Optical coherence tomography (OCT) is a technique that gives us the ability to obtain images with resolutions better than 20 micrometers. A large interest in OCT development creates the need for a rapid programming method that allows for easy hardware integration and image processing. The solution should be easy to apply not only for engineers but also for scientists with limited engineering backgrounds.
The OCT group at Gdańsk University of Technology (GUT) focuses on developing new OCT solutions and applications. We possess three different OCT systems. Two are presented in Figure 1.
The effective implementation of new OCT image processing methods for all systems requires an easily adjustable application framework. In this case study, we describe LabVIEW application frameworks for two OCT systems: one built at GUT (system A) and the other a commercial Santec IVS-2000 (system B).
The core part of both of the presented systems is a tunable laser source. The examined sample is illuminated by the laser and light reflected from different parts of the sample structure. The tomographic image is obtained in a way similar to ultrasonography (USG), by measuring the time-of-flight of the reflected light using the optical interferometry (see Figure 2).
Interference between the reflected light and the other part of the laser light beam causes modulation of the light spectrum with modulation frequency proportional to the time-of-flight of reflected light. Scanning the laser wavelength and recording the interference signal allows us to obtain the waveform containing harmonic oscillation with frequencies corresponding to positions of sample structure elements. The tomographic image is then generated using the Fourier transform of these waveforms.
OCT imaging requires high-speed DAQ cards: NI PCI-5922 (system A) and Signatec PDA14 (system B) with sampling rates 15 MHz and 100 MHz, respectively. The advantage of PCI-5922 is a built-in antialiasing filter that compensates a lower sampling rate and can provide a clearer OCT image. Both systems allow for polarization analysis using the two-channel measurement (for two orthogonal polarization states of light).
To obtain the full image, the sample surface must be scanned with the laser beam. Both systems are equipped with a two-mirror galvanometer scanner controlled by the analog waveform generators based on NI USB-6363 (system A) and NI PCI-6221 (system B).
The presented application framework is a LabVIEW solution with the main loop of the program composed of three stages: checking the user controls, data acquisition, and image processing.
During the data acquisition stage, the light spectra are acquired synchronously by scanning the sample with the laser beam. At the same time, the data from the previous loop iteration is preprocessed using the pipelining technique.
The preprocessing transforms acquired data into two black and white images for each polarization channel. During the next stage, these images are joined into a color image that we can further process and analyze. LabVIEW IMAQ and digital signal processing (DSP) libraries were very useful for this task. The framework also allows us to perform three-dimensional OCT scans. Figure 3 shows the application GUI with the sample scan.
NI products helped us meet the project requirements with:
- High-level programming with robust access to the hardware
- Readable code
- Access to many DSP and image processing libraries
- Easy implementation of pipelining and parallel computing
The described framework gave us the ability to program two different OCT systems with the time and cost being only a little higher than programming one of these systems. It is expected to greatly facilitate future research projects and the programming of new OCT setups with different hardware solutions.
Gdańsk University of Technology, Department of Metrology and Optoelectronics
G. Narutowicza 11/12
Tel: +48 58 347 1361
Fax: +48 58 347 1848