PCI-Based Control System for Laser Etching an Intraocular Lens

Author(s):
Roy E. Williams - FEO Medical, Inc

Industry:
Medical/ Medical Instrumentation, Life Science

Products:
Vision, LabVIEW

The Challenge:
Developing a high-resolution, deep ultraviolet (UV) laser etching system to produce highly complex, custom acrylic or silicone intraocular lenses for implantation in the human eye after cataract removal or for refractive correction, allowing the physician to better fit patients.

The Solution:
Creating a high-speed, mixed-signal PCI data acquisition and control system that interfaces to an ophthalmology-based topographic wavefront measurement system, a UV-enabled digital micromirror device, and a deep UV laser system, all through National Instruments LabVIEW software and PCI hardware.

Introduction

The human visual system is complex, consisting of the cornea, the lens, and the retina. Over time, the lens can form cataracts, preventing it from focusing light on the retina, resulting in vision loss. Surgeons correct cataracts by removing the lens and inserting an artificial lens (IOL). Current methods for manufacturing IOLs are limited to producing simple refractions – spherical and cylindrical. Physicians can now measure higher-order aberrations of the eye, but cannot correct them with currently available manufactured products. It would be advantageous to produce IOLs to correct these aberrations to provide patients with superior vision. No systems previously existed to accomplish this.

A well-known cataract surgeon commissioned FEO Solutions to develop a prototype of such a system, providing mass manufacturing capability, interfacing to commercial eye measurement systems, data import from remote locations, and scalable, maintainable, user-friendly software. The customer required delivery of the prototype in nine months. We could only spare one physicist and one electro-optical engineer for the project. It took a commercial refractive surgery company nearly two years, with multiple engineers, to interface one wavefront measurement system to their refractive laser surgery system. Based on our previous work with National Instruments hardware and NI LabVIEW software for a laser refractive surgery system eye tracker, we felt confident that we could meet the challenge.

System Description

Each part of the system had a unique requirement met by NI hardware and LabVIEW software. In the eye measurement interface section, the software had to interface with many systems using different communication protocols and data sets. In the profile generation section, the software had to calculate several equations and merge the results to construct a 3D model of the etch pattern. The IOL is etched using the laser control section, where the software had to precisely control the UV laser fire and laser energy, and to monitor the beam uniformity; and the laser beam shaping section, where the software had to control 786,432 micromirrors on the UV-enabled DLP device (UV-DLP).

Eye Measurement Interface

In treating a patient, the first task is to measure the patient’s visual system, which is performed with systems in clinics located remotely from the IOL etching system. Systems for measuring corneal topography, or wavefront components, output their data in a proprietary format, which is available from the manufacturer. The remote clinics upload the patient’s measurement data to a common FTP-based database. The IOL etching system, which is based on LabVIEW, accesses the FTP database (which was written using the LabVIEW Database Connectivity Toolkit) via the Internet to get the patient’s data (using LabVIEW Internet Toolkit).

The data is then parsed based on the manufacturer’s format. This format is contained in a LabVIEW configuration file, allowing for easy addition of other measurement systems’ files as they become available. The parsed information is saved to the hard disk and then passed to the profile generation code.

Profile Generation

Next, the system must generate a 3D volumetric profile based on the measurement data. Profile generation begins with a basic IOL formula to calculate the IOL power. The IOL equations are implemented using LabVIEW math functions. Once the IOL power is determined, the code converts the power to a radius-of-curvature (ROC) based on an index of refraction. From the ROC, a 3D shape defining the basic IOL shape is generated. This profile is then altered by adding the higher-order aberrations captured by the wavefront system (for example, spherical aberration and coma). This 3D profile is displayed at every step, using LabVIEW 3D surface graph functions so the surgeon can review the profile. The resulting 3D profile is then sent to the laser beam shaping code.

IOL Etch – Laser Beam Shaping

Based on the etch depth per laser pulse for each IOL material, the code “slices” the 3D profile into a number of individual 2D layers. (For example, for a spherical shape, each layer is represented by a “circle” with a different diameter.) Each individual 2D slice is presented as an “image” to the UV-DLP device via a high-speed USB port. Each mirror is then set to reflect or not reflect the laser energy, based on the “image.” The 2D mirror configuration is read back to confirm correct positioning. Each mirror in the device’s 1024 x 768 array is 13 µm², and thus, when the laser is fired, nearly 768,000 individual 13 µm² laser beams are directed to the IOL surface.

IOL Etch – Laser Control

With the mirror configuration set, the excimer UV laser is ready to fire. The LabVIEW code first sends the desired energy setting to the laser via the RS232 link. When the laser is ready, it enables a digital “ready” signal. The code reads the “ready” signal and sends a digital “fire” signal to the laser. Once the laser fires, it enables a “fired” signal. Upon receiving the “fired” signal, the code reads the laser energy output via the RS232 link, and a second energy reading from the deep UV imager via the National Instruments PCI-6036E data acquisition device. The code continually monitors these two energy readings and adjusts the laser accordingly to maintain the required energy (via a PID loop).

When the laser pulse is fired and reflected from the UV-DLP device mirrors to the IOL blank, the surface is etched as described by each 2D slice “image.” For the example above, the result is an IOL with a spherical shape.

Note that this laser uses digital I/O and RS232 for monitoring and control. Future lasers may use Ethernet or USB. With LabVIEW, we can easily modify the system to use these new laser interfaces.

IOL Etch – Laser Beam Uniformity

Laser beam uniformity is checked before each procedure begins. It may also be checked during the procedure to ensure optimum results. The system checks beam uniformity using the deep UV imager. Here, a National Instruments PCI-1409 captures a single laser shot from a custom-designed UV camera. Using the NI Vision Development Module for LabVIEW, the system calculates laser beam intensity at every point (via an individual camera sensor). The system then uses this intensity information in the laser beam shaping section to decide which micromirrors to turn off and how long they will be turned off to compensate for any beam non-uniformities. With this method, the system achieves the smoothest etch of the IOL surface.

Conclusion

We successfully constructed the system and etched an IOL blank in August of 2005, thus meeting the customer’s deadline. We continue to refine the system while preparing it for manufacture. We are also implementing an automated laser alignment and calibration tool for the system, which is also simplified by due to the scalability of LabVIEW.

For more information, contact:

Roy E. Williams

FEO Solutions

1025 Crosswinds Cove

Collierville, TN 38017

Tel: (901) 853-2244

E-mail: roy_williams@feomedical.com

 

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