Purpose
To evaluate temporal changes and predictors of accuracy in the alignment between simultaneous near-infrared image and optical coherence tomography (OCT) scan on the Heidelberg Spectralis using a model eye.
Design
Laboratory investigation.
Methods
After calibrating the device, 6 sites performed weekly testing of the alignment for 12 weeks using a model eye. The maximum error was compared with multiple variables to evaluate predictors of inaccurate alignment. Variables included the number of weekly scanned patients, total number of OCT scans and B-scans performed, room temperature and its variation, and working time of the scanning laser. A 4-week extension study was subsequently performed to analyze short-term changes in the alignment.
Results
The average maximum error in the alignment was 15 ± 6 μm; the greatest error was 35 μm. The error increased significantly at week 1 ( P = .01), specifically after the second imaging study ( P < .05); reached a maximum after the eighth patient ( P < .001); and then varied randomly over time. Predictors for inaccurate alignment were temperature variation and scans per patient ( P < .001). For each 1 unit of increase in temperature variation, the estimated increase in maximum error was 1.26 μm. For the average number of scans per patient, each increase of 1 unit increased the error by 0.34 μm.
Conclusion
Overall, the accuracy of the Heidelberg Spectralis was excellent. The greatest error happened in the first week after calibration, and specifically after the second imaging study. To improve the accuracy, room temperature should be kept stable and unnecessary scans should be avoided. The alignment of the device does not need to be checked on a regular basis in the clinical setting, but it should be checked after every other patient for more precise research purposes.
Spectral-domain optical coherence tomography (SDOCT) has emerged as the gold-standard noninvasive technique to visualize fine retinal details and to evaluate retinal structural changes. High axial scanning speeds and axial resolution of 5-7 μm allow histologic-like cross-sectional images of the posterior structures. Many SDOCT devices are commercially available, with different features including eye-tracking, image averaging, 3-dimensional imaging, and choroid enhancement.
The Heidelberg Spectralis (Heidelberg Engineering, Heidelberg, Germany) is a widely used confocal scanning laser ophthalmoscope (cSLO). The device simultaneously performs and correlates SDOCT scans with multiple imaging modalities such as near-infrared (NIR), fundus autofluorescence, red free, fluorescein angiography, and indocyanine green angiography. Its unique ability to co-localize posterior structures on bi-dimensional fundus images and on cross-sectional scans helps ophthalmologists in the diagnosis and the management of many ocular disorders for both clinical care and research. The platform of the device simultaneously images the eye with 2 beams of light; 1 beam captures an image of the retina and maps over 1000 points to track eye movement. Using the mapped image as a reference, the second beam is directed to the desired location despite blinks or saccadic eye movements. The eye-tracking dual-beam technology (TruTrack Active Eye Tracking software; Heidelberg Engineering) mitigates eye motion artifact and ensures point-to-point correlations between the OCT scan and fundus images. The eye-tracking technology also permits precise scanning of the same location over consecutive visits, and has been proven to provide repeatable macular thickness measurements. The infrared light (830 nm) of the cSLO is largely invisible to the patient; thus NIR is the best-tolerated and most commonly used imaging modality when acquiring simultaneous OCT scans. Moreover, this wavelength does not necessitate pupil dilation in order to obtain good-quality OCT scans at any arbitrary location of the posterior pole.
The correlation of bi-dimensional fundus images with cross-sectional images allows precise evaluation of posterior structures, as well as analysis of structural changes over micrometric areas of interest. Therefore, precise alignment between the NIR image and the OCT scan is necessary to avoid errors when performing microstructural analysis of the retinal anatomy. The purpose of this multicenter study was to evaluate temporal changes in the alignment between the simultaneous NIR image and OCT scan when using the Heidelberg Spectralis, which would further assess the inter-instrument variability and determine predictors of inaccurate alignment.
Methods
Six ophthalmologic sites were involved in this multicenter study beginning in March of 2012. Sites included were: (1) Jacobs Retina Center and (2) Hamilton Glaucoma Center at the Shiley Eye Center, University of California San Diego, La Jolla, California, USA; (3) Fondazione IRCCS Ca’ Granda-Ospedale Maggiore Policlinico and (4) Luigi Sacco Hospital, University of Milan, Milan, Italy; (5) Azienda Ospedaliera Sant’Anna, Como, Italy; and (6) Department of Ophthalmology, Ludwig-Maximilians-University, Munich, Germany.
Before starting the study, all sites checked the alignment of their own device and then performed a calibration procedure, using a dedicated alignment/calibration tool as model eye and a dedicated software developed by Heidelberg Engineering. The tool consisted of a lens and a 3-dimensional target. The target was an inverse square pyramid with a slight rotational offset at each step. The software was written to use the 3-dimensional nature of the target to find proper alignment between the cSLO image and the OCT image. The procedure to check the alignment required approximately 60 seconds, and the calibration procedure required between 2 and 3 minutes. The alignment procedure was repeated every week for a total of 12 weeks. This check was done after the last scheduled patient was scanned, and the printout of the procedure was collected. There was no direct contact with patients’ eyes during the alignment/calibration procedure.
All sites collected daily data about turn-on and turn-off times of the laser of the devices, as well as lowest and highest temperature in the area where the Heidelberg Spectralis was located. Using the Heidelberg Eye Explorer software (Heyex; Heidelberg Engineering), the number of weekly scanned patients, total number of OCT scans performed, and total number of B-scans performed were exported and collected retrospectively. The average number of weekly scans performed per patient was calculated; each scan (including line, circle, star, or raster scans) was considered to count as 1 unit. The average number of weekly B-scans performed per patient was calculated as well; line and circle scans consist of 1 B-scan, star scan consists of 6 B-scans, and raster scans consist of multiple B-scans according to the settings chosen by the operator. Patient identifying information was not recorded and patient imaging results were not analyzed. During the 3-month study, sites were asked not to delete any OCT scans acquired on the device, nor to turn off the laser of the device until scanning the last scheduled patient for each day.
After completing the 12-week study, a 4-week extension study was performed at the Jacobs Retina Center (site 1) to analyze the short-term (daily) change in the alignment. Calibration was performed before scanning the first patient of the week for each of the 4 weeks. The alignment of the device was then rechecked right after completing the image acquisition of each of these patients. During the following days of the week, the alignment was checked only after scanning the last patient of the day. The same protocol was repeated for all 4 weeks.
Statistical analyses were performed applying the Generalized Estimating Equations (GEE) test using SAS statistical software version 9.2 (SAS Inc, Cary, North Carolina, USA). A P value <.05 was considered to be statistically significant.
Results
A summary of all results of the 12-week study is presented in the Table . After the calibration procedure at baseline, the mean horizontal error among the 6 sites was 0.54 ± 0.31 pixels, equivalent to 3.10 ± 1.80 μm; the mean vertical error was 0.69 ± 0.12 pixels, equivalent to 3.93 ± 0.68 μm. During the 12-week study, the mean horizontal error among the 6 sites was 2.43 ± 1.20 pixels, equivalent to 13.90 ± 6.89 μm; the mean vertical error was 2.46 ± 1.14 pixels, equivalent to 14.08 ± 6.55 μm. The average maximum error among all measurements was 2.60 ± 1.01 pixels (range, 0.41-6.07 pixels), equivalent to 15.35 ± 6.29 μm (range, 2.35-34.78 μm).
| Sites | ||||||
|---|---|---|---|---|---|---|
| (1) Jacobs Retina Center | (2) Hamilton Glaucoma Center | (3) Ospedale Maggiore Policlinico | (4) Luigi Sacco Hospital | (5) Azienda Ospedaliera Sant’Anna | (6) Department of Ophthalmology Munich | |
| Horizontal error, μm (±SD) | 18.79 ± 5.39 | 16.87 ± 1.73 | 15.99 ± 8.42 | 8.16 ± 4.35 | 15.26 ± 7.52 | 13.74 ± 3.24 |
| Vertical error, μm (±SD) | 18.39 ± 4.92 | 17.21 ± 1.53 | 17.76 ± 7.96 | 8.30 ± 4.73 | 14.42 ± 6.48 | 13.51 ± 2.66 |
| Maximum error, a μm (±SD) | 18.96 ± 5.30 | 17.42 ± 1.70 | 17.63 ± 7.95 | 8.72 ± 4.61 | 15.35 ± 7.45 | 14.03 ± 3.12 |
| Lowest temperature, C (±SD) | 18.75 ± 0.45 | 20.08 ± 0.79 | 20.17 ± 2.17 | 20.17 ± 1.53 | 21.33 ± 0.65 | 19.6 ± 1.31 |
| Highest temperature, C (±SD) | 23.25 ± 1.14 | 23.5 ± 0.52 | 25.3 ± 1.37 | 25.25 ± 1.71 | 25.00 ± 0.43 | 26.08 ± 0.79 |
| Temperature excursion, C (±SD) | 4.25 ± 1.06 | 2.92 ± 0.79 | 4.42 ± 2.11 | 3.67 ± 0.49 | 3.42 ± 0.90 | 6.00 ± 1.28 |
| Working time, hours (±SD) | 26.06 ± 4.80 | 43.57 ± 10.31 | 33.47 ± 7.26 | 41.65 ± 7.24 | 44.88 ± 6.91 | 39.10 ± 5.10 |
| Scanned patients, n (±SD) | 40.50 ± 7.86 | 15.25 ± 7.28 | 140.50 ± 30.99 | 155.58 ± 49.38 | 62.25 ± 7.75 | 24.58 ± 7.60 |
| OCT scans, n (±SD) | 874.92 ± 278.99 | 176.42 ± 88.69 | 997.58 ± 221.44 | 626.67 ± 215.68 | 380.75 ± 63.10 | 64.33 ± 18.20 |
| Scans per patient, n (±SD) | 21.27 ± 3.45 | 11.35 ± 1.09 | 7.15 ± 0.57 | 4.03 ± 0.43 | 6.11 ± 0.57 | 2.65 ± 0.42 |
| B-scans, n (±SD) | 4,374 ± 999 | 6,029 ± 2,739 | 7,662 ± 1,94 | 11,402 ± 3,845 | 4,927 ± 820 | n/a |
| B-scans per patient, n (±SD) | 109.64 ± 28.91 | 409.26 ± 70.76 | 54.26 ± 3.90 | 72.46 ± 6.76 | 79.02 ± 7.15 | n/a |
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