Magnification changes in scanning laser tomography

The Evaluation of Anatomic and Functional Changes in Unilateral Moderate Amblyopic Eyes Using Optical Coherence Tomography and Pupil Cycle Time

PURPOSE

To investigate whether macular anatomic structure and afferent visual system function differ among amblyopic eyes, non-amblyopic fellow eyes, and controls, using spectral-domain optical coherence tomography (SD-OCT), and pupil cycle time (PCT).

METHODS

This observational, cross-sectional study included 30 patients with unilateral amblyopia and 30 healthy subjects. Optical coherence tomography (OCT) and pupil cycle time (PCT) were used to evaluate patients with unilateral amblyopia and were compared with their non-amblyopic fellow eyes and age- and gender-matched healthy eyes (30 participants). The amblyopic eyes were separated into two groups: anisometropic amblyopia (n = 16) and strabismic amblyopia (n = 14). OCT maps were used to calculate central macular thickness (CMT), retinal nerve fiber layer thickness (RNFLT), and ganglion cell-inner plexiform layer (GC-IPL) thickness.

RESULTS

The average RNFLT, GC-IPL thickness, and CMT did not show any significant differences among the amblyopic eyes, non-amblyopic fellow eyes and controls (p > 0.05, Kruskal-Wallis test). Mean PCT was 773.57 ± 64 msn in strabismic eyes, 771.25 ± 58 msn in anisometropic eyes, 778.00 ± 72 msn in non-amblyopic fellow eyes, and 774.75 ± 69 msn in control eyes. The differences among the amblyopic eyes, its fellow and control eyes were not statistically significant (p > 0.05, Kruskal-Wallis test).

CONCLUSIONS

In this study, we investigated morphological and functional differences among amblyopic eyes, non-amblyopic fellow eyes and controls by using SD-OCT and PCT. We did not find anatomical or functional changes in amblyopic eyes.

The effects of ocular magnification on Spectralis spectral domain optical coherence tomography scan length

PURPOSE

The purpose of this study was to assess the effects of incorporating individual ocular biometry measures of corneal curvature, refractive error, and axial length on scan length obtained using Spectralis spectral domain optical coherence tomography (SD-OCT).

METHODS

Two SD-OCT scans were acquired for 50 eyes of 50 healthy participants, first using the Spectralis default keratometry (K) setting followed by incorporating individual mean-K values. Resulting scan lengths were compared to predicted scan lengths produced by image simulation software, based on individual ocular biometry measures including axial length.

RESULTS

Axial length varied from 21.41 to 29.04 mm. Spectralis SD-OCT scan lengths obtained with default-K ranged from 5.7 to 7.3 mm, and with mean-K from 5.6 to 7.6 mm. We report a stronger correlation of simulated scan lengths incorporating the subject's mean-K value (ρ = 0.926, P < 0.0005) compared to Spectralis default settings (ρ = 0.663, P < 0.0005).

CONCLUSIONS

Ocular magnification appears to be better accounted for when individual mean-K values are incorporated into Spectralis SD-OCT scan acquisition versus using the device's default-K setting. This must be considered when taking area measurements and lateral measurements parallel to the retinal surface.

Agreement between retinal nerve fiber layer measures from Spectralis and Cirrus spectral domain OCT

PURPOSE

An assessment of the retinal nerve fiber layer (RNFL) provides important information on the health of the optic nerve. There are several non-invasive technologies, including spectral domain optical coherence tomography (SD OCT), that can be used for in vivo imaging and quantification of the RNFL, but often there is disagreement in RNFL thickness between clinical instruments. The purpose of this study was to investigate the influence of scan centration, ocular magnification, and segmentation on the degree of agreement of RNFL thickness measures by two SD OCT instruments.

METHODS

RNFL scans were acquired from 45 normal eyes using two commercially available SD OCT systems. Agreement between RNFL thickness measures was determined using each instrument's algorithm for segmentation and a custom algorithm for segmentation. The custom algorithm included ocular biometry measures to compute the transverse scaling for each eye. Major retinal vessels were identified and removed from RNFL measures in 1:1 scaled images. Transverse scaling was also used to compute the RNFL area for each scan.

RESULTS

Instrument-derived global RNFL thickness measured from the two instruments correlated well (R(2) = 0.70, p < 0.01) but with significant differences between instruments (mean of 6.7 μm; 95% limits of agreement of 16.0 μm to -2.5 μm, intraclass correlation coefficient = 0.62). For recentered scans with custom RNFL segmentation, the mean difference was reduced to 0.1 μm (95% limits of agreement 6.1 to -5.8 μm, intraclass correlation coefficient = 0.92). Global RNFL thickness was related to axial length (R = 0.24, p < 0.01), whereas global RNFL area measures were not (R(2) = 0.004, p = 0.66). Major retinal vasculature accounted for 11.3 ± 1.6% (Cirrus) or 11.8 ± 1.4% (Spectralis) of the RNFL thickness/area measures.

CONCLUSIONS

Sources of disagreement in RNFL measures between SD-OCT instruments can be attributed to the location of the scan path and differences in their retinal layer segmentation algorithms. In normal eyes, the major retinal vasculature accounts for a significant percentage of the RNFL and is similar between instruments. With incorporation of an individual's ocular biometry, RNFL area measures are independent of axial length, with either instrument.

The effect of cataract surgery on imaging optic nerve head topography with the Heidelberg Retina Tomograph

Purpose. To determine whether phacoemulsification cataract surgery with intraocular lens (IOL) implantation interferes with measuring optic nerve head (ONH) topography using the Heidelberg Retina Tomograph (HRT). Methods. The study population consisted of 31 women and 12 men aged 63-81 years with no previous history of eye diseases, surgery, laser procedures, or medication. The patients underwent first eye cataract surgery for senile cataract. The HRT II with software version 1.6 was used to obtain 3-dimensional images of the ONH as a part of a comprehensive ocular examination. The quality of the HRT image was assessed using topography standard deviation (TSD). Topography measurements are considered to be unreliable if TSD is more than 50 µm. Re-examination took place 1 month after surgery. Results. Before surgery, the topography measurements were unreliable in 35% (15/43) of the eyes; in 3 of these cases, ONH topography could not be calculated at all. One month after cataract surgery, the ONH topography could be calculated in all eyes and only one displayed unreliable topography measurements. The mean TSD was 40 µm before and 22 µm after surgery when calculated for all eyes with measurable topographies. The change in TSD was statistically significant (p<0.0005). The image alignment between the HRT examinations before and after surgery was excellent in 67% (26/39). Magnification changes occurred in 21% (8/39). Conclusions. Phacoemulsification cataract surgery with IOL implantation improves the image quality of the HRT. However, because of magnification changes and image misalignment, HRT follow-up of the ONH after cataract surgery is often unreliable.

Retinal nerve fiber layer thickness in amblyopic eyes

PURPOSE

To compare the peripapillary retinal nerve fiber layer (RNFL) thickness of amblyopic and fellow eyes. We hypothesized that the RNFL of the amblyopic eye may be thinner.

DESIGN

Prospective, cross-sectional, observational case series.

METHODS

Optical coherence tomography of the peripapillary RNFL thickness of amblyopic and fellow eyes was performed in 37 patients 7 to 12 years of age (mean age +/- standard deviation, 9.2 +/- 1.5 years) with unilateral strabismic, anisometropic, or combined-mechanism amblyopia enrolled in a randomized treatment trial.

RESULTS

Mean global RNFL thickness of the amblyopic and fellow eyes was 111.4 and 109.6 microm, respectively (mean difference, 1.8 microm thicker in the amblyopic eyes; 95% confidence interval, -0.6 to 4.3 microm). The amblyopic eye was 8 microm or more thicker than the fellow eye in 9 patients (24%); the fellow eye was 8 microm or more thicker than the amblyopic eye in 2 patients (5%); and the difference was within test-retest variability (7 microm) in 26 patients (70%).

CONCLUSIONS

Our findings do not indicate that peripapillary RNFL thickness is thinner in eyes with moderate amblyopia compared with their fellow eyes.

Magnification characteristics of the Optical Coherence Tomograph STRATUS OCT 3000

PURPOSE

To evaluate the magnification characteristics of an Optical Coherence Tomograph (STRATUS OCT 3000) used to measure optic disc size.

METHODS

An eye model was designed, built and used to measure changes in the magnification of the Optical Coherence Tomograph (OCT) with variation in the axial length of the eye and OCT correction lenses. Theoretical calculations using ray-tracing were used to compare the experimental and theoretical results.

RESULTS

OCT-performed fundus measurements have a magnification that depends on two factors: eye axial length and optical correction with the focusing dial of the OCT. Theoretical calculations showed that the size of the optic disc image for greater axial lengths of the eye model (myopic eye) was less than that for smaller axial lengths (hyperopic eye). The disc diameter measurements of the image obtained with the OCT did not agree with the real disc size. By focussing with the focusing dial of the OCT, the power of the optics can be varied from -12.0 D to +12.0 D and the image size is decreased for all axial lengths. The maximum variation of the disc area was 2.1%. Differences in area measured at intervals of 2.0 D after correcting the eye refraction for measurements with the OCT were equivalent to the repeatability error at that point.

CONCLUSIONS

Our results confirm that any variation in the optical system of the eye, in the OCT, and/or in the distance between them, induces a change in the magnification of the tomograph images. These variations will affect the measurements of any of the structures in the retinal plane.