Velocity model for Heidelberg Retina Flowmetry

Background: An application of scanning-laser ophthalmoscopic flowmetry, Hei delberg Retina Flowmetry (HRF) is reported to faithfully quantify retinal p erfusion. Indeed, without convincing regard for the fact that the method pr oduces numerical values for such so-called "VELOCITY" land subsequently "ca lculates" "FLOW", and "VOLUME" with no physical units or proven real-life m eaning, current clinical drug studies and patient care are being based on t his method. To test the physical "fidelity" of the HRF method, the author h ence devised a simple, reliable mechanical model to generate known velociti es of movement of a test surface. Materials and Methods: Movement of the circumferential surface of an 89mm ( 31/2-inch) empty set cylinder, belt-driven by a brass spindle with segments of increasing diameter, was "analyzed" with the HRF method. The true veloc ities (mm/sec) with which the surface passed the HRF's focal point were the n determined using a stopwatch. A 10 degrees x2,5 degrees measuring field a nd a 20x20-pixel analysis window were used. Measurements were made for hori zontal, diagonal (45 degrees and 135 degrees), and vertical motion at all v elocity settings of the model. Results: The relationship between real velocities of cylinder-surface motio n and the corresponding HRF-"VELOCITY" values was nonlinear, an effect whic h increased significantly for diagonal and vertical motion. Conclusions: Considering the dependency on orientation of motion and the no n-linearity of the relationship between HRF results and true velocity, as w ell as several other weaknesses discussed herein, the question arises wheth er the validity of the HRF method should be reconsidered.