SDOCT Opens New Avenue in Choroidal Imaging

Applications in clinical practice are beginning to emerge.

BY JOHN W. KITCHENS, MD

Given its far posterior location and complex vasculature obscured by the retinal pigment epithelium (RPE) and other ocular structures, it’s no wonder the choroid seems to be a “final frontier” in our understanding of vitreoretinal disease. We know it plays a crucial role in retinal function and therefore a role in dysfunction as well. Yet, the ability to obtain detailed images of the choroid has eluded even our most sophisticated imaging modalities. Fortunately, that’s beginning to change. While we’ve been able to evaluate some features of the choroid using ultrasound, indocyanine green (ICG) angiography or Doppler flowmetry, a more recent goal has been to obtain useful cross-sectional images and measure thickness using spectral domain optical coherence tomography (SDOCT). All of the features that make SDOCT an improvement over time-domain OCT, such as its speed, sensitivity and resolving power, also make it capable of imaging the choroid.

“The ability to obtain detailed images of the choroid has eluded even our most sophisticated imaging modalities. Fortunately, that’s beginning to change.” —John W. Kitchens, MD

Connecting the Choroid With Retinal Disease

Spaide and colleagues¹ were the first to report on a technique for imaging the choroid using SDOCT. SDOCT instruments are designed so that the tissue to be imaged, the inner retina, is closest to the zero delay line, which is the point where the interferometric signals are strongest. By positioning the instrument closer to the eye than would be done in practice, Spaide and his team obtained an inverted image. In the inverted image, the choroid is closer to the zero delay line, which means it’s the structure benefiting from optimal signal strength and sensitivity. In their initial 2008 paper describing this technique,¹ known as enhanced depth imaging (EDI), they noted that using the images they obtained, independent observers were able to measure choroidal thickness, and their measurements correlated well. Spaide’s group has also reported on using EDI to evaluate the association of choroidal thickness with age and to measure mean choroidal thickness in healthy and myopic eyes. Beyond their initial work, they’ve used the technique to describe the morphological features of the choroid in several conditions, including central serous chorioretinopathy (CSC),² Vogt-Koyanagi-Harada disease³ and RPE detachment in AMD.⁴ Spaide also described a new clinical entity, age-related choroidal atrophy, based on observations with EDI.⁵

Figure 1. This 76-year-old female patient was referred for evaluation of exudative AMD. Enhanced-depth imaging revealed a thickened choroid. Based on the choroidal imaging in combination with her clinical findings (primarily subretinal fluid) and failure to respond to anti-VEGF therapy, she was diagnosed with central serous chorioretinopathy and treated with photodynamic therapy.

Figure 2. In this 81-year-old female patient with unexplained longstanding vision loss (20/70 OD and 20/150 OS), enhanced-depth imaging showed choroidal atrophy. This finding helped to explain her level of acuity in the absence of significant geographic atrophy or exudative changes.

Figure 3. This 29-year-old female patient, with a history of retinal detachment repair 7 years ago, presented with visual obscurations. Her daily dose of the oral anti-seizure/migraine prophylaxis medication topiramate had recently been increased. Topiramate has been associated with choroidal folds and choroidal detachments, and enhanced-depth imaging showed a thickened choroid.

Research by other groups is progressing along similar lines. For example, investigators at the New England Eye Center determined that choroidal thickness is altered in patients with diabetes, and diabetic macular edema is associated with a significant decrease in subfoveal choroidal thickness.⁶ They also described choroidal changes associated with retinitis pigmentosa⁷ and observed significant thinning in the choroids of wet AMD patients following 6 months of anti-VEGF therapy, which wasn’t seen in control eyes.⁸ Still others have used EDI to link reduced choroidal thickness with idiopathic macular hole.⁹

“Although enhanced depth imaging with SDOCT is a new area of concentration, it has vast potential for enhancing our understanding of the choroid’s role in vision and retinal diseases and therefore improving our diagnoses and treatments.” —John W. Kitchens, MD

A New Piece to Clinical Puzzles

At least three SDOCT devices that enable imaging of the choroid in some manner are now available in the United States: the Cirrus HD-OCT (Carl Zeiss Meditec), the Spectralis (Heidelberg Engineering) and the RTVue SD-OCT (Optovue). Other manufacturers, such as Nidek and Topcon, have incorporated this capability into their devices outside the U.S. market. Topcon has introduced the DRI OCT-1 for what it calls Deep Range Imaging for research purposes. And according to Nidek, its RS-3000 captures 13,250 A-scans per second in its highest sensitivity Ultra-Fine mode.

I have used both the Cirrus and Spectralis devices to image the choroid. In my practice, it has been most useful for helping to differentiate older CSC patients from patients who have AMD so the appropriate management, whether it be observation, photodynamic therapy or anti-VEGF treatment, can be determined (Figures 1 and 2). On average, I obtain choroidal images with SDOCT for two to five patients each day. In addition to the CSC vs. AMD cases, I obtain the images in cases where a cause of decreased vision or visual symptoms aren’t readily apparent (Figures 3). In some cases, I’ve been able to gauge response to treatment for polypoidal choroidal vasculopathy and pigment epithelial detachments.

A Glimpse into the Future

Although EDI with SDOCT is a new area of concentration, it has vast potential for enhancing our understanding of the choroid’s role in vision and retinal diseases and therefore improving our diagnoses and treatments. The technique will continue to become more useful as advances are made in related processes such as image averaging to reduce noise. Even more significant improvements will likely come from newer applications of OCT technology, such as full-depth imaging, which can provide EDI and retinal OCT in one view, and swept-source imaging. Swept-source OCT utilizes a longer wavelength (1,050 nm vs. 850 nm), which means it can penetrate more deeply. It can also register more images in less time and provide wider line scans (12 mm). Swept-source also allows a more detailed view of the vitreous (another “final frontier” in vitreoretinal imaging).

Other imaging modalities, such as ICG angiography and perhaps photoacoustics, will improve alongside SDOCT and help us to clarify what we are seeing with EDI. Eventually, we will be able to set our sights on analyzing the choroid’s large and medium blood vessels, as well as the choriocapillaris and Bruch’s membrane. ■