Ординатура / Офтальмология / Учебные материалы / Uveitis Text and Imaging Text and Imaging Text and Imaging 2009

Figure 19: Overlay image of in patient with Birdshot retinopathy. Overlay of the OCT C-scan (magenta, presented in Figure 15) and fluorescein angiogram (green) in which the arrows indicate the areas of fluorescein leakage correspond to the small areas of retinal thickening

With appropriate, standalone software conventional angiographic images can be spatially transformed and superimposed over the confocal image.16 Given the pixel-to-pixel correspondence between the confocal and OCT channel, the converted angiographic image can be directly superimposed over the OCT image as well. In Figure 19, the late FA image of a Birdshot patient (Figure 15) shows leakage along the vascular arcades and in the fovea. An overlay image of this FA and the OCT C-scan in Figure 15 shows that the temporal area of retinal thickening on the OCT exactly matches the temporal, parafoveal area of leakage on the angiographic image. On closer examination the overlay image shows the correlation between the smaller areas of leakage at the nasal side of the fovea and along the vascular arcades on the angiographic image and the somewhat irregularly shaped retinal contour on the OCT C-scan.16 In case of unexplained leakage on an angiographic image or an unexplained patchy surface in the OCT C-scan, this overlay feature can be used to combine the functional and morphological information provided by the imaging techniques separately and help elucidate the findings in either one or both. A similar feature as described above is available in the system’s acquisition program.

CASE PRESENTATIONS

VOGT-KOYANAGI-HARADA’S DISEASE

Figure 20 shows consecutive visits of a patient diagnosed with Harada’s disease. At presentation, the patient had bilateral involvement of the retina showing enormous serous neurosensory detachments in the

Figure 20: OCT/SLO images of a patient with Vogt-Kayanaki- Harada’s disease (A1) OCT B-scan through fovea taken at presentation. Arrows indicate serous detachments. (A2-4) Consecutive OCT C-scans in depth taken at presentation. Arrows indicate the extent of the serous detachments in the various areas. Note that the serous detachments are irregularly shaped. (B1) OCT B-scan taken one week later. Arrows indicated decreased serous detachments. (B2-4) Consecutive OCT C-scans taken one week later. Arrows indicate the various serous detachments, that are less irregular in shape. (C1,2) OCT B- and C-scan taken two weeks later show complete resolution of the serous detachments

macular area (Figure 20A, only showing the left eye). In the longitudinal OCT scan (Figure 20A1) this serous detachment is seen in the horizontal axis. The successive OCT C-scan (Figures 20A2-4) show the extraordinary expansion of this serous detachment, not showing a symmetrical circular or oval detachment as seen in central serous retinopathy, but a lobular serous detachment in the foveal region and more detachments along the vascular arcades. OCT C-scans taken one week later (Figure 20B) show partial resolution of the serous detachments, as they appear less lobular and more symmetrical. After two weeks (Figure 20C) the

Figure 21: OCT/SLO images of choroidal folds in an uveitic patient (A-C) Consecutive OCT C-scans, showing first surface wrinkling (arrows in A), followed by a coarser wrinkling of the retina and retinal pigment epithelium (arrows in B,C) suggestive of choroidal folding, and a small serous detachment (*). (D) Horizontal OCT B-scan showing a small serous detachment (*) and a big wavy retinal pigment epithelial border. (E) Vertical OCT B-scan showing the small serous detachment (*) and the wavy pattern of the outer retina caused by the choroidal folding (arrowheads)

serous detachments have resolved, but with some loss of structure in the outer retina (Figure 20C1).

CHOROIDAL FOLDS IN UVEITIC PATIENT

The images of a uveitic patient in Figure 21 show extensive choroidal folding. In the confocal image the folding pattern, extending from the ONH, is visible, but could easily be mistaken for an epiretinal membrane. However, this folding pattern is seen in all retinal layers, including the RPE. Interestingly, this choroidal folding, causing the visible folding of the RPE and retina, can not be appreciated in the horizontal OCT B-scan, and is only seen in the vertical scan.

ACUTE POSTERIOR MULTIFOCAL PLACOID PIGMENT EPITHELIOPATHY (APMPPE)

As one of the “white dot” syndromes, APMPPE was one of the first to be image by OCT.17 Figure 22 shows

Figure 22: OCT/SLO images in a patient with acute posterior multifocal placoid pigment epitheliopathy (APMPPE) (A) Compilation of two consecutive OCT C-scans and two OCT B- scans. Arrows indicate the hyper-reflective, pathological areas located in the outer retina, as confirmed by the OCT B-scans.

(B) Compilation of OCT C-scan and two OCT B-scans taken two weeks later. Arrows indicate the cicatrised pathological areas, with loss of the inner/outer segments hyper-reflective layer in these areas, shown in the OCT B-scans. (red line in right bottom corner of OCT B-scan indicates its position)

images of the left eye of a patient with APMPPE at presentation and after two weeks of expectative management. At presentation, multiple hyperreflective spots located in the macular region within the vascular arcade are seen in both the confocal image and the coronal OCT. Judging by the surrounding tissue these spots are located in the outer retina. The localisation in the outer retina is confirmed in the OCT B-scan. Some dissociation of the outer segments from the underlying RPE appears to be present. After two weeks, the hyper-reflective spots disappeared. But both the OCT C- and B-scan reveal loss of the inner/ outer segments in the photoreceptor layer in the areas where the spots were located.

CONCLUSION

The strength of the OCT/SLO lies in its ability to provide a real-time overview of the area of interest, to quickly localize abnormal areas and to appreciate the extent of any pathology in the central retinal area. The pixel-to-pixel correspondence between the OCT and confocal images allows for accurate follow-up over time. Combined with its ability to provide accurate overlays between angiographic images and OCT, this OCT technology will provide us with new anatomic clues in understanding the pathophysiology of retinal diseases. The only major hurdle as with any new technology is learning to interpret coronal images. As a new appreciation for the patterns characteristic to this technology develops, the superiority of this approach will become self evident.

REFERENCES

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14.Hee MR, Izatt JA, Swanson EA, et al. Optical coherence tomography of the human retina. Arch Ophthalmol 1995;113:325-32.

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Vishali Gupta, Amod Gupta

INTRODUCTION

Optical coherence tomography has become a standard diagnostic technique and is widely used for imaging by the uveitis experts for measurement of retinal thickness, monitoring cystoid macular oedema and for studying the vitreoretinal interface (see previous sections of this chapter). Stratus OCT, that is most commonly used, is based on the depth information of the several layers of retina that is acquired as a sequence of samples and is termed as ‘Time-domain’. In a time-domain technology, measurements of the echo delay and magnitude of light are performed by mechanically scanning the reference path length so that light echoes with sequentially different delays are detected at different times as this reference path length is scanned. Tomographic images are then constructed by rapid successive axial measurement at different transverse points resulting in the formation of a twodimensional picture, which is a cross-sectional map of underlying structures. However, this technology had significant limits in signal acquisition time and thus comprehensive three-dimensional imaging of the retina was not possible. In addition, there was no proper recording of the eye movement during acquisition and movement of the device or patient during scanning would reduce the quality and also markedly decrease the spatial registration (the ability to determine the precise location of the B scan OCT image). Also, there was no comprehensive coverage of retina that could result in missing a pathology if the scan line did not pass through the area of pathology.

Recently, these shortcomings of the time-domain technology have been overcome by the introduction of spectral-domain OCT1,2 that allows cross-sectional and three dimensional tomograms of the posterior segment of the eye.

PRINCIPLE

Spectral/Fourier domain detection techniques measure the echo time delay of light by measuring the spectrum of the interference between light from the

tissue and light from a stationary unscanned reference arm. Light returning from the sample and reference paths is combined at the detector, which is a spectrometer in SD-OCT. The spectrometer resolves the interference signals throughout the depth of each A-scan immediately by means of a Fourier transformation. This is possible because the spectrometer resolves the relative amplitudes and phases of the spectral components scattered back from all depths of each A-scan tissue sample, without varying the length of the reference path. Eliminating the necessity of moving a mechanical reference arm makes it possible to acquire OCT image data about 70 times faster than conventional (time domain) OCT. The vast increase in scan speed makes it possible for Cirrus HD-OCT to acquire three-dimensional data sets, or entire cubes of data in about the same time (depending on the selected scan type) than conventional OCT.

THE CIRRUS OCT MACHINE

The Cirrus high definition (HD) OCT (Carl zeiss Meditec) is a computerized instrument that acquires and analyses OCT scans using the spectral domain technology (Figure 1). Besides HD OCT system, Cirrus also employs two additional live imaging systems namely a CCD video camera to monitor eye movements and line scanning laser ophthalmoscope (LSLO) that provides clear image of the retinal area being scanned.

The Cirrus HD-OCT integrates all the hardware components in a unit which includes the scan acquisition optics, the interferometer and spectrometer, the system computer and video monitor.

TECHNIQUE OF SCANS ACQUISITION

Switching on the system leads to a screen that requires user login, followed by patient ID mode that is a default mode and is the launch point for clinical functions of Cirrus OCT, i.e. ‘Acquire’ or ‘Analyse’ the scans.

PATIENT PREPARATION

OCT scans can be performed through undilated pupil, though it is advisable to dilate the pupils for better quality scans. Cirrus HD OCT provides internal fixation that is easy to use for the patient. The external fixation device may be used by a patient with poor visual acuity. The patient is seated comfortably with his chin on the chin-rest and is asked to look into the imaging aperture where he sees a green star shaped target against black background. When scanning begins, the background changes into bright flickering red. The patient must be instructed to look at the centre of the green target throughout the scan acquisition.

SCAN ACQUISITION

For a new patient, the data entry is made before seating the patient and for old patients, the patient past records are retrieved by searching the database. Identify the desired patient and click ‘Acquire’. All scans is the default protocol and include the following:

FIVE-LINE RASTER (FIGURE 2)

This protocol scans through five closely spaced horizontal lines. Each line is 6 mm long and composed of 4096 A-scans, and the 5 lines together cover 1 mm vertically. This scan gives the greatest resolution.

MACULAR CUBE 200 × 200 COMBO (FIGURE 3)

A 6 mm square grid by acquiring a series of 200 horizontal scan lines each composed of 200 A-scans.

MACULAR CUBE 512 × 128 COMBO (FIGURE 4)

Generates a cube of data through a 6 mm square grid by acquiring a series of 128 horizontal scan lines each composed of 512 A-scans. This scan has greater resolution in each line from left to right, but the lines are spaced further apart, giving less resolution from top to bottom.

Any desired scan type can be selected by clicking on it or skipped if so desired. After selecting the scan acquisition protocol, the patient is asked to put his head on the chinrest and look into the green star. The automated chinrest will go to the default position for the selected scan. The iris view port is used to align the eye (Figure 5). Centre the pupil in the iris view port by clicking the centre of the pupil. Click Optimise, just above the Capture button. This automatically optimises first the scan image centering (Z-offset), and then optimises the scan image quality (polarisation). The scans can be reviewed and saved.

ANALYSIS OF SCAN

The ‘analyse screen’ enables one to view anatomical structures depicted in the scan images. Two types of analysis can be done:

•High definition image analyses for 5 Line raster scan

•Advanced Interactive analysis for cube scans.

HIGH DEFINITION IMAGE ANALYSES (HIDA) FOR FIVE LINE RASTER SCAN (FIGURE 6)

This analysis allows morphological study of the retinal layers with 5 μ transverse resolution.

Figure 6: Normal Raster Line scan showing different retinal layers. Abbreviations: RPE: retinal pigment epithelium; IS: inner segment of photoreceptors; OS: outer segment of photoreceptors; ELM: external limiting membrane; IS/OS: inner and outer photoreceptor junction; ONL: outer nuclear layer; OPL: outer plexiform layer; INL: inner nuclear layer; IPL: inner plexiform layer; GCL: ganglion cell layer; ILM: inner limiting membrane; RNFL: retinal nerve fibre layer; post hyaloid: posterior hyaloid

ADVANCED INTERACTIVE ANALYSIS (AIA)

AIA presents an interactive multi-planar reformat (MPR) that enables three-dimensional cross sectional view (Figure 7).

MPR allows the visualisation of retinal layers at different depths by taking a transverse C scan. Due to the curvature of the eye, the various retinal layers would be arranged inside out, with the inner and outer retinal layers visible like “onion rings” (Figures 8 A- C). At the level of the ganglion and inner plexiform layers, the retinal vessels are easily recognised. They initially appear white in the more superficial layer and appear dark (shadow) when OCT C-scans are taken at deeper layers of the retina (Figure 8D).

Figure 7: Multi-planar-reformat. Reproduced from cirrus HD OCT user manual (with permission)

Figure 8A : Fundus image with cube showing the area scanned

Figure 8B: Horizontal line scan showing Vitreofoveal traction with normal retinal thickness. Slice navigator indicates the plane through which C-scan section is taken

Figure 8C: Transverse C-scan showing different retinal layers. Abbreviations: RPE: retinal pigment epithelium; IS: inner segment of photoreceptors; OS: outer segment of photoreceptors; ELM: external limiting membrane; IS/OS: inner and outer photoreceptor junction; ONL: outer nuclear layer; OPL: outer plexiform layer; INL: inner nuclear layer; IPL: inner plexiform layer; GCL: ganglion cell layer; ILM: inner limiting membrane; RNFL: retinal nerve fibre layer; post hyaloid: posterior hyaloid

Figure 8D: Transverse C-scan showing retinal blood vessels that appear as dark lines in deeper layers of the retina (arrows)

Three-dimensional single layer mapping allows visualisation of internal limiting membrane and retinal pigment epithelium surface maps (Figure 9A-C).

Like time-domain Stratus OCT (see chapter 10A), high-definition Cirrus OCT too has been found to be useful in the imaging of intraocular inflammations. It is commonly done for the following conditions:

CYSTOID MACULAR OEDEMA (CME)

CME is commonly associated with intraocular inflammation due to various aetiologies. HD-OCT is useful in depicting the cystic spaces that are seen as hyporeflective spaces with intervening hyper-reflective septae (Figures 10A-C). The retinal thickness can be measured in microns in the central 6 mm and response to an intervention monitored.

SEROUS RETINAL DETACHMENTS

Spectral domain OCT is very useful in studying serous retinal detachments in patients of sympathetic ophthalmia and Vogt-Koyanagi-Harada (VKH) syndrome. In our experience, the retina inner to external limiting membrane did not show any remarkable structural alteration in VKH patients. The most common finding seen in these eyes is the serous retinal detachment with bands of moderate hyperreflectivity with detached retina (Figures 11 A-D). Because of its high resolution, Cirrus OCT is also useful

Figure 9A : Horizontal Line scan showing Epiretinal membrane on the surface of retina (arrows)

Figure 9B: Internal Limiting membrane surface map showing tent-like elevation of the surface

Figure 9C: Retinal pigment epithelium surface map showing rippled pattern (normal)

in studying the photoreceptor layer that may show irregularity of IS/OS junction in the acute phase.

The single layer RPE surface map shows dome-like elevation corresponding to serous retinal detachment

Figure 10C: Horizontal line scan shows intraretinal cystoid spaces (arrows) and a pocket of subretinal fluid (arrowhead)

Figure 11A: Fundus photograph R/E showing exudative retinal detachment in a patient with VKH disease

Figure 11B: Fluorescein angiogram in the late phase shows pooling of dye in the subretinal space with disc hyperfluorescence

on line-scan and multiple bumps on the RPE surface both in VKH disease and sympathetic ophthalmia (Figure 12). Dome-like elevation shows early resolution on systemic corticosteroids therapy, whereas bumps persist longer (Figures 12 and 13).3

CHOROIDAL NEOVASCULAR MEMBRANE (CNVM)

CNVM can develop in any pathologic process that affects RPE-Bruch’s membrane. Multifocal choroiditis, POHS, serpiginous choroiditis, Birdshot chorioretinopathy, toxoplasmic retinochoroiditis and several other