Constructing a Custom-Built Laser Surgery System

Author(s):
Roy E.. Williams, PhD - FEO Medical
Jerre M.. Freeman, MD - MECA Surgery Center
Brian Callies - FEO Medical

Industry:
Life Science

Products:
LabVIEW

The Challenge:
Finding a flexible, low-cost, readily available eye surgery alternative.

The Solution:
Constructing a custom-built refractive laser surgery system for Laser In Situ Keratomileusis (LASIK) and Photo-Refractive Keratectomy (PRK) based on individual ophthalmologist specifications.

A Flexible, Friendly, Off-the-Shelf Answer
In 1997, before excimer refractive laser systems were released, the medical community tapped FEO Medical to develop a custom-built eye surgery system based on an opthalmologist custom design. Since system user requirements would change as newly emerging procedural information became available, the system would need to be made in a short amount of time using off-the-shelf technology. After considering several software development platforms, we decided to use LabVIEW because it boasts easy-to-use interfaces, minimal maintenance, upgradeability, and smooth hardware integration. We also chose NI SCXI hardware for our application.

Precise Algorithm Controls Surgery from Start to Finish
LabVIEW and SCXI hardware monitors and controls multiple system components, including:

• The excimer laser fire, energy monitoring, and gas evacuation and fill
• Laser energy sensors
• Pressure and vacuum gauges
• Flow sensors
• Safety switches on cabinet doors and laser access panels
  
  

LabVIEW interfaces with PC-based Newport motor controller hardware, which controls DC motors that drive the laser-beam-shaping hardware – a mechanical iris and slit on a rotating stage.

To perform surgery, the LabVIEW-based system:

• Interfaces with surgical personnel to accept patient demographic information and refraction-correction information in diopters
• Converts the refraction information to the appropriate laser etching profile (etch depth in microns per etch diameter in millimeters)
• Converts the etching profile into a beam-shaping motor profile
• Controls the necessary hardware to etch the information onto the patient cornea
• Produces a surgery report hardcopy

The easy-to-use LabVIEW interface helps a non-skilled computer technician input patient data (name, address, age, sex) and the surgeon-defined refractive correction. To assist the technician, the system includes popup help for buttons and data fields, and an online user manual.

The refractive correction information generates the etch profile based on a modified Munnerlyn equation, which requires a numerical solution by root-finding (i.e., not a closed-form solution). A LabVIEW built-in Ridders Zero Finder function solves the equation, provides the two-dimensional etch profile, and visually displays it to the surgeon.

The program converts the 2D etch profile into a 3D volume and separates it into discrete layers, where each layer corresponds to the cornea etch depth per each excimer laser pulse (~0.17 microns per laser pulse). Each layer describes a 2D shape that must be delivered to the cornea by shaping the homogenized excimer laser beam into a circular pattern for spherical correction, a rectangular pattern for astigmatism, or a combination of both. A motorized iris provides the circular pattern and converts the diameter at each layer to program the appropriate motor steps to open or close the iris. A motorized slit provides the rectangular pattern and converts the width at each layer to program the appropriate motor steps to open or close the slit. The system saves this information to a file so that it can be sent to the motors during the actual laser etching procedure.

To begin surgery, the algorithm must be able to complete its processing cycle at the laser maximum firing rate (20 Hz, or a cycle rate of 50 ms). The algorithm begins by sending an “activate” laser signal to the excimer laser and reading the motor step data from the 3D profile file for the first etch layer to be delivered. It then sends this information to the motor controllers and checks several inputs in sequence before allowing the laser to fire. At a minimum, it checks the safety switches, the laser “ready” feedback signal, the patient fixation-light “activated” signal, the surgeon footswitch (closed or open), and finally, the iris and slit motor placement. If all these conditions pass, the laser fires. The system then initiates the next sequence by retrieving the next etch layer information. During this process, the algorithm updates the user display with time and pulses remaining, and etch depth.

Once the system processes all laser etch data and delivers it to the cornea, the algorithm returns to a main menu, where the surgeon can input new patient information, perform another surgery, perform diagnostic functions, or shut the system down.When the procedure is complete, the surgical technician can request a surgery report, which includes patient demographics, refraction correction, and laser etch profile information.

LabVIEW Provides Timely Update Capability for Emerging Technology
National Instruments hardware and LabVIEW software have proven extremely useful in constructing this custom refractive laser surgery system. Aside from LabVIEW easily processing multiple inputs and outputs, it rapidly accepts and implements an ever-changing set of user requirements as more and more procedural information becomes available.

Even with customized features and capabilities, our system cost less than half of what a commercially available system would have cost. Coding in LabVIEW took a third of the time it would have taken to code the system in C. And, whereas another software code development platform would have monopolized 24 to 48 months of our time and resources, we developed our LabVIEW-based system in approximately 18 months.

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