Abstract:
To quantify the in vitro energy transmission and scatter by blood of the laser used in verteporfin photodynamic therapy.
The spot size of a 689 nm laser was adjusted until a power sensor showed the output through air was 100 milliwatts (mW). A purple top blood collection tube containing ETDA was filled with balanced salt solution (BSS). The ETDA/BSS mixture was placed in cuvettes 10, 50, 100, 200, and 500 microns thick. Power readings were taken at 1, 2, 3, 4, and 30 centimeters (cm) posterior to each cuvette. A second purple top tube was filled with blood. The anticoagulated blood was substituted for the EDTA/BSS mixture in each cuvette, and power readings repeated. All measurements are the mean of six readings + standard deviation.
The power readings for all ETDA/BSS filled cuvettes at all distances varied from 100 to 95 milliwatts (mw). No correction for this slight, variable decrease was made in further analyses. For blood filled cuvettes, extrapolation of the power at zero distance from the cuvette (=transmitted power) was accomplished by performing a weighted least–squares fit of a second–order polynomial to the power at 1, 2, 3, and 4 cm for each cuvette thickness (Table1).
Quantification of scatter was determined by percentage of transmitted power that is scattered beyond the edges of the detector at 30 cm [(Power at 0 cm– Power 30 at cm)/(Power at 0 cm)] x 100. The percents of scatter for the 10, 50, 100, 200, and 500 microns thick cuvettes were 73%, 99%, 100%, 100%, and 100%, respectively. Blood in amounts frequently encountered clinically decreases transmission of the 689 nm laser. Most of the transmitted light is scattered, even by small amounts of blood. Maneuvers to displace the blood over choroidal neovascular membranes should be considered prior to verteporfin photodynamic therapy unless the layer is very thin.
Keywords: photodynamic therapy • age-related macular degeneration • laser