Abnormal retinal vascular oxygen tension response to light flicker in diabetic rats.

PURPOSE: To test the hypothesis that the intravascular oxygen response to light flicker is abnormal in diabetes. METHODS: Ten eyes of normal rats and 10 eyes of rats made diabetic with streptozotocin were examined. Oxygen tension (PO(2)) was measured noninvasively in the retinal arteries and veins on optical section retinal images. PO(2) was estimated based on the quenching by oxygen of the phosphorescence of an intravenously injected palladium porphyrin molecular probe. Measurements were conducted with and without light flicker at 10 Hz. Oxygen saturation (SO(2)) was calculated with adjustment for the arterial pH. RESULTS: In the normal rats flicker induced an increase in arterial PO(2) and in the difference in arterial and venous (A-V difference) PO(2) from 51 +/- 5 (mean and SD) to 55 +/- 7 mm Hg and from 22 +/- 3 to 26 +/- 5 mm Hg, respectively (P < 0.002 and 0.015, respectively). Flicker induced an increase of arterial SO(2) and A-V SO(2) difference from 64% +/- 8% to 68% +/- 7% and from 34% +/- 4% to 38% +/- 6%, respectively (P < 0.002 and 0.035, respectively). No changes in PO(2) or SO(2) were observed with flicker in the veins. In the diabetic rats, no significant flicker-induced changes were seen in PO(2) or SO(2) in the retinal arteries, veins, or A-V differences. CONCLUSIONS: The diabetic rats lacked the flicker induced increase in arterial PO(2) and SO(2) and also the A-V difference in PO(2) and SO(2) observed in the normal rats. The best explanation appeared to be that diabetes impairs the increase in oxygen consumption normally provoked by light flicker.

Three-dimensional mapping of chorioretinal vascular oxygen tension in the rat.

PURPOSE: An optical section phosphorescence lifetime imaging system was developed for three-dimensional mapping of oxygen tension (P(O2)) in chorioretinal vasculatures. METHODS: A laser line was projected at an oblique angle and scanned on the retina after intravenous injection of an oxygen-sensitive molecular probe to generate phosphorescence optical section images. An automated software algorithm segmented and combined images from spatially adjacent locations to construct depth-displaced en face retinal images. Intravascular P(O2) was measured by determining the phosphorescence lifetime. Three-dimensional chorioretinal P(O2) maps were generated in rat eyes under varying fractions of inspired oxygen. RESULTS: Under an air-breathing condition, mean P(O2) in the choroid, retinal arteries, capillaries, and veins were 58+/-2 mm Hg, 47+/-2 mm Hg, 44+/-2 mm Hg, and 35+/-2 mm Hg, respectively. The mean arteriovenous P(O2) difference was 12+/-2 mm Hg. With a lower fraction of inspired oxygen, chorioretinal vascular P(O2) and mean arteriovenous P(O2) differences decreased compared with measurements under an air-breathing condition. Retinal venous P(O2) was statistically lower than P(O2) measured in the retinal artery, capillaries, and choroid (P<0.004). CONCLUSIONS: Three-dimensional mapping of chorioretinal oxygen tension allowed quantitative P(O2) measurements in large retinal blood vessels and in retinal capillaries. This method has the potential to facilitate better understanding of retinal oxygenation in health and disease.

Effect of aberrations and scatter on image resolution assessed by adaptive optics retinal section imaging.

The effect of increased high-order wavefront aberrations on image resolution was investigated, and the performance of adaptive optics (AO) for correcting wavefront error in the presence of increased light scatter was assessed in a model eye. An AO section imaging system provided an oblique view of a model retina and incorporated a wavefront sensor and deformable mirror for measurement and compensation of wavefront aberrations. Image resolution was quantified by the width of a Lorentzian curve fitted to a laser line image. Wavefront aberrations were significantly reduced with AO, resulting in improvement of image resolution. In the model eye, image resolution was degraded with increased high-order wavefront aberrations (horizontal coma and spherical) and improved with AO correction of wavefront error in the presence of increased light scatter. The findings of the current study suggest that AO imaging systems can potentially improve image resolution in aging eyes with increased aberrations and scatter.

A method for three-dimensional imaging of the retina in human eyes.

BACKGROUND AND OBJECTIVES: An optical system for three-dimensional imaging of the retinal tissue in human eyes is described. PATIENTS AND METHODS: A laser beam was projected at an oblique angle on the retina and scanned to acquire 40 optical section images in a 1.0 x 1.5 mm retinal area. Because the incident laser beam was not coaxial with the viewing system, structures at various retinal depths appeared laterally displaced according to their depth location on the optical section image. The optical section images were segmented to construct a series of en face retinal images, parallel to the retinal surface and displaced in depth. Imaging was performed in three control subjects. RESULTS: A series of 8 depth-displaced en face images of retinal layers was reconstructed in each eye, which allowed enhanced visualization of the retinal structures and vasculatures. En face depth-displaced retinal images provided improved contrast compared with fundus images and delineated the foveal depression and the surrounding retinal vasculatures. CONCLUSIONS: An optical system for three-dimensional retinal imaging was developed that has potential as a tool for evaluation of retinal pathologies associated with chorioretinal diseases.

Chorioretinal vascular oxygen tension changes in response to light flicker.

PURPOSE: To investigate oxygen tension (P(O2)) changes in the retinal and choroidal vasculatures in response to visual stimulation by light flicker. METHODS: A previously developed optical section phosphorescence imaging system was used to measure P(O2) separately in the retinal veins, arteries, and capillaries and in the choroid before and during light flicker. Imaging was performed in rats during light flicker at frequencies between 0 and 14 Hz. Light flicker-induced changes in the chorioretinal vasculature P(O2) and arteriovenous P(O2) differences were determined. Retinal arterial and venous P(O2) were measured along blood vessels as a function of the distance from the optic nerve head. RESULTS: Retinal arterial P(O2) and arteriovenous P(O2) differences increased with increasing light flicker at frequencies up to 10 Hz, after which no further increase was observed. Significant increases in retinal arterial P(O2) (P = 0.009; n =10) and in retinal capillary P(O2) (P = 0.04, n = 10) were measured in response to light flicker at 10 Hz. Retinal arteriovenous P(O2) differences during light flicker were significantly greater than differences before light flicker (P = 0.01; n = 10). Retinal arterial P(O2) decreased significantly with increased distance from the optic nerve head (P < or = 0.004), whereas retinal venous P(O2) remained relatively unchanged (P > or= 0.4). CONCLUSIONS: Measurement of changes in the chorioretinal vasculature P(O2) can potentially advance the understanding of oxygen dynamics in challenged physiological states and in animal models of human retinal diseases.

A method for chorioretinal oxygen tension measurement.

PURPOSE: To report an optical imaging system that was developed to measure oxygen tension (pO2) in the chorioretinal vasculatures. The feasibility of the system for the measurement of changes in pO2 separately in the retinal and choroidal vasculatures was established in rat eyes by varying the fraction of inspired oxygen and inhibiting nitric oxide activity. METHODS: Our optical section phosphorescence imaging system was modified to provide quantitative measurements of pO2 separately in the retinal and choroidal vasculatures. A narrow laser line was projected at an angle on the retina after intravenous injection of an oxygen-sensitive probe (Pd-porphyrin), and phosphorescence emission was imaged. A frequency-domain approach allowed measurements of the phosphorescence lifetime by varying the phase relationship between the modulated excitation laser light and sensitivity of the imaging camera. Chorioretinal pO2 was measured while varying the fraction of inspired oxygen and during intravenous infusion of Nomega-nitro-L-arginine (Nomega-NLA), a nonselective nitric oxide synthase inhibitor. RESULTS: The systemic arterial pO2 varied according to the fraction of inspired oxygen. The pO2 in the retinal and choroidal vasculatures increased as the fraction of inspired oxygen was increased. Compared with baseline, choroidal pO2 decreased during infusion of Nomega-NLA, whereas the pO2 in the retinal vasculatures remained relatively unchanged. The choroidal pO2 decreased markedly with each incremental increase in Nomega-NLA infusion rate, in the range 1-6 mg/min, and there was no additional change in the choroidal pO2 at Nomega-NLA infusion rates above 6 mg/min. CONCLUSIONS: An optical method combining pO2 phosphorescence imaging with chorioretinal optical sectioning was established that can potentially be applied for better understanding of retinal and choroidal oxygen dynamics in physiologic and pathologic states.

Noninvasive assessment of chorioretinal oxygenation changes in experimental carotid occlusion.

PURPOSE: To demonstrate the capability of our optical imaging system to assess oxygenation changes in chorioretinal vasculatures due to experimentally induced carotid occlusion. METHODS: Chorioretinal oxygenation was assessed by projecting a narrow laser line at an angle on the retina after intravenous injection of an oxygen sensitive probe and imaging phosphorescence emission. Optical section phosphorescence imaging was performed in rats, under steady-state conditions and during unilateral occlusion of the common carotid artery. Phosphorescence intensity was measured in the retinal vein, artery, capillaries, and choroid vascular areas. Oxygenation was defined as the inverse of phosphorescence intensity. Oxygenation changes in the four vascular areas were determined relative to initial preocclusion oxygenation values and compared to measured changes under steady-state conditions. RESULTS: Under steady-state conditions, phosphorescence intensity in chorioretinal vasculatures remained constant, displaying a change of < or = 8% over time. At 12 +/- 5 s from initiation of occlusion, oxygenation decreased in the retinal venous, arterial, capillary, and choroidal circulations by -41 +/-19%, -10 +/- 5%, -20 +/- 18%, -10 +/- 5%, respectively (p < or = 0.05; n = 6). At 30 +/- 10 s from initiation of occlusion, oxygenation change in the retinal vein, artery, capillaries, and choroid was -9 +/- 12%, -2 +/- 4%, -11 +/- 21%, -1 +/- 8%, respectively, and not statistically different as compared to steady-state oxygenation changes (p > or = 0.3; n = 6). CONCLUSIONS: Optical section phosphorescence imaging technique can be used to assess intravascular oxygenation changes and may be a valuable tool for studying disease-related oxygen dynamics in the chorioretinal vasculatures.

Optical section retinal imaging and wavefront sensing in diabetes.

PURPOSE: To investigate differences in higher-order ocular aberrations and in optical section retinal image resolution between healthy normal and diabetic subjects. METHODS: An optical imaging system was established for combined retinal optical section imaging and wavefront sensing. A laser beam was expanded and focused to a point on the retina by the optics of the eye. For optical section retinal imaging, a cylindrical lens was placed in the path of the incident laser beam to form a focused line on the retina. Because of the angle between the incident laser and imaging path, an optical section image of the retina was captured. For wavefront sensing, a Shack-Hartmann aberrometer was incorporated in the imaging system. Twenty-two subjects with diabetes (average age, 52 +/- 12 years) and 13 normal subjects (average age, 47 +/- 9 years) were imaged. Retinal depth resolution was determined from the width of the laser line on the retina. Higher-order ocular aberrations were determined from the root mean square of the third to seventh Zernike terms, characterizing the wavefront aberration function. The data were analyzed statistically using Student's t-test and linear regression. RESULTS: Higher-order ocular aberrations in diabetic subjects were significantly higher than in normal subjects (p=0.03). The retinal image depth resolution in diabetic subjects was significantly lower than in normal subjects (p <0.001). The retinal image depth resolution was inversely correlated with higher-order aberrations (r=-0.5; p=0.007; N=35). CONCLUSIONS: The results demonstrate disease-related increases in higher-order ocular aberrations that influence retinal image resolution in diabetic eyes. This information is useful for designing high-resolution retinal imaging systems applicable for eyes with retinal disease.

Feasibility of noninvasive imaging of chorioretinal oxygenation.

BACKGROUND AND OBJECTIVE: The feasibility of an optical system for noninvasive imaging of chorioretinal oxygenation was evaluated. Due to its depth discrimination, this optical section imaging technique has potential for differential imaging of oxygen tension in the chorioretinal vasculatures. MATERIALS AND METHODS: The method consisted of projecting a narrow laser line obliquely on the retina after intravenous injection of an oxygen-sensitive probe and imaging the phosphorescence emission. Due to the angle between the incident laser and imaging path, a phosphorescence optical section image of the retina was captured. The phosphorescence intensity was measured in the chorioretinal vasculatures. The method was tested in three rats while breathing 10% oxygen, 50% oxygen, and room air. RESULTS: On the phosphorescence optical section image, vasculatures appeared laterally displaced according to their depth location, displaying probe phosphorescence separately in the chorioretinal vasculatures. Oxygenation increased in all vasculatures with increased inhaled percent oxygen. Oxygenation in the retinal artery was 2.3, 1.9, and 1.6 times oxygenation in the retinal vein, capillary, and choroid, respectively. During hypoxia, oxygenation decreased by 28%, 18%, 22%, and 14% in the retinal vein, artery, capillary, and choroid, respectively. During hyperoxia, oxygenation increased by 30%, 45%, 36%, and 28% in the retinal vein, artery, capillary, and choroid, respectively. CONCLUSION: The results demonstrate the feasibility of this technique for noninvasive and separate imaging of chorioretinal oxygenation and its potential for three-dimensional oxygen tension imaging.

Chorioretinal topography and histopathology in laser-induced choroidal neovascularization.

OBJECTIVE: To investigate the correspondence between topographic mapping of the vitreoretinal and chorioretinal surfaces in vivo and histopathology findings. MATERIALS AND METHODS: Choroidal neovascularization was induced in the retina of a primate by an argon laser. Serial optical section images of the retina were obtained using an optical imaging system based on the Retinal Thickness Analyzer. Topography of the vitreoretinal and chorioretinal surfaces was mapped. The animal was killed and the eyes enucleated for histopathologic examination. RESULTS: In the normal retina, the topography of the vitreoretinal surface showed a depression at the center of the fovea while the chorioretinal surface was relatively flat, corresponding to normal anatomy. In the retina with choroidal neovascularization, the topography of the vitreoretinal surface indicated a smooth elevation while there were irregular elevations in the topography of the chorioretinal surface. Histological sections displayed focal serous retinal detachment, metaplasia of retinal pigment epithelium, and choroidal neovascularization. CONCLUSION: Topographic mapping of the vitreoretinal and chorioretinal surfaces in vivo corresponds with histological findings and shows promise for quantitative evaluation of pathologic alterations caused by chorioretinal diseases.

Relation between retinal thickening and clinically visible fundus pathologies in mild nonproliferative diabetic retinopathy.

BACKGROUND AND OBJECTIVE: To determine the association of retinal thickening (RT) with clinically observable retinal pathologies in eyes with mild nonproliferative diabetic retinopathy. PATIENTS AND METHODS: Using an objective quantitative imaging method (Retinal Thickness Analyzer), the ratio relative to normal RT (RTI) was measured in 23 eyes with and 35 eyes without clinically observable diabetic fundus pathology. RTI was analyzed in relation to presence of mild diabetic retinal lesions in the +/-0.5 mm vicinity. RESULTS: The percent of eyes with RTI significantly above normal values did not differ significantly between eyes with and without retinopathy (30% vs 34%). Mean RTI was not associated with local presence of microaneurysms (P=0.92), soft exudates (P=0.55), or retinal hemorrhages (P=0.31). Areas without hard exudates had significantly greater mean RTI (1.10) than areas with exudates (0.97, P=0.009). CONCLUSION: In diabetic patients with mild retinopathy, areas with and without clinically observable retinal pathologies had similar retinal thickness. We conclude that clinical strategies for detection of retinal thickening should not be limited to areas with visible fundus pathologies.