Advanced Cancer Research Using Next-Generation Medical Imaging With PXI Modular Instrumentation and NI LabVIEW
"The PXI platform from National Instruments enabled the high-channel-count acquisition because of its synchronization capabilities, small size, and modularity."
- Dr. Kohji Ohbayashi,
Kitasato University, Center for Fundamental Sciences
Creating a medical instrument that can detect cancer during medical checkups by improving on traditional methods that do not provide sufficient resolution or require the patient to undergo severe stress during the checkup.
Using optical coherence tomography (OCT) and a patented light-source technology along with a high-speed (60 MS/s) data acquisition system with 32 NI PXI-5105 digitizers to provide 256 simultaneously sampled channels.
Dr. Kohji Ohbayashi - Kitasato University, Center for Fundamental Sciences
D. Choi - Kitasato University, Center for Fundamental Sciences
H. Hiro-Oka - Kitasato University, center for Fundamental Sciences
H. Furukawa - Kitasato University, Center for Fundamental Sciences
R. Yoshimura - Kitasato University, Center for Fundamental Sciences
M. Nakanishi - Kitasato University, Center for Fundamental Sciences
K. Shimizu - Kitasato University, Center for Fundamental Sciences
OCT is a noninvasive imaging technique that provides subsurface, cross-sectional images of translucent or opaque materials. OCT images enable us to visualize tissues or other objects with resolution similar to that of some microscopes. In the academic community, 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). Additionally, the method does not require us to prepare and is extremely safe for the patient because we use very low laser outputs and do not need to use ionizing radiation.
OCT uses a low-power light source and the corresponding light reflections to create images, which is a method similar to ultrasound, but we monitor light instead of sound. When we project a beam into a sample, much of the light is scattered, but there is a small amount that reflects as a collimated beam, which can be detected and used to create an image.
High-Level System Overview
Our task was to create a high-speed Fourier domain OCT system using optical demultiplexers to enable separation of 256 narrow spectral bands from a broadband incident light in a 25.0 GHz frequency interval centered at 192.2 THz (1559.8 nm wavelength). Separation of the spectrum enabled simultaneous detection of all the bands using the 60 MS/s sample rate of the 256 high-speed analog-to-digital converter (ADC) channels incorporated in the PXI-5105 digitizers for data acquisition.
Our system includes 32 of the eight-channel PXI-5105 digitizers distributed throughout three 18-slot NI PXI-1045 chassis. We synchronized the digitizers in different chassis using NI PXI-6652 timing and synchronization modules and NI-TClk synchronization technology, which provided phase coherency among channels in tens of picoseconds. We selected the PXI-5105 for its high-channel density of eight inputs per board, which enabled the 256 high-speed-channel system to maintain a small footprint. After we acquired the data, we used LabVIEW for processing and visualization of the data.
Using the optical demultiplexers in a Fourier domain OCT system as spectral analyzers, we achieved OCT imaging of 60 million axial scans per second. Using a resonant scanner for lateral scan, 16 kHz frame rate, 1400 A-lines per frame, and a 3 mm depth range, our OCT imaging demonstrated a 23 µm resolution.
In-Depth System Description
In our system, the light source is a broadband superluminescent diode (SLD, prototype by NTT Electronics). We amplify output light from the SLD with a semiconductor optical amplifier (SOA, COVEGA, BOA-1004 type) and divide it equally into the sample arm and reference arm with the coupler (CP1). We adjust the output intensity of light from SOA1 so that the power illuminating the sample is 9 mW, which conforms to the ANSI safety limit. Our system directs the sample arm light onto the sample (S) with a collimator lens (L1) and an objective lens (L2). We use a resonant scanner (RS, Electro-Optical Products, SC-30 type) and a galvano mirror (G, Cambridge Technology, 6210 type) to scan the light beam on the sample. Our system collects back-scattered or back-reflected light from the sample with the light illuminating optics and directs it to SOA2 (COVEGA, BOA 1004 type) with an optical circulator C1. We combine the output of SOA2 and the reference light with a coupler CP2 (50:50 coupling ratio). The reference arm comprises the optical circulator C2, the collimator lens L3, and reference mirror RM.
Our system demultiplexes the outputs from CP2 with two optical demultiplexers (OD1 and OD2) for balanced detection. It detects outputs at the same optical frequency from the two ODs with balanced photoreceivers (New Focus, 2117 type), 256 photoreceivers in total. It detects the outputs of the photoreceivers with the fast multichannel ADC system of 32 PXI-5105 digitizers described above. It stores data in the deep onboard memory of the digitizers during the single-shot acquisition and then transfers it to a computer for analysis.
The OD-OCT is similar to SD-OCT in that it detects the interference spectrum simultaneously. The difference is that it detects a whole interferogram at the speed of data acquisition at different frequencies simultaneously, instead of accumulating it into a CCD detector during certain time duration as in SD-OCT. It therefore determines the axial scan rate by the data acquisition speed of the data acquisition system, which is as fast as 60 MHz in the present system. The 16 KHz speed of the resonant scanner determines the frame rate. We use only one scanning direction for data acquisition (50 percent duty), which led to the sampling time of 31.25 μs per frame. The system acquires 1875 axial scans per frame; however, the lateral scanning with a resonant scanner is highly nonlinear and we use only 1,400 axial scans, discarding 475 axial scans.
We determine the dynamic range as the ratio between the peak value of the point spread function (PSF) and the noise floor when the sample arm is not blocked. From the result, we estimate the dynamic range to be about 40 dB at all depths, slightly decreasing as the depth increases. A merit of OD-OCT is that the spectral width detected at each channel of the optical demultiplexer is narrower than the frequency step of 25 GHz. The dynamic range of 40 dB is marginally sufficient for biological tissue measurements.
In Figure 5, the left vertical axis indicates signal power. We attenuate the reflection 39.3 dB with neutral density filters. The thick solid curve is the noise floor measured with the sample light blocked. Sensitivity determined by these values is scaled at the right-hand side vertical scale.
The penetration depth of the image is about 1 mm, which is shallow compared to the 2 mm penetration depth usually obtained with SS-OCT or SD-OCT. This is due to the low sensitivity. To render a 3D image, a number of OCT cross sections are required. Due to limited memory, we reduced the sampling rate to 10 MHz.
Our team at Kitasato University was able to create the fastest OCT system in the world, achieving a 60 MHz axial scan rate. The end goal of this research is to help detect cancer sooner in patients and increase their quality of life. To create this system, we combined three innovative technologies. The first was the technology from NTT Technologies, which we used as a broadband light source. The second was the signal conditioning system of optical demultiplexers and balanced photoreceivers, which enabled our system to detect the 256 narrow spectrum bands.
Finally, the PXI platform from National Instruments enabled the high-channel-count acquisition because of its synchronization capabilities, small size, and modularity. With the modularity of the PXI platform, our team was able to initially scale from 128 to 256 channels. The platform also provided the ability to scale the system to even higher-channel counts. As the platform adds capabilities with higher-performance instruments and faster data transfer speeds with PXI Express, we can meet future requirements and continue to advance our research.
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