Tag Archives: spectralis

Celestial Bodies – The Eye and Space

First time viewers of ophthalmic images frequently make the observation that the photos look like something from outer space. Especially when reviewing the round orange retinal photos with their eye doctor, patients often comment, “That looks like the planet Mars.”

Every time it happens I get a chuckle out of it. As if we all truly know what the planet Mars really looks like! But to most people, images of the inside of an eye are foreign and amazing. And there does seem to be a little science fiction aspect to both the appearance of the eye when viewed at high magnification, as well as the technology used to capture these amazing images. There are however, several space analogies that really seem to ring true. Among eye-care professionals, the eyeball is routinely referred to as the “globe”.

Macular Star in a patient with cat scratch neuroretinitis

Many clinical findings are named by their appearance rather than an underlying cause, and several conditions have names derived from their similarity in appearance to objects in space: asteroid hyalosis, macular star, star folds, starry sky, astrocytoma, stellate pattern, etc.

Asteroid hyalosis is comprised of calcium soaps suspended in the vitreous cavity behind the iris. Despite their appearance, most patients with this condition are asymptomatic.

In fact, there are enough conditions like this, that I’ve been able to compile them into the Ophthalmic Jeopardy category: Celestial Bodies.

Transillumination of a thinly pigmented iris in a patient with ocular albinism.

Like images from space, there does seem to be an element of wonder and mystery when we peer inside the globe, so in some ways the analogy makes sense.


Many ophthalmic images seem reminiscent of photographs from NASA. Or they may stir our imagination or perception of how objects in space might appear.

Lisch nodules on the iris of this patient with neurofibromatosis are reminiscent of peaks, valleys and craters seen in NASA photographs from planets or moons in our solar system.

There are other connections as well. Some of the photographic techniques used by both astronomers and ophthalmic photographers are actually similar. IR capture, interferometry and stereo imaging are common techniques in both fields.  The principles of  rotational stereo imaging can be applied to both subjects. Filters or lasers of different wavelengths are commonly used to enhance visibility of certain features in both subject types.

Most of these analogies between the eye and outer space, are loose associations rather than a direct connection. There is however, at least one eye condition that can be directly associated with a celestial body. Solar retinopathy is a type of photic injury to the retina that is the result of staring at the sun. This condition typically occurs in patients with psychiatric disorders or under the influence of hallucinogenic drugs.

A case of solar retinopathy with a subtle yellow-white foveal lesion with associated early pigmentary changes.

Some scholars believe that early astronomers, especially Gallileo, went blind as the result of solar retinopathy from viewing the sun through a telescope. It’s important to note that this condition can also occur from viewing a solar eclipse without protective eyewear. The upcoming solar eclipse visible in the U.S. on August 21, may cause a spike in cases of solar retinopathy presenting to emergency rooms and eye clinics. The American Academy of Ophthalmology offers some tips for safe viewing of the eclipse.

In recent years, another connection between outer space and vision has been discovered. It turns out that space travel can have some damaging effects on the human eye. Long-term exposure to microgravity can lead to a hyperopic shift in vision from flattening of the globe. This condition is believed to be related to increased intracranial pressure and is sometimes associated with optic disc edema, cotton wool spots and choroidal folds. Optical coherence tomography (OCT) is used to document  changes in thickness of the retinal nerve fiber layer of astronauts before, during, and after space flight.

I took this OCT selfie a few years back when we had a scientist from NASA visiting our clinic while exploring the possibility of putting an OCT on the International Space Station. She wanted to see the Heidelberg Spectralis in clinical use. After demonstrating on several patients, the scientist asked me if I thought it were possible for someone to take an OCT image of themselves. I pivoted the monitor, control panel, and footswitch around so I could operate the OCT from the patient chair and then captured some images of my own retina. I was showing off a little and smugly cautioned the NASA doctor that this was a difficult feat that only an experienced ophthalmic imager could perform. After all, I’ve been doing this for over thirty years. She paused for a moment and then said, “With all due respect, astronauts are some of the smartest and most talented people on earth. They shouldn’t have any difficulty performing OCTs on themselves after a some brief training.” Suddenly I didn’t feel so smug.

A year or so later, the Spectralis arrived at the International Space Station and it looks like she was right. I heard from some colleagues at Heidelberg that the astronauts were given less than 30 minutes of training on the instrument and mastered it quickly!

It’s pretty cool knowing that astronauts are performing ophthalmic imaging on the International Space Station. I wonder if they ever see any resemblance between the eye and celestial bodies?

Disclosure: I have no financial or proprietary interest in the Heidelberg Spectralis.

Here are some links on the condition that’s effecting the vision of astronauts and the use of diagnostic imaging on the space station:







From Blogger to Scholar?

I always look forward to receiving my copy of the the Journal of Ophthalmic Photography in the mail. Even after reading digital proofs as an editor or contributing author it’s great to see the final product in print. There’s just something tangible and authentic about turning the pages of a high quality publication printed on good stock.

I was especially looking forward to the Spring 2016 issue of the JOP. It contains an article I contributed on The Confocal Tonal Shift. What’s unique about this article is that evolved from a blog post here on eye-pix into a scholarly article (of sorts).

The blog was based on an observational series of photographs that documents the tonal changes that occur when focusing the Heidelberg Spectralis, a confocal scanning laser ophthalmoscope (cSLO). I put the blog together to further my own understanding of just what I was seeing with the cSLO, and share my observations with fellow imagers. When asked to convert the blog into a piece for the JOP, I initially felt it was too informal, opinionated, and lacking strong references to be included in the professional literature.

web image2-640 crop

But the kernel of a good idea was definitely there. So I worked backwards and did a literature search after the fact. Then I reworked the article and was able to place my observations into the context of other clinical findings described in the ophthalmic literature. Probably not the ideal way to develop an idea into an article, but in this case it worked.

A mentor of mine has always encouraged me to get as much mileage as possible out of a project. The Confocal Tonal Shift is a good example of that. It’s gone from a blog post to a formal article in a well-respected professional journal. I’ve also adapted it into an expanded lecture topic (The Quirks of Confocal Imaging) that  I presented at a recent educational program. It received good feedback and will work it’s way into my lecture rotation this year.

The full journal article can be viewed here.

Bennett TJ. The confocal tonal shift.
Journal of Ophthalmic Photography, 38(1):17-22, Spring, 2016

The Confocal Tonal Shift

The Heidelberg Spectralis confocal scanning laser ophthalmoscope (cSLO) is a commonly used diagnostic imaging device that uses monochromatic laser illumination to image the eye. It can be used for several retinal imaging modalities including infrared reflectance (IR), fluorescein angiography, ICG angiography and fundus autofluorescence (FAF). The confocal capability of the cSLO allows it to capture high-contrast, finely detailed images.

But what does confocal actually mean and how does it work? The word confocal simply means “having the same focus”. In this case it refers to the confocal pinhole or aperture that is optically located at the same plane of focus as the subject. The cSLO utilizes a focused laser to scan the subject point-by-point and then captures the reflected light after it passes through a confocal pinhole. The pinhole suppresses out-of-focus light from reaching the image detector resulting in very sharp images. The confocal pinhole is especially effective at eliminating unwanted scatter from cataracts or corneal opacities since these structures fall far outside the plane of focus.

Left: Patient with a cataract obscuring the view of the retina and optic nerve through a fundus camera. Right: The confocal pinhole of the cSLO suppresses the scatter from the cataract improving the view of the fundus.

When imaging a patient, you can see the confocal effect as you adjust the focus to the plane of the retina where it is most light efficient. The image on screen will get brightest just as you come into sharpest focus. A secondary effect of the confocal aperture is how it effects the appearance of elevated or out of focus retinal structures.

The plane of focus effects reflectivity and appearance of retinal tissues based on depth due to the confocal aperture. All three images are at the same wavelength. Focus is on the elevated optic disc on the left image. Tonality changes as focus is shifted to the retinal surface.

Adjusting the focus knob of the Spectralis can have a dramatic effect on the tonal appearance of elevated structures such as papilledema or vitreous floaters as seen here in this video.

Note the optic nerve get progressively darker as focus is adjusted from the peak of the nerve to the surface of the surrounding retina, which starts to appear brighter. The opposite occurs in the second example. Vitreous floaters from asteroid hyalosis appear as dark shadows when focus is set on the optic nerve. As focus is shifted up into the vitreous, the floaters begin to brighten and the retina fades to  dark. The brightness/exposure has not been adjusted during this tonal shift.  The only change is the focus.

So what does this mean for us as diagnostic imagers? Because of the inherently shallow depth of focus of the cSLO, some ocular structures may appear dark simply because they are slightly out of focus. Elevated serous detachments or papilledema are examples of this phenomenon that I call the confocal tonal shift.

serous detachment 2
A case of central serous chorioretinopathy with a classic serous detachment that can be seen ophthalmoscopically. The cSLO image is dramatic in it’s appearance due to the confocal shift. The dark area is elevated and filled with clear fluid (not blood).

In some cases the confocal tonal shift can enhance the diagnostic information by clearly outlining the borders of an elevated area or lesion. The effect is most notable with the IR laser and in red free mode.

wavelength small
The confocal tonal shift in three different modalities. From left to right: IR, monochromatic blue, fundus autofluosescence. The effect seems to be most pronounced in IR mode.

The confocal tonal shift also has the potential to create tonal “artifacts” which can confound the appearance of findings like blood or hemorrhage that inherently appear dark.  Vitreous opacities will appear dark because they are usually out of focus and blocked by the confocal pinhole. But are they from blood or vitreous debris? It’s impossible to tell with the cSLO since they appear the same even though one is translucent and one is more opaque when viewed ophthalmoscopically or with a fundus camera.

vitreous floaters-640
Vitreous debris can appear very dark in cSLO images even though it is almost completely transparent. Without a frame of reference, it is impossible to know if these dark areas represent floaters or vitreous hemorrhage.

Similarly, it is difficult to distinguish between blood and elevation within retinal tissues in conditions such as macular degeneration, retinal vein occlusions and diabetic macular edema.

Patient with bilateral diabetic macular edema (DME). There are some associated dot and blot hemorrhages present, but the dark patches are a result of elevation from the DME. Each of these dark areas correspond to elevation on OCT.
Branch retinal vein occlusion (BRVO). The dramatic dark lesion is a result of both hemorrhage and elevation.

It is important to note that elevated lesions can appear dark regardless of the pathologic location in the fundus.  cSLO imaging alone can’t always differentiate the anatomic location. OCT imaging or angiography may be necessary to further investigate the location of the pathology.

Left: choroidal tumor. Right: serous retinal detachment. Two very different disease processes but they appear quite similar because of the confoccal shift.

In some cases, the the confocal pinhole may suppress light that is reflected from the actual plane of focus, but is slightly blurred because of scattering from a lesion in tissue that is normally clear.

IR imaging with the cSLO can help identify paracentral acute middle maculopathy (PAMM). Although these lesions aren’t elevated, light scatter from the slight thickening in the middle retinal layers are suppressed by the confocal pinhole making the lesionappear dark. The findings are far more subtle in the color fundus photographs.

Although originally designed to image the retina, the cSLO can also be used to image the front of the eye. The confocal tonal shift may also effect the appearance of some anterior segment findings.

iris atrophy3
Patient with iris atrophy. The cSLO is focused on the surface of the iris which make these dark brown irides appear light at the plane of focus. The dark areas represent absence or thinning of the anterior iris surface. The deeper, out-of-focus, layers appear dark.

In addition to the confocal shift, light scattering from some corneal lesion types may also be suppressed by the pinhole contributing to the dark appearance of the lesion.

corneal opacity2 small
Corneal opacities shown with diffuse illumination and sclerotic scatter at the slit lamp. The cSLO image on the right more clearly delineates the extent of the lesion. Focus is at the level of the iris with the cornea being out of focus. Where the cornea is clear, there is no blocking effect from the confocal pinhole. But where there is scatter and reflectivity from the (out-of-focus) corneal lesion, this light is rejected by the confocal pinhole causing the dark appearance.

It is important to understand the confocal density shift when capturing or interpreting cSLO images and differentiate between structures that truly are dark from those that are simply out-of-focus. In some cases the tonal shift enhances areas of interest that may not be easily identified by other means. In others it may confound the documentation of blood  or hemorrhage. A second imaging modality such as color fundus photography, OCT or angiography is often needed to present a more complete diagnostic imaging study.

Here are a few more examples:

Diabetic macular edema (DME) appears dark on the IR image from the tonal shift and corresponds to the red (increased thickness) area on the OCT false-color thickness map.
Left: cystoid macular edema (CME). Right: central serous chorioretinopathy.
Dark lesions on two separate patients diagnosed with one of the phakomatoses. Left: a patient with tuberous sclerosis and multiple hamartomas that appear dark from elevation. Right: a retinal hemangioma in von Hippel-Lindau disease. This blood filled lesion is dark from the blood itself rather than elevation
WagnerB not dark640
This case represents a rare exception to the tonal shift in an elevated lesion. Adjusting the focus up and down had little effect on the tonal density, except at the borders of the lesion. Note the very high reflectivity of the inner retina on OCT. Presumably the reflective surface of the elevated area was bright enough to attenuate the normal confocal shift.