MEDICAL IMAGING: OCT provides nondestructive testing at high resolution

Optical coherence tomography (OCT) is a noninvasive imaging technique that enables visualization of tissue or other objects with resolution similar to that of some microscopes. Recently, there has been an increasing interest in OCT because it provides much greater resolution than other imaging techniques such as magnetic resonance imaging (MRI) or positron emission tomography (PET).

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Optical coherence tomography (OCT) is a noninvasive imaging technique that enables visualization of tissue or other objects with resolution similar to that of some microscopes. Recently, there has been an increasing interest in OCT because it provides much greater resolution than other imaging techniques such as magnetic resonance imaging (MRI) or positron emission tomography (PET).

In OCT systems, light is optically split into two paths: a sample path to illuminate the object and a reference path. Light reflected from the object is then combined with the reference beam, resulting in an interference pattern that can be used to generate sub-surface images of translucent or opaque materials.

To compute the amplitude of the signal at each point, data are converted to frequency space and an inverse fast Fourier transform (IFFT) performed on the signal. This process results in a signal that represents the amplitude at different depths within the sample. At National Instruments (NI; Austin, TX, USA), senior systems engineer Brent Runnels recently demonstrated how this technique is used to develop a fast OCT system based around the company's LabVIEW FPGA module by coding the IFFT in the FPGA (see "FPGA-based processing transforms high-speed OCT," Vision Systems Design, April 2011).

At NI Week, held in Austin, TX (Aug. 2–4, 2011), presenters from Santec Corp. (Aichi, Japan), also showed how they had used the LabVIEW FPGA module to prototype the design of a portable OCT system.

"In swept-source OCT," says Changho Chong, director and business unit manager, "reflected data from a laser that scans the sample must be digitized and processed at high speed. As a result, the system must be capable of high-speed acquisition, complex image processing, and accurate control of the laser scanning. In addition, acquisition and control portions of the system must be tightly synchronized."

To prototype the architecture, Chong and his colleagues used NI FlexRIO modular FPGA hardware and NI LabVIEW FPGA module, a LabVIEW graphical development environment that allows FPGAs to be programmed on NI's reconfigurable hardware. For the I/O, a custom adapter module was developed that allowed 100 MS/s, 12-bit resolution to be used for data acquisition and 50 kS/s, 12-bit laser scanner control. Initially, algorithms were developed using LabVIEW and then ported to the FPGA to accelerate processing. After prototyping the hardware and firmware, a PCI Express board was used for final deployment.

"Moving the processing to the FPGA from the PC allowed frame rates as fast as 40 frames/s to be achieved," says Chong. "In our previous system configuration, a digitizer for data acquisition and a DAC board were required to control the scanner. In the latest configuration, these were combined into a single module and both functions synchronized using the FPGA," he says.

At NI Week, Chong demonstrated how the system could be used to visualize cross-sections of both skin and teeth. In performing an analysis on his own front tooth, Chong showed how the system could display the barrier between the enamel and the dentin of his tooth. When the same analysis was performed on my false ceramic front tooth, no enamel or dentin barrier could be seen (see figure).

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OCT analysis of an original healthy tooth (a) clearly shows the enamel/dentin barrier while that of a false ceramic tooth (b) shows no such barrier.
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