Intraocular Laser Surgery Research Articles

基于双模态图像信息的眼内激光手术联合标定方法

基于双模态图像信息的眼内激光手术联合标定方法

Techniques for robot-aided intraocular surgery using monocular vision

This paper presents techniques for robot-aided intraocular surgery using monocular vision in order to overcome erroneous stereo reconstruction in an intact eye. We propose a new retinal surface estimation method based on a structured-light approach. A handheld robot known as the Micron enables automatic scanning of a laser probe, creating projected beam patterns on the retinal surface. Geometric analysis of the patterns then allows planar reconstruction of the surface. To realize automated surgery in an intact eye, monocular hybrid visual servoing is accomplished through a scheme that incorporates surface reconstruction and partitioned visual servoing. We investigate the sensitivity of the estimation method according to relevant parameters and also evaluate its performance in both dry and wet conditions. The approach is validated through experiments for automated laser photocoagulation in a realistic eye phantom in vitro. Finally, we present the first demonstration of automated intraocular laser surgery in porcine eyes ex vivo.

Comparative Evaluation of Handheld Robot-Aided Intraocular Laser Surgery.

This paper presents robot-aided intraocular laser surgery using a handheld robot known as Micron. The micromanipulator incorporated in Micron enables visual servoing of a laser probe, while maintaining a constant distance of the tool tip from the retinal surface. The comparative study was conducted with various control methods for evaluation of robot-aided intraocular laser surgery.

Toward Automated Intraocular Laser Surgery Using a Handheld Micromanipulator.

This paper presents a technique for automated intraocular laser surgery using a handheld micromanipulator known as Micron. The novel handheld manipulator enables the automated scanning of a laser probe within a cylinder of 4 mm long and 4 mm in diameter. For the automation, the surface of the retina is reconstructed using a stereomicroscope, and then preplanned targets are placed on the surface. The laser probe is precisely located on the target via visual servoing of the aiming beam, while maintaining a specific distance above the surface. In addition, the system is capable of tracking the surface of the eye in order to compensate for any eye movement introduced during the operation. We compared the performance of the automated scanning using various control thresholds, in order to find the most effective threshold in terms of accuracy and speed. Given the selected threshold, we conducted the handheld operation above a fixed target surface. The average error and execution time are reduced by 63.6% and 28.5%, respectively, compared to the unaided trials. Finally, the automated laser photocoagulation was demonstrated also in an eye phantom, including compensation for the eye movement.

Semiautomated intraocular laser surgery using handheld instruments.

In laser retinal photocoagulation, hundreds of dot-like burns are applied. We introduce a robot-assisted technique to enhance the accuracy and reduce the tedium of the procedure. Laser burn locations are overlaid on preoperative retinal images using common patterns such as grids. A stereo camera/monitor setup registers and displays the planned burn locations overlaid on real-time video. Using an active handheld micromanipulator, a 7 x 7 grid of burns spaced 650 microm apart is applied to both paper slides and porcine retina in vitro using 30 milliseconds laser pulses at 532 nm. Two scenarios were tested: unaided, in which the micromanipulator is inert and the laser fires at a fixed frequency, and aided, in which the micromanipulator actively targets burn locations and the laser fires automatically upon target acquisition. Error is defined as the distance from the center of the observed burn mark to the preoperatively selected target location. An experienced retinal surgeon performed trials with and without robotic assistance, on both paper slides and porcine retina in vitro. In the paper slide experiments at an unaided laser repeat rate of 0.5 Hz, error was 125+/-62 microm with robotic assistance and 149+/-76 microm without (P < 0.005), and trial duration was 70+/-8 seconds with robotic assistance and 97+/-7 seconds without (P < 0.005). At a repeat rate of 1.0 Hz, error was 129+/-69 microm with robotic assistance and 166+/-91 microm without (P < 0.005), and trial duration was 26+/-4 seconds with robotic assistance and 47+/-1 seconds without (P < 0.005). At a repeat rate of 2.0 Hz on porcine retinal tissue, error was 123+/-69 microm with robotic assistance and 203+/-104 microm without (P < 0.005). Robotic assistance can increase the accuracy of laser photocoagulation while reducing the duration of the operation.

Plasma formation in water by picosecond and nanosecond Nd:YAG laser pulses. I. Optical breakdown at threshold and superthreshold irradiance

We investigated plasma formation in distilled water by 30-ps and 6-ns Nd:YAG laser pulses of 1064-nm and 532-nm wavelength for focusing angles between 1.7° and 32°. We determined the optical thresholds and analyzed the plasma length achieved at superthreshold irradiance, The parameter range investigated covers the parameters used for intraocular laser surgery. The experimental results are compared to theoretical models for the calculation of thresholds and the description of plasma growth for superthreshold breakdown. We found that at λ=1064 nm the measured thresholds for both pulse durations coincide with the calculated thresholds for the generation of seed electrons by multiphoton ionization. The process is completed by avalanche ionization. The seed electron density required for is about 4×10/sup 9/ cm for the 6-ns pulses, and 1.4×10/sup 11/ cm/sup -3/ for the 30-ps pulses. No spot size dependence of the irradiance threshold for was observed. The average threshold is by a factor of 5.9 higher for 30-ps pulses (I/sub th/=4.5×1011 W/cm2) than for 6-ns pulses (Ith=0.76×10/sup 11/ W/cm/sup 2/). At angles below approximately 2°, the threshold is influenced by self-focusing effects. The thresholds at 532 nm are slightly lower than at 1064 nm. Here, multiphoton ionization contributes considerably to the generation of free electrons throughout the whole process of plasma formation. Our results for plasma formation at superthreshold energies support a breakdown wave mechanism of plasma growth. For picosecond pulses, the threshold can be considered to be time-invariant, but for nanosecond pulses there is probably a decrease of the threshold during the laser pulse which may be due to UV-radiation emitted from plasma created at the beginning of the pulse.

Shock wave emission and cavitation bubble generation by picosecond and nanosecond optical breakdown in water

Shock wave emission and cavitation bubble expansion after optical breakdown in water with Nd:YAG laser pulses of 30-ps and 6-ns duration is investigated for energies between 50 μJ and 10 mJ which are often used for intraocular laser surgery. Time-resolved photography is applied to measure the position of the shock front and the bubble wall as a function of time. The photographs are used to determine the shock front and bubble wall velocity as well as the shock wave pressure as a function of time or position. Calculations of the bubble formation and shock wave emission are performed using the Gilmore model of cavitation bubble dynamics and the Kirkwood–Bethe hypothesis. The calculations are based on the laser pulse duration, the size of the plasma, and the maximally expanded cavitation bubble, i.e., on easily measurable parameters. They yield the dynamics of the bubble wall, the pressure evolution inside the bubble, and pressure profiles in the surrounding liquid at fixed times after the start of the laser pulse. The results of the calculations agree well with the experimental data. A large percentage of the laser pulse energy (up to 72%) is transformed into the mechanical energy ES and EB of the shock wave and cavitation bubble, whereby the partitioning between ES and EB is approximately equal. 65%–85% of ES is dissipated during the first 10 mm of shock wave propagation. The pressure at the plasma rim ranges from 1300 MPa (50 μJ, 30 ps) to 7150 MPa (10 mJ, 6 ns). The calculated initial shock wave duration has values between 20 and 58 ns, the duration measured 10 mm away from the plasma is between 43 and 148 ns. A formation phase of the shock front occurs after the ns pulses, but not after the ps pulses where the shock front exists already 100 ps after the start of the laser pulse. After shock front formation, the pressure decays approximately proportional to r−2, and at pressure values below 100 MPa proportional to r−1.06. The maximum bubble wall velocity ranges from 390 to 2450 m/s. The calculations of bubble and shock wave dynamics can cover a large parameter range and may thus serve as a tool for the optimization of laser parameters in medical laser applications.

Plasma formation in water by picosecond and nanosecond Nd:YAG laser pulses. II. Transmission, scattering, and reflection

For pt.I see ibid., vol.2, no.4, p.847-60 (1996) We investigated the transmission, scattering, and reflection of plasmas produced in water by Nd:YAG laser pulses of 6-ns and 30-ps duration. The transmission measurements comprise a large energy range at wavelengths of 1064 and 532 nm and various focusing angles between 1.7/spl deg/ and 22/spl deg/. This parameter range covers the parameters used for intraocular microsurgery, but also allows one to assess the influence of self-focusing on plasma shielding, which is only relevant at small focusing angles. We found that most of the laser light is either absorbed or transmitted; scattering and reflection amount to only a few percent of the incident laser energy. The transmission is considerably higher for picosecond pulses than for nanosecond pulses, regardless of the focusing angle. The plasma transmission increases with decreasing focusing angle. Self-focusing, which occurs at focusing angles below 2/spl deg/, leads to a further increase of transmission. The experimental results were compared with the predictions of the moving breakdown distributed shielding model. Only partial agreement could be achieved, because the model assumes a spatially and temporally constant absorption coefficient within the plasma which is not realistic. The model can, however, be used to determine the average absorption coefficient. Fits of calculated transmission curves to the experimental data at /spl theta/=22/spl deg/ yielded 900 cm/sup -1//spl les//spl alpha//spl les/1800 cm/sup -1/ nanosecond plasmas and 360 cm/sup -1//spl les//spl alpha//spl les/570 cm/sup -1/ picosecond plasmas. The efficacy of plasma-mediated intraocular laser surgery is higher with 6-ns pulses than with 30-ps pulses, because with the nanosecond pulses nearly 50% of the laser pulse energy is absorbed already at threshold, whereas it is only 8% with the picosecond pulses.