Mark E. Arnoldussen

Theory concerning the ablation of corneal tissue with large-area, 193-nm excimer laser beams

Excimer laser beams (193 nm) of uniform fluence were studied to find out why they produce corneal ablations deeper at the edge than the center. Ablation depth profiles were taken of porcine corneas, including five dehydrated samples. Hydrated corneas and polymethyl methacrylate were ablated with and without central masks. Ablation plumes were photographed. Hydrated porcine corneas showed patterns of central underablation. As the incident beam increased, the crater exhibited increasingly shallower central ablation while maintaining nearly constant depth at the edges. Dehydrated corneas did not vary significantly. Masks did not alter the depth or shape of craters near ablation edges, but depth adjacent to the images of the masks was more than twice that with no mask. Depth adjacent to the mask image was nearly the same as at the edge of the zone. The rate of change in depth with position was nearly equal in both areas. Maximum plume density was centered over the entire ablation with and without the mask. Redeposition of plume particles is not the major cause of central underablation. Propagating transverse energy from the absorption of photons by peptide bonds increases pressure on excited components within the irradiated area, increasing recombination, which raises the ablation threshold.

Consequences of scattering for spectral imaging of turbid biologic tissue

Spectral imaging permits two-dimensional mapping of the backscattering properties of biological systems. Such mapping requires broadband illumination of the entire area of interest. However, imaging of turbid biological media under these conditions often involves mean photon path lengths that exceed the pixel size. Using a numerical Monte Carlo model, we have studied the effects of photon scattering in a hemoglobin-bearing model system. We find that photon migration and the resulting wavelength-dependent optical coupling between pixels can complicate the analysis of imaging spectroscopy data. In fact, the wavelength dependence of photon trajectories also alters the distribution of photon exit angles at the tissue surface. We therefore find that the finite optical field of view of an imaging spectrometer can affect the measured spectra in the absence of chromatic aberrations.

Evaluation of retinal image degradation by higher-order aberrations and light scatter in chick eyes after photorefractive keratectomy (PRK)

Intraocular light scatter can severely degrade retinal image quality, a key determinant of visual performance and eye growth regulation, yet it is often ignored in analyses of retinal image quality. A method for characterizing ocular light scatter using Shack–Hartmann images is described here. A local light scatter map is derived by de-convolving the dots in the Shack–Hartmann image recorded from a real eye with those from a model eye, and used to estimate overall ocular light scatter. As an index of retinal image quality, the modulation transfer function (MTF) of the eye is derived as the product of MTFs for light scatter and aberrations alone and for comparative purposes, an MTF volume ratio, equivalent to the Strehl ratio computed in the frequency domain, is computed. These analyses were applied to Shack–Hartmann images collected from three chick eyes, one day before and four days after photorefractive keratectomy (PRK) surgery. PRK was performed on one eye of each of three 4-week-old chicks; ablation parameters for a clinical argon fluorine excimer laser system from Visx® were adjusted based on the results of ablations performed on enucleated chick eyes of the same age. While all three eyes showed increased higher order aberrations after PRK, only two eyes showed significantly increased light scatter. The proposed method for characterizing retinal image quality was able to capture these interocular differences.

Fluorescence-guided laser removal of chemically damaged cornea

Purpose: To correlate the observed fluorescence spectrum with the depth of ablation during 193 nm argon−fluoride excimer laser ablation of chemically damaged corneas. Setting: Laser facility, Cedars‐Sinai Medical Center, Los Angeles, California, USA. Methods: Three cadaver New Zealand white rabbit corneas were exposed to 1 N hydrogen chloride for 10 seconds. The resultant opaque corneas were ablated to perforation using the excimer laser. Laser‐induced fluorescence was collected at 45 degrees from incidence and channeled into an ultraviolet‐visible spectrometer coupled to an optical multichannel analyzer reading a diode array detector. The detector recorded single‐shot fluorescence spectra. The data were examined by principal component analysis, and the evolution of eigenvectors and their weighting coefficients were used to compare data among corneas. The results were correlated with histopathological sections. Results: The eigenvalues of 3 principal components corresponded to 88.9%, 10.0%, and 0.4% of the data in acid‐burned corneas. Compared to that in undamaged corneas, more information was stored in the first principal component and the third eigenvector was distinctly altered. Acid‐scarred tissue blue shifted the dominant fluorescence peak compared to that in normal corneal tissue. Conclusions: After severe hydrogen chloride burn to the rabbit corneal surface, monitoring the dominant peak wavelength shift of excimer‐laser‐induced fluorescence can detect the transition between severely acid‐damaged and underlying tissue.

Imaged backscatter from three-dimensional tissue structure

When imaging the backscattered light from turbid tissue using a broadband illumination source, the random scattering of photons within the tissue causes wavelength-dependent optical coupling between pixels. That is, a photon may exit the tissue surface an extended distance away from its entry point. The resulting spectral crosstalk in the detected image can be explained by studying the mean photon path lengths through the tissue. Considering complex tissue geometries with features such as cylindrical vessels, these photons not only travel multiple paths due to wavelength- dependent absorption and scattering, but may also travel through multiple chromophores. To study the effects of 3D features in object space on backscattered light into the image plane, we have constructed a Monte Carlo simulation capable of modeling 3D photon propagation for a tissue slab with an embedded cylinder. The results of hemoglobin-bearing vessels as a primary chromophore are investigated. Because of the relationship between mean photon path length and photon exit angle, we have shown that the choice of entrance pupil in the imaging system plays an important role on the detected backscatter for the specific case of embedded cylinders.

Effects of scattering in layered biological tissue on imaging spectroscopy data

Spectral imaging permits two-dimensional mapping of the reflectance properties of biological systems. However, imaging in turbid media involves pixel sizes that are comparable to or smaller than the mean photon path length. This implies that the spectrum measured at a given pixel in the image plane will be determined by manifold photon trajectories through an extended volume in the object, so there is not a uniquely defined path length. In addition, this implies nonlinear spectral mixing for systems with multiple layers and chromophores. Using Monte Carlo model, we have studied photon path distributions in the case of layered turbid systems and their effects on spectral imaging. In particular, we emphasize the effect of hemoglobin on imaging reflectance-mode hyperspectral data.