The choroidal circulation assessed by laser-targeted angiography

The choroid plays an important role in supplying nutrients to and removing waste products from the outer region of the retina. Abnormal choroidal blood flow can disrupt normal retinal function and lead to alterations in visual function. Visualization of the choriocapillaris in vivo is a great challenge to understanding its normal physiology and involvement in the disease process. Laser-targeted angiography (LTA) is a relatively new method used to visualize and analyze the choroidal circulation. Carboxyfluorescein (CF), encapsulated in heat-sensitive liposomes, is released locally in the choroid through the application of a heat beam provided by an infrared laser. Video angiograms are generated with excitation illumination provided by an argon laser. Obtained images are highly selective to the choriocapillaris and are sharply contrasted against underlying and overlying structures. The images can be obtained repetitively, during which period the circulating liposome concentration is sufficient to generate adequate angiograms. These high-quality images have revealed three distinct phases (filling, plateau, and draining) of the choriocapillaris. In the plateau phase, a cluster of lobules fed by a common arteriole has been uniformly illuminated. This defined cluster area does not change in size while an infrared laser is continuously applied to the same spot, which demonstrates that each cluster is functionally independent and no physiological communication exists between them. Only in posterior regions do the angiograms demonstrate during the filling and draining phases that each lobule is filled from a central spot and drained along a peripheral ring, showing honeycomb flow patterns. The regional differences in choriocapillaris flow patterns revealed by LTA suggests that the choriocapillaris provides a more highly efficient system of outflow in posterior regions than in peripheral regions. LTA is useful in analyzing choroidal circulation in vivo and has the potential for clinical application in the future. Additionally, LTA has a unique capability to image choroidal neovascularization in animal models and it promises potential application in age-related macular degeneration (AMD).