15.8  Volume Rendering

to arise from increased VEGF levels in the outer retina [55]. VEGF is released in response to a number of stimuli, so the growth of the vessels with eventual anastomotic connection could occur by an active process rather than the passive one, as described by Gass. The EDI OCT findings in PEDs so far has not supported the pathophysiologic processes and stages, as proposed to occur in RAP.

Summary for Clinicians

■There are numerous opposing theories concerning the formation of PEDs in AMD

■Each theory is limited by incomplete information concerning the contents of PEDs

■Untreated PEDs appear to have evidence of fibrovascular proliferation, oftentimes coursing up the back surface of the detached RPE

■Contracture of the fibrovascular material may lead to tears of the RPE

■Treated cases appear to contain material consistent with multilamellar scar-containing vessels

15.7  Additional Diseases such as Choroidal

Tumors and Glaucoma

EDI OCT has the ability to image the choroid, but still suffers the problems of decreased signal secondary to absorption and scattering from blood and pigment.

For example, it is not particularly helpful in examining pigmented tumors. On the other hand, it is possible to visualize the choroidal infiltration caused by nonpigmented tumors. For example, it is possible to see and measure lymphomatous collections and amelanotic melanomas. EDI OCT imaging can be helpful in ruling out the possibility of choroidal thickening. Glaucoma may involve both vascular and structural abnormalities. The choroidal contribution from the choroid was mentioned previously. The optic nerve can be imaged using the EDI technique to gain information about the lamina cribosa, the pores of which can easily be visualized.

15.8  Volume Rendering

OCT pictures are generally shown as 2D b-scan images acquired through the thickness of the retina. Some older time-domain OCT instruments created en face images that allowed inspection of lateral relationships among various retinal structures. The problem with simple en face imaging is that the eye is a curved structure and so an en face image cut across multiple retinal planes. This made image interpretation exceedingly difficult. In addition, small changes in position or scan depth from one image to the next produced vastly different representation of the eye. However, it is simple to obtain a large number of adjacent b-scan images with any typical SD OCT. After the successive images are aligned, a 3D

volume can be reconstructed. The noise reduction through averaging can be done to each component of b-scan prior to creating the 3D volume.

Volume rendering in this instance takes the advantage of the regular rectangular size of the successive 2D images, which were acquired at a fixed interval by the OCT instrument. Many SD OCT instruments rely on their inherently fast scan rate to assemble a series of b-scans with the assumption that the patient did not make any significant saccades during the scan interval. A projection image of the successive b-scans can be

made, and this projection image looks similar to a monochromatic image of the fundus, such as one acquired by a fundus camera or scanning laser ophthalmoscope. Usually, a series of b-scans obtained without eye tracking will show artifacts in the projection image showing that eye movement occurred during the scan interval. With mathematical techniques such as crosscorrelation between b-scans, it is possible to partially compensate for eye saccades. However, image averaging is very difficult because there is little in the way of any feedback to know where in the eye the scan is taking

place during any particular b-scan. Eye-tracking techniques can assure that the successive b-scans are repeatedly acquired from the same location. A common method to render volume images uses a technique called ray casting. Uniform resampling of nonuniform sampled information when the sampling intervals are known (such as the information derived from conventional OCT instruments) is often best done with polynomial techniques or the use of band-limited sinc functions. The resultant ray is assigned gradients of shading based on the density and location of the resampled points which are composited to create a planar set of data representing the 3D attributes as seen from the viewing

point. This planar data is used to make the display image shown, for example, on a computer monitor. An example of 3D volume rendering of the choroid is shown in Fig. 15.22. With faster OCT scanning and different scanning strategies, it will be possible to easily generate choroidal volume scans in the future.

15.9Summary

The choroid, although difficult to image, accounts for the majority of blood flow in the eye, and is central to several major diseases affecting visual function. A simple method

to visualize the choroid was presented, and this technique allows visualization in nearly every patient. The choroidal thickness in normal eyes can be measured and used as a comparison to that found in various disease states. Primary atrophy of the choroid occurs with age and produces a condition differentiable from typical AMD. Myopic

degeneration may have a significant contribution from choroidal thinning induced by both the degree of myopia as well as age. Volume-rendering techniques allow visualization and evaluation of the volume of the choroid. The same imaging techniques can be applied to the optic nerve to investigate glaucoma.