Fundamentals of Fluorescein Angiography
Equipment and Technique

Timothy J. Bennett, CRA FOPS
Department of Ophthalmology
Penn State University
Hershey, Pennsylvania

Fundus Camera

Angiography typically requires the use of a specialized fundus camera equipped with a matched pair of exciter and barrier filters along with a fast recycling electronic flash tube that allows a capture rate of up to 1 frame per second. Narrow band-pass interference filters are utilized to allow maximum transmission of peak wavelengths, while minimizing any crossover of transmission curves. The exciter filter transmits blue-green light at 465-490nm, the peak excitation range of fluorescein. The barrier filter transmits a narrow band of yellow at fluorescein's peak emission range of 520-530nm. The barrier filter effectively blocks all visible wavelengths but the specific color of fluorescein. The fundus-illuminating beam is delivered axially, through the image forming optics and filters of the fundus camera. Fundus cameras equipped for angiography have a timer that records the angiographic sequencing on each frame of the study.

Fundus cameras are often described by their optical angle of view. A 30-degree field of view (with an magnification of approximately 2.5x on 35 mm film) is the most common field for documenting macular detail. Wide-angle cameras of 50 or 60 degrees are useful for documenting larger areas of the retina. Images are captured either with high-speed black-and-white 35mm panchromatic film or electronically, with a charge-coupled device (CCD) and computerized system for digital imaging. Film-based angiography requires either the use of a processing service, or access to a darkroom for processing films on-site.

Stereo Imaging

Stereo imaging techniques can be employed during angiography to enhance diagnostic information. Stereo separation is achieved by laterally shifting the fundus camera a few millimeters between sequential photographs. The lateral shift causes the illuminating beam of the fundus camera to fall on opposite slopes of the cornea. The resulting cornea-induced parallax creates a hyperstereoscopic effect that is evident when the sequential pair of photographs is viewed together. Stereo pairs are particularly useful in identifying the histopathologic location of angiographic findings within the retina.

Digital Imaging

Digital imaging offers several distinct advantages over traditional film-based angiography. Having instant access to the electronic images increases clinical efficiency and promotes patient education through image review on large display monitors. Software tools provide adjustments for brightness, contrast and sharpness. Digital analysis enables measurement of pathologic structures, digital overlays can be used to identify potential changes in lesion size in serial photographs, and multiple fields can be linked together to form composite wide-field images. Images can be stored on magnetic or optical media like CD or DVD-ROMs and transmitted electronically across computer networks for remote viewing or storage on file servers. Digital systems also offer the additional advantage of shortening the learning curve for novice angiographers. Having instant feedback allows the angiographer to quickly adjust exposure settings and camera alignment to correct any flaws in technique. Despite these advantages, the high initial cost of digital systems has prevented them from being employed universally, although they are now more common than film-based systems.

Fluorescein angiography can also be recorded using a confocal scanning laser ophthalmoscope (cSLO) in place of the conventional fundus camera. The confocal SLO uses a laser beam of the appropriate excitation wavelength to scan across the fundus in a raster pattern to illuminate successive elements of the retina, point-by-point. The laser can deliver a very narrow wavelength band for more efficient excitation of fluorescence than the flash illumination generated by a fundus camera flash tube. A confocal aperture is positioned in front of the image detector at a focal plane conjugate to the retina, effectively blocking non image-forming light. The confocal optical system and laser illumination combine to produce high contrast, finely detailed images. The laser scan rate is synchronized at a frame rate compatible with digital video display, providing a continuous high-speed representation of the flow dynamics of the retina and choroid. This can be especially useful when documentation of the very early filling stages is necessary, such as in identification of choroidal neovascular feeder vessels. The SLO lessens the need for pupillary dilation and patients can easily tolerate the low light level of the laser. The major drawback of scanning laser technology is the high cost of the equipment.(8)

Sequencing

Proper sequencing of the angiographic series is essential in obtaining maximum diagnostic information. Color fundus photographs as well as black-and-white monochromatic green filter (red free) images are routinely taken as baseline views before administering the dye. The early transit phase is the most critical part of the angiogram and usually lasts less than a minute. Before injecting the dye, the illuminating beam of the fundus camera is centered within the dilated pupil. The angiographer then pre-focuses the camera on the appropriate area of interest and takes a “control photo” at normal fluorescein exposure settings to document the effectiveness of the barrier filter in blocking excitation wavelengths. The dye is administered as a bolus injection, typically through a scalp vein needle into the antecubital vein. The timer is started and photography commences. The arm-to-retina circulation time varies, but in a normal patient takes 10-12 seconds. The experienced angiographer anticipates the initial appearance of the dye and begins rapid sequence photography before the dye is visible. Images are routinely captured at a rate of one frame per second until maximum fluorescence occurs, usually about one minute post injection. During this dynamic early phase only one eye can be captured. In the seconds it takes to switch from one eye to the other, valuable information could easily be missed. After completion of the early phase, photographs of the fellow eye or other areas of interest in the retina can be taken.

Over the next few minutes the appearance of the dye stabilizes and begins to slowly fade. The angiographer can capture appropriate views as necessary without the urgency needed during the early phase. Late phase photographs are taken as the dye dissipates, typically between 7 to 15 minutes after injection.

Many angiographers implement disease-specific protocols for sequencing and appropriate fields of view. For example, peripheral shots of the retina are routinely taken after the early transit in diabetic retinopathy, while the macula or posterior pole is the major area of interest in macular degeneration and peripheral views are not usually necessary. The angiographer often adjusts the specific protocol based on changes that may occur as the angiogram progresses.

In rare cases when venous access is severely compromised or the patient is known to be highly allergic to the dye, fluorescein can be administered orally. Due to the slow absorption rate, an early transit sequence is not possible. The resulting images are less than ideal, but have traditionally provided limited diagnostic information in conditions where late phase photographs are helpful, such as cystoid macular edema. The current use of optical coherence tomography to detect the presence of edema or fluid within the retinal tissues has essentially eliminated the use of oral fluorescein in recent years.