Treating Retinal Disease with FPGA Controlled Lasers
- Friday, 01 May 2009
At the highest level, FPGAs are reprogrammable silicon chips. Using prebuilt logic blocks and programmable routing resources, users can configure these chips to implement digital computing tasks in software and compile them down to a configuration file or bit-stream that contains information on how the components should be wired together. Traditionally, using FPGA technology required expertise in hard- ware description languages (HDLs) like VHDL or Verilog. Now with higher-level design tools such as LabVIEW that abstract the complexities of digital design, engineers and scientists of any discipline can take advantage of FPGA technology. OptiMedica engineers, whose expertise is in ophthalmology and photocoagulation, used graphical block diagrams in LabVIEW FPGA to successfully embed their laser treatment algorithms in hardware.
In previous methods of retinal laser photocoagulation, the physician used a mechanical joystick and foot pedal to deliver single 100-millisecond laser pulses to abnormal blood vessels of the peripheral retina. The PAS-CAL Photocoagulator essentially automates and improves this process by taking advantage of FPGA technology in National Instruments intelligent data acquisition (DAQ) devices. In the NI PCI-7833R, built-in digital-to-analog converters are used to generate the control signal for the laser position and built-in analog-to-digital converters ensure that the actual laser position matches the desired position. Other inputs and outputs, such as the laser power and the foot pedal, are read and written using isolated digital inputs and outputs. With this approach, OptiMedica engineers can achieve a closed-loop control system with a response time of four microseconds.
As with all medical devices, PASCAL incorporates redundancy with additional sensors for an increased level of safety. In addition to feedback built into the galvanometer, the laser beam is split and directed to an internal photo detector, which is constantly read by another analog input channel on the PCI-7833R. This photo detector gives PASCAL the ability to constantly test and monitor the laser power coming from the output beam. If the power is too low or too high, a second galvanometer deflects the beam away from the final lens and prevents faulty lasers from entering into the patient’s eye. PASCAL also uses this internal photo detector to ensure that the power is off when necessary, such as during the start-up sequence or when the system is in standby mode. Using an FPGA-based data acquisition board, PAS-CAL can make hardware-timed decisions in microseconds.
Although the laser control logic and safety logic are embedded within the same FPGA chip, dedicated blocks of I/O and silicon operate independently. These dedicated blocks of I/O give PAS-CAL the ability to fully take advantage of the parallelism that FPGAs offer. Engineers at OptiMedica chose to use NI LabVIEW for all software development of the host and FPGA applications. The graphical nature of LabVIEW makes it simpler to program and visualize tasks running in parallel on the block diagram. LabVIEW FPGA code can be migrated between PCI, PXI, and PCI Express off-the-shelf hardware devices. Each part of the LabVIEW FPGA application functions simultaneously with all other pieces of the application, allowing redundant logic to become more than just another set of commands that procedurally execute. PASCAL can continuously test and monitor itself and immediately identify possible component failures before undesirable results have occurred. Reading the photo detector, for example, will happen every five microseconds, while the voltage output loop to the galvanometer happens every 50 microseconds. Because each of the safety routines is independent of all other tasks, each part of the safety code runs quickly and reliably.
For the user interface, OptiMedica engineers chose an off-the-shelf touch panel monitor running on a Windows system. From the main control panel, an LCD display with a touch screen control is used to select from predetermined pattern types and administer up to 25 spots at a time. All other equipment is very similar to conventional, single-shot photocoagulation, with a mechanical joystick and foot pedal, so physician training is minimal. Once the pattern is selected, a separate red beam is used for aiming and visualizing placement before delivery. The touch screen interface is also used for selecting various parameters, such as aim beam intensity, treatment laser power, exposure time, system status, and shutdown. Because each pulse size and pattern is predictable, PASCAL photocoagulation is a more precise way to evenly distribute patterned spots, which also increases the reproducibility of results for each treatment.
In the past, FPGA technology was available only to engineers with a deep understanding of digital hardware design. The rise of high-level design tools, however, has opened up this technology to a new class of users — domain experts with no embedded design experience. The result was that OptiMedica was able to quickly and efficiently prototype and deploy their final system using off-the-shelf hardware.