Purpose. The overall goals of this research were to (1) develop and test direct and non-invasive methods that will allow for the characterization of parafoveal capillary anatomy and hemodynamics and (2) perform the first large scale study of parafoveal capillary hemodynamics and anatomy using these new methods thus providing researchers with a database of normal subject hemodynamic and anatomy results that could be useful in future research of sight threatening diseases such as diabetes, age related macular degeneration, and glaucoma. To achieve these goals we (1) studied and developed new ways to improve image contrast and image processing strategies of parafoveal capillaries and leukocyte movement, (2) developed a method to non-invasively and directly assess capillary leukocyte velocity, (3) investigated the role of the cardiac pulse on leukocyte velocity variability, (4) developed a technique to non-invasively assess leukocyte pulsatility, (5) devised methods to non-invasively assess the foveal avascular zone (FAZ) and adjacent capillary density, (6) collected and processed videos of 29 normal healthy subjects in order to assemble a parafoveal capillary hemodynamic and anatomy database.
Methods. In all experiments the subjects were imaged using an adaptive optics scanning laser opthalmoscope (AOSLO). There were 29 subjects in total ranging from 18–39 years of age. All subjects were dilated with tropicamide 1% and phenylephrine hydrochloride 2.5% prior to imaging. The imaging wavelengths used were 660 nm or 532 nm. The field of view was 1.4° x 1.5° or 2.35° x 2.5°. In Experiment (1) the subject's capillaries were imaged in their best focal plane with both wavelength lasers and equal average intensities of the images at the photodetector. Samples of varying diameter capillaries and surrounding retina were selected from the same retinal location per 532 nm and 660 nm wavelength, green and red respectively, for Michelson contrast calculations, Experiment (2) leukocyte velocity was determined in the parafoveal capillaries including the foveal avascular zone border for 16 subjects. Leukocyte velocity was measured directly from movie segments where leukocytes were clearly visible, Experiment (3) a photoplethysmograph was used to record the subject's pulse synchronously with each AOSLO video. Each peak in the subject's pulse was encoded onto the bottom of each corresponding video frame. Parafoveal capillary leukocyte velocities and pulsatility were determined for two capillaries in each of the 8 subjects, and Experiment (4) the FAZ area and diameter in all 20 subjects and surrounding parafoveal cumulative capillary density in 10 subjects were measured.
Results. Experiment (1). Equal contrast measurements were found with the red and green wavelength for larger diameter capillaries. Smaller diameter capillaries had a contrast gain that ranged from 1.22–2.37 with the use of 532 nm vs. 660 nm. Experiment (2). The mean parafoveal capillary leukocyte velocity was 1.27 mm/sec ranging from 0.34–3.28 mm/sec. Experiment (3). There was a statistically significant difference between the leukocyte velocities, Vmax and Vmin for each subject, p < 0.05. The mean parafoveal capillary leukocyte velocity for all subjects was Vmean = 1.29 mm/sec and mean pulsatility was Pmean = 0.47. Experiment (4). The mean FAZ diameter for all subjects was 582.68 μm. The range of FAZ diameters was from 262.37 μm to 1051.92 μm. The mean FAZ area was 305790.07±163033 μm 2 and ranged from 77532.27 μm2 to 689499.20 μm 2. The cumulative capillary density for 10 subjects at 1° and 2° eccentricities respectively was 5.1852 and 11.9167 mm−1 .
Conclusions. Experiment (1). There is an improvement in contrast when imaging the smaller diameter parafoveal capillaries with a 532 nm vs. 660 nm laser in the AOSLO. The 532 nm, green wavelength, laser is the best choice for further studies to allow for increased visualization of retinal capillaries and leukocyte movement through the parafoveal capillaries. Experiment (2). Parafoveal capillary leukocyte velocity can be directly and non-invasively measured without the use of contrast dyes using an AOSLO. There is a large amount of variation in leukocyte velocity in normal subjects. Thus, a large change in mean velocity would probably be necessary to be useful in disease detection. Experiment (3). It is possible to directly and non-invasively assess parafoveal capillary leukocyte pulsatility. A substantial amount of the variation found in leukocyte velocity is due to the pulsatility that is induced by the cardiac cycle. By controlling for the variation in leukocyte velocity caused by the cardiac cycle, we can better detect other changes in retinal leukocyte velocity induced by disease or pharmaceutical agents. Experiment (4). It is possible to measure the FAZ and adjacent parafoveal cumulative capillary density non-invasively and directly using the AOSLO. The measurements from these experiments provide the most comprehensive description to date of parafoveal capillary hemodynamics ever collected using non-invasive and objective methods in the human eye.
|School:||University of Houston|
|School Location:||United States -- Texas|
|Source:||DAI-B 68/04, Dissertation Abstracts International|
|Keywords:||Hemodynamics, Noninvasive evaluation, Ophthalmoscope, Parafoveal capillaries|
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