Intraluminal fiber-optic Doppler imaging catheter for structural and functional optical coherence tomography

In Vivo Quantitative Imaging of Coronary Blood Flow by Intracoronary Doppler OCT

OBJECTIVE

This work aims to develop a novel approach for in vivo quantitative imaging of coronary blood flow using intracoronary Doppler OCT.

METHODS

A 200 kHz swept-source OCT system with an endoscopic catheter probe was used to acquire both structural and Doppler OCT images. In the Doppler measurement, raw OCT phase signal was processed with timing variation and motion compensation to reduce the phase noise. Furthermore, the Doppler measurement angle was inferred by pullback image data and used to correct the measurement error due to oblique imaging caused by non-coaxial alignment of the imaging catheter. To validate our system, the flow rate measurement was performed using an ex vivo phantom and in vivo experiments were conducted in living swine coronary arteries.

RESULTS

After oblique imaging compensation, the measurement error was reduced from 3.0±11.4% to 0.7±8.1%. A minor systematic difference of 4.2±8.7% was found between preset (0 - 240 ml/min) and calculated flow rate for all the measurements. The feasibility of imaging coronary flow in vivo was successfully demonstrated in a living swine.

CONCLUSION

The experimental results show that Doppler OCT can be an accurate approach for quantitative assessment of the intracoronary flow in vivo.

SIGNIFICANCE

To our knowledge, this is the first in vivo catheter-based Doppler OCT coronary blood flow measurement.

Optimized light delivery probe using ball lenses for co-registered photoacoustic and ultrasound endo-cavity subsurface imaging

An optimized hand-held photoacoustic and ultrasound probe suitable for endo-cavity tumor subsurface imaging was designed and evaluated. Compared to previous designs, the prototype probe, consisting of four 1 mm multi-mode optical fibers attached with 1.5 mm diameter ball-shaped fiber tips sandwiched between a transvaginal ultrasound transducer and a custom-made sheath, demonstrated a higher light output and better beam homogeneity on tissue subsurface. The output power and fluence profile were simulated for different design parameters. A camera recorded fluence profiles through calibrated intralipid solution at various imaging depths. The light delivery efficiency was experimentally compared with and without the ball lenses, based on ex-vivo imaging of human colorectal cancer and in-vivo imaging of a palmar vein proximal to the human wrist. The simulations and experiments demonstrated that ball-shaped fiber tip design can achieve homogeneous fluence distribution on tissue subsurface with acceptable light output efficiency, suggesting its clinical potential for in-vivo endo-cavity imaging.

Endoscopic optical coherence tomography: technologies and clinical applications [Invited]

In this paper, we review the current state of technology development and clinical applications of endoscopic optical coherence tomography (OCT). Key design and engineering considerations are discussed for most OCT endoscopes, including side-viewing and forward-viewing probes, along with different scanning mechanisms (proximal-scanning versus distal-scanning). Multi-modal endoscopes that integrate OCT with other imaging modalities are also discussed. The review of clinical applications of endoscopic OCT focuses heavily on diagnosis of diseases and guidance of interventions. Representative applications in several organ systems are presented, such as in the cardiovascular, digestive, respiratory, and reproductive systems. A brief outlook of the field of endoscopic OCT is also discussed.

Intravascular optical coherence tomography [Invited]

Shortly after the first demonstration of optical coherence tomography for imaging the microstructure of the human eye, work began on developing systems and catheters suitable for intravascular imaging in order to diagnose and investigate atherosclerosis and potentially to monitor therapy. This review covers the driving considerations of the clinical application and its constraints, the major engineering milestones that enabled the current, high-performance commercial imaging systems, the key studies that laid the groundwork for image interpretation, and the clinical research that traces intravascular optical coherence tomography (OCT) from early human pilot studies to current clinical trials.

Ultrathin lensed fiber-optic probe for optical coherence tomography

We investigated and validated a novel method to develop ultrathin lensed fiber-optic (LFO) probes for optical coherence tomography (OCT) imaging. We made the LFO probe by attaching a segment of no core fiber (NCF) to the distal end of a single mode fiber (SMF) and generating a curved surface at the tip of the NCF using the electric arc of a fusion splicer. The novel fabrication approach enabled us to control the length of the NCF and the radius of the fiber lens independently. By strategically choosing these two parameters, the LFO probe could achieve a broad range of working distance and depth of focus for different OCT applications. A probe with 125μm diameter and lateral resolution up to 10μm was demonstrated. The low-cost, disposable and robust LFO probe is expected to have great potential for interstitial OCT imaging.

Heartbeat OCT: in vivo intravascular megahertz-optical coherence tomography

Cardiac motion artifacts, non-uniform rotational distortion and undersampling affect the image quality and the diagnostic impact of intravascular optical coherence tomography (IV-OCT). In this study we demonstrate how these limitations of IV-OCT can be addressed by using an imaging system that we called "Heartbeat OCT", combining a fast Fourier Domain Mode Locked laser, fast pullback, and a micromotor actuated catheter, designed to examine a coronary vessel in less than one cardiac cycle. We acquired in vivo data sets of two coronary arteries in a porcine heart with both Heartbeat OCT, working at 2.88 MHz A-line rate, 4000 frames/s and 100 mm/s pullback speed, and with a commercial system. The in vivo results show that Heartbeat OCT provides faithfully rendered, motion-artifact free, fully sampled vessel wall architecture, unlike the conventional IV-OCT data. We present the Heartbeat OCT system in full technical detail and discuss the steps needed for clinical translation of the technology.

En face Doppler total retinal blood flow measurement with 70 kHz spectral optical coherence tomography

An automated algorithm was developed for total retinal blood flow (TRBF) using 70-kHz spectral optical coherence tomography (OCT). The OCT was calibrated for the transformation from Doppler shift to speed based on a flow phantom. The TRBF scan pattern contained five repeated volume scans (2 x 2 mm) obtained in 3 s and centered on central retinal vessels in the optic disc. The TRBF was calculated using an en face Doppler technique. For each retinal vein, blood flow was measured at an optimal plane where the calculated flow was maximized. The TRBF was calculated by summing flow in all veins. The algorithm tracked vascular branching so that either root or branch veins are summed, but never both. The TRBF in five repeated volumes were averaged to reduce variation due to cardiac cycle pulsation. Finally, the TRBF was corrected for eye length variation. Twelve healthy eyes and 12 glaucomatous eyes were enrolled to test the algorithm. The TRBF was 45.4 ± 6.7 μl/min for healthy control and 34.7 ± 7.6 μl/min for glaucomatous participants (p-value = 0.01). The intravisit repeatability was 8.6% for healthy controls and 8.4% for glaucoma participants. The proposed automated method provided repeatable TRBF measurement.

Quantitative technique for robust and noise-tolerant speed measurements based on speckle decorrelation in optical coherence tomography

Intensity-based techniques in optical coherence tomography (OCT), such as those based on speckle decorrelation, have attracted great interest for biomedical and industrial applications requiring speed or flow information. In this work we present a rigorous analysis of the effects of noise on speckle decorrelation, demonstrate that these effects frustrate accurate speed quantitation, and propose new techniques that achieve quantitative and repeatable measurements. First, we derive the effect of background noise on the speckle autocorrelation function, finding two detrimental effects of noise. We propose a new autocorrelation function that is immune to the main effect of background noise and permits quantitative measurements at high and moderate signal-to-noise ratios. At the same time, this autocorrelation function is able to provide motion contrast information that accurately identifies areas with movement, similar to speckle variance techniques. In order to extend the SNR range, we quantify and model the second effect of background noise on the autocorrelation function through a calibration. By obtaining an explicit expression for the decorrelation time as a function of speed and diffusion, we show how to use our autocorrelation function and noise calibration to measure a flowing liquid. We obtain accurate results, which are validated by Doppler OCT, and demonstrate a very high dynamic range (> 600 mm/s) compared to that of Doppler OCT (±25 mm/s). We also derive the behavior for low flows, and show that there is an inherent non-linearity in speed measurements in the presence of diffusion due to statistical fluctuations of speckle. Our technique allows quantitative and robust measurements of speeds using OCT, and this work delimits precisely the conditions in which it is accurate.

Fiber-optic polarization diversity detection for rotary probe optical coherence tomography

We report a polarization diversity detection scheme for optical coherence tomography with a new, custom, miniaturized fiber coupler with single mode (SM) fiber inputs and polarization maintaining (PM) fiber outputs. The SM fiber inputs obviate matching the optical lengths of the X and Y OCT polarization channels prior to interference and the PM fiber outputs ensure defined X and Y axes after interference. Advantages for this scheme include easier alignment, lower cost, and easier miniaturization compared to designs with free-space bulk optical components. We demonstrate the utility of the detection system to mitigate the effects of rapidly changing polarization states when imaging with rotating fiber optic probes in Intralipid suspension and during in vivo imaging of human airways.

High speed, wide velocity dynamic range Doppler optical coherence tomography (Part IV): split spectrum processing in rotary catheter probes

We report a technique for blood flow detection using split spectrum Doppler optical coherence tomography (ssDOCT) that shows improved sensitivity over existing Doppler OCT methods. In ssDOCT, the Doppler signal is averaged over multiple sub-bands of the interferogram, increasing the SNR of the Doppler signal. We explore the parameterization of this technique in terms of number of sub-band windows, width and overlap of the windows, and their effect on the Doppler signal to noise in a flow phantom. Compared to conventional DOCT, ssDOCT processing has increased flow sensitivity. We demonstrate the effectiveness of ssDOCT in-vivo for intravascular flow detection within a porcine carotid artery and for microvascular vessel detection in human pulmonary imaging, using rotary catheter probes. To our knowledge, this is the first report of visualizing in-vivo Doppler flow patterns adjacent to stent struts in the carotid artery.

Sapphire ball lensed fiber probe for common-path optical coherence tomography in ocular imaging and sensing

We describe a novel common-path optical coherence tomography (CP-OCT) fiber probe design using a sapphire ball lens for cross-sectional imaging and sensing in retina vitrectomy surgery. Single mode Gaussian beam (TEM00) simulation was used to optimize lateral resolution and working distance (WD) of the common-path probe. A theoretical sensitivity model for CP-OCT was prosed to assess its optimal performance based an unbalanced photodetector configuration. Two probe designs with working distances (WD) 415μm and 1221μm and lateral resolution 11μm and 18μm, respectively were implemented with sensitivity up to 88dB. The designs are also fully compatible with conventional Michelson interferometer based OCT configurations. The reference plane of the probe, located at the distal beam exit interface of the single mode fiber (SMF), was encased within a 25-gauge hypodermic needle by the sapphire ball lens facilitates its applications in bloody and harsh environments. The performances of the fiber probe with 11μm of lateral resolution and 19μm of axial resolution were demonstrated by cross-sectional imaging of a cow cornea and retina in vitro with a 1310nm swept source OCT system. This probe was also attached to a piezoelectric motor for active compensation of physiological tremor for handheld retinal surgical tools.

Sapphire ball lens-based fiber probe for common-path optical coherence tomography and its applications in corneal and retinal imaging

We describe a common-path swept source optical coherence tomography fiber probe design using a sapphire ball lens for cross-sectional imaging and sensing for retina vitrectomy surgery. The high refractive index (n=1.75) of the sapphire ball lens improves the focusing power and enables the probe to operate in the intraocular space. The highly precise spherical shape of the sapphire lens also reduces astigmatism and coma compared to fused nonspherical ball lenses. A theoretical sensitivity model for common-path optical coherence tomography (CP-OCT) was developed to assess its optimal performance based on an unbalanced photodetector configuration. Two probe designs-with working distances 415 and 1221 μm and lateral resolution 11 and 18 μm-were implemented with sensitivity up to 88 dB, which is significantly higher than previously reported CP-OCT probes. We assessed the performances of the fiber probes by cross-sectional imaging a bovine cornea and retina in air and in vitreous gel with a 1310 nm swept source OCT system. To the best of our knowledge, this is the first demonstration of sapphire ball lens-based CP-OCT probes directly inserted into the vitreous gel of a bovine eyeball for ocular imaging with a sensitivity approaching the theoretical limitation of CP-OCT.

In vivo feasibility of endovascular Doppler optical coherence tomography

Feasibility of detecting intravascular flow using a catheter based endovascular optical coherence tomography (OCT) system is demonstrated in a porcine carotid model in vivo. The effects of A-line density, radial distance, signal-to-noise ratio, non-uniform rotational distortion (NURD), phase stability of the swept wavelength laser and interferometer system on Doppler shift detection limit were investigated in stationary and flow phantoms. Techniques for NURD induced phase shift artifact removal were developed by tracking the catheter sheath. Detection of high flow velocity (~51 cm/s) present in the porcine carotid artery was obtained by phase unwrapping techniques and compared to numerical simulation, taking into consideration flow profile distortion by the eccentrically positioned imaging catheter. Using diluted blood in saline mixture as clearing agent, simultaneous Doppler OCT imaging of intravascular flow and structural OCT imaging of the carotid artery wall was feasible. To our knowledge, this is the first in vivo demonstration of Doppler imaging and absolute measurement of intravascular flow using a rotating fiber catheter in carotid artery.

Ex vivo imaging of human thyroid pathology using integrated optical coherence tomography and optical coherence microscopy

We evaluate the feasibility of optical coherence tomography (OCT) and optical coherence microscopy (OCM) for imaging of benign and malignant thyroid lesions ex vivo using intrinsic optical contrast. 34 thyroid gland specimens are imaged from 17 patients, covering a spectrum of pathology ranging from normal thyroid to benign disease/neoplasms (multinodular colloid goiter, Hashimoto's thyroiditis, and follicular adenoma) and malignant thyroid tumors (papillary carcinoma and medullary carcinoma). Imaging is performed using an integrated OCT and OCM system, with <4 microm axial resolution (OCT and OCM), and 14 microm (OCT) and <2 microm (OCM) transverse resolution. The system allows seamless switching between low and high magnifications in a way similar to traditional microscopy. Good correspondence is observed between optical images and histological sections. Characteristic features that suggest malignant lesions, such as complex papillary architecture, microfollicules, psammomatous calcifications, or replacement of normal follicular architecture with sheets/nests of tumor cells, can be identified from OCT and OCM images and are clearly differentiable from normal or benign thyroid tissues. With further development of needle-based imaging probes, OCT and OCM could be promising techniques to use for the screening of thyroid nodules and to improve the diagnostic specificity of fine needle aspiration evaluation.

High power wavelength linearly swept mode locked fiber laser for OCT imaging

We report a long coherence length, high power, and wide tuning range wavelength linearly swept fiber mode-locked laser based on polygon scanning filters. An output power of 52.6 mW with 112 nm wavelength tuning range at 62.6 kHz sweeping rate has been achieved. The coherence length is long enough to enable imaging over 8.1 mm depth when the sensitivity decreases by 8.7 dB (1/e(2)). The Fourier components are still distinguishable when the ranging depth exceeds 15 mm, which corresponds to 30 mm optical path difference in air. The parameters that are critical to OCT imaging quality such as polygon filter linewidth, the laser coherence length, output power, axial resolution and the Fourier sensitivity have been investigated theoretically and experimentally. Since the wavelength is swept linearly with time, an analytical approach has been developed for transforming the interference signal from equidistant spacing in wavelength to equidistant spacing in frequency. Axial resolution of 7.9 microm in air has been achieved experimentally that approaches the theoretical limit.

Super-Achromatic Rapid Scanning Microendoscope for Ultrahigh-Resolution OCT Imaging

Microendoscope is a critical technology to enable high-resolution imaging of internal luminal organs with optical coherence tomography. This paper reports the development of an achromatic compound microlens and a rapid scanning microendoscope based on the microlens that offers an ultrahigh transverse resolution of 4 μm (and an axial resolution of 2.2 μm when using a low-coherence light source with a broad spectrum bandwidth of 150 nm). The rapid scanning endoscope is capable of ultrahigh-resolution (UHR) optical coherence tomography (OCT) imaging in real time at an imaging speed of about 1220 lateral scans/s, with the image quality comparable to a slow bench-top UHR-OCT system (~100 scans/s). The superior performance of a scanning endoscope made of a compound microlens over an endoscope made of a conventional GRIN (gradient index) lens in imaging biological tissues has also been demonstrated.

In vivo ultrahigh-resolution optical coherence tomography of mouse colon with an achromatized endoscope

Endoscopic ultrahigh-resolution optical coherence tomography (OCT) enables collection of minimally invasive cross-sectional images in vivo, which may be used to facilitate rapid development of reliable mouse models of colon disease as well as assess chemopreventive and therapeutic agents. The small physical scale of mouse colon makes light penetration less problematic than in other tissues and high resolution acutely necessary. In our 2-mm diameter endoscopic time domain OCT system, isotropic ultrahigh-resolution is supported by a center wavelength of 800 nm and full-width-at-half-maximum bandwidth of 150 nm (mode-locked titanium:sapphire laser) combined with 1:1 conjugate imaging of a small core fiber. A pair of KZFSN5/SFPL53 doublets provides excellent color correction to support wide bandwidth throughout the imaging depth. A slight deviation from normal beam exit angle suppresses collection of the strong back reflection at the exit window surface. Our system achieves axial resolution of 3.2 microm in air and 4.4-microm lateral spot diameter with 101-dB sensitivity. Microscopic features too small to see in mouse tissue with conventional resolution systems, including colonic crypts, are clearly resolved. Resolution near the cellular level is potentially capable of identifying abnormal crypt formation and dysplastic cellular organization.

Inverse scattering for rotationally scanned optical coherence tomography

Optical coherence tomography of luminal structures, such as for intravascular or gastrointestinal imaging, is performed by using a fiber-optic catheter as a beam-delivery probe. The interrogating beam is scanned angularly by rotating the fiber around a fixed central axis. Because the beam is focused only at a fixed distance from the center of the fiber, only scatterers near this distance are resolved. We present a solution of the inverse scattering problem that provides an estimate of the susceptibility of the sample for an angularly scanned Gaussian beam focused at a fixed distance from the origin. This solution provides quantitatively meaningful reconstructions while also extending the volume of the sample that is resolvable by the instrument.

Technology Insight: optical coherence tomography--current status and future development

The understanding of concepts in coronary artery disease, such as the vulnerable or high-risk plaque, which accounts for many acute coronary events arising from non-flow-limiting coronary lesions, has advanced remarkably. Although coronary angiography is an established imaging technique for visualizing atherosclerotic disease, it is limited by its two-dimensional imaging aspect and a low sensitivity for identifying lesions in the presence of positive remodeling and diffuse disease. Moreover, coronary atherosclerotic plaques cannot be characterized. Although intravascular ultrasound is currently the most commonly employed adjunctive method to better define lesions, it is limited by low resolution. The development of new technologies for improved coronary plaque characterization has, thus, been desired. Optical coherence tomography is a developing technique that uses near-infrared light for the cross-sectional visualization of the vessel wall at the microscopic level. It enables excellent resolution of coronary architecture and precise characterization of plaque architecture. Quantification of macrophages within the plaque is also possible. These capabilities allow precise identification of the most common type of vulnerable plaque, the thin-cap fibroatheroma. Here, we discuss results from clinical studies which indicate that optical coherence tomography is a promising imaging technique for improved characterization of the coronary atherosclerotic plaque.

High speed, wide velocity dynamic range Doppler optical coherence tomography (Part III): in vivo endoscopic imaging of blood flow in the rat and human gastrointestinal tracts

We previously described a fiber based Doppler optical coherence tomography system [1] capable of imaging embryo cardiac blood flow at 4~16 frames per second with wide velocity dynamic range [2]. Coupling this system to a linear scanning fiber optical catheter design that minimizes friction and vibrations, we report here the initial results of in vivo endoscopic Doppler optical coherence tomography (EDOCT) imaging in normal rat and human esophagus. Microvascular flow in blood vessels less than 100 microm diameter was detected using a combination of color-Doppler and velocity variance imaging modes, during clinical endoscopy using a mobile EDOCT system.