Importing and Calibrating a Fundus Collage

If possible, have the Ophthalmologist provide the completed collage as a .jpg file or as a print that you can digitize on a flatbed scanner. Many fundus cameras provide collages now. If you receive a set of fundus pictures, the recommended method for creating a collage for Plaque Simulator is to use the Adobe Photoshop program to assemble the collage manually and export it as a .jpg file.

The fundus collage is used to digitize the location, appearance, and perimeter of the tumor base on the retinal surface with respect to retinal landmarks that can also be identified in CT imaging space.

Two landmarks that are readily identifiable in fundus photography are the optic disc and macula. The location of the optic disc can be closely estimated in 3D CT (or MR) imaging space by reconstructing a meridian plane which bisects the eye and passes through the center of the optic nerve. This meridian plane, by definition, will also pass through the posterior pole of the eye. The posterior pole can be closely estimated in the fundus photos because the macula is adjacent to the pole.

Therefore, in order to accomplish the fundus-CT fusion, the fundus collage MUST contiguously include as much of the tumor as can be photographed as well as the macula and optic disc.

In the Image window...
  • If the Image window is not visible, bring it to the front by selecting the Images item from the Window menu.

Importing and Calibrating a Fundus Collage
  • In the Image window, select the fundus image buffer by clicking the Fundus button in the Fundus controls group. If there is already an image loaded in this buffer, PS will simply display that image in the window.
  • To clear the current image, control-click (or right mouse button) the fundus button and select Clear from the contextual menu.
  • To load a new image, control-click (or right mouse button) the fundus button and select Load Fundus... from the contextual menu.
  • Navigate to the folder in which you have saved the prepared images and select the fundus image (e.g. Fundus.jpg). Click the Open button in the OSX file navigator to continue.
  • You may optionally simply drag and drop an image file into the Image window. When a single file is dragged into the window it will be loaded into the currently selected image buffer irrespective of the file's name.

Using the fundus ruler tool
  • The fundus calibration is critical because it establishes how the fundus image will be mapped onto the retinal surface of the 3D model, and thus the size and shape of the tumor base and where it is located within the eye.
  • After the fundus collage has loaded, click the Ruler button in the Fundus controls group to display the PS fundus calibration ruler tool.
  • FundusGroup
  • The Ruler button in the Fundus controls group also features a contextual menu for customization of ruler properties and related parameters.
  • FundusGroupRuler
  • The Angle button in the Fundus controls group adds a merdian compass to the fundus calibration ruler tool.
  • FundusGroupFundus
  • The primary PS fundus ruler tool is a magenta line with yellow sizing control points (aka handles) at each end. One end is labeled Pole.
  • Click the cursor in the 'pole' handle and drag it over the posterior pole in the fundus collage.
  • Click the cursor in the unlabeled handle and drag it over the center of the optic disc in the fundus collage.
  • Select the number of collage photo centers from the menu in the Fundus controls (e.g. 3 in this example). Additional photo center markers will appear.
  • Important: The posterior pole marker is always considered as photo center #1, even if it is not exactly centered in a photo.
  • Drag the additional photo center markers (labeled 2 and 3 in this example) over the center of each of the photos that make up the collage.
  • Adjust the photo size slider (located just below the collage centers menu) so that the magenta circles surrounding each photo center marker match the photo edges.
  • Next, click the Calibrate button in the fundus group to open the Calibration sheet.
  • Enter the chord distance in mm from the center of the optic disc to the posterior pole.
  • Click the Import from eye model button to estimate this distance from a previously calibrated CT image.
  • You may also customize the image transparency color in this sheet. The default transparency color is black. All black pixels in the photo, such as the rectangular background padding surrounding the circular fundus photos, will become transparent when applied to the eye or retinal diagram.
  • Click the OK button to exit the sheet.

A properly calibrated fundus collage

When properly calibrated, the end of the fundus ruler centered on the optic disc should now have a 1 mm radius magenta circle surrounding it, just outside the border of the optic disc.


The number of collage centers
  • It is not necessary to digitize every photo center. In this example, although the collage was assembled from 4 photos, it is sufficient to digitize only 3 centers (the pole and centers #2 & #3) which adequately cover the region of interest which includes the tumor, the pole and the disc.
  • When the number of collage centers comprising the collage is set properly, the collage will map onto the 3D retinal surface with less distortion near the equator.
Only 1 collage center located at the posterior pole
3 centers reduces distortion near the equator