1Department of Ophthalmology, Oregon Health and Science University, 2Department of Ophthalmology, University of Southern California
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Tan, O., Wang, Y., Konduru, R. K., Zhang, X., Sadda, S. R., Huang, D. Doppler Optical Coherence Tomography of Retinal Circulation. J. Vis. Exp. (67), e3524, doi:10.3791/3524 (2012).
Noncontact retinal blood flow measurements are performed with a Fourier domain optical coherence tomography (OCT) system using a circumpapillary double circular scan (CDCS) that scans around the optic nerve head at 3.40 mm and 3.75 mm diameters. The double concentric circles are performed 6 times consecutively over 2 sec. The CDCS scan is saved with Doppler shift information from which flow can be calculated. The standard clinical protocol calls for 3 CDCS scans made with the OCT beam passing through the superonasal edge of the pupil and 3 CDCS scan through the inferonal pupil. This double-angle protocol ensures that acceptable Doppler angle is obtained on each retinal branch vessel in at least 1 scan. The CDCS scan data, a 3-dimensional volumetric OCT scan of the optic disc scan, and a color photograph of the optic disc are used together to obtain retinal blood flow measurement on an eye. We have developed a blood flow measurement software called "Doppler optical coherence tomography of retinal circulation" (DOCTORC). This semi-automated software is used to measure total retinal blood flow, vessel cross section area, and average blood velocity. The flow of each vessel is calculated from the Doppler shift in the vessel cross-sectional area and the Doppler angle between the vessel and the OCT beam. Total retinal blood flow measurement is summed from the veins around the optic disc. The results obtained at our Doppler OCT reading center showed good reproducibility between graders and methods (<10%). Total retinal blood flow could be useful in the management of glaucoma, other retinal diseases, and retinal diseases. In glaucoma patients, OCT retinal blood flow measurement was highly correlated with visual field loss (R2>0.57 with visual field pattern deviation). Doppler OCT is a new method to perform rapid, noncontact, and repeatable measurement of total retinal blood flow using widely available Fourier-domain OCT instrumentation. This new technology may improve the practicality of making these measurements in clinical studies and routine clinical practice.
1. Protocol Text
Figure 1a. Measuring total blood flow with the circumpapillary double-circular scan and the 3D disk scan using DOCTORC.
2. Representative Results
Normal and glaucomatous eyes are selected from the Advanced Imaging for Glaucoma Study (AIGS, www.aigstudy.net). 48 eyes were scanned by the "dual-angle" protocol and produced scans that passed the image quality check. Using the DOCTORC software, valid flow measurements could be obtained from 83% of the eyes.
To evaluate the reproducibility of the DOCTORC system, another small dataset with 20 eyes was graded by 3 graders. This dataset was also used to train and test graders. 2 graders used the semi-automated DOCTORC software and 1 used an earlier totally manual software employed in previous publications.2,3 The total retinal blood flows (Table 1) determined by the two graders using DOCTORC software are similar to one another and to flow rates determined by the other grader using the manual software. Only 65% of the eyes had valid results because some of the data were not based on dual angle protocol, but single angle protocol.2 The single angle protocol includes 5 Doppler scans obtained with the OCT beam passing through the center of the pupil. Therefore the Doppler angle is more often small and therefore a greater portion of vessels are usually not gradable.
For all graders, the inter-grader reproducibility, as measured by the coefficient of variation, is similar for both glaucomatous and normal eyes (Table 2). Likewise, the reproducibility measurements for the two methods, DOCTORC and the Manual software,1-5 are similar (Table 2). For three graders, a good correlation exists between total blood flow and the pattern standard deviation from visual field tests (Figure 2) for glaucomatous eyes.
|DOCTORC software Manual software3|
|Condition||Grader 1||Grader 2|
Table 1. Total Retinal Blood Flow using 2 Different Software.
|Coefficient of Variation|
|Glaucoma (7 eyes)|
|Grader 1 vs. Grader 2(DOCTORC)||9.58%|
|DOCTORC vs. manual method3|
|Normal (6 eyes)|
|Grader 1 vs. Grader 2 (DOCTORC)||5.99%|
|DOCTORC vs. manual method|
Table 2. Reproducibility of Total Retinal Blood Flow Measurements.
Figure 2. Correlation between Total Retinal Blood Flow and Visual Field in Glaucoma. a. Grader 1 using DOCTORC software. Visual field loss is summarized by pattern standard deviation (p=0.048). b. Grader 2 using DOCTORC software. Visual field loss is summarized by pattern standard deviation (p=0.032).
Blood flow abnormalities occur in glaucoma and vascular diseases of the retina such as diabetic retinopathy.6-10 Volumetric measurement of retinal blood flow gives valuable information about the disease process.4-6,11,12 DOCTORC provides a practical way to estimate total retinal blood flow based on the measurements in individual vessels determined by Doppler OCT using the double circle scan pattern.1-5
The mean total retinal blood flow measured by Doppler OCT in normal eyes is 47-49 μl/min, comparable to literature values of 34-65 μl/min obtained using laser Doppler techniques.13,14 Doppler OCT measurements made with the newer semi-automated DOCTORC software agreed closely with the results of manual measurements that we published previously.1-5 The difference between DOCTORC measurements and manual measurements in individual cases, as measured by CV, is similar to inter-grader differences. This indicates that the difference was primarily associated with subjective portion of the grading process, and not the difference between software. With the manual method and DOCTORC, we measure only veins with diameter larger than 33 μm. Veins with diameters less than 33 μm were usually not detectable using DOCTORC. These veins constitute only a very small portion of the total venous area (0.2%), and they contribute even less to total retinal blood flow because the flow speed in these vessels is less than in larger vessel.2 Thus, the difference between including and excluding very small vessels is not significant for the determination of total retinal blood flow. The high correlation between visual field tests and total retinal blood flows agrees with our previous result, indicating a close link between perfusion and visual function. Glaucoma eyes also have significantly lower blood flow than normal groups, which agrees with other studies.15-17 Thus total retinal blood flow determined by DOCTORC will be useful in the diagnosis and monitoring of the progression of glaucoma. In addition to blood flow measurements, DOCTORC also provides vessel area and vessel velocity measurements, which may also be useful in the clinic.
Other techniques are also available for measuring retinal blood flow; however each has some limitations. Laser Doppler techniques need many measurements over a long session as it tests only one vessel at a time. Ultrasound color Doppler evaluates only velocity in larger retrobulbar vessels, and it cannot determine volumetric blood flow. Ultrasound Doppler results vary with operator and subject anatomy. The variability makes it problematic to compare results between subjects and study centers.18 These instruments are also expensive and only available in major research centers. Other techniques such as fluorescein and indocyanine green angiography require intravenous injection, and they do not provide quantitative results. Fourier domain (or spectral domain) OCT is popular in ophthalmology and only software upgrade is required to enable Doppler blood flow measurement in these equipments. Our Doppler OCT method is the only means to measure blood flow with clinically available FD-OCT instruments. The prevalence and relatively low cost of this instrumentation make possible large multicenter studies of retinal blood flow in health and disease.
There are several limitations for the current version of DOCTORC. The grading process is still not fully automated, and the grading time of one eye is up to 30 min. This grading time is acceptable for large scale clinical studies but not fast enough for daily clinical usage. Direct arterial flow measurement is not available for DOCTORC because the high flow rate in artery is beyond the measurement range of the selected OCT system with speed of 26,000 a-scans/sec. Faster OCT systems would enable the measurement of arterial flow. About 17% of eyes scanned did not yield valid blood flow measurement due to poor Doppler angles on major vessels.
In summary, we provide a practical method to measure total retinal blood flow with a commercially available Fourier-domain OCT instrument. It will have wide applications for optic nerve and retinal diseases, such as glaucoma, diabetic retinopathy, and non-arteritic ischemic optic neuropathy.
Dr. Huang receives grant support, patent royalty, stock options, travel support and lecture fees from Optovue, Inc.; Dr. Tan and Dr. Wang receives patent royalty and grant support from Optovue, Inc.; Dr. Koduru and Dr. Sadda received grant support from Optovue.
This study is supported by NIH grant RO1 013516 and a grant form Optovue.
|RTVue Fourier Domain optical coherence tomography||Optovue||N/A||Version 188.8.131.52 or higher Installed with blood flow double ring scan pattern|