HRT Software Research Articles

Comparison of optic nerve head topography in normal eyes using confocal laser scanning tomograph and retinal thickness analyzer

Background: To compare optic nerve head topography measurements by Heidelberg retina tomograph I (HRT) and retinal thickness analyser (RTA) and thereby to determine clinical agreement between the devices. To investigate reproducibility of measurements for HRT and RTA. To obtain our own HRT and RTA database for comparison of healthy and glaucomatous eyes. Methods: The study included 100 normal eyes of 50 white adults of both sexes (mean age of 55.5 [SD 9.5] years). Ophthalmologic examination, fundus photography, HRT and RTA examinations were performed. To determine reproducibility 10 volunteers repeated the examination after a week. For HRT and RTA measurements mean (SD), median, 5th and 95th percentile, range and coefficient of variation were calculated. Differences in topographic parameters between HRT and RTA were tested for significance. Clinical agreement was assessed with »limits of agreement« and reproducibility expressed by repeatability coefficient and intraclass correlation coefficient (ICC). Results: Significant differences (p 0.10 for RTA parameters cup area (0.15), rim area (0.19), maximum cup depth (0.13), height variation contour (0.11) and RNFL cross-sectional area (0.14). ICC was good (> 90%) for all parameters, except for mean RNFL thickness (89%) for HRT and height variation contour (84%) for RTA. Conclusions: The two devices cannot be used interchangeably. At present, clinical usefulness of HRT is superior to RTA due to better reproducibility of measurements, better software support, shorter duration of the examination and good comparability of our HRT database with the database included in the latest HRT software version. At the moment RTA is using the same database as HRT. Therefore, our RTA database provides a new basis for comparison of optic disc characteristics between healthy and glaucomatous eyes.

Detecting abnormality in optic nerve head images using a feature extraction analysis.

Imaging and evaluation of the optic nerve head (ONH) plays an essential part in the detection and clinical management of glaucoma. The morphological characteristics of ONHs vary greatly from person to person and this variability means it is difficult to quantify them in a standardized way. We developed and evaluated a feature extraction approach using shift-invariant wavelet packet and kernel principal component analysis to quantify the shape features in ONH images acquired by scanning laser ophthalmoscopy (Heidelberg Retina Tomograph [HRT]). The methods were developed and tested on 1996 eyes from three different clinical centers. A shape abnormality score (SAS) was developed from extracted features using a Gaussian process to identify glaucomatous abnormality. SAS can be used as a diagnostic index to quantify the overall likelihood of ONH abnormality. Maps showing areas of likely abnormality within the ONH were also derived. Diagnostic performance of the technique, as estimated by ROC analysis, was significantly better than the classification tools currently used in the HRT software - the technique offers the additional advantage of working with all images and is fully automated.

Relationship between Diastolic Perfusion Pressure and Progressive Optic Neuropathy as Determined by Heidelberg Retinal Tomography Topographic Change Analysis

To determine through retrospective file analysis which clinical factors best predict glaucomatous optic neuropathy as evaluated by Heidelberg retinal tomography (HRT II) imaging. One HUNDRED twenty-two records from patients referred for HRT imaging at the University of Waterloo Ocular Heath Clinic met inclusion criteria for this study and were reviewed. Topographic change analysis (TCA) data generated by HRT were examined in addition to the following clinical information: diastolic blood pressure, right arm sitting, intraocular pressure, and central corneal thickness. All HRT scans included were required to have 20 μm or better standard deviation (SD) on acquisition and deemed "very good" quality or "excellent" by HRT software. Based on previously defined published HRT TCA change criteria, each patient was allocated to one of the following groups: stable, borderline, or progressive. Diastolic perfusion pressure (DPP) was found to be significantly lower in the borderline and progressive groups compared with the stable group (P < 0.001). DPP was also lower significantly lower in the progressive group compared with the borderline group (P < 0.001). Low DPP appears to be a reasonable predictor of progressive optic neuropathy as determined using scans of <20 μm SD on the HRT TCA platform. DPP of 56 mm Hg or lower appears to be a clinically useful threshold to identify patients at increased risk of progressive optic neuropathy.

Comparison of Neuroretinal Rim Area Measurements Made by the Heidelberg Retina Tomograph I and the Heidelberg Retina Tomograph II

Purpose:To investigate the agreement between neuroretinal rim area (RA) measurements using the Heidelberg Retina Tomograph I (HRT Classic) and Heidelberg Retina Tomograph II (HRT II). To compare apparent RA changes in follow-up series of HRT II topographies when using either an HRT Classic or HRT II mean topography as baseline.Design:Cross-sectional study and “no-change,” short time series study.Participants:Forty-three ocular hypertensive and 31 primary open angle glaucoma subjects.Methods:Five HRT Classic and 5 HRT II examinations were acquired from 1 eye of each subject, across 2 visits within 6 weeks. For the cross-sectional study, follow-up RA measurements from HRT Classic and HRT II were compared, using the same HRT Classic mean topography as the baseline. The linear rates of RA change were compared in 2 short time series with either an HRT Classic or an HRT II mean topography as baseline, and 4 follow-up HRT II mean topographies. Intervals between topographies were arbitrarily set at 1 year for meaningful comparisons of rates. Rates of RA change over time were calculated by linear regression. Separate analyses were performed using 3 available reference planes (RP).Main Outcome Measures:Global and sectoral RA measurements in HRT Classic and HRT II mean topographies; linear rates of RA change.Results:HRT Classic minus HRT II mean differences (95% limits of agreement) were 0.09 (−0.17, 0.35) mm2, 0.09 (−0.13, 0.32) mm2, and 0.11 (−0.24, 0.46) mm2 for the Moorfields, 320 µm, and standard RPs, respectively (P<0.001 for all RPs, Wilcoxon rank sum test). In the time series, the mean differences (95% limits of agreement) of RA rates of change (HRT Classic baseline minus HRT II baseline) were −0.01 (−0.06, 0.03) mm2/y, −0.01 (−0.06, 0.04) mm2/y, and −0.02 (−0.09, 0.05) mm2/y using the Moorfields, 320 µm, and standard RPs, respectively.Conclusion:Although HRT software is backward-compatible, follow-up RA measurements made in the same eye using HRT Classic and HRT II devices display statistically and clinically meaningful systematic differences when HRT Classic topographies are used as a baseline.

Agreement between Heidelberg Retina Tomograph-I and -II in detecting glaucomatous changes using topographic change analysis

To evaluate the suitability of including both Heidelberg Retina Tomograph-I (HRT-I) and HRT-II examinations in the same longitudinal series for HRT topographic change analysis (TCA) and to evaluate parabolic error correction (PEC) to improve the agreement between HRT-I and HRT-II examinations. A total of 66 eyes from the University of California San Diego Diagnostic Innovations in Glaucoma Study with baseline HRT-I and HRT-II examinations obtained on the same day and ≥ 3 HRT-II follow-up examinations were included. Two TCA analyses, HRT-I examination at baseline (HRT-I-mixed series) and HRT-II examination at baseline (HRT-II-only series) were compared. Agreement between the HRT-I-mixed and HRT-II-only series were estimated using Bland-Altman plots. Agreement was assessed: (1) using the current HRT software settings (PEC applied only to HRT-II-only series), and (2) modified HRT settings (PEC also applied to HRT-I-mixed series). With current HRT software settings, the HRT-I-mixed series significantly overestimated change locations (ie, red pixels) compared with the HRT-II-only series as indicated by statistically significant proportional biases in the Bland-Altman analysis. By applying PEC to HRT-I-mixed series there were no statistically significant biases in the TCA parameter estimates compared with the HRT-II-only series. In some eyes, HRT-I and HRT-II baseline examinations are not interchangeable in TCA analysis without parabolic error correction. HRT-I-mixed series detected more changes characteristic of glaucoma when there were only minimal changes in the HRT-II-only series. Our results suggest that in the majority of cases, with PEC, HRT-I examinations may be included in a longitudinal series containing HRT-II examinations.

AnIn SilicoModel of Scanning Laser Tomography Image Series: An Alternative Benchmark for the Specificity of Progression Algorithms

There is no gold-standard measurement of glaucomatous structural progression against which to validate software progression algorithms. A computer model was developed and validated to simulate stable series of Heidelberg Retina Tomograph II (HRT; Heidelberg Engineering, Heidelberg, Germany) images, with realistic topographic variability, suitable for benchmarking false-positive rates of progression algorithms. Three confocal image stacks were selected from each of five sets of HRT II scans, obtained within 6 weeks in 127 eyes of 66 patients. For each eye, a simulated series was propagated from one baseline confocal stack by adding fixational eye movements, photon-counting, and electronic measurement noise. Simulated confocal stacks were imported into the HRT software to generate topography images. Real and simulated image comparisons were quantified with the mean pixel height standard deviation (MPHSD), image cross-correlation (CC) of pixel-wise variability maps, and the rim area (RA) coefficient of variation (CV). The mean difference (95% limits of agreement; LoA) in MPHSD between real and simulated images was 3.5 μm (-20.9 to 28.8 μm) within mean topographies and 2.0 μm (-5.4 to 9.3 μm) between mean topographies. The mean CC between real and simulated spatial variability maps was 0.58 within mean topographies and 0.54 between mean topographies. The mean difference (95% LoA) between real and simulated mean topography RA CV was -2.1% (-17.6% to +13.4%). Variability about anatomic features was well reproduced. Simulation realistically reproduces variability in real, stable images acquired over a short period. Stability in clinical datasets is uncertain, whereas in these modeled series, it is certain. This method provides benchmark datasets on which the specificity of progression algorithms can be tested.

Reproducibility of the Heidelberg Retina Tomograph III Glaucoma Probability Score

To evaluate the intrasession reproducibility of the Heidelberg Retina Tomograph III (HRT III) Glaucoma Probability Score (GPS) and analyze the factors that may affect it. Twenty normal subjects and 20 glaucomatous patients underwent 10 HRT III examinations of 1 randomly selected eye during the same session. The reproducibility of the GPS and its parameters was assessed by calculating the intraclass correlation coefficients (ICCs) and 95% confidence intervals of the study population as a whole, and of the 2 subgroups. The factors influencing individual reproducibility were evaluated by means of regression analysis. The reproducibility of the GPS parameters (at a confidence level of 95%) was almost perfect in the population as a whole (ICCs 0.964-0.856) and in the normal subjects (ICCs 0.976-0.897), but decreased in the glaucoma patients in whom it ranged from almost perfect to substantial (ICCs 0.922-0.705). The parameters were mainly influenced by image quality, but also by diagnosis and age, although the probability of glaucoma did not seem to depend on these variables. GPS is a highly reproducible diagnostic tool, especially in the case of "good" or better quality images as evaluated by the HRT software; moreover, the reproducibility of the GPS parameters was significantly better in normal subjects than in glaucoma patients.

Analysis of HRT Images: Comparison of Reference Planes

The values of Heidelberg Retinal Tomograph (HRT; Heidelberg Engineering, Heidelberg, Germany) stereometric parameters depend on the reference plane (RP), the instability of which results in parameter variability. Identification of change depends on RP stability. This study was undertaken to evaluate the influence of various RPs on rim areas (RAs) in a longitudinal image series. A longitudinal image series of 31 subjects with ocular hypertension who had reproducible visual field loss and 19 normal subjects was analyzed using five different RPs: the standard RP (HRT software version 3.1.2.0), two 320-mum RPs (software ver. 3.1.2.0 and 1.7.0), a previously described experimental RP, and a new Moorfields RP. The Moorfields RP takes the standard RP at baseline and then is fixed relative to the reference ring for subsequent images. Linear regression of RA over time was performed, and the slope and residual SD (RSD) were calculated for each RP. Comparisons between RPs were made by paired t-tests. In eyes with progressing disease, application of the standard RP resulted in significantly greater variability (as measured by the RSD) compared with other RPs (mean 0.057 mm(2) vs. 0.035-0.038 mm(2)). There was a trend toward faster RA change/time (mean, -0.0123 mm(2)/y) for the standard RP and slower (-0.0095 mm(2)/y) for the experimental RP. There was a trend for the Moorfields RP to result in the best signal-to-noise ratio (speed of RA change/variability): mean RA slope -0.0118 mm(2)/y and mean global RSD 0.037mm(2). Compared to the standard RP, the Moorfields RP has significantly lower variability and probably provides a greater facility for discriminating RA change from measurement variability in a longitudinal HRT image series.

Comparison of Retinal Nerve Fiber Layer and Optic Disc Imaging for Diagnosing Glaucoma in Patients Suspected of Having the Disease

To compare retinal nerve fiber layer (RNFL) and optic disc topographic imaging for detection of optic nerve damage in patients suspected of having glaucoma. Observational cohort study. A cohort of 82 patients suspected of having glaucoma based on the appearance of the optic nerve. All patients were imaged using the GDx VCC scanning laser polarimeter and HRT (software version 3.0) confocal scanning laser ophthalmoscope. All patients had normal standard automated perimetry visual fields at the time of imaging and were classified based on history of documented stereophotographic evidence of progressive glaucomatous change in the appearance of the optic nerve occurring before the imaging sessions. Areas under the receiver operating characteristic (ROC) curves were used to evaluate the diagnostic accuracies of GDx VCC and the HRT. Forty eyes with progressive glaucomatous optic nerve change were included in the glaucoma group, and 42 eyes without any evidence of progressive damage to the optic nerve followed untreated for an average time of 8.97+/-3.08 years were included in the normal group. The area under the ROC curve for the best parameter from GDx VCC (nerve fiber indicator [NFI]) was significantly larger than that of the best parameter from the HRT (rim volume) (0.83 vs. 0.70; P = 0.044). The NFI parameter also had a larger ROC curve area than that of the contour line-independent parameter glaucoma probability score (0.83 vs. 0.68; P = 0.023). Assuming borderline results as normal, the Moorfields regression analysis classification had a sensitivity of 48% for specificity of 69%. For a similar specificity (70%), the parameter NFI had a significantly larger sensitivity (83%) (P = 0.003). Retinal nerve fiber layer imaging with GDx VCC had a superior performance versus topographic optic disc assessment with the HRT for detecting early damage in patients suspected of having glaucoma. For glaucoma diagnosis, these results suggest that GDx VCC may offer advantage over the HRT when these tests are combined with clinical examination of the optic nerve.

Interobserver variability in confocal optic nerve analysis (HRT)

To study the observer-related variability of optic nerve head (ONH) measurements using confocal laser scanning tomography (HRT I) in a screening setting. Six experienced glaucoma specialists independently evaluated 50 ONH topographies from 25 adults using HRT software ver. 2.01 in a masked fashion. ONH topographies were obtained from a cohort study of 882 healthy adults and additionally included one patient with glaucomatous eyes. A glaucoma-screening-like setting was intended. The mean interobserver difference was defined as the mean percentual difference between an observer's analysis and the mean of all six observers for all eyes and all observers. The interobserver range was calculated for each eye as the percentual difference between the lowest and highest measurement, with the highest measurement as denominator. Additionally, Kendall's coefficient of rank concordance was assessed for the main HRT parameters. Mean disc area ranged from 1.83 +/- 0.49 to 2.21 +/- 0.40 mm(2) (mean interobserver difference: 8.3%; interobserver range: 5-50%; rank concordance: 0.86). The lowest mean interobserver differences were found for mean retinal nerve fibre layer thickness (RNFLT; 6.5%), maximum cup depth (2.9%) and cup shape (6.8%). An increased interobserver range was significantly correlated to a low cup to disc area ratio (r=0.64, P<0.0001). The observer-dependent diagnostic variability of HRT measurements can lead to divergent diagnostic evaluation of the ONH in a screening setting. Any HRT software relying on a reference database is exposed to relevant observer-related variability of the disc area. For screening purposes, HRT measurements should be completed by other diagnostic methods to compensate for possible diagnostic uncertainty.

A New Statistical Approach for Quantifying Change in Series of Retinal and Optic Nerve Head Topography Images

To describe and evaluate new statistical techniques for detecting topographic changes in series of retinal and optic nerve head images acquired by scanning laser tomography (Heidelberg Retinal Tomograph [HRT]; Heidelberg Engineering, Heidelberg, Germany). Proven quantitative techniques, collectively referred to as statistic image mapping (SIM), are widely used in neuroimaging. These techniques are applied to HRT images. A pixel-by-pixel analysis of topographic height over time yields a statistic image that is generated by using permutation testing, derives significance limits for change wholly from the patient's own data, and removes the need for reference data sets. These novel techniques were compared to the Topographic Change Analysis (TCA super-pixel analysis) available in the current HRT software, by means of an extensive series of computer experiments. The SIM and TCA techniques were further tested and compared to linear regression of rim area (RA) against time, in real longitudinal HRT series of eyes of 20 normal subjects and 30 ocular hypertensive (OHT) patients that were known to have converted to glaucoma, on the basis of visual field criteria. Computer simulation indicated that SIM has better diagnostic precision at detecting change. In the real longitudinal series, SIM flagged false-positive structural progression in two (10%) of normal subjects, whereas TCA identified three (15%), and linear regression of RA against time identified two (10%). SIM identified 22 (73%) of the OHT converters as having structural progression, whereas the TCA and linear regression of RA against time each identified 16 (53%) over the course of the follow-up. SIM has better diagnostic precision in detecting change in series of HRT images when compared to current quantitative techniques. The clinical utility of these techniques will be established on further longitudinal data sets.

Arterial narrowing as a predictive factor in glaucoma.

To evaluate whether retinal arteriolar widths change in normal patients and in different glaucoma types. We measured the arteriolar width at the merge in the optic nerve (MD), at the edge of the optic nerve (ED) and at the peripapillary area (1 disc diameter distance) (disc distance D). HRT software 1.11, Interactive Means program was used. One hundred forty three eyes of seventy-two patients divided in four subgroups normal, low-tension glaucoma (LTG), primary open angle glaucoma (POAG) and ocular hypertensive (OH). No statistically significant was found in each studied group. The normal patients showed no changes in the retinal arteriolar width compared with all the glaucoma groups.