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Module 26 Section 1 |
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Module 26 |
Fluorescein Angiography, Part 2 |
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Section 1: |
The Angiogram | ||||
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You have the needle in a patent vein and you have your camera set up for fluorescein angiography. You are ready for the injection.
The normal angiogram shows a quick choroidal flush, a fast, bright filling and then fading of the arteries, and then a slower filling and fading of the venous circulation. The whole sequence normally lasting less than 30 seconds. The recirculation phase shows grey retinal vessels against a grey choroidal background with no grey scale difference between arteries and veins. The grey gradually fades to black over time as the fluorescein leaves the body. Early, mid phase, and late phase photos The arm-to-retina circulation time for a teenager can be as little as 8 seconds. The arm-to-retina circulation time for an eighty year old with poor circulation may be as long as 30 to 40 seconds. An antecubital injection will have a significantly shorter circulation time than an injection in the back of the hand. You must estimate this time for your patient and you must begin to take photos before you see any dye enter the bloodstream of the retina. If you wait until the dye is visible to you, you have already missed the arterial phase of the angiogram, which typically lasts only 1 to 3 seconds. The Choroidal Flush and The Arterial Phase There are two vascular systems that feed the retina. The choroidal vessels lie beneath the retinal pigment epithelium (RPE). The circulation from these vessels reaches the eye first and produces a "choroidal flush" just before the retinal arteries start filling. This flush is so short in duration that it is missed in many FAs. Fluorescein dye normally leaks from these choroidal vessels. If dense enough, the RPE layer will block our view of the fluorescein in the choridal circulation. A mottled grey choroidal background shows through the RPE layer of most patients. White patients have a less pigmented RPE, and black patients have a more pigmented RPE layer.
Above: Arterial phase, the arteries are white (dye present) and the veins are still dark. The background has illuminated because of the choroidal flush. The choroidal arteries fill first and the choridal fluorescence can be seen through the RPE layer, more so with patients who are less pigmented. The retinal vascular system branches across the retina from the optic nerve head. Fluorescein dye does not normally leak from these vessels. An intravenous injection of fluorescein first travels to the heart, where it is then pumped through the central retinal artery, through the branch retinal arteries, into the retinal capillary bed where it is taken up by venous capillaries. The dye travels through the venous branching vessels into the central retinal vein and out through the optic nerve from whence it came. Next is the capillary filling phase, which usually is over within 5 seconds. The venous phases, up to the recirculation phase, have usually passed within 20 seconds. So, there's not much time to get photos of the early phases of the angiogram. Below: The early venous phase is marked by dye beginning to enter the veins (right arrow). Notice the laminar flow. The dye fills the vein starting at the outside walls and filling toward the center. This is a characteristic of fluid entering a flowing stream from a tributary. Also notice the larger vessels in the background (left arrow). This patient is lightly pigmented and choroidal vessels can be easily seen through the RPE layer in this stage of the angiogram.
Above: Late venous phase of the angiogram. Notice that the vein has filled with dye (arrow). You will want to take photos at a fairly rapid rate in the first few seconds of the angiogram. Some cameras will allow you to press the trigger button or footpedal and hold it down to take photos at a rate of 3 or more frames per second. Capturing good photos in this early part of the angiogram requires good patient cooperation, meaning no movement and no blinking. Coach your patient about this when you are taking color photos. If you don't get many early phase photos, it is usually not a big deal. The ophthalmologist can usually make good diagnosis and treatment decisions with only a few photos in the early phases. The Heidelberg Retinal Analyzer is a scanning laser ophthalmoscope that captures a real-time movie of the early phases of the angiogram, at 6+ frames per second. You line up the camera, inject the dye, and push a button to begin recording. At the end of the angiogram, you can play the movie or you can view and print individual frames from the angiogram. With this camera, you don't miss anything. I suppose every retina practice would have one of these if it weren't for the price-tag ($100K for the latest model). If using a digital system, pay attention to the brightness and grayscale (range between black and white) of the images in the venous phase of the angiogram. If the images are dim and gray, try turning up the flash power to get a better image. If the flash power is already high, or if the flash recycle time would be affected by a higher flash power, then try adjusting the gain (or ISO) to a higher setting. The tradeoff is that the image quality (increased noise) usually decreases at higher gain settings. Be familiar with how to adjust the gain setting on your digital system. Adjusting the illumination light has little effect upon the exposure of the image. Leave the needle in the arm until the end of the initial phase of the angiogram. Universal precautions reminders:
Any photos taken after 1 minute from the start of the injection are from the latter stages of the angiogram, meaning the dye is re-circulating, going around for the 2nd, 3rd, 4th time. You can take your time with these photos. You should wait for a full 10 minutes to get the last of the late phase photos. Sometimes abnormal leakage does not show up well until this10 minute mark. Some ophthalmologists consider 5 minute late photos to be sufficient to catch 99 percent of the significance of fluorescein angiography. As a photographer, this is not your call. Ask the ophthalmologist how long you should wait before taking the last of the photos. Below: Late phase photo at 5 minutes. The dye has re-circulated several times by now. Notice uniform coloration and the reduction in fluorescent intensity in the arteries and veins. The white areas are from dye leakage (in the macula) and staining (at the optic nerve head). The optic nerve head has a capillary circulation that is part of the choroidal circulation. These capillaries normally leak dye and may cause a late fluorescence of the optic nerve head that is a normal appearance.
Ask your ophthalmologist about her philosophy on the number of photos required in an angiogram (and color photography). There is such a thing as phototoxicity of the retina. The retina can become damaged from overexposure or repeated exposure to bright light. The idea is that you should limit the number of flash exposures to the retina, based upon what the doctor feels is necessary for diagnosis and treatment. With digital photography available to confirm the quality of the photography, it is possible to produce a good angiographic study with a relatively few number of photos. What parts of the retina do I photograph? The vast majority of fluorescein angiograms are of the posterior pole (optic nerve, macula, superior and inferior vessel arcades). It is helpful to see a drawing of what your doctor wants to photograph. Specific instructions are always helpful. The key is to communicate with your doctor. You should always take at least one photo of the posterior pole, even if your primary subject matter is a nevus in the periphery. Always look around the retina with the camera before you stop taking photos. Your primary interest may the posterior pole, but you may discover abnormal fluorescence elsewhere on the retina that should be documented and may otherwise be missed. When possible, try to include a landmark such as the optic nerve in the photograph. This helps to orient the viewer as to the location of the pathology. When taking photos of the fellow eye, make the first photo one of the posterior pole to mark the transition from one eye to the other. What magnification (angle) should you use? The older Zeiss fundus cameras have a fixed angle of 30 degrees, but the newer models (e.g. Topcon) have a choice of 50 degree, 30 degree, and 20 degree fields of view. The narrower angles provide a more magnified field of view. The increased magnification comes at the expense of decreased depth-of-field and more difficulty with alignment. The Topcon is easiest to use at the 50 degree setting. The 30 degree angle is useful for fine focusing and for macular pathology. The 50 degree field is particularly useful for documenting diabetic retinopathy and large areas of pathology. I only use the 20 degree field if a doctor requests that magnification for optic nerve photos. If you get lost when performing fluorescein angiography with one of the narrower fields of view, switch to the 50 degree angle to regain alignment and focus, and then switch back to the other angle. When taking FA or color photos of large areas of pathology, be sure to photographically document all of the edges of the pathology. Examples would be choroidal nevi, a malignant melanoma, or a large subretinal hemorrhage. Photos of all edges will aid in determining if the lesion is getting larger or smaller when compared to future photos. The doctor-patient-photographer relationship After gaining experience performing fluorescein angiography and listening to your doctor talk to patients, you will become a pretty good diagnostician of retinal diseases. Most of time you will know the correct diagnosis, and you will be able to predict the course of action that your doctor will take. And most patients know that you know, and they may ask your opinion, thinking that you will give them more information than the doctor will. This is a dangerous game to play, particularly if you are not thinking the same thing that the doctor is thinking. Always be respectful of patient questions, but only respond in general terms. Say something like: "We won't know the extent of your problem until the doctor can review your angiogram." Never offer your opinion of the diagnosis, prognosis, or course of action. Only communicate what your doctor has authorized you to communicate. Most patients will respect your role. Documentation (printing, etc.)Documentation will depend upon your system set-up and the preferences of your doctor. Film based systems usually involve contact printing the negative onto positive film or paper, although some doctors prefer to read the negatives. Digital systems have options that allow you to print any number of frames in different templates. You should have a system in place for archiving and/or backing up the patient data on your digital system. |
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