Indocyanine green angiography

Diagnostic procedure

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Choroidal blood flow revealed with indocyanine green angiography

Indocyanine green angiography (ICGA) is a diagnostic procedure used to examine choroidal blood flow and associated pathology. Indocyanine green (ICG) is a water soluble cyanine dye which shows fluorescence in near-infrared (790'805 nm) range, with peak spectral absorption of 800-810 nm in blood.[1][2] The near infrared light used in ICGA penetrates ocular pigments such as melanin and xanthophyll, as well as exudates and thin layers of sub-retinal vessels.[3] Age-related macular degeneration is the third main cause of blindness worldwide, and it is the leading cause of blindness in industrialized countries.[4] Indocyanine green angiography is widely used to study choroidal neovascularization in patients with exudative age-related macular degeneration.[5] In nonexudative AMD, ICGA is used in classification of drusen and associated subretinal deposits.[5]

Indications

Indications for indocyanine green angiography include:

Procedure

Fundus camera-based indocyanine green angiography techniques and scanning laser ophthalmoscope-based indocyanine green angiography techniques are there.[10] The concentration of indocyanine green dye may vary according to instrument used. For fundus cameras, 25 ml ICG dissolved in 5 ml solvent is used, it may be increased to 50 ml in patients with poorly dilated pupil and high pigmentation.[6] In case of iodine allergy, instead of ICG, iodine-free dye Infracyanine green should be used.[11]

To perform test, pupil should be dilated. The dye is injected through the antecubital vein as bolus.[12] Images are taken in several second intervals until the retinal and choroidal circulations are maximally hyperfluorescent.[10] Then for first few minutes, take photos at approximately 30 to 60 second intervals. Pictures taken are classified under three phases:[13]

1. Early phase at 60 seconds: large choroidal arteries and veins are highlighted in this phase.

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2. Mid phase at 5'15 minutes: in this phase choroidal vasculature become less distinct and more diffuse, and hyperfluorescent lesions appear bright against the fading background.

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3. Late phase at 15'30 minutes: in this phase hyperfluorescent lesions appear bright against the dark background.

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   The choroidal neovascularization are best detected in this phase.

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Advantages over fluorescein angiography

Indocyanine green angiography has many advantages over commonly used fundus fluorescein angiography (FFA). Because of its protein-binding properties, its leakage from choriocapillaries is less and thus it will remain longer in choroidal vessels compared to fluorescein dye.[1] Choroidal neovascularization is better visualized by ICGA, than fluorescein angiography.[5] The patient toleration is also better compared to FFA.[12]

History

Physical and physiological properties of indocyanine green dye were first described by Fox and Wood, in .[15] Indocyanine green angiography was developed by Kodak Research Laboratories for determining cardiac output. In , Kogure et al. performed intra-arterial choroidal absorption angiography using indocyanine green dye in monkeys.[16] In the year , using ICGA, Kogure and Choromokos studied cerebral circulation in a dog. In , Hochhimer replaced color film with black and white infrared film. First human ICG angiogram was of carotid artery. First intravenous ICGA in human eye was performed by Flower and Hochheimer in .[12][6] In Hayashi et al. used infrared-sensitive video camera to perform ICGA.[10] In the year , Guyer et al. introduced the use of high resolution ( × ) digital imaging system coupled with infrared video cameras to produce better high resolution images.[17]

See also

References

Indocyanine green angiography (ICGA) is an imaging technique used to evaluate choroidal vasculature and circulation in which indocyanine green (ICG) dye is injected intravenously. ICG was developed to study cardiac output and hepatic circulation, applied to the field of ophthalmology, and has helped overcome the limitations of sodium fluorescein, the dye used in fluorescein angiography (FA), in the study of choroidal vessels. FA provides great resolution of retinal circulation, but it minimally penetrates the retinal pigment epithelium (RPE), and choroidal circulation is not clearly visualized with this technique.

ICG emits light in the near-infrared spectrum, improving considerably the amount of light that is able to go through the RPE and choroidal pigment, fluid, blood, or lipids. In addition, the majority of the dye binds to plasma proteins, and this property prevents the diffusion of ICG through the fenestrated choriocapillaris, so the dye can accurately delineate choroidal vessels. Also, it is considered a very safe technique, with a very low incidence of adverse reactions. All of these chemical and biophysical properties of ICG make it a very useful tool for the study of choroidal vasculature, especially in those cases in which fluid or blood obscure choroidal details in fluorescein angiogram.

The choroid is a thick and multilayered structure, which constitutes the major blood supply of the eye and receives 90% of the ocular blood flow. The visualization of this complex structure by ICGA can help us expand our current knowledge and understanding of the choroid in the normal eye and also in pathologic conditions. For these reasons, ICGA is a very useful imaging technique in clinical practice.