Choroidal Imaging Techniques: Past, Current, and Future

Introduction

The choroid is a highly vascular, pigmented tissue located between the retina and the sclera. The term is derived from the Greek words “form” and “membrane.” It measures 0.22 mm at the posterior pole and from 0.1 to 0.15 mm in its most anterior aspect in a postmortem pathologic sample study. Blood enters the choroid through the short posterior ciliary arteries and is distributed in three layers: Haller’s, Sattler’s, and choriocapillaris. Arterial blood flows consecutively through each of these layers and is then collected by venules. These venules converge in ampoules, which form the vorticose veins, and leave the eye through its equator. The choroid is responsible for the oxygenation and nourishment of the outer retina.

Few years ago, ultrasound and indocyanine green (ICG) angiography were the only imaging techniques available to assess the choroid. However, the understanding of the choroid has rocketed during the past decade followed by several advances in imaging technology that have granted it faster and easier visualization and measurement.

Ultrasonography

In 1956, ultrasonic techniques were first used for the diagnosis of ocular diseases. Some used the so-called intensity modulation technique (B-scan) that required the immersion of the eyes in water, whereas an acoustic tomogram of the eye could be obtained by a scanning movement of a crystal in front of the eye. Some others used time–amplitude methods (A-scan), in which the axis x of the screen forms the time axis, and axis y forms the amplitude axis of the echo.

Although the normal choroid could not be measured, and some authors stated that lesions under 4.0 mm in height could not be fully evaluated, choroidal melanomas and detachments could be found in cases they penetrated into the vitreous cavity for at least 1.5 to 2.0 mm. Subretinal coagulated blood presented a difficult differential diagnosis at that time.

Contact US scanners were introduced in the 1970s, with continuous evolutions and sensitivity improvements. During the last two decades, the revolution of digital format technology brought changes in examination technique, storage of data, and further improvements in image quality ( Fig. 4.1 ).

Choroidal melanoma is the most frequent intraocular malignant tumor, and despite the appearance of new imaging techniques, ultrasound is still of great use. Before its advent, melanomas were only suspected when a visible mass could be seen through clear media, and even in cases where a mass is visible, diagnosis may not be so clear. But it is in case of opaque media when ultrasound is of greatest use. Ultrasound patterns of choroidal masses are still critical to establish a good differential diagnosis. Low resolution, unavailability of quantitative data, and more importantly, poor reproducibility and operator dependency are major limitations of ultrasound.

Angiography

Fluorescein angiography (FA) and ICG angiography have been performed for decades to obtain useful clinical information about the retina and the choroid.

FA was developed in the 1960s for the study of choroidal tumors, and was mainly used to study retinal vasculature, so some of the first authors to use this dye studied choroidal circulation during the earliest phases of the angiogram or through areas of retinal atrophy, which made choroid vessels more visible and easier to distinguish.

Meanwhile, ICG was the first dye used in photographic industry and was first applied for clinical purposes in 1972, when Flower et al . tried to image and describe choroidal vasculature. Indocyanine is a lipophilic and hydrophilic substance with high protein-binding properties (up to 98%). These show greater molecular weight than albumin, which grants indocyanine a lower vascular permeability and tissue penetrance. This differentiates it from fluorescein and allows us for a better study of choroid vasculature. It is metabolized by the liver and suffers biliary excretion.

It is injected intravenously using concentrations of 5 mg/mL and in order to capture its circulation both an excitation and a barrier filter with peaks of 805 and 835 nm, respectively, are necessary. Pictures are usually taken from 8–10 up to 40 min after the injection of the dye. Later studies suggested that earlier times of the ICG could be useful to locate feeder vessels of choroidal neovascularization (CNV) complexes, and so, help focal treatment guidance.

It was, in fact, age-related macular degeneration (AMD) CNV that centered the vast majority of the studies performed using ICG, but has also been helpful for the investigations on the physiology of certain chorioretinal inflammatory disorders, as well as anterior segment disorders.

The choroidal vasculature is best demonstrated on ICG fluorescence angiograms and is not limited to cases of diseased pigment epithelium and choriocapillaris as in the case of FA ( Fig. 4.2 ). Given the choroid is a tridimensional tissue and the fact that the images that are obtained are displayed in a two dimensions, it is mandatory to have a proper knowledge of choroid anatomy in order to perform an accurate interpretation of the angiogram. Normal anatomic variations of blood drainage show an asymmetric pattern in up to 50% of patients, with preference for one of the vorticose veins, which may lead to misinterpretations.

ICG proves itself especially useful for the detection of recurrent CNV in difficult cases, such as pigment epithelial detachments (PED) or areas adjacent to laser scars. It is more valuable than FA for the location of CNV beneath subretinal or subRPE hemorrhages because of the greater penetration properties that infrared light grants. ICG angiography has become the most useful tool for the detection macular, extramacular, or peripapillary polyps. It can also be useful to identify the feeder vessel to a CNV or choroidal leakage/dilated choroidal vessels in CSC patients.