Multiplexed imaging in live cells using pulsed interleaved excitation spectral FLIM

Multiplexed fluorescence detection has become increasingly important in the fields of biosensing and bioimaging. Although a variety of excitation/detection optical designs and fluorescence unmixing schemes have been proposed to allow for multiplexed imaging, rapid and reliable differentiation and quantification of multiple fluorescent species at each imaging pixel is still challenging. Here we present a pulsed interleaved excitation spectral fluorescence lifetime microscopic (PIE-sFLIM) system that can simultaneously image six fluorescent tags in live cells in a single hyperspectral snapshot. Using an alternating pulsed laser excitation scheme at two different wavelengths and a synchronized 16-channel time-resolved spectral detector, our PIE-sFLIM system can effectively excite multiple fluorophores and collect their emission over a broad spectrum for analysis. Combining our system with the advanced live-cell labeling techniques and the lifetime/spectral phasor analysis, our PIE-sFLIM approach can well unmix the fluorescence of six fluorophores acquired in a single measurement, thus improving the imaging speed in live-specimen investigation.

Generative adversarial network enables rapid and robust fluorescence lifetime image analysis in live cells

Fluorescence lifetime imaging microscopy (FLIM) is a powerful tool to quantify molecular compositions and study molecular states in complex cellular environment as the lifetime readings are not biased by fluorophore concentration or excitation power. However, the current methods to generate FLIM images are either computationally intensive or unreliable when the number of photons acquired at each pixel is low. Here we introduce a new deep learning-based method termed flimGANE (fluorescence lifetime imaging based on Generative Adversarial Network Estimation) that can rapidly generate accurate and high-quality FLIM images even in the photon-starved conditions. We demonstrated our model is up to 2,800 times faster than the gold standard time-domain maximum likelihood estimation (TD_MLE) and that flimGANE provides a more accurate analysis of low-photon-count histograms in barcode identification, cellular structure visualization, Förster resonance energy transfer characterization, and metabolic state analysis in live cells. With its advantages in speed and reliability, flimGANE is particularly useful in fundamental biological research and clinical applications, where high-speed analysis is critical.

BMEntored: Enhancing the First-Year Experience in a BME Doctoral Program

We describe our experiences with the first offering of a new program, BMEntored, for supporting first-year doctoral students in Biomedical Engineering (BME) during their first semester. The goal of BMEntored was to enhance the first-semester experience of first-year doctoral students in BME with an emphasis on guiding students in selecting a research supervisor and promoting cross-cohort, cross-lab social connections.

Path-length-multiplexed scattering-angle-diverse optical coherence tomography for retinal imaging

A low-resolution path-length-multiplexed scattering angle diverse optical coherence tomography (PM-SAD-OCT) is constructed to investigate the scattering properties of the retinal nerve fiber layer (RNFL). Low-resolution PM-SAD-OCT retinal images acquired from a healthy human subject show the variation of RNFL scattering properties at retinal locations around the optic nerve head. The results are consistent with known retinal ganglion cell neural anatomy and principles of light scattering. Application of PM-SAD-OCT may provide potentially valuable diagnostic information for clinical retinal imaging.

Thickness, phase retardation, birefringence, and reflectance of the retinal nerve fiber layer in normal and glaucomatous non-human primates

PURPOSE

We identified candidate optical coherence tomography (OCT) markers for early glaucoma diagnosis. Time variation of retinal nerve fiber layer (RNFL) thickness, phase retardation, birefringence, and reflectance using polarization sensitive optical coherence tomography (PS-OCT) were measured in three non-human primates with induced glaucoma in one eye. We characterized time variation of RNFL thickness, phase retardation, birefringence, and reflectance with elevated intraocular pressure (IOP).

METHODS

One eye of each of three non-human primates was laser treated to increase IOP. Each primate was followed for a 30-week period. PS-OCT measurements were recorded at weekly intervals. Reflectance index (RI) is introduced to characterize RNFL reflectance. Associations between elevated IOP and RNFL thickness, phase retardation, birefringence, and reflectance were characterized in seven regions (entire retina, inner and outer rings, and nasal, temporal, superior and inferior quadrants) by linear and non-linear mixed-effects models.

RESULTS

Elevated IOP was achieved in three non-human primate eyes with an average increase of 13 mm Hg over the study period. Elevated IOP was associated with decreased RNFL thickness in the nasal region (P = 0.0002), decreased RNFL phase retardation in the superior (P = 0.046) and inferior (P = 0.021) regions, decreased RNFL birefringence in the nasal (P = 0.002) and inferior (P = 0.029) regions, and loss of RNFL reflectance in the outer rings (P = 0.018). When averaged over the entire retinal area, only RNFL reflectance showed a significant decrease (P = 0.028).

CONCLUSIONS

Of the measured parameters, decreased RNFL reflectance was the most robust correlate with glaucomatous damage. Candidate cellular mechanisms are considered for decreased RNFL reflectance, including mitochondrial dysfunction and retinal ganglion cell apoptosis.

Complex polarization ratio to determine polarization properties of anisotropic tissue using polarization-sensitive optical coherence tomography

Complex polarization ratio (CPR) in materials with birefringence and biattenuance is shown as a logarithmic spiral in the complex plane. A multi-state Levenberg-Marquardt nonlinear fitting algorithm using the CPR trajectory collected by polarization sensitive optical coherence tomography (PS-OCT) was developed to determine polarization properties of an anisotropic scattering medium. The Levenberg-Marquardt nonlinear fitting algorithm using the CPR trajectory is verified using simulated PS-OCT data with speckle noise. Birefringence and biattenuance of a birefringent film, ex-vivo rodent tail tendon and in-vivo primate retinal nerve fiber layer were determined using measured CPR trajectories and the Levenberg-Marquardt nonlinear fitting algorithm.

The relationship between retinal ganglion cell axon constituents and retinal nerve fiber layer birefringence in the primate

PURPOSE

To determine the degree of correlation between spatial characteristics of the retinal nerve fiber layer (RNFL) birefringence (Delta n(RNFL)) surrounding the optic nerve head (ONH) with the corresponding anatomy of retinal ganglion cell (RGC) axons and their respective organelles.

METHODS

RNFL phase retardation per unit depth (PR/UD, proportional to Delta n(RNFL)) was measured in two cynomolgus monkeys by enhanced polarization-sensitive optical coherence tomography (EPS-OCT). The monkeys were perfused with glutaraldehyde and the eyes were enucleated and prepared for transmission electron microscopy (TEM) histologic analysis. Morphologic measurements from TEM images were used to estimate neurotubule density (rho(RNFL)), axoplasmic area (A(x)) mode, axon area (A(a)) mode, slope (u) of the number of neurotubules versus axoplasmic area (neurotubule packing density), fractional area of axoplasm in the nerve fiber bundle (f), mitochondrial fractional area in the nerve fiber bundle (x(m)), mitochondria-containing axon profile fraction (m(p)), and length of axonal membrane profiles per unit of nerve fiber bundle area (L(am)/A(b)). Registered PR/UD and morphologic parameters from corresponding angular sections were then correlated by using Pearson's correlation and multilevel models.

RESULTS

In one eye there was a statistically significant correlation between PR/UD and rho(RNFL) (r = 0.67, P = 0.005) and between PR/UD and neurotubule packing density (r = 0.70, P = 0.002). Correlation coefficients of r = 0.81 (P = 0.01) and r = 0.50 (P = 0.05) were observed between the PR/UD and A(x) modes for each respective subject.

CONCLUSIONS

Neurotubules are the primary source of birefringence in the RNFL of the primate retina.

Quality assessment for spectral domain optical coherence tomography (OCT) images

Retinal nerve fiber layer (RNFL) thickness, a measure of glaucoma progression, can be measured in images acquired by spectral domain optical coherence tomography (OCT). The accuracy of RNFL thickness estimation, however, is affected by the quality of the OCT images. In this paper, a new parameter, signal deviation (SD), which is based on the standard deviation of the intensities in OCT images, is introduced for objective assessment of OCT image quality. Two other objective assessment parameters, signal to noise ratio (SNR) and signal strength (SS), are also calculated for each OCT image. The results of the objective assessment are compared with subjective assessment. In the subjective assessment, one OCT expert graded the image quality according to a three-level scale (good, fair, and poor). The OCT B-scan images of the retina from six subjects are evaluated by both objective and subjective assessment. From the comparison, we demonstrate that the objective assessment successfully differentiates between the acceptable quality images (good and fair images) and poor quality OCT images as graded by OCT experts. We evaluate the performance of the objective assessment under different quality assessment parameters and demonstrate that SD is the best at distinguishing between fair and good quality images. The accuracy of RNFL thickness estimation is improved significantly after poor quality OCT images are rejected by automated objective assessment using the SD, SNR, and SS.

Fibre orientation contrast for depth-resolved identification of structural interfaces in birefringent tissue

Incorporation of polarimetric sensitivity into optical coherence tomography can provide additional image contrast when structures of interest are optically anisotropic (e.g., fibrous tissue). We present a generalized technique based on polarization-sensitive optical coherence tomography to detect changes in depth-resolved fibre orientation and thus increase image contrast in multiple-layered birefringent tissues. A high contrast B-scan image of collagen fibre orientation is shown for a porcine intervertebral disc cartilage specimen that exhibited low backscattering intensity contrast. Interfaces in the annulus fibrosus identified using depth-resolved fibre orientation allowed quantification of lamellae thickness. Moreover, the technique detects changes in fibre orientation without intense processing needed to effectively quantify tissue retardation and diattenuation.

Differential geometry of normalized Stokes vector trajectories in anisotropic media

Trajectory of the normalized Stokes vector on the Poincaré sphere corresponding to light propagation in anisotropic tissues with birefringence and biattenuance is derived. Analytic expressions are determined from the Serret-Frenet formulas and derivatives of arc length for five quantities including the tangent, normal, and binormal vectors with curvature and torsion. Depth variation of curvature and torsion of normalized Stokes vector trajectories corresponding to light propagating in rodent tail tendon are given. Use of analytic expressions for depth variation of curvature and torsion of the normalized Stokes vector trajectories on the Poincaré sphere is discussed for analysis of polarization-sensitive optical coherence tomography data recorded from anisotropic biological tissues with birefringence and biattenuance.

Birefringence of the primate retinal nerve fiber layer

The purpose of this study was to measure the peripapillary retinal nerve fiber layer (RNFL) thickness, phase retardation (PR), and depth-resolved birefringence (Deltan) of the normal primate eye using Enhanced Polarization-Sensitivity Optical Coherence Tomography (EPS-OCT). Both eyes of two rhesus monkeys were imaged with EPS-OCT. A multiple incident polarization state nonlinear fitting algorithm was used to determine RNFL phase retardation. RNFL thickness (RNFLT) was determined from the corresponding EPS-OCT intensity image and phase retardation per unit depth (PR/UD, proportional to Deltan) was calculated by dividing PR by RNFLT. Peripapillary area maps consisting of pixels uniformly distributed along a radius from 0.8 to 1.8 mm from the center of the optic nervehead were constructed for RNFLT, PR, and PR/UD. Average PR/UD in the superior and inferior quadrants was 18 degrees /100 mivrom (Deltan=4.2 x 10(-4)) and average PR/UD in the nasal and temporal quadrants was 6.3 degrees /100 microm (Deltan=1.5 x 10(-4)). Relative magnitude of PR radial gradient is similar to that of RNFLT radial gradient and no radial gradient was observed for PR/UD. Polarization-dependent amplitude attenuation per unit depth (PDAA/UD) was 0.02 rad/100 microm in thick RNFL regions. RNFL birefringence was higher in the arcuate bundles compared to nasal and temporal fibers (P=0.001). Birefringence was nearly equal in nasal and temporal quadrants. No statistically significant (P=0.01) radial gradient of birefringence was observed in any quadrant. RNFL birefringence is believed to originate from anisotropic structures within the cytoskeleton of the parallel axons. Birefringence differences presented in this study cannot be explained by the known axon diameter distribution around the optic nervehead and suggest other sources of the birefringence signal including neurotubules and neurofilaments.

High-sensitivity determination of birefringence in turbid media with enhanced polarization-sensitive optical coherence tomography

Polarization-sensitive optical coherence tomography provides high-resolution cross-sectional characterization of birefringence in turbid media. Weakly birefringent biological tissues such as the retinal nerve fiber layer (RNFL) require advanced speckle noise reduction for high-sensitivity measurement of form birefringence. We present a novel method for high-sensitivity birefringence quantification by using enhanced polarization-sensitive optical coherence tomography (EPS-OCT) and introduce the polarimetric signal-to-noise ratio, a mathematical tool for analyzing speckle noise in polarimetry. Multiple incident polarization states and non-linear fitting of normalized Stokes vectors allow determination of retardation with +/-1 degrees uncertainty with invariance to unknown unitary polarization transformations. Results from a weakly birefringent turbid film and in vivo primate RNFL are presented. In addition, we discuss the potential of EPS-OCT for noninvasive quantification of intracellular filamentous nanostructures, such as neurotubules in the RNFL that are lost during the progression of glaucoma.

Imaging tissue response to electrical and photothermal stimulation with nanometer sensitivity

BACKGROUND AND OBJECTIVES

Tissue response to thermal, electrical, or chemical stimuli are important in the health and survival of tissue. We report experimental results to assess tissue response to various stimuli using a low coherence differential phase interferometer.

STUDY DESIGN/MATERIALS AND METHODS

The optical system utilized to measure tissue response is a novel fiber-based phase sensitive optical low coherence reflectometer (PS-OLCR). Inasmuch as the PS-OLCR works with back-reflected light, noninvasive sensing of tissue response to stimuli is possible. In addition to high lateral (approximately 10 microm) and longitudinal (approximately 10 microm) resolution, PS-OLCR can measure sub-wavelength changes in optical path-length (Angstrom/nanometer range) by extracting the phase difference between interference fringes in two channels corresponding to orthogonal polarization modes.

RESULTS

When light spatially splits into two polarization states, precise analysis of surface topography or tissue surface response such as swelling or collapse are possible. Time resolved measurements of nanometer-scale path length changes in response to electrical and thermal stimuli are demonstrated using longitudinally delayed polarization channels.

CONCLUSIONS

Since PS-OLCR is a useful tool to detect ultra-small path length changes, the system has potential to aid scientists in investigating important phenomena in biomaterials and developing useful diagnostic and therapeutic imaging modalities. Applications include tissue surface profilometry, measurement of tissue, and cell response to various stimuli, high-resolution intensity and phase imaging.

Investigation of the transduction mechanism of infrared detection in Melanophila acuminata: photo-thermal-mechanical hypothesis

Differential phase optical low coherence reflectometry (OLCR) was used to detect sub-wavelength displacements in the infrared-sensitive thoracic pit organ of Melanophila acuminata (Coleoptera: Buprestidae) upon absorption of infrared radiation at 3.39 microm. The displacement had more complex morphology but similar amplitude ( approximately 100 nm at 1 W cm(-2)) when compared to the displacement measured from the exocuticle in an alternate region on the beetle's body. In addition, a simplified finite difference model was developed to predict the temperature distribution and resultant thermal expansion in the pit organ tissue. The experimental and model results were interpreted to help clarify the mechanism by which the sensilla in the pit organ convert infrared radiation to neural signals. The results of this paper are discussed in relation to the photo-thermal-mechanical transduction hypothesis. This is the first experimental examination of the transduction mechanism in Melanophila acuminata.

Primate retina imaging with polarization-sensitive optical coherence tomography

Polarization-sensitive optical coherence tomography (PSOCT) is applied to determine the depth-resolved polarization state of light backreflected from the eye. The birefringence of the retinal nerve fiber layer (RNFL) was observed and measured from PSOCT images recorded postmortem in a Rhesus monkey. An image-processing algorithm was developed to identify birefringent regions in acquired PSOCT retinal images and automatically determine the thickness of the RNFL. Values of the RNFL thickness determined from histology and PSOCT were compared. PSOCT may provide a new method to determine RNFL thickness and birefringence for glaucoma diagnostics.

Maintaining the cornea and the general physiological environment in visual neurophysiology experiments

Neurophysiologists have been investigating the responses of neurons in the visual system for the past half-century using monkeys and cats that are anesthetized and paralyzed, with the non-blinking eyelids open for prolonged periods of time. Impermeable plastic contact lenses have been used to prevent dehydration of the corneal epithelium, which would otherwise occur in minutes. Unfortunately, such lenses rapidly introduce a variety of abnormal states that lead to clouding of the cornea, degradation of the retinal image, and premature termination of the experiment. To extend the viability of such preparations, a new protocol for maintenance of corneal health has been developed. The protocol uses rigid gas permeable contact lenses designed to maximize gas transmission, rigorous sterile methods, and a variety of methods for sustaining and monitoring the overall physiology of the animal. The effectiveness of the protocol was evaluated clinically by ophthalmoscopy before, during, and after the experiments, which lasted 8-10 days. Histopathology and quantitative histology were performed on the corneas following the experiment. Our observations showed that this protocol permits continuous contact lens wear without adversely affecting the corneas. Thus, it is possible to collect data 24 h each day, for the entire duration of the experiment.

Sensitivity threshold and response characteristics of infrared detection in the beetle Melanophila acuminata (Coleoptera: Buprestidae)

The minimum detection threshold of the infrared sensitive beetle, Melanophila acuminata, was measured with a helium-neon laser that emitted light at a wavelength of 3.39 microm. Extracellular recordings were taken both at the pit organ responsible for detection and at the interganglionic connectives in the thorax of the beetle. At the pit organ, generator and action potentials from single neurons were measured with a sharpened tungsten electrode. At the connectives that linked the fused second meso-/metathoracic and prothoracic ganglia, compound action potentials were measured with a tungsten hook electrode that encircled the connective. The latter recordings confirmed conveyance of infrared information through specific pathways to rostrally-situated sites in the nervous system of the beetle. The 50% probability irradiance threshold at which action potentials were elicited from the receptor and connectives occurred at 17.3 and 14.6 mW/cm(2), respectively. In addition to sensitivity threshold, several other characteristics of the response were quantified including dependence of generator potential latency, generator potential duration, spike frequency, and spike latency on irradiance, dependence of response strength (spike count) on exposure time, and flicker fusion frequency. The ability to detect infrared radiation is rare in nature, and these results provide valuable information necessary to understand this unique sensitivity.

Digital integrated retinal surgical laser system

The year is 2001--ophthalmic retinal surgery is now fully computer assisted. Patients arriving for scheduled treatments of diabetic retinopathy, retinal tears, or macular degeneration have their retina digitally mapped by a technician. From the retinal map, the ophthalmologist plots therapeutic lesion sites with a light pen on the computer screen that will automatically be placed by a computer controlled argon laser. The treatment only requires 100 ms per lesion placement thus reducing office calls to approximately 45 minutes freeing the ophthalmologist for other pressing cases. This paper reports on the development of a clinically significant prototype system that will help bring this scenario to fruition.

Hybrid retinal photocoagulation system using analog tracking

We describe initial in vivo experimental results of a new hybrid digital and analog design for retinal tracking and laser beam control. An overview of the design is given. The results show in vivo tracking rates which exceed the equivalent of 38 degrees per second in the eye, with automated lesion pattern creation. Robotically-assisted laser surgery to treat conditions such as diabetic retinopathy and retinal breaks may soon be realized under clinical conditions with requisite safety using standard video hardware and inexpensive optical components based on this design.

Use of an agent to reduce scattering in skin

BACKGROUND AND OBJECTIVE

A method to increase light transport deeply into target areas of tissue would enhance both therapeutic and diagnostic laser applications. The effects of a hyperosmotic agent on the scattering properties of rat and hamster skin were investigated.

STUDY DESIGN/MATERIALS AND METHODS

A hyperosmotic agent, glycerol, was applied in vitro and in vivo to rat and hamster skin to assess the changes in tissue optical properties. Changes in the reduced scattering coefficient after application of the agent in vitro to rat skin and after the skin has been rehydrated were assessed to evaluate the effect of the agent on tissue.

RESULTS

Experimental results showed a transient change in the optical properties of in vitro rat skin. A 50% increase in transmittance and decrease in diffuse reflectance occurred within 5-10 min after the introduction of anhydrous glycerol. In addition, reduction of light scattering with this technique increased depth of visibility with optical coherence tomography. Injection of glycerol under the skin allowed in vivo visualization of blood vessels.

CONCLUSIONS

The application of the agent reduces the amount of refractive mismatch found in the tissue and markedly reduces random scattering, thereby making the skin less turbid for visible wavelengths for a controlled period of time.

Initial in vivo results of a hybrid retinal photocoagulation system

We describe initial in vivo experimental results of a new hybrid digital and analog design for retinal tracking and laser beam control. An overview of the design is given. The results show in vivo tracking rates which exceed the equivalent of 38 degrees/s in the eye. A robotically assisted lesion pattern is created for laser surgery to treat conditions such as diabetic retinopathy and retinal breaks.

Optical model for light distribution during transscleral cyclophotocoagulation

Transscleral cyclophotocoagulation (TSCPC) is currently performed clinically as an effective treatment for end-stage glaucoma. We develop a theoretical model for the analysis of optical attenuation phenomena during TSCPC as a basis for selection of an optimal wavelength. A multilayered Monte Carlo model was developed to calculate the fluence and the rate of heat generation in each tissue layer for the wavelengths of Nd:YAG, diode, ruby, krypton yellow, and argon lasers. Of the five wavelengths under study, our theoretical results suggest that the diode laser wavelength offers the best penetration through the conjunctiva, sclera, and ciliary muscle and highest absorption within the ciliary pigment epithelium.

Optical properties of conjunctiva, sclera, and the ciliary body and their consequences for transscleral cyclophotocoagulation: erratum

Corrections are given to the reported [Appl. Opt. 35, 3321 (1996)] units of the absorption and scattering coefficients mu (a) and mu (s), respectively. The corrections pertain to two figures, several tables, and the text describing results.

Development of an integrated automated retinal surgical laser system

Researchers at the University of Texas and the USAF Academy have worked toward the development of a retinal robotic laser system. The overall goal of this ongoing project is to precisely place and control the depth of laser lesions for the treatment of various retinal diseases such as diabetic retinopathy and retinal tears. Separate low speed prototype subsystems have been developed to control lesion depth using lesion reflectance feedback parameters and lesion placement using retinal vessels as tracking landmarks. Both subsystems have been successfully demonstrated in vivo on pigmented rabbits using an argon continuous wave laser. Preliminary testing on rhesus primate subjects have been accomplished with the CW argon laser and also the ultrashort pulse laser. Recent efforts have concentrated on combining the two subsystems into a single prototype capable of simultaneously controlling both lesion depth and placement. We have designated this combined system CALOSOS for Computer Aided Laser Optics System for Ophthalmic Surgery. Several interesting areas of study have developed in integrating the two subsystems: 1) "doughnut" shaped lesions that occur under certain combinations of laser power, spot size, and irradiation time complicating measurements of central lesion reflectance, 2) the optimal retinal field of view (FOV) to achieve both tracking and lesion parameter control, and 3) development of a hybrid analog/digital tracker using confocal reflectometry to achieve retinal tracking speeds of up to 100 dgs. This presentation will discuss these design issues of this clinically significant prototype system. Details of the hybrid prototype system are provided in "Hybrid Eye Tracking for Computer-Aided Retinal Surgery" at this conference. The paper will close with remaining technical hurdles to clear prior to testing the full-up clinical prototype system.

Corneal photocoagulation with continuous wave and pulsed holmium: YAG radiation

In this study, the effectiveness of pulsed and continuous wave (CW) holmium: YAG lasers in coagulating in vitro pig corneas was analyzed. With the CW laser, irradiance and exposure time were varied; irradiance, from 162 to 324 W/cm2 and exposure time, from 200 to 800 ms. With the pulsed laser, number of pulses and radiant exposure were varied; number of pulses per lesion, from 4 to 30 and radiant exposure, from 10 to 25 J/cm2. Laser-induced corneal damage was determined by analyzing histological cross sections of each lesion. Depth and diameter of the lesions were plotted against the varying laser parameters. Light and birefringent photomicrographs of typical lesion histology show that the pulsed laser significantly damaged superficial layers of the cornea and could not achieve the coagulation depths produced by the CW laser. Additional histology demonstrates that minimal surface damage (intrastromal coagulation) occurred when the CW laser beam was delivered with a sapphire-tipped contact probe. The results provide empirical data on the sensitivity of each parameter in producing a range of coagulation end points. In addition, the experimental results describe trends between the parameters of either laser and the extent of coagulation.

Automated retinal robotic laser system

Researchers at the University of Texas and the USAF Academy have worked toward the development of a retinal robotic laser system. The overall goal of this ongoing project is to precisely place and control the depth of laser lesions for the treatment of various retinal diseases such as diabetic retinopathy and retinal tears. Separate low speed prototype subsystems have been developed to control lesion depth using lesion reflectance feedback parameters and lesion placement using retinal vessels as tracking landmarks. Both subsystems have been successfully demonstrated in vivo on pigmented rabbits using an argon continuous wave laser. Recent efforts have concentrated on combining the two subsystems into a single prototype capable of simultaneously controlling both lesion depth and placement. We have designated this combined system CALOSOS for Computer Aided Laser Optics System for Ophthalmic Surgery. Following the dual-use concept, this system is being adapted for clinical use as a retinal treatment system as well as a research tool for military laser-tissue interaction studies.

Automated lesion placement in the rabbit eye

BACKGROUND AND OBJECTIVE

The objective of this research was to build a prototype feedback control system to precisely place argon laser lesions on the retina for treatment of retinal disorders. STUDY DESIGN/MATERIALS, AND METHODS: The prototype feedback control system was tested by placing lesions at specific locations on the retina of pigmented rabbits to simulate the treatment of diabetic retinopathy, retinal breaks or tears, and a pre-programmed, two-dimensional array of lesions was placed at a specific site.

RESULTS

Results of feedback-controlled lesion placement performed in vivo on pigmented rabbits are presented. The ability to place lesions with automated feedback control is demonstrated.

CONCLUSION

Automated feedback control placement of argon laser lesions is possible at a reasonable cost and has numerous therapeutic and safety benefits over current ballistic delivery.

Preliminary results on reflectance feedback control of photocoagulation in vivo

The size of therapeutic laser-induced retinal lesions is critical for effective treatment and minimal complications. Due to tissue variability, the size of a lesion that results from a given set of laser irradiation parameters cannot be predicted. Real time feedback control of lesion size is implemented based on two-dimensional reflectance images acquired during irradiation. Preliminary results of feedback controlled lesions formed in pigmented rabbits demonstrate an ability to produce uniform lesions despite variations in tissue absorption or changes in laser power.

Real-time algorithm for retinal tracking

Conventional retinal laser photocoagulation is presently performed by an ophthalmologist manually aiming a low-power laser beam at a desired site and firing a high-power laser for a preselected interval of time. To automate this process a retinal tracker must acquire a target, track small saccades, and identify loss of track during a large saccade. The authors successfully implemented a real-time algorithm that used a simple computer, video digitizing card, low light video camera, and fundus camera to perform rudimentary tracking on a photograph of a retina undergoing smooth circular motion. The algorithm tracked speeds up to 5 Hz, or 27 degrees/s, which equated to the retina moving in a 525 microns diameter circle.

Reflectance feedback control of photocoagulation in vivo

OBJECTIVE

The objective of this research was to build a real-time feedback system that controlled lesion size. Two-dimensional reflectance images were acquired with a charge-coupled device camera during irradiation, and argon laser exposure was ended when parameters of the image reached prespecified values.

METHODS

The real-time feedback control system was tested by creating lesions at different power levels in pigmented rabbits. Laser exposure time was controlled by monitoring the central reflectance.

RESULTS

Results of feedback-controlled lesions formed in vivo in pigmented rabbits are presented. An ability to produce uniform lesions despite variation in tissue absorption or changes in laser power is demonstrated.

CONCLUSIONS

Reflectance control of photocoagulation is possible; incorporation of feedback during photocoagulation has numerous therapeutic and safety benefits over current ballistic delivery.

Dynamic optical property changes: implications for reflectance feedback control of photocoagulation

During laser treatment, coagulation affects the optical properties of the tissue. In particular, the formation of a white lesion significantly increases the scattering coefficient. This change in the optical properties in turn affects the laser light distribution in the tissue. The white lesion formed during photocoagulation of the retina has a dynamic effect upon reflection and fluence rate. This problem has been simulated on a model medium consisting of a thin absorbing layer covered with a 1 cm thick layer of albumin. The albumin layer is subdivided into coagulated (white) and uncoagulated (clear) layers. The optical properties of each layer have been determined and these values have been used to model light distribution in the medium. One-dimensional adding-doubling and three-dimensional Monte Carlo methods have provided light distributions in the medium for varying thicknesses of the coagulated albumin. Computed fluence reaching the absorbing layer decreased in the presence of a 275 microns or thicker coagulated layer. The coagulated layer attenuates light because it is highly scattering; however, this scattering also leads to a sub-surface peak in fluence rate at a level higher than the incident fluence. The latter effect outweighed the former for coagulated layer thicknesses less than 275 microns. Computed reflectance of argon laser light from a semi-infinite coagulated region initially increased linearly as a function of thickness. As the coagulation thickness increased beyond 4-5 optical depths, the reflectance approached a constant value, R infinity, at 9 optical depths (2 mm). Experimentally measured total reflectance is shown to be an inadequate indicator of the thickness of a lesion (finite coagulated volume); however, central reflectance from a lesion measured with a CCD camera confirmed the computed trends. These results provide a theoretical foundation for control of lesion thickness using reflectance images.

Rate process parameters of albumen

Both egg white and egg yolk are excellent media for studying photocoagulation due to thermal response of tissue to laser light. They are readily available, consistently the same, and provide a standard biological medium for different labs to compare results. The irradiation parameters for laser coagulation of these media depend upon their optical properties and their temperature-dependent rate reactions. Estimates of the rate process damage coefficients were obtained from constant-temperature water bath experiments. The activation energy E for egg white was estimated to be 92,000 (cal/M*K] and the integration constant A was found to be 3.8 x 10(57) (second-1).

Reflectance as an indirect measurement of the extent of laser-induced coagulation

Two-dimensional reflectance images and surface thermal distributions were recorded during argon laser induced coagulation. During laser irradiation, coagulated egg yolk formed a white lesion. The whiteness, or reflectance caused by backscattering of light from the forming lesion, would be measured after a short delay from the onset of laser irradiation. In the experiments which covered exposure time from 4.5 to 17.0 s, we found that it started slowly, the reflectance increased rapidly once the surface temperature of the lesion reached approximately 90 degrees C. After this rapid rise, the reflectance began to taper off until no change in reflectance was recorded. There was a 0.98 correlation between lesion diameter measured from a two dimensional reflectance image and the lesion diameter that was measured microscopically. There was a 0.92 correlation between reflectance at the center of the lesion and microscopically measured depth of coagulation at the same point. However, the correlation between microscopically measured coagulation depth and width was only 0.88.

The effect of radial keratotomy in the rupture strength of pig eyes

Freshly enucleated pig eyes are significantly weakened following radial keratotomy surgery and are more susceptible to rupture under lateral or axial compression than their unoperated fellow eyes. Patients should be cautioned to restrict hazardous activity in the post-operative period to prevent disastrous globe rupture.

Performance of chronically implanted induction-powered oscillator epidural pressure transducers

✓ Chronic measurements of epidural pressure made with an induction-powered oscillator transducer revealed several problems with the current design of the device. All transducers failed over a period of months due to the diffusion of extracellular fluid through epoxy seals, which resulted in corrosion of the electrical components. Significant fibrosis occurred around the circumference of the burr hole, which reduced the sensitivity of the transducers. Design modifications are suggested to circumvent these problems.

Photocoagulation of the fovea

The visual outcome, histopathology, and electrodiagnostic tests of a man with a malignant melanoma of the choroid who received foveal argon laser photocoagulation prior to enucleatiion are described. The low intensity foveal burn destroyed only the outer retinal layers and spared the inner retina. Vision was reduced to 20/50 immediately, but recovered to 20/30 after one day. The visually-evoked-potential (VEP) was altered in both amplitude and waveform by the small foveal burn. A patient with solar retinopathy and permanent visual loss demonstrated only waveform changes on VEP testing. Monkeys with mild experimental foveal burns demonstrated outer layer retinal damage and waveform changes on VEP testing. The retinal energy density threshold for argon laser photocoagulation of the fovea is lower than 0.13 joules/mm.2.

Chronic measurement of epidural pressure with an induction-powered oscillator transducer

An induction-powered oscillator transducer (IPOT) was designed for the chronic measurement of epidural pressure. The transducer was completely implantable so all pressure measurements were made through the intact skin. The IPOT had a linear pressure range from -50 to +200 cm H2O, was sensitive to 1 mm H2O and had a zero drift of less than 1 mm H2O/day under full load. Zero drift was minimized by using a hermetically-sealed metal bellows transducing element which was chemically treated to prevent corrosion and creep. The correlation between epidural pressure and intraventricular pressure was determined during the first 24 hours after implantation in six dogs. Epidural pressure was found to be a linear function of intraventricular fluid pressure. Epidural pressure and intraventricular pressure were essential equal provided the epidural wedge pressure was minimized by proper insertion of the transducer. The correlation between epidural pressure and intraventricular pressure was determined after chronic implantation in five dogs. Epidural pressure was a linear function of intraventricular pressure in the chronically implanted dogs, but epidural pressure was not equal to intraventricular pressure. After chronic implantation, the epidural pressure transducer was not responsive to changes in intraventricular pressure because of mechanical changes in the dura. The dura became stiff and non-compliant. Maximum correlation between epidural pressure and intraventricular fluid pressure in chronic implantations will depend on judicious material selection and mechanical design at the transducer-dura interface.