Maya Ratna Jerath

Digital tracking and control of retinal images

Abstract : Laser induced retinal lesions are used to treat a variety of eye diseases such as diabetic retinopathy and retinal detachment. Both the location and size of the retinal lesions are critical for effective treatment and minimal complications. Currently, once an irradiation is begun, no attempt is made to alter the laser beam location on the retina. However, adjustments are desirable to correct for patient eye movements. Lesions form in much less than one second and typical treatment for a disease such as diabetic retinopathy requires as many as 2000 lesions per eye. This type of tedious task is ideally suited for computer implementation. A system has been developed to track a specific lesion coordinate on the retinal surface and provide corrective signals to maintain laser position on the coordinate. Six distinct retinal landmarks are tracked on a high contrast retinal image using two dimensional blood vessel templates. Use of therapeutic lesions as tracking algorithm landmarks is also investigated. An X and Y laser correction signal is derived from the landmark tracking information and provided to a pair of galvanometer steered mirrors to maintain the laser on a prescribed location. Once the laser position has been corrected, a function checks the terminal laser position for minor corrections. A development speed tracking algorithm has been implemented and tested using both vessel and lesion templates. Closed loop feedback control of laser position is demonstrated with calibrated retinal velocities and in vivo testing of the development system. Trade off analysis of parameters affecting tracking system performance is provided. The analysis is used to specify requirements and implementation details for a real time system.

Computer-aided retinal photocoagulation system

Researchers at the University of Texas at Austins Biomedical Engineering Laser Laboratory and the U. S. Air Force Academy’s Department of Electrical Engineering are developing a computer-assisted prototype retinal photocoagulation system. The project goal is to rapidly and precisely automatically place laser lesions in the retina for the treatment of disorders such as diabetic retinopathy and retinal tears while dynamically controlling the extent of the lesion. Separate prototype subsystems have been developed to control lesion parameters (diameter or depth) using lesion reflectance feedback and lesion placement using retinal vessels as tracking landmarks. Successful subsystem testing results in vivo on pigmented rabbits using an argon continuous wave laser are presented. A prototype integrated system design to simultaneously control lesion parameters and placement at clinically significant speeds is provided.

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.<<ETX>>

Reflectance feedback control of photocoagulation in vivo

Laser induced retinal lesions are used to treat a variety of eye diseases such as diabetic retinopathy and retinal detachment. In this treatment, an argon laser beam is directed into the eye through the pupil onto the fundus where the heat resulting from the absorbed laser light coagulates the retinal tissue. This thermally damaged region is highly scattering and appears as a white disk. The size of the retinal lesions is critical for effective treatment and minimal complications. A real time feedback control system is implemented that monitors lesion growth using two-dimensional reflectance images acquired by a CCD camera. The camera views the lesion formation on axis with the coagulating laser beam. The reflectance images are acquired and processed as the lesion forms. When parameters of the reflectance images that are correlated to lesion dimensions meet certain preset thresholds, the laser is shuttered. Results of feedback controlled lesions formed in vivo in pigmented rabbits are presented. An ability to produce uniform lesions despite variation in the tissue absorption or changes in laser power is demonstrated. This lesion control system forms part of a larger automated system for retinal photocoagulation.

Automated retinal robotic laser system instrumentation

Researchers at the University of Texas at Austins Biomedical Engineering Laser Laboratory investigating the medical applications of lasers have worked toward the development of a retinal robotic laser system. The ultimate 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. Researchers at the USAF Academys Department of Electrical Engineering have also become involved with this research due to similar interests. 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. Work is ongoing to build a prototype system to simultaneously control lesion depth and placement. The instrumentation aspects of the prototype subsystems were presented at SPIE Conference 1877 in January 1993. Since then our efforts have concentrated on combining the lesion depth control subsystem and the lesion placement subsystem into a single prototype capable of simultaneously controlling both parameters. We have designed this combined system CALOSOS for Computer Aided Laser Optics System for Ophthalmic Surgery. An initial CALOSOS prototype design is provided. We have also investigated methods to improve system response time. The use of high speed non-standard frame rate CCD cameras and high speed local bus frame grabbers hosted on personal computers are being investigated. A review of system testing in vivo to date is provided in SPIE Conference proceedings 2374-49 (Novel Applications of Lasers and Pulsed Power, Dual-Use Applications of Lasers: Medical session).

Instrumentation for feedback-controlled retinal photocoagualation

Laser induced retinal lesions are used to treat a variety of eye diseases. The size and location of these retinal lesions are critical for effective treatment and minimal complications. An automated system is under development for retinal photocoagulation to improve the accuracy of this treatment. Separate instrumentation systems have been developed to monitor and control lesion growth in real time to compensate for tissue inhomogeneity, and to track and compensate for retinal movement during irradiation. A real time lesion feedback control system is implemented on a UNIX based workstation. A CCD camera (30 frames/second) and coagulating laser are coaxially aligned such that images of the lesion can be acquired during laser irradiation. Parameters of these reflectance images are extracted by an image processor in real time and when certain preset thresholds are exceeded, the laser is shut off. The camera and laser legs are alternately shuttered during irradiation by a high speed spinning wheel to prevent the reflected light from the laser from interfering with the reflectance signal. This system is coupled to a fundus camera for delivery to the eye.

Photodynamic therapy for endometrial ablation: a study of treatment parameters and effects

The use of PDT for endometrial ablation has been the focus of much recent research. However, the mechanism of action, optimal treatment parameters, and long-term clinical effect are still poorly understood. This study was undertaken to further the understanding of the endometrial response to this drug/light- induced damage. Postpartum rat (Charles River) uterine horns were used as the animal model for fluorescence and treatment studies. Aminolevulinic acid was administered topically (intrauterine), and following a 0.5- to 3-hour drug incubation time, the endometrium was either removed and processed for fluorescence microscopy to assess drug localization or exposed to 150-200 J/cm2 of 630-nm laser light via a 1-cm cylindrical diffusing tip. The light=treated uterine horns were removed and histologically examine 7 to 10 days following treatment. The extent and character of uterine and endometrial damage (gross and histological analysis) were recorded for the varying light doses and incubation times. With topical (intrauterine) application of photosensitizer, incubation time and penetration ability of drug were found to be crucial factors. The use of a drug penetration enhancing vehicle produced greater tissue effects (endometrial ablation). These preliminary studies also showed that tissue effect is drug and light dose related and that the most profound effects may be vascular mediated. The study provided preliminary information for the use of PDT in gynecological applications such as endometrial ablation and female sterilization through Fallopian tube occlusion.

Automated placement of retinal laser lesions in vivo

Researchers at the University of Texas at Austins Biomedical Engineering Laser Laboratory investigating the medical applications of lasers have worked toward the development of a retinal robotic laser system. The overall goal of the 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. Researchers at the USAF Academys Department of Electrical Engineering and the Optical Radiation Division of Armstrong Laboratory have also become involved with this research due to similar related interests. 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. Work is ongoing to build a prototype system to simultaneously control lesion depth and placement. 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. Specifically, the system is being adapted for use with an ultra-short pulse laser system at Armstrong Laboratory and Frank J. Seiler Research Laboratory to study the effects of ultra-short laser pulses on the human retina. The instrumentation aspects of the prototype subsystems were presented at SPIE Conference 1877 in January 1993. Since then our efforts have concentrated on combining the lesion depth control subsystem and the lesion placement subsystem into a single prototype capable of simultaneously controlling both parameters. We have designated this combined system CALOSOS for Computer Aided Laser Optics System for Ophthalmic Surgery. We have also investigated methods to improve system response time. Use of high speed nonstandard frame rate CCD cameras and high speed frame grabbers hosted on personal computers featuring the 32 bit, 33 MHz PCI bus have been investigated. Design details of an initial CALOSOS prototype design is provided in SPIE Conference proceedings 2396B-32 (Biomedical Optics Conference, Clinical Laser Delivery and Robotics Session). This paper will review in vivo testing to date and detail planned system upgrades.