High-speed Spectral Domain Optical Coherence Research Articles

Optical Coherence Tomography Angiography of the Intestine: How to Prevent Motion Artifacts in Open and Laparoscopic Surgery?

(1) Introduction. The problem that limits the intraoperative use of OCTA for the intestinal circulation diagnostics is the low informative value of OCTA images containing too many motion artifacts. The aim of this study is to evaluate the efficiency and safety of the developed unit for the prevention of the appearance of motion artifacts in the OCTA images of the intestine in both open and laparoscopic surgery in the experiment; (2) Methods. A high-speed spectral-domain multimodal optical coherence tomograph (IAP RAS, Russia) operating at a wavelength of 1310 nm with a spectral width of 100 μm and a power of 2 mW was used. The developed unit was tested in two groups of experimental animals-on minipigs (group I, n = 10, open abdomen) and on rabbits (group II, n = 10, laparoscopy). Acute mesenteric ischemia was modeled and then 1 h later the small intestine underwent OCTA evaluation. A total of 400 OCTA images of the intact and ischemic small intestine were obtained and analyzed. The quality of the obtained OCTA images was evaluated based on the score proposed in 2020 by the group of Magnin M. (3) Results. Without stabilization, OCTA images of the intestine tissues were informative only in 32-44% of cases in open surgery and in 14-22% of cases in laparoscopic surgery. A vacuum bowel stabilizer with a pressure deficit of 22-25 mm Hg significantly reduced the number of motion artifacts. As a result, the proportion of informative OCTA images in open surgery increased up to 86.5% (Χ2 = 200.2, p = 0.001), and in laparoscopy up to 60% (Χ2 = 148.3, p = 0.001). (4) Conclusions. The used vacuum tissue stabilizer enabled a significant increase in the proportion of informative OCTA images by significantly reducing the motion artifacts.

Evaluation of choriocapillaris with high-speed spectral-domain optical coherence tomography angiography and image averaging

To study the capabilities of high-speed spectral-domain optical coherence tomography angiography (SD-OCTA) with image averaging in the evaluation of choriocapillaries in healthy individuals and patients with central serous chorioretinopathy (CSC). All participants underwent OCTA examination on the SOLIX tomograph (Optovue, USA) using quadruplicate 3×3 mm scan with image averaging. Image analysis was performed in 9-µm custom slab with Phansalkar auto local threshold algorithm and calculation of flow voids larger than 5000 µm2 and 10 000 µm2. The optimal slab depth was determined in healthy eyes by consecutive analysis of the slabs with a 3-µm shift from zero position to 33 µm below Bruch's membrane. The study included 18 eyes of 18 healthy volunteers (40.4±6.0 years old) and 18 fellow eyes of 18 unilateral CSC patients (37.4±10.7 years old). The slab 12 µm below the Bruch's membrane showed the minimal number of flow voids (23.1±7.0 of >5000 µm2 voids per scan) and was chosen for further analysis. The number of flow voids of >5000 µm2 in healthy paired eyes of CSC patients was statistically significantly higher than in the eyes of healthy individuals (32.7±10.7 and 25.3±8.1 voids/scan, respectively; p=0.022). The number of flow voids of >10000 µm2 was also statistically significantly higher in CSC eyes compared to healthy eyes (5.6±3.3 and 3.5±1.9 voids/scan, respectively; p=0.045). There was no statistically significant difference in total area of the voids between the eyes of healthy individuals and CSC patients (96406.1±3924.5 µm2 and 95395.7 ± 3615.1 µm2, respectively; p=0.42). The optimal settings for choriocapillaris imaging on the SOLIX tomograph include 9-µm slab 9 to 18 µm below the Bruch's membrane. Using a 9-µm slab 12 µm below the Bruch's membrane, a substantial difference was found in choriocapillaris perfusion between eyes of healthy individuals and CSC patients.

In situ measurements of layer roughness during laser powder bed fusion additive manufacturing using low coherence scanning interferometry

Layer-to-layer height measurements of additively manufactured 316L stainless steel using high speed spectral-domain optical coherence tomography (SD-OCT) are presented. Layers are built up using an open architecture laser powder bed fusion machine while height measurements are made in-line along the process laser path following each layer print. Printed cubes, with and without an internal ‘overhang’ channel, were built to investigate the effect of scanning parameters on surface structure. Layer-to-layer scan rotation strategy significantly impacts surface roughness between layers which in turn can influence porosity. Spatter particles, which have been correlated with numerous defect modalities, generate high points in the powder bed and can persist on a melted surface for many layers. Laser power significantly affects overhang morphology, as measured by SD-OCT. Large dross occurs in the high energy density regime, while balling and a capillary-driven coalescence of unstable melt pools perpendicular to the scanning direction occurs in the low energy density regime. High fidelity powder-scale simulations of deep powder layers were used to further elucidate the underlying physics revealed by SD-OCT measurements and high speed imaging, yielding insight to defect formation mechanisms which can lead to improved process parameters.

Measuring flow dynamics in a microfluidic chip using optical coherence tomography with 1 µm axial resolution

An experimental verification of simulated flow dynamics in a novel design of microfluidic device can be difficult, especially, in devices such as mixers or deterministic lateral displacement (DLD) arrays. Doppler optical coherence tomography (DOCT) is one of the visualization modalities used to measure flow velocity profiles in microfluidic channels, capillaries and microvascular networks. Previously conducted flow measurements with high speed spectral-domain optical coherence tomography (SD-OCT) devices have had typical axial resolution in the range few microns which can be insufficient for accurate mapping of flow dynamics in microchannels. This paper introduces a high-speed SD-OCT system with an axial resolution of 1.2µm in air (below 1µm in water) to evaluate flow in a microfluidic chip. The A-line acquisition rate of the device is 123kHz. Flow velocities from 1% Intralipid suspension are obtained across the microfluidic channel with the specified height of 20µm and the width of 50µm. The results show that the utilized SD-OCT device can visualize flows as close as 1µm from the channel wall. The flow rate of 0.63µl/min, determined from the cross-sectional velocity map, agreed well with the value of 0.67µl/min set to the syringe pump.

Ultrahigh-resolution, high-speed spectral domain optical coherence phase microscopy

We present an ultrahigh-resolution, high-speed spectral domain optical coherence phase microscopy (SD-OCPM) system that combines submicrometer transverse spatial resolution and subnanometer optical path length sensitivity, with an acquisition speed of over 217,000 voxels/s. The proposed SD-OCPM system overcomes two significant drawbacks of traditional common-path interferometers-limited transverse spatial resolution and suboptimal detection sensitivity-while maintaining phase stability that is comparable with common-path interferometer setups. The transverse and axial spatial resolution of the setup is measured to be 0.6 and 1.9 μm, respectively, with a phase sensitivity of 0.0027 rad (corresponds to optical path length sensitivity of 110 pm). High-speed acquisition allows for phase-sensitive 4D imaging of biological samples with subcellular resolution.

Noninvasive imaging of pulsatile movements of the optic nerve head in normal human subjects using phase-sensitive spectral domain optical coherence tomography

We report use of high-speed spectral domain optical coherence tomography to noninvasively image pulsatile axial movement of the optic nerve head (ONH) in normal human subjects. Time-lapse B-scan mode is used to image the ONH at 500 frames per second. Capture of phase differences between adjacent B-scans permits extraction of axial ONH movement. We find the ONH experiences continuous oscillatory axial motion that is strongly correlated with simultaneously measured pulsatile blood flow in the central retinal artery.

High-speed spectral domain optical coherence tomography using non-uniform fast Fourier transform

The useful imaging range in spectral domain optical coherence tomography (SD-OCT) is often limited by the depth dependent sensitivity fall-off. Processing SD-OCT data with the non-uniform fast Fourier transform (NFFT) can improve the sensitivity fall-off at maximum depth by greater than 5dB concurrently with a 30 fold decrease in processing time compared to the fast Fourier transform with cubic spline interpolation method. NFFT can also improve local signal to noise ratio (SNR) and reduce image artifacts introduced in post-processing. Combined with parallel processing, NFFT is shown to have the ability to process up to 90k A-lines per second. High-speed SD-OCT imaging is demonstrated at camera-limited 100 frames per second on an ex-vivo squid eye.

Volumetric in vivo imaging of intracochlear microstructures in mice by high-speed spectral domain optical coherence tomography.

There is considerable interest in developing new methods for in vivo imaging of the complex anatomy of the mammalian cochlea for clinical as well as fundamental studies. In this study, we explored, the feasibility of spectral domain optical coherence tomography (SD-OCT) for 3-D in vivo imaging of the cochlea in mice. The SD-OCT system employed in this study used a broadband light source centered at 1300 nm, and the imaging speed of the system was 47,000 A-scans per second using the InGaAs camera. The system was capable of providing fully processed, high-resolution B-scan images [512 (axial) x 128 (lateral) pixels] at 280 frames per sec. The 3-D imaging acquisition time for a whole cochlea was approximately 0.45 sec. The traditional SD-OCT structural imaging algorithm was used to reconstruct 3-D cochlear morphology. We demonstrated that SD-OCT can be successfully used for in vivo imaging of important morphological features within the mouse cochlea, such as the otic capsule and structures within, including Reissner's membrane, the basilar membrane, tectorial membrane, organ of Corti, and modiolus of the apical and middle turns.

Analysis of the Outer Retina Reconstructed by High-Resolution, Three-Dimensional Spectral Domain Optical Coherence Tomography

A retrospective cross-sectional study was conducted to demonstrate an analysis of an outer retinal layer reconstructed by the three-dimensional and high-speed spectral domain optical coherence tomography (SD-OCT) instrument. New measurement protocols for SD-OCT and methods of analysis and visualization of the individual segmented retinal layer reconstructed by SD-OCT were proposed. Three contour maps representing mutual distances between the basal part of the retinal pigment epithelium, the junction between the inner and outer segments of photoreceptors, and a reference contour representing the shape of a healthy retina were introduced. The analysis of the outer retina was performed on pathological eyes. Three cases of central serous chorioretinopathy, age-related macular degeneration, and acute zonal occult outer retinopathy are demonstrated. Three contour maps reconstructed for clinical cases demonstrate high variability of observed patterns depending on analyzed pathology. The authors believe this can help to present OCT data simultaneously in a more comprehensive and convenient way to assist in everyday clinical diagnosis.

Linear-wavenumber spectrometer for high-speed spectral-domain optical coherence tomography

An equidistant (in the wavenumber) spectrometer based on a diffraction grating, a compensation prism, and a CCD linear array is developed and implemented for spectral-domain optical coherence tomography. A criterion is introduced for estimating the level of residual nonequidistance. This criterion allows one to determine the threshold compensation level necessary for obtaining the spectrally limited spatial resolution. The method is tested in spectral-domain optical coherent tomography systems at wavelengths of 1270 and 830 nm.

Phase-Contrast OCT Imaging of Transverse Flows in the Mouse Retina and Choroid

To test the hypothesis that a novel phase-contrast optical coherence tomography (OCT) system can image retinal and choroidal vessels in the living mouse. A high-speed spectral domain optical coherence tomography (SDOCT) system, which measures the reflections for the entire depth of the retina at once with each axial scan (A-scan), was developed for mouse retinal imaging. Acquiring multiple A-scans over a transverse line across the mouse retina offers a two-dimensional cross-sectional image (B-scan); several neighboring B-scans can be assembled into a three-dimensional OCT image. To visualize mobility and transverse flow in retinal vessels, the statistical variance of phase for each location was calculated from multiple B-scans acquired successively for the same retinal cross-section. Such measures of phase variance offer a direct measure of motions over a large dynamic range of flow velocities. Three-dimensional phase-contrast images of the live mouse retina were created using multiple two-dimensional cross-sectional image slices through the retina. For the data presented here, each cross-sectional phase-contrast slice resulted from five images of 100 or 200 transverse pixels, acquired over 25 ms or 50 ms, respectively. The approach offered clear identification of motion regions at different depths, including flow in the retinal microvasculature and in the choroidal vessels. Phase-contrast OCT enables three-dimensional visualization of retinal and choroidal vasculature in vivo.

Optical coherence angiography

Noninvasive angiography is demonstrated for the in vivo human eye. Three-dimensional flow imaging has been performed with high-speed spectral-domain optical coherence tomography. Sample motion is compensated by two algorithms. Axial motion between adjacent A-lines within one OCT image is compensated by the Doppler shift due to bulk sample motion. Axial displacements between neighboring images are compensated by a correlation-based algorithm. Three-dimensional vasculature of ocular vessels has been visualized. By integrating volume sets of flow images, two-dimensional images of blood vessels are obtained. Retinal and choroidal blood vessel images are simultaneously obtained by separating the volume set into retinal part and choroidal parts. These are corresponding to fluorescein angiogram and indocyanine angiogram.

Simultaneous acquisition of sectional and fundus ophthalmic images with spectral-domain optical coherence tomography

A high-speed spectral-domain optical coherence tomography (OCT) system was built to image the human retina in vivo. A fundus image similar to the intensity image produced by a scanning laser ophthalmoscope (SLO) was generated from the same spectra that were used for generating the OCT sectional images immediately after the spectra were collected. This function offers perfect spatial registration between the sectional OCT images and the fundus image, which is desired in ophthalmology for monitoring data quality, locating pathology, and increasing reproducibility. This function also offers a practical way to detect eye movements that occur during the acquisition of the OCT image. The system was successfully applied to imaging human retina in vivo.

High-speed spectral-domain optical coherence tomography at 13 μm wavelength

We demonstrate a high-speed spectral domain optical coherence tomography (SD-OCT) system capable of acquiring individual axial scans in 24.4 micros at a rate of 19,000 axial scans per second, using an InGaAs line scan camera and broadband light source centered at 1.31microm. Sensitivity of >105 dB over a 2-mm depth range was obtained with a free-space axial resolution of 12-14 microm, in agreement with our signal-to-noise ratio predictions. Images of human tissue obtained in vivo with SD-OCT show similar penetration depths to those obtained with state-of-the-art time domain OCT despite the ten-fold higher image acquisition speed. These results demonstrate the potential of 1.3 microm SD-OCT for high-speed and high-sensitivity imaging in patients.