Corneal Vertex Location Attribute
| Tag | (0046,0202) |
|---|---|
| Type | Required (1) |
| Keyword | CornealVertexLocation |
| Value Multiplicity | 2 |
| Value Representation | Single (FL) |
Location of the corneal vertex. This is used for the reference point for other Attributes within this SOP Instance and determined by the Image referenced in the Attribute Source Image Sequence (0008,2112). Given as column\row. Column is the horizontal offset and row is the vertical offset.
Image relative position specified with sub-pixel resolution such that the origin at the Top Left Hand Corner (TLHC) of the TLHC pixel is 0.0\0.0, the Bottom Right Hand Corner (BRHC) of the TLHC pixel is 1.0\1.0, and the BRHC of the BRHC pixel is Columns\Rows (see Figure C.10.5-1). The values must be within the range 0\0 to Columns\Rows.
This location shall anchor the corneal vertex at the x, y and z coordinates of 0.0, 0.0, 0.0, in mm. See Section C.8.30.3.1.4 for further explanation.
Figure C.10.5-1
Figure C.10.5-1. Sub-pixel Addressing Units in PIXEL Space
Section C.8.30.3.1.4
C.8.30.3.1.4 Corneal Vertex Location
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.
Note
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