroid of the pupil relative to location of the cornea vertex specified in the Attribute Corneal Vertex Location (0046,0202), in mm, such that up is positive. See Section C.8.30.3.1.4 for further explanation.

Required if Attribute Corneal Topography Surface (0046,0201) is A (Anterior).

This Attribute is conveyed in mm from the corneal vertex because display applications will have zoom and 3-D rotational capabilities.

The Corneal Vertex Location (0046,0202) establishes the reference point for the corneal vertex, the origin of the Ophthalmic Coordinate System. The Ophthalmic Coordinate System is used as the Frame of Reference that establishes the spatial relationship for the corneal vertex (i.e., used within corneal topography maps) for a set of Images within a Series. It also allows Images across multiple Series to share the same Frame Of Reference. The corneal vertex is the point located at the intersection of the patient's line of sight (visual axis) and the corneal surface. It is represented by the corneal light reflex when the cornea is illuminated coaxially with fixation.

Since the criteria used to group images into a Series is application specific, it is possible for imaging applications to define multiple Series within a Study that share the same imaging space. Therefore the images with the same Frame of Reference UID (0020,0052) Attribute value share the same corneal vertex location within the patient's eye.

Figure C.8.30.3.1-3 illustrates the representation of corneal topography. The corneal vertex lies at the center of the rulers. Typical circular grids are 3, 5, 7, and 9 mm diameters centered on the vertex. The annotations in Figure C.8.30.3.1-3 are R, right; L, left; H = Head; F = Foot.

**Figure C.8.30.3.1-3. Representation of Corneal Topography**

Numerical position data shall use the Cartesian (i.e., two dimensional rectangular) coordinate system. The direction of the axes are determined by Patient Orientation (0020,0020), see Section C.7.6.1.1.1 for further explanation.

Devices that internally capture data in polar coordinates will need to convert to Cartesian coordinates, see Figure C.8.30.3.1-4.

**Figure C.8.30.3.1-4. Sample Coordinate Data Points**

When using the 3 dimensional coordinates (X, Y, Z), the Z axis shall represent corneal elevation. Z shall be measured from the length of a vector normal to the plane that is normal to and intersects the corneal vertex at the intersection of the x, y, z, axes. It is shown in the diagram as "+" (0.0, 0.0, 0.0). The Z axis shall be positive towards the anterior direction of the eye; (i.e., it is a right-hand rule coordinate system. Thus the Z values (see Figure C.8.30.3.1-5 and Figure C.8.30.3.1-6) will be predominantly negative, as they are posterior to the plane of the corneal vertex.

**Figure C.8.30.3.1-5. Schematic of the 3-Dimensional Representation of Corneal Elevation**

**Figure C.8.30.3.1-6. Schematic of the Ophthalmic Coordinate System of the 3-Dimensional Representation used in Wide Field Measurements**