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

INTRODUCTION

Autofluorescence has become one of the centers of interest in fundus imaging since the confocal scanning laser opththalmoscope (cSLO) allows to detect low intensity (auto)fluorescence such as lipofuscin levels in the retinal pigment epithelial cells.1 This imaging technique in the field of inflammatory diseases is especially useful for the group of diseases that affect the external retina and/or the retinal pigment epithelium (RPE) such as the inflammatory diseases of the choriocapillaris including Multiple Evanescent White Dot Syndrome (MEWDS), Multifocal Choroiditis (MFC) or Acute Posterior Multifocal Placoid Pigment Epitheliopathy (APMPPE).

PRINCIPLES OF FUNDUS

AUTOFLUORESCENCE ANALYSIS

The only system that presently allows to routinely obtain autofluorescence images is the Heidelberg retina angiograph (HRA).1 Autofluorescence images can be obtained in the short wave mode (488 nm) or in the near-infrared mode (787 nm). Fundus autofluorescence obtained with the cSLO in the short-wave mode monitors basically the content of lipofuscin in the RPE. The near-infrared auto-fluorescence mode reveals melanin contaning cells such as RPE-filled cells in the fovea. Other structures or pathological conditions such as drusen and basal membrane deposits are also detected using cSLO auto-fluorescence short-wave mode.

CSLO AUTOFLUORESCENCE IN

INFLAMMATORY DISEASES

Fundus autofluorescence (FAF) is principally produced by the lipofuscin present in the RPE cell.2 This material is originating from the photoreceptor outer segments and its degradation process or accumulation in the RPE cell lysosomes seems to be an indicator on the quality of the RPE cell metabolism. In inflammatory diseases of the fundus, FAF analysis can contribute additional information to other imaging methods to study the lesion process. The inflammatory diseases that produce patterns of increased or decreased auto-fluorescence are mainly those entities whose inflammatory process involves the chorio- capillaris-RPE complex and the outer retina comprising most of the primary and secondary inflammatory choriocapillaropathies.

The information we have so far is scarce and difficult to understand, still awaiting standard interpretation rules. The FAF pattern may be different in the same disease entity and probably depends on the severity and the stage of the disease. It is not sure whether autofluorescence changes are due to direct RPE damage, which seems probable, or indirect damage coming from damaged outer retina or both. In the group of the different diseases of the choriocapillaris, FAF is more pronounced on the more benign side of the spectrum such as in MEWDS and similar choriocapillaropathies (Figure 1) or in entities with smaller chorioretinal lesions such as in MFC (Figure 2).

Figure 1A

Figure 1C: Fluorescein angiography shows only slight mottled fluorescence and does the disease process occuring at the immediate subretinal and not shows outer retinal level

Figure 1B

Figures 1A-B: Unclassifiable case of primary inflammatory choriocapillaropathy. Numerous hyperautofluorescent points can be seen scattered in the mid-periphery fundus (A). ICGA clearly shows choriocapllaris non-perfusion or hypoperfusion typical of choriocapillaritis (B). ICGA hypofluorescence cannot be strictly made to overlap autofluorescence, indicating that the two methods show different aspects of the disease. It is possible that in the peripheral areas where hyperautofluorescent points predominate choriocapillaris perfusion dysfunction is less important , producing moderate RPE cell dysfunction nevertheless sufficient to cause lipofuscin accumulation. In the center more severe hypoperfusion is causing more pronounced RPE dysfunction with its metabolism further reduced rendering the cell incapable to accumulate lipofuscin

On the other side, in conditions with more widespread zones of involvement such as in APMPPE and serpiginous choroiditis, FAF is very often decreased within the active lesions being sometimes visible on the border of active or progressing lesions (Figures 3A- E). In these conditions bright FAF is often present in the center scarred lesions, but this type of autofluorescnce is not reflecting RPE dysfunction but accumulation of cellular fluorophore debris (Figures 3A-E).

CONCLUSION

Fundus autofluorescence might become a useful complementary imaging method in addition to fundus examination and photography, fluorescein angiography and indocyanine green angiography useful for the investigation of inflammatory choriocapillaris diseases: It might also bring additional information on the lesion mechanism involved in this spectrum of inflammatory diseases.

KEY POINTS: FUNDUS AUTOFLUORESCENCE (FAF)

•Low intensity FAF obtained by cSLO is an imaging modality gives information on the accumulation of lipofuscin in RPE cells indicating RPE cell dysfunction.

•In inflammatory diseases, FAF is indicated when the RPE is involved primarily or secondarily such as in inflammatory disases of the choriocapillaris

•The information obtained by FAF still has to be understood to determine what it tells us on the disease process and it has to be standardized to be used for practical purposes.

REFERENCES

1.Schmitz-Valckenberg S, Fitzke FW, Holz FG. Fundus autofluorescence imaging with the confocal scanning laser ophthalmoscope. In: Holz FG, Schmitz-Valckenberg S, Spaide RF, Brid AC (Eds): Atlas of fundus autofluorescence imaging. Springer, Heidelberg 2007;31-6.

2.Sparrow JR. Lipofuscin of the retinal pigment epithelium. In: Holz FG, Schmitz-Valckenberg S, Spaide RF, Brid AC (Eds): Atlas of fundus autofluorescence imaging. Springer, Heidelberg 2007;3-16.

INTRODUCTION – HISTORY

Ultrasonography offers the clinician a safe, noninvasive, dynamic tool with instant feed-back for evaluation of vitreoretinal disorders. Ultrasound examination is indicated when opacification of the ocular media precludes adequate clinical examination of the posterior segment or when the clinical presentation is atypical.

Ultrasonography was first applied in ophthalmology in 1956 by Mundt and Hughes.1 They used time amplitude-mode (A-scan) to evaluate an intraocular tumor. The first two-dimensional (immersion) brightnessmode (B-scan) ultrasound instrument was developed by Baum and Greenwood in 1958.2 With the development of the portable contact B-scan machine by Bronson, ultrasound began to be a useful component of the everyday practice of ophthalmology.3

Ossoinig, in 1960 was the first person to highlight the need for standardizing the ocular ultrasound instrument and technique, so that other people can rely on the result of the fellow investigator using similar instrumentation and technique. He devised the first standardized A-scan instrument, the Kretztechnik 7200 MA, to allow reliable differentiation of tissue.4,5 He further added B-scan instrument with his machine and then described examination techniques for the use of two instruments. From his work, the concept of Standardized Echography came into existence, for the detection and differentiation of both intraocular and orbital disorders.6 Recently with advances in digitalization and computerization, three dimensional

ultrasound imaging has become possible in ophthalmology.7

PRINCIPLE

Ultrasound wave has frequencies greater than 20-KHz (i.e. 20,000 oscillations/sec), rendering them inaudible to humans. The machine uses frequency in the range of 8 to10 MHz. These high frequency and short wavelength waves allow minute resolution of ocular and orbital structures. New techniques use even higher frequencies (i.e. 20 MHz and 50 MHz) for to obtain very fine resolution within the posterior or anterior segment. There are two important properties of ultrasound that make it useful in ophthalmic practice. Ultrasound is propagated as a longitudinal wave that consists of alternating compressions and rarefactions of molecules as the wave passes through a medium. Velocity of the ultrasound wave is dependent on the medium through which it passes. It travels faster in solid medium than liquid and gases. Secondly, like light wave it may also be refracted and reflected back once it travels through a medium (Figure 1). This reflected beam is known as echo. Ultrasonography uses the system in which the piezoelectric crystal, located near the face of the probe, produces ultrasound waves. It emits ultrasound waves and then detects and processes the returning echos. Returning echos are received by the transducer system, which converts them into electrical signal that is transmitted to the receiver and displayed on the screen (Figure 2). Returning echos are affected by many factors including

absorption and refraction, the angle of the sound incidence and the size, shape, and smoothness of acoustic interfaces.

A and B-scan are the important ultrasound displays being used in ophthalmology. A-scan echography consists of representation of echos in a one-dimensional display where spacing between the vertical spikes denotes time required by the wave to reach and come back from interface (Figure 3). It gives us the information regarding the lesion’s character as well as its size.

B-scan echography consists of a two-dimensional display i.e using both the vertical and horizontal displays to denote location and configuration (Figure 4). It yields information primarily about the topographic nature of intraocular and orbital structures and lesions. Reflected wave i.e. echo is represented as a dot on the screen and brightness of the spot depends on the strength of the echo. Multiple dots coalesce on the screen to form a two-dimensional representation of the tissue image being scanned.

Figure 3: Normal standardized A-scan echogram. Probe is placed on cornea with sound beam directed through the lens. Reflectivity is determined on A-scan by estimating the height of the spike C: Initial high spike (100%) corresponding to the cornea; AC, anterior lens capsule; PC, posterior lens capsule; 0% spike from vitreous V; R, retina; S, sclera; O, orbital soft tissue

Factors that can affect the quality of B-scan include poor signal processing, angle of scanning section, the speed of transducer oscillation, and the gray scale.

Figure 4: Normal standardized 10 MHz B-scan with A-scan echogram: C, cornea; AC, anterior lens capsule; PC, posterior lens capsule; R, retina; ON, optic nerve. The vitreous (V) is echo lucent and the sclera (S) echo dense

A typical 10 MHz B-scan ultrasonogram along an axial scan plane in normal eye shows both the anterior and posterior surfaces of the cornea, the echoes from the posterior iris surface usually merge from the anterior lens surface, the posterior curvature of the lens is demonstrated, the vitreous compartment appears as an anechoic cavity with no interval sound reflections, the final echoes produced by retina merge with echoes from the choroid and sclera (Figure 4).

A recently introduced three-dimensional B-scan imaging uses multiple sections taken by a mechanically rotating probe and software to make threedimensional images (Figure 5).7-8

THE ULTRASONOGRAPHY MACHINE

Standardized echography uses a combined A-scan and B-scan (along with the Doppler for orbit). Hardware

consists of a patient table, probe, and computer unit with a flat screen video monitor, keyboard, mouse and printer. The software necessary to operate and analyze is pre-installed and varies with the make of the machine.

TECHNIQUE OF ACQUIRING A SCAN

Scan can be acquired after positioning the patient in reclining position on examination chair with adjustable height. The examiner is seated besides the patient on examining stool. The head of the patient, probe and screen are close together so that screen can be viewed simultaneously while acquiring scans.

Acquiring A-scan

A-scan is usually done after the pathology has been detected with B-scan. A-scan is used to measure the size of the lesion in various directions. Apart from that its important use lies in knowing the character of the lesion i.e. point like, band like, membrane like or mass like and to evaluate exact location and extent of the lesion (Figure 6).

Acquiring B-scan

All B-scan probes have a marker. The upper part of the scan represents the position of the marker in relation to globe. Methylcellulose is applied to the probe as a coupling medium. The probe can be placed on conjunctiva or cornea. Examination through lids can be done using other eye as the target fixator but it can attenuate sound. Three basic probe orientations are used to evaluate the intraocular lesions i.e. transverse, longitudinal and axial.

Figure 6: Membrane like echo source shown with B and A-scan echograms (arrow)

1.Transverse scans are acquired by placing probe tangential to limbus. This gives a circumferential slice of the opposite side of globe. The direction of the marker will be nasal in horizontal transverse scans and superior in vertical and oblique scans. Transverse scan in four major quadrants at high gain is initial screening scan (Figure 7).

Figure 8: Technique for longitudinal B-scan approach. Probe marker oriented towards the clock hour to be examined and patient fixates in that clock hour. Normal globe; Scan provides image from optic nerve (ON) to the anterior part in that clock hour

2.Longitudinal scan – In these scans probe is kept on conjunctiva and marker is always towards the meridian being examined. The optic disc or posterior aspect of the globe is always placed inferiorly in these scans. Longitudinal scan along with transverse scan can screen whole of the eye, including peripapillary and macular lesions (Figure 8).

3.Axial scan – In this scan, the probe is centered on the cornea. Horizontal axial scans can be acquired by placing the marker nasally, while in vertical and oblique axial scans, marker points superiorly. These are useful in picking the pathologies of the posterior retina, easiest to read because they show lens in the anterior aspect and optic nerve in the posterior part, however sound attenuation and refraction from lens can hinder the resolution of the scan (Figure 9).

Figure 7: Technique for transverse B-scan approach. Probe marker oriented nasally and patient fixates superiorly. Normal globe; Scan provides images of the superior retina from 3.00 to 9.00

Figure 9: Technique for Axial B-scan approach. Probe marker oriented vertically up or nasally depending on the vertical or horizontal axial scan. Patient fixates in primary gaze. Normal globe; Scan provides image with optic nerve (ON) in center in vertical axial scan

4.Para-axial scan —In this scan, the probe is centered on the cornea. It is useful in evaluating the peripapillary region. The scans are very similar to axial scans in that the probe is again centered on the cornea and wave passes through the lens. The sound beam is slightly shifted so that the regions surrounding the optic disc are visualized.

ANALYSIS/INTERPRETATION

When an intraocular lesion is detected, the following properties are used to analyze the lesion.

Topographic Analysis

Location, extension and shape of the lesion can be determined using A and B-scan. B-scan can give twodimensional display of the lesion. A-scan topography is important in order to carry out quantitative and kinetic techniques.

Quantitative Analysis

Quantitative topography is performed to determine the reflectivity, internal structure, and sound attenuation of the lesion.

1.Reflectivity: It is measured by noting the spike height on A- scan and signal brightness on B-scan. On A-scan, reflectivity is determined by using the tissue sensitivity gain. Height of a lesion’s spike is measured in relation to the vitreous baseline (0%) and the top of the initial spike (100%) (Figure 3A). On B scan, the assessment of signal brightness is a gross estimation. In order to assess the significance of a lesion’s signal brightness on B-scan, the signal must be compared with that of either the normal highly reflective (echo-dense) sclera or the very low reflective (echo-lucent) vitreous cavity (Figure 3B). A lesion’s internal reflectivity, in comparison to these known tissues, is assessed in different degrees of echo-density.

2.Internal structure: Internal structure refers to degrees of variation in histologic architecture within a mass like lesion. This feature is evaluated by noting down the differences in height and length of the A-scan spikes and, to a limited extent by different echograms obtained from the same lesion. Homogenous architecture is represented by little

Figure 10: A homogeneous mass lesion showing regular spikes on A-scan and uniform echogenicity on B-scan

or no variation in the height and length of spikes on A-scan and uniform appearance of echoes on B- scan (Figure 10). While heterogeneous structure is represented by marked differences in echos appearance, showing irregular internal structure (Figure 11).

3.Sound attenuation: Sound attenuation occurs when the sound energy is scattered, reflected or absorbed by a given medium. Sound attenuation may be seen on A and B-scan and is indicated by a progressive decrease in the strength of echoes either within or posterior to lesion. This spike is called angle kappa on A scan, and is determined by drawing an imaginary line through the peaks of the lesion spikes and estimating the angle that is then formed with the vitreous baseline (Figure 12A). The steeper the angle, the greater is the sound attenuation. On B-scan, sound attenuation is indicated by a decrease in the brightness of the echoes (Figure 12B).Various substances like bone, calcium and most foreign bodies, typically produce strong sound attenuation.

Kinetic Echography

One of the unique abilities of conventional ultrasound instruments is the real-time echogram. Kinetic echography is used to dynamically assess the movement of the lesion. Three types of movements can be noted i.e. after movements, vascularity and convection movements.

Figure 11: A heterogeneous mass lesion showing irregular spikes on A-scan (Figure 11A) and variable echogenicity on B- scan (Figure11B)

1.After movements: After movements, indicative of mobility are determined by observing motion of the lesion echoes following cessation of eye movements.

2.Vascularity: Vascularity is represented by fast spontaneous movements of echoes i.e. blood flow within vessels.

3.Convection movement: Slow spontaneous movement of echoes i.e. convection occurs due to convection

current of fine particles with in a large cavity i.e. cholesterol debris in vitreous cavity.

In the learning curve, topographic techniques are performed first, followed by quantitative and then kinetic echography. But with experience, one can combine all the techniques together with greater efficiency.

ULTRASONOGRAPHY AND UVEITIS

Ultrasonography may be useful in the evaluation of intraocular inflammatory conditions, especially when

Figure 12: A choroidal melanoma showing strong internal sound attenuation; A- scan (Figure 12A) showing moderately steep angle kappa, indicative of internal sound attenuation. Transverse B-scan (Figure 12B) shows homogenous echogenic lesion

visualization of the fundus is poor or when the clinical presentation is atypical. It also may be used to follow the ophthalmic response to therapy. Ultrasonography may provide useful information on vitreous, retina, choroid, sclera, optic disc, and periocular structures.

VITREOUS INVOLVEMENT

In the normal eye, the vitreous appears as acoustically clear cavity (anechoic). On the A-scan, no echoes are seen between the posterior lens capsule and the retina (Figure 3). On B-scan, normal vitreous is almost ultrasonographically clear, appearing as a uniformly sonolucent area (Figure 4).

In eyes with vitritis, cellular agregates produce low reflectivity homogenous echoes (Figures 13-15).8