ICG Angiography

Window to the Choroid

Laser imaging is sparking renewed interest in ICG angiography. Instead of the cloudy images traditionally associated with ICG, the confocal laser excites the dye, avoids the scattered light from flash photography, and produces images of stunning clarity.

SPECTRALIS provides crisp images of the choroidal circulation, including dynamic (movie) images of the uptake phase, adding to the diagnostic utility of ICG imaging.

ICG: The Heavyweight vs. Fluorescein

Indocyanine green (ICG) angiography enables imaging of the choroidal circulation below the RPE. This contrasts with fluorescein angiography which captures blood flow above the RPE, highlighting retinal vessels and capillary networks against the dark background of the RPE.

Two properties of ICG dye enable this different view of ocular circulation. The first is that ICG has a high molecular weight (775 kD) compared to fluorescein (375 kD) which helps explain its slower rate of leakage. Because it is largely protein bound, ICG remains in larger blood vessels longer and does not easily diffuse into smaller vessels or interstitial space, particularly in the choriocapillaris.

Peering through the RPE

The second key property of ICG is wavelength. It has peak absorption and emission wavelengths in the near infrared range (790-800 nm absorption and 830-840 nm emission). Infrared light penetrates deeper into tissue than visible light which enables excitation and reflected imaging through the RPE. Fluorescein has much different light properties, absorbing and emitting light in the visible blue-green range (494 nm absorption, 521 nm emission), a range of light which is blocked by the RPE.

One of the strongest applications of ICGA is in helping to visualize and confirm CNV. ICGA can often reveal occult CNV which may not appear on FA and can more clearly image CNV, potentially leading to a change in conclusion from no CNV to occult CNV or from occult CNV to classic CNV. Combing the properties of longer wavelength fluorescence with limited diffusion within the choriocapillaris mean that areas of CNV beneath blood, exudate or PED’s can be imaged more readily with ICG angiography (Yannuzzi 1992).

Identification of RAP Lesions Increasing

A form of neovascular AMD known as retinal angiomatous proliferation (RAP), alternatively called retinal choroidal anastamosis (RCA), was characterized in 1995 and has been identified as a potential indicator of a more aggressive form of CNV. RAP is difficult to distinctly identify in fluorescein angiograms, while the ability of ICG to “light up” the choroidal circulation has enabled easier identification of this lesion type. As a result, estimates of the frequency of RAP have been increasing, but vary widely, ranging from 10% to 28% of all CNV cases.

Even with the advent of anti-VEGF therapy, several investigators have suggested alternative treatment regimens when encountering this lesion type, including combination therapy using lasers, PDT, and other alternative therapeutics.

Central Serous Chorioretinopathy and Choroidal Hyperpermeability

ICGA may also provide useful information about central serous chorioretinopathy (CSCR). While CSCR is a relatively common disorder, FA does not provide evidence as to whether the initial abnormality is at the level of the choroid or RPE. However, due to its unique properties, ICGA enables assessment of the underlying choroidal hyperpermeability.

ICG angiography is not a replacement for fluorescein. It is adjunctive information which assists in defining the choroidal circulatory involvement in retinal pathology.