OCT imaging method offers measurement of retinal blood flow

Studies suggest a host of sight-threatening diseases may be associated with blood flow abnormalities, highlighting the relevance of detection.

A vascular imaging upgrade may offer ophthalmologists the ability to quantify blood flow, an attribute that studies suggest may signify progression of numerous sight-threatening retinal diseases as well as glaucoma.

In a presentation at Retina 2012, SriniVas R. Sadda, MD, told colleagues that accumulating evidence indicates that identification of blood flow abnormalities may be useful in the detection and monitoring of diabetic retinopathy, retinal vein occlusion, age-related macular degeneration and glaucoma. Doppler OCT, which supplies unique information compared to other imaging techniques such as angiography, may offer surgeons this capability.

“While we have good vascular imaging methods, in particular angiography, which provides excellent visualization of the microvasculature, provides an idea of the status of the blood-retinal barrier in the form of leakage and gives some dynamic information … there are still some limitations. You have relatively limited quantitative information, and certainly you do not have information such as blood flow velocity or the total amount of blood passing through the retina,” Dr. Sadda said.

Additionally, other strategies for blood flow measurement are associated with considerable setbacks. According to Dr. Sadda, Doppler ultrasound is plagued by relatively poor resolution, which hinders accuracy, and laser Doppler flowmetry is a slow approach that requires scanning one vessel at a time, making it a farfetched choice in a clinical setting.

SriniVas R. Sadda

“For blood flow measurement to be clinically relevant, the technology should be rapid, reliable and broadly available. It would seem that spectral-domain OCT would be the appropriate venue that would fit the bill for achieving those types of goals,” he said.

Relevant clinical data

In his presentation, Dr. Sadda discussed various case reports and studies suggesting potential applications of the Doppler OCT upgrade. For example, one case control comparison demonstrated that eyes with glaucoma may have lower total retinal blood flow and that flow could more strongly correlate with glaucoma than retinal nerve fiber thickness.

“There is also a lot of interest as to whether changes in blood flow may be an early feature of someone who is going to develop progressive retinopathy,” he said.

A pilot analysis published in Investigative Ophthalmology & Visual Science demonstrated that eyes with glaucoma, non-arteritic ischemic optic neuropathy, treated proliferative diabetic retinopathy and branch retinal vein occlusion all had significantly decreased retinal blood flow, compared with normal eyes (P < .001).

In terms of glaucoma and optic neuropathies, Doppler OCT may offer screening and early detection, identification of high-risk cases and evaluation of therapeutic efficacy, Dr. Sadda said. These same possibilities may apply to retinal vascular diseases as well.

“Perhaps this will be a new tool to rate or stage the severity of disease. … We may also look at targets that actually improve retinal blood flow; that may be a new endpoint in future studies,” he said.

Areas for improvement

In order to compute blood flow values, two factors must be known. Surgeons require measurement of Doppler shift, which is easy to extract from OCT data, and the incidence angle at which a beam of light strikes the blood vessel, which poses more of a challenge, Dr. Sadda said. Once these elements are known, an algorithm is applied.

Numerous artifacts, including motion, a poor-quality scan or a weak signal due to low Doppler angle, could disturb blood flow measurement, Dr. Sadda noted. For a low Doppler angle, he advised angling toward the vessel rather than perpendicularly, thereby increasing the Doppler angle.

Identifying vessels remains the greatest challenge, according to Dr. Sadda, and considerable errors are associated with automatic software. However, in a study published in Ophthalmic Surgery, Lasers & Imaging, a semi-automatic grading strategy using Doppler OCT of Retinal Circulation software achieved reasonably reproducible measurements of total retinal blood flow.

“I should emphasize that clearly some technological improvements are still required or are in progress to make the software better, so surgeons do not need a reading center,” he said. “That is the critical element needed for widespread clinical use.”

Larger, prospective trials are needed to validate the role of Doppler OCT, which does not make angiography obsolete, Dr. Sadda said.

“I do not think this technology necessarily kills angiography. … If you are going to be looking for areas of capillary non-profusion outside of the optic nerve or leakage, you still do not get that [information],” he said. – by Michelle Pagnani

References:

Konduru RK, Tan O, Nittala MG, Huang D, Sadda SR. Reproducibility of retinal blood flow measurements derived from semi-automated Doppler OCT analysis. Ophthalmic Surg Lasers Imaging. 2012;43(1): 25-31.
Wang Y, Fawzi AA, Varma R, et al. Pilot study of optical coherence tomography measurement of retinal blood flow in retinal and optic nerve diseases. Invest Ophthalmol Vis Sci. 2011;52(2):840-845.

For more information:

SriniVas R. Sadda, MD, can be reached at the Keck Medical Center of USC Pasadena, 10 Congress St., Pasadena, CA 91105; 626-395-0777; email: sadda@usc.edu.
Disclosure: Dr. Sadda performs contract research for Optos and Optovue.