Automated noise estimation in polarization-sensitive optical coherence tomography

Advanced signal reconstruction in polarization-sensitive optical coherence tomography (OCT) frequently relies on an accurate determination of the signal noise floor. However, current methods for evaluating the noise floor are often impractical and subjective. Here we present a method using the degree of polarization uniformity and known speckle intensity statistics to model and estimate the OCT noise floor automatically. We establish the working principle of our method with a series of phantom experiments and demonstrate the robustness of our noise estimation method across different imaging systems and applications in vivo.

Interobserver agreement for the detection of Barrett's esophagus with optical frequency domain imaging

BACKGROUND

Optical frequency domain imaging (OFDI) is a second-generation form of optical coherence tomography (OCT) providing comprehensive cross-sectional views of the distal esophagus at a resolution of ~7 μm.

AIM

Using validated OCT criteria for squamous mucosa, gastric cardia mucosa, and Barrett's esophagus (BE), the objective of this study was to determine the inter- and intra-observer agreements by a large number of OFDI readers for differentiating these tissues.

METHODS

OFDI images were obtained from nine subjects undergoing screening and surveillance for BE. Sixty-four OFDI image regions of interest were randomly selected for review. A training set of 19 images was compiled distinguishing squamous mucosa from gastric cardia and BE using previously validated OCT criteria. The ten readers then interpreted images in a test set of 45 different images of squamous mucosa (n = 15), gastric cardia (n = 15), or BE (n = 15). Interobserver agreement differentiating the three tissue types and BE versus non-BE mucosa was determined using multi-rater Fleiss's κ value. The images were later randomized again and four readers repeated the test 3 weeks later to assess intraobserver reliability.

RESULTS

All ten readers showed excellent agreement for the differentiation of BE versus non-BE mucosa (κ = 0.811 p < 0.0001) and for differentiating BE versus gastric cardia versus squamous mucosa (κ = 0.866, p < 0.0001). For the four readers who repeated the test, the median intraobserver agreement (BE vs. non-BE) was high (κ = 0.975, IQR: 0.94, 1.0).

CONCLUSIONS

Trained readers have a high interobserver agreement for differentiating BE, squamous, and gastric cardia mucosa using OFDI.

Flexible transbronchial optical frequency domain imaging smart needle for biopsy guidance

Transbronchial needle aspiration (TBNA) is a procedure routinely performed to diagnose peripheral pulmonary lesions. However, TBNA is associated with a low diagnostic yield due to inappropriate needle placement. We have developed a flexible transbronchial optical frequency domain imaging (TB-OFDI) catheter that functions as a "smart needle" to confirm the needle placement within the target lesion prior to biopsy. The TB-OFDI smart needle consists of a flexible and removable OFDI catheter (430 µm dia.) that operates within a standard 21-gauge TBNA needle. The OFDI imaging core is based on an angle polished ball lens design with a working distance of 160 µm from the catheter sheath and a spot size of 25 µm. To demonstrate the potential of the TB-OFDI smart needle for transbronchial imaging, an inflated excised swine lung was imaged through a standard bronchoscope. Cross-sectional and longitudinal OFDI results reveal the detailed network of alveoli in the lung parenchyma suggesting that the TB-OFDI smart needle may be a useful tool for guiding biopsy acquisition to increase the diagnostic yield.

Co-registered spectrally encoded confocal microscopy and optical frequency domain imaging system

Spectrally encoded confocal microscopy and optical frequency domain imaging are two non-contact optical imaging technologies that provide images of tissue cellular and architectural morphology, which are both used for histopathological diagnosis. Although spectrally encoded confocal microscopy has better transverse resolution than optical frequency domain imaging, optical frequency domain imaging can penetrate deeper into tissues, which potentially enables the visualization of different morphologic features. We have developed a co-registered spectrally encoded confocal microscopy and optical frequency domain imaging system and have obtained preliminary images from human oesophageal biopsy samples to compare the capabilities of these imaging techniques for diagnosing oesophageal pathology.

Compensation of motion artifacts in catheter-based optical frequency domain imaging

A novel heterodyne Doppler interferometer method for compensating motion artifacts caused by cardiac motion in intracoronary optical frequency domain imaging (OFDI) is demonstrated. To track the relative motion of a catheter with regard to the vessel, a motion tracking system is incorporated with a standard OFDI system by using wavelength division multiplexing (WDM) techniques. Without affecting the imaging beam, dual WDM monochromatic beams are utilized for tracking the relative radial and longitudinal velocities of a catheter-based fiber probe. Our results demonstrate that tracking instantaneous velocity can be used to compensate for distortion in the images due to motion artifacts, thus leading to accurate reconstruction and volumetric measurements with catheter-based imaging.

Doppler imaging using spectrally-encoded endoscopy

The capability to image tissue motion such as blood flow through an endoscope could have many applications in medicine. Spectrally encoded endoscopy (SEE) is a recently introduced technique that utilizes a single optical fiber and miniature diffractive optics to obtain endoscopic images through small diameter probes. Using spectral-domain interferometry, SEE is furthermore capable of three-dimensional volume imaging at video rates. Here we show that by measuring relative spectral phases, this technology can additionally measure Doppler shifts. Doppler SEE is demonstrated in flowing Intralipid phantoms and vibrating middle ear ossicles.

Single-detector polarization-sensitive optical frequency domain imaging using high-speed intra A-line polarization modulation

We demonstrate a novel high-speed polarization-sensitive optical frequency domain imaging system employing high-speed polarization modulation. Rapid and continuous polarization modulation of light prior to illumination of the sample is accomplished by shifting the frequency of one polarization eigenstate by an amount equal to one quarter of the digitization sampling frequency. This approach enables polarization-sensitive imaging with a single detection channel and overcomes artifacts that may arise from temporal variations of the birefringence in fiber-optic imaging probes and spatial variation of birefringence in the sample.

Volumetric sub-surface imaging using spectrally encoded endoscopy

Endoscopic imaging below tissue surfaces and through turbid media may provide improved diagnostic capabilities and visibility in surgical settings. Spectrally encoded endoscopy (SEE) is a recently developed method that utilizes a single optical fiber, miniature optics and a diffractive grating for high-speed imaging through small diameter, flexible endoscopic probes. SEE has also been shown to provide three-dimensional topological imaging capabilities. In this paper, we have configured SEE to additionally image beneath tissue surfaces, by increasing the system's sensitivity and acquiring the complex spectral density for each spectrally resolved point on the sample. In order to demonstrate the capability of SEE to obtain subsurface information, we have utilized the system to image a resolution target through intralipid solution, and conduct volumetric imaging of a mouse embryo and excised human middle-ear ossicles. Our results demonstrate that real-time subsurface imaging is possible with this miniature endoscopy technique.

High-speed polarization sensitive optical frequency domain imaging with frequency multiplexing

Polarization sensitive optical coherence tomography (PS-OCT) provides a cross-sectional image of birefringence in biological samples that is complementary in many applications to the standard reflectance-based image. Recent ex vivo studies have demonstrated that birefringence mapping enables the characterization of collagen and smooth muscle concentration and distribution in vascular tissues. Instruments capable of applying these measurements percutaneously in vivo may provide new insights into coronary atherosclerosis and acute myocardial infarction. We have developed a polarization sensitive optical frequency domain imaging (PS-OFDI) system that enables high-speed intravascular birefringence imaging through a fiber-optic catheter. The novel design of this system utilizes frequency multiplexing to simultaneously measure reflectance of two incident polarization states, overcoming concerns regarding temporal variations of the catheter fiber birefringence and spatial variations in the birefringence of the sample. We demonstrate circular cross-sectional birefringence imaging of a human coronary artery ex vivo through a flexible fiber-optic catheter with an A-line rate of 62 kHz and a ranging depth of 6.2 mm.

Increased ranging depth in optical frequency domain imaging by frequency encoding

A technique for increasing the ranging depth in optical frequency domain imaging utilizing frequency encoding is presented. Ranging depth is enhanced by using two interferometer reference arms with different path lengths and independent modulation frequencies (25 and 50 MHz). With this configuration, the sensitivity decreases by 6 dB over a depth range of 7 mm, approximately a threefold improvement over the conventional optical frequency domain imaging technique. We demonstrate that the reference arm frequency separation, tuning speed, center wavelength, and instantaneous coherence length determine the signal-to-cross-talk ratio.

Estimation of the scattering coefficients of turbid media using angle-resolved optical frequency-domain imaging

Noninvasive measurements of the scattering coefficients of optically turbid media using angle-resolved optical frequency-domain imaging (OFDI) are demonstrated. It is shown that, by incoherently averaging OFDI reflectance signals acquired at different backscattering angles, speckle noise is reduced, allowing scattering coefficients to be extracted from a single A-line with much higher accuracy than with measurements from conventional OFDI and optical coherence tomography systems. Modeling speckle as a random phasor sum, the relationship between the measurement accuracy and the number of compounded angles is derived. The sensitivity analysis is validated with measurements from a tissue phantom.

Angle-resolved optical coherence tomography with sequential angular selectivity for speckle reduction

We present a novel method for rapidly acquiring optical coherence tomography (OCT) images at multiple backscattering angles. By angularly compounding these images, high levels of speckle reduction were achieved. Signal-to-noise ratio (SNR) improvements of 3.4 dB were obtained from a homogeneous tissue phantom, which was in good agreement with the predictions of a statistical model of speckle that incorporated the optical parameters of the imaging system. In addition, the fast acquisition rate of the system (10 kHz A-line repetition rate) allowed angular compounding to be performed in vivo without significant motion artifacts. Speckle-reduced OCT images of human dermis show greatly improved delineation of tissue microstructure.

Large area confocal microscopy

Imaging large tissue areas with microscopic resolution in vivo may offer an alternative to random excisional biopsy. We present an approach for performing confocal imaging of large tissue surface areas using spectrally encoded confocal microscopy (SECM). We demonstrate a single-optical-fiber SECM apparatus, designed for imaging luminal organs, that is capable of imaging with a transverse resolution of 2.1 microm over a subsurface area of 16 cm2 in less than 1 min. Due to the unique probe configuration and scanning geometry, the speed and resolution of this new imaging technology are sufficient for comprehensively imaging large tissues areas at a microscopic scale in times that are appropriate for clinical use.

Image formation in fluorescence coherence-gated imaging through scattering media

Recently, we have experimentally demonstrated a new form of cross-sectional, coherence-gated fluorescence imaging referred to as SD-FCT ('spectral-domain fluorescence coherence tomography'). Imaging in SD-FCT is accomplished by spectrally detecting self-interference of the spontaneous emission of fluorophores, thereby providing depth-resolved information on the axial positions of fluorescent probes. Here, we present a theoretical investigation of the factors affecting the detected SD-FCT signal through scattering media. An imaging equation for SD-FCT is derived that includes the effects of defocusing, numerical-aperture, and the optical properties of the medium. A comparison between the optical sectioning capabilities of SD-FCT and confocal microscopy is also presented. Our results suggest that coherence gating in fluorescence imaging may provide an improved approach for depth-resolved imaging of fluorescently labeled samples; high axial resolution (a few microns) can be achieved with low numerical apertures (NA<0.09) while maintaining a large depth of field (a few hundreds of microns) in a relatively low scattering medium (6 mean free paths), whereas moderate NA's can be used to enhance depth selectivity in more highly scattering biological samples.

Speckle Reduction in OCT using Massively-Parallel Detection and Frequency-Domain Ranging

Speckle noise significantly limits the information content provided by coherent optical imaging methods such as optical coherence tomography and its recent derivative, optical frequency-domain imaging (OFDI). In this paper, we demonstrate a novel OFDI system that simultaneously acquires hundreds of angularly resolved images, which can be compounded to reduce speckle noise. The system comprises an InGaAs line-scan camera and an interferometer, configured so that the elements of the detector array simultaneously capture light spanning a backscattering angular range of 32 degrees. On successive read-outs of the array, the wavelength of the laser source was stepped through a range of 130 nm centered at 1295 nm to concurrently generate 400 angle-resolved OFDI images. A theory of angle-resolved OFDI and the design equations of the system are presented. Incoherent averaging of the angle-resolved data is shown to yield substantial speckle reduction (as high as an 8 dB SNR improvement) in images of a tissue phantom and esophageal tissue ex vivo.

Numerical study of wavelength-swept semiconductor ring lasers: the role of refractive-index nonlinearities in semiconductor optical amplifiers and implications for biomedical imaging applications

Recent results have demonstrated unprecedented wavelength-tuning speed and repetition rate performance of semiconductor ring lasers incorporating scanning filters. However, several unique operational characteristics of these lasers have not been adequately explained, and the lack of an accurate model has hindered optimization. We numerically investigated the characteristics of these sources, using a semiconductor optical amplifier (SOA) traveling-wave Langevin model, and found good agreement with experimental measurements. In particular, we explored the role of the SOA refractive-index nonlinearities in determining the intracavity frequency-shift-broadening and the emitted power dependence on scan speed and direction. Our model predicts both continuous-wave and pulse operation and shows a universal relationship between the output power of lasers that have different cavity lengths and the filter peak frequency shift per round trip, therefore revealing the advantage of short cavities for high-speed biomedical imaging.

Elimination of depth degeneracy in optical frequency-domain imaging through polarization-based optical demodulation

A novel optical frequency-domain imaging system is demonstrated that employs a passive optical demodulation circuit and a chirped digital acquisition clock derived from a voltage-controlled oscillator. The demodulation circuit allows the separation of signals from positive and negative depths to better than 50 dB, thereby eliminating depth degeneracy and doubling the imaging depth range. Our system design is compatible with dual-balanced and polarization-diverse detection, important techniques in the practical biomedical application of optical frequency-domain imaging.

Ultrahigh-resolution full-field optical coherence microscopy using InGaAs camera

Full-field optical coherence microscopy (FFOCM) is an interferometric technique for obtaining wide-field microscopic images deep within scattering biological samples. FFOCM has primarily been implemented in the 0.8 mum wavelength range with silicon-based cameras, which may limit penetration when imaging human tissue. In this paper, we demonstrate FFOCM at the wavelength range of 0.9 - 1.4 mum, where optical penetration into tissue is presumably greater owing to decreased scattering. Our FFOCM system, comprising a broadband spatially incoherent light source, a Linnik interferometer, and an InGaAs area scan camera, provided a detection sensitivity of 86 dB for a 2 sec imaging time and an axial resolution of 1.9 mum in water. Images of phantoms, tissue samples, and Xenopus Laevis embryos were obtained using InGaAs and silicon camera FFOCM systems, demonstrating enhanced imaging penetration at longer wavelengths.

Ultra-high speed and ultra-high resolution spectral-domain optical coherence tomography and optical Doppler tomography in ophthalmology

We present ultra-high resolution optical coherence tomography (OCT) structural intensity and optical Doppler tomography (ODT) flow velocity images of the human retina in vivo. The ultra-high speed OCT system is based on Spectral Domain or Fourier Domain technology, which provides a sensitivity advantage over conventional OCT of more than 2 orders of magnitude. This sensitivity improvement allows video rate OCT and ODT cross sectional imaging of retinal structures. Images will be presented with axial resolutions of 6 and 3.5 microns. We observed small features in the inner and outer plexiform layers, which are believed to be small blood vessels. Flow velocity images will be presented showing pulsatile flow in retinal arteries and veins.

115 kHz tuning repetition rate ultrahigh-speed wavelength-swept semiconductor laser

We demonstrate an ultrahigh-speed wavelength-swept semiconductor laser using a polygon-based wavelength scanning filter. With a polygon rotational speed of 900 revolutions per second, a continuous wavelength tuning rate of 9200 nm/ms and a tuning repetition rate of 115 kHz were achieved. The wavelength tuning range of the laser was 80 nm centered at 1325 nm, and the average polarized output power was 23 mW.

Three-dimensional imaging using spectral encoding heterodyne interferometry

We present a novel heterodyne approach for performing fast, three-dimensional spectrally encoded imaging. Volumetric data of a volunteer's finger and of coin surfaces were acquired at a rate of 5 volume sets per second with a depth resolution of 145 microm.

Wide Tuning Range Wavelength-Swept Laser With Two Semiconductor Optical Amplifiers

We demonstrate a wide tuning range high-speed wavelength-swept semiconductor laser based on a polygon scanning filter that is common to two laser cavities. Linear wavelength tuning was achieved over 145 nm around 1310 nm at a tuning repetition rate of 20 kHz. The wavelength tuning filter is expandable to accommodate multiple semiconductor optical amplifiers for further widening of the laser wavelength tuning range.

Double-clad fiber for endoscopy

Endoscopes employing a single optical fiber may have advantages over conventional fiber-bundle or CCD array imaging techniques, including the potential for greater flexibility and miniaturization. Although single-mode fibers can provide superior resolution compared with multimode fibers, they are prone to increased speckle noise and suffer from limited optical throughput and reduced depth of field. We demonstrate the use of a double-clad fiber for single-mode illumination and multimode detection to achieve high-resolution, reduced-speckle imaging with high optical throughput and a large depth of field.

Extended-Cavity Semiconductor Wavelength-Swept Laser for Biomedical Imaging

We demonstrate a compact high-power rapidly swept wavelength tunable laser source based on a semiconductor optical amplifier and an extended-cavity grating filter. The laser produces excellent output characteristics for biomedical imaging, exhibiting >4-mW average output power, <0.06-nm instantaneous linewidth, and >80-dB noise extinction with its center wavelength swept over 100 nm at 1310 nm at variable repetition rates up to 500 Hz.

Three-dimensional spectrally encoded imaging

A method for three-dimensional surface measurements with phase-sensitive spectrally encoded imaging is demonstrated. Both transverse and depth information is transmitted through a single-mode optical fiber, allowing this scheme to be incorporated into a miniature probe. This approach is demonstrated by measurement of the profile of a lens surface and by three-dimensional imaging of the face of a small doll.

High-speed optical frequency-domain imaging

We demonstrate high-speed, high-sensitivity, high-resolution optical imaging based on optical frequency-domain interferometry using a rapidly-tuned wavelength-swept laser. We derive and show experimentally that frequency-domain ranging provides a superior signal-to-noise ratio compared with conventional time-domain ranging as used in optical coherence tomography. A high sensitivity of -110 dB was obtained with a 6 mW source at an axial resolution of 13.5 microm and an A-line rate of 15.7 kHz, representing more than an order-of-magnitude improvement compared with previous OCT and interferometric imaging methods.

High-speed wavelength-swept semiconductor laser with a polygon-scanner-based wavelength filter

Ultrahigh-speed tuning of an extended-cavity semiconductor laser is demonstrated. The laser resonator comprises a unidirectional fiber-optic ring, a semiconductor optical amplifier as the gain medium, and a novel scanning filter based on a polygonal scanner. Variable tuning rates up to 1150 nm/ms (15.7-kHz repetition frequency) are demonstrated over a 70-nm wavelength span centered at 1.32 microm. This tuning rate is more than an order of magnitude faster than previously demonstrated and is facilitated in part by self-frequency shifting in the semiconductor optical amplifier. The instantaneous linewidth of the source is <0.1 nm for 9-mW cw output power and a low spontaneous-emission background of -80 dB.

Speckle reduction in optical coherence tomography by "path length encoded" angular compounding

Speckle, the dominant factor reducing image quality in optical coherence tomography (OCT), limits the ability to identify cellular structures that are essential for diagnosis of a variety of diseases. We describe a new high-speed method for implementing angular compounding by path length encoding (ACPE) for reducing speckle in OCT images. By averaging images obtained at different incident angles, with each image encoded by path length, ACPE maintains high-speed image acquisition and requires minimal modifications to OCT probe optics. ACPE images obtained from tissue phantoms and human skin in vivo demonstrate a qualitative improvement over traditional OCT and an increased SNR that correlates well with theory.

Evaluation of intracoronary stenting by intravascular optical coherence tomography

BACKGROUND

Conventional contrast cineangiography and intravascular ultrasound (IVUS) provide a limited definition of vessel microstructure and are unable to evaluate dissection, tissue prolapse, and stent apposition on a size scale less than 100 micro m.

OBJECTIVE

To evaluate the use of intravascular optical coherence tomography (OCT) to assess the coronary arteries in patients undergoing coronary stenting.

METHODS

OCT was employed in patients having percutaneous coronary interventions. Images were obtained before initial balloon dilatation and following stent deployment, and were evaluated for vessel dissection, tissue prolapse, stent apposition, and stent asymmetry. IVUS images were obtained before OCT, using an automatic pull back device.

RESULTS

42 stents were imaged in 39 patients without complications. Dissection, prolapse, and incomplete stent apposition were observed more often with OCT than with IVUS. Vessel dissection was identified in eight stents by OCT and two by IVUS. Tissue prolapse was identified in 29 stents by OCT and 12 by IVUS; the extent of the prolapse (mean (SD)) was 242 (156) microm by OCT and 400 (100) microm by IVUS. Incomplete stent apposition was observed in seven stents by OCT and three by IVUS. Irregular strut separation was identified in 18 stents by both OCT and IVUS.

CONCLUSIONS

Intracoronary OCT for monitoring stent deployment is feasible and provides superior contrast and resolution of arterial pathology than IVUS.

Spectrally encoded miniature endoscopy

A method for performing miniature endoscopy with a high number of resolvable points is presented. This approach, spectrally encoded endoscopy (SEE), uses a broad-bandwidth light source and a diffraction grating to simultaneously detected the reflectivity at multiple points along a transverse line within the sample. As opposed to images from miniature optical fiber bundle endoscopes, the number of resolvable points in SEE images is dependent on the spectral width and the groove density of the diffraction grating. We acquired images of a human finger in vivo, using a 550-mu;m -diameter SEE system to demonstrate the feasibility of this technique.

Diagnosis of specialized intestinal metaplasia by optical coherence tomography

BACKGROUND AND AIMS

Optical coherence tomography (OCT) is an imaging technique that produces high-resolution cross-sectional images in vivo. The aim of this study was to establish the sensitivity and specificity of OCT for diagnosing specialized intestinal metaplasia (SIM).

METHODS

OCT was used to image the stomach and esophagus of 121 patients. A total of 288 biopsy-correlated OCT images were acquired. OCT criteria for SIM were formulated by analyzing 75 images of SIM. The SIM image criteria were retrospectively tested by applying them to images of gastric, squamous, SIM, and cardiac epithelium. The criteria were then tested prospectively to determine the sensitivity and specificity of OCT for diagnosing SIM.

RESULTS

OCT images of SIM are characterized by (1) absence of the layered structure of normal squamous epithelium and the vertical "pit and crypt" morphology of gastric mucosa, (2) disorganized architecture with inhomogeneous tissue contrast and an irregular mucosal surface, and (3) presence of submucosal glands. These criteria were 100% sensitive and 93% specific for SIM when applied retrospectively and 97% sensitive and 92% specific when tested prospectively.

CONCLUSIONS

OCT is highly sensitive and specific for SIM and may aid in the diagnosis and surveillance of this preneoplastic lesion.

Optical coherence tomography in the gastrointestinal tract

Optical coherence tomography (OCT) is a high-resolution, cross-sectional optical imaging technique that allows in situ imaging of tissue by measuring back-reflected light. OCT provides images in real time with a resolution approaching that of conventional histopathology, but without the need for tissue removal. OCT imaging can be performed endoscopically to visualize gastrointestinal tissue using a fiberoptic catheter passed through the instrument channel of a conventional endoscope. The resolution of OCT allows visualization of the different layers of gastrointestinal epithelium and the differentiation of Barrett's epithelium from normal gastric and squamous mucosa. OCT has also been used to image esophageal adenocarcinoma and colonic polyps. Recent developments include Doppler OCT, spectroscopic OCT, and ultrahigh-resolution OCT, which can visualize nuclei within single cells. Although still in its infancy as a clinical tool, OCT currently provides high-resolution images over the same imaging depth as conventional mucosal biopsy, and may prove to be a useful and minimally invasive technique for evaluating gastrointestinal tissue, particularly for early neoplastic changes.

Porcine coronary imaging in vivo by optical coherence tomography

OBJECTIVE

A high-resolution coronary artery imaging modality has the potential to address important diagnostic and management problems in cardiology. Optical coherence tomography (OCT) is a promising new optical imaging technique with a resolution of approximately 10 microm. The purpose of this study was to use a new OCT catheter to demonstrate the feasibility of performing OCT imaging of normal coronary arteries, intimal dissections, and deployed stents in vivo.

METHODS AND RESULTS

Normal coronary arteries, intimal dissections, and stents were imaged in five swine with OCT and compared with intravascular ultrasound (IVUS). In the normal coronary arteries, visualization of all of the layers of the vessel wall was achieved with a saline flush, including the intima which was not identified by IVUS. Following dissection, detailed layered structures including intimal flaps, intimal defects, and disruption of the medial wall were visualized by OCT. IVUS failed to show clear evidence of intimal and medial disruption. Finally, the microanatomic relationships between stents and the vessel walls were clearly identified only by OCT.

CONCLUSIONS

In this preliminary experiment, we have demonstrated that in vivo OCT imaging of normal coronary arteries, intimal dissections, and deployed stents is feasible, and allows identification of clinically relevant coronary artery morphology with high-resolution and contrast.

High-resolution imaging of the human esophagus and stomach in vivo using optical coherence tomography

BACKGROUND

Optical coherence tomography is a new, high spatial-resolution, cross-sectional imaging technique. We investigated the ability of optical coherence tomography to provide detailed images of subsurface structures in the upper gastrointestinal (GI) tract.

METHODS

Optical coherence tomography was performed during routine upper GI endoscopy on 32 patients including 20 patients with Barrett's esophagus. An endoscopic mucosal biopsy was obtained immediately after imaging and was used for histopathologic correlation.

RESULTS

Optical coherence tomography provided clear delineation of layers of the normal human esophagus extending from the epithelium to the longitudinal muscularis propria. Gastric mucosa was differentiated from esophageal mucosa, Barrett's esophagus was differentiated from normal esophageal mucosa, and esophageal adenocarcinoma was distinguished from normal esophagus and Barrett's esophagus.

CONCLUSIONS

Optical coherence tomography allows visualization of the subsurface architectural morphology of the upper GI tract. The diagnostic information provided by this new imaging modality suggests that it may be a useful adjunct to endoscopy.

High resolution in vivo intra-arterial imaging with optical coherence tomography

BACKGROUND

Optical coherence tomography (OCT) is a new method of catheter based micron scale imaging. OCT is analogous to ultrasound, measuring the intensity of backreflected infrared light rather than sound waves.

OBJECTIVE

To demonstrate the ability of OCT to perform high resolution imaging of arterial tissue in vivo.

METHODS

OCT imaging of the abdominal aorta of New Zealand white rabbits was performed using a 2.9 F OCT imaging catheter. Using an ultrashort pulse laser as a light source for imaging, an axial resolution of 10 micrometer was achieved.

RESULTS

Imaging was performed at 4 frames/second and data were saved in either super VHS or digital format. Saline injections were required during imaging because of the signal attenuation caused by blood. Microstructure was sharply defined within the arterial wall and correlated with histology. Some motion artefacts were noted at 4 frames/second.

CONCLUSIONS

In vivo imaging of the rabbit aorta was demonstrated at a source resolution of 10 micrometer, but required the displacement of blood with saline. The high resolution of OCT allows imaging to be performed near the resolution of histopathology, offering the potential to have an impact both on the identification of high risk plaques and the guidance of interventional procedures.

Power-efficient nonreciprocal interferometer and linear-scanning fiber-optic catheter for optical coherence tomography

A nonreciprocal fiber-optic interferometer is demonstrated in an optical coherence tomography (OCT) system. The increased power efficiency of this system provides a 4.1-dB advantage over standard Michelson implementations. In addition, a new linear-scanning fiber-optic catheter is demonstrated that avoids the rotary optical junction that is required in circumferential scanning systems. These advancements have permitted the clinical implementation of OCT imaging in the human gastrointestinal tract.

Low-repetition-rate high-peak-power Kerr-lens mode-locked TiAl(2)O(3) laser with a multiple-pass cavity

We demonstrate a novel, long, multiple-pass cavity (MPC) to obtain low repetition rates and high peak intensities from Kerr-lens mode-locked lasers. We show that the MPC provides a zero effective length by a unity transformation of the q parameter after a given number of transits of the laser beam. Pulse durations of 16.5 fs with 0.7 MW of power at a 15-MHz repetition rate are achieved. This is, to our knowledge, the lowest repetition rate ever achieved directly from a femtosecond laser resonator without use of additional active devices and cavity dumping. The combination of low repetition rates and high peak intensity is extremely useful for femtosecond pump-probe and other nonlinear experiments because it permits the application of high peak intensity without excessive average power.

High resolution imaging of normal and osteoarthritic cartilage with optical coherence tomography

OBJECTIVE

We describe optical coherence tomography (OCT), a high resolution micron scale imaging technology, for assessment of osteoarthritic articular cartilage microstructure. OCT is analogous to ultrasound, measuring the intensity of backreflected infrared light rather than acoustical waves.

METHODS

OCT imaging was performed on over 100 sites on 20 normal and osteoarthritic cartilage specimens in vitro.

RESULTS

Microstructures that were identified included fibrillations, fibrosis, cartilage thickness, and new bone growth at resolutions between 5 and 15 microm. In addition, the polarization sensitivity of imaging suggested a diagnostic role of polarization spectroscopy.

CONCLUSION

OCT represents an attractive new technology for intraarticular imaging due to its high resolution (greater than any available clinical technology), ability to be integrated into small arthroscopes, compact portable design, and relatively low cost.

Spectrally encoded confocal microscopy

An endoscope-compatible, submicrometer-resolution scanning confocal microscopy imaging system is presented. This approach, spectrally encoded confocal microscopy (SECM), uses a quasi-monochromatic light source and a transmission diffraction grating to detect the reflectivity simultaneously at multiple points along a transverse line within the sample. Since this method does not require fast spatial scanning within the probe, the equipment can be miniaturized and incorporated into a catheter or endoscope. Confocal images of an electron microscope grid were acquired with SECM to demonstrate the feasibility of this technique.

Intraoperative assessment of microsurgery with three-dimensional optical coherence tomography

PURPOSE

To evaluate three-dimensional optical coherence tomography (OCT) for use in the assessment of the microsurgical anastomoses of vessels and nerves.

MATERIALS AND METHODS

OCT is an optical analogue of ultrasonography and is capable of imaging nontransparent biologic tissue by detecting backscattered infrared light. Cross-sectional in vitro images of rabbit and human vessels and nerves were obtained in as little as 125 msec at 10-micron resolution by using a solid-state laser as a light source. A surgical microscope was integrated with OCT to perform simultaneous imaging with en face visualization. Cross-sectional images were assembled to produce three-dimensional reconstructions of microsurgical specimens.

RESULTS

Three-dimensional OCT reconstructions depicted the structure within an arterial anastomosis and helped identify sites of luminal obstruction. The longitudinal spatial orientation of individual nerve fascicles was tracked in three dimensions to identify changes in position. In vitro human arteries and nerves embedded in highly scattering tissue and not visible at microscopy were located and imaged with OCT at eight frames per second.

CONCLUSION

The three-dimensional, micrometer-scale, diagnostic imaging capabilities of OCT permit rapid feedback for assessment of microsurgical procedures. OCT technology can be readily integrated with surgical microscopes and has potential for intraoperative monitoring to improve patient outcome.

Two- and three-dimensional high-resolution imaging of the human oviduct with optical coherence tomography

OBJECTIVE

To evaluate the feasibility of optical coherence tomography, a new method of micron-scale imaging, for high-resolution assessment of the oviduct. Optical coherence tomography is analogous to ultrasound except that it measures the backreflection of infrared light rather than acoustical waves.

DESIGN

The ampulla of a human fallopian tube was imaged in vitro using optical coherence tomography. Images were generated in 2 and 3 dimensions.

SETTING

University.

PATIENT(S)

Samples were obtained from women who had undergone hysterectomy for leiomyomatosis.

INTERVENTION(S)

None

MAIN OUTCOME MEASURE(S)

The ability to perform imaging on a micron scale, which is a level of resolution higher than that of any currently available clinical technology.

RESULT(S)

Two- and three-dimensional data sets of the reflectance of a human fallopian tube were acquired. A volume of 5 x 5 x 2.5 mm (length x width x depth) was scanned. The axial resolution was 11 microm, and the lateral resolution at the focus was 20 microm. The data sets showed detailed structures of the fallopian tube.

CONCLUSION(S)

Our ability to obtain micron-scale two- and three-dimensional images of an in vitro oviduct suggests that it may be possible to identify and surgically treat tubal causes of infertility.

Optical biopsy in human pancreatobiliary tissue using optical coherence tomography

Optical coherence tomography (OCT) is a new technique for performing high-resolution, cross-sectional tomographic imaging in human tissue. OCT is analogous to ultrasound B mode imaging except that it uses light rather than acoustical waves. As a result, OCT has over 10 times the resolution of currently available clinical high-resolution cross-sectional imaging technologies. In this work, we investigate the capability of OCT to differentiate the architectural morphology of pancreatobiliary tissues. Normal pancreatobiliary tissues, including the gallbladder, common bile duct, pancreatic duct, and pancreas were taken postmortem and imaged using OCT. Images were compared to corresponding histology to confirm tissue identity. Microstructure was delineated in different tissues, including tissue layers, glands, submucosal microvasculature, and pancreatic islets of Langerhans. The ability of OCT to provide high-resolution imaging of pancreatobiliary architectural morphology suggests the feasibility of using OCT as a powerful diagnostic endoscopic imaging technology to image early stages of pancreatobiliary disease.

High resolution imaging of the upper respiratory tract with optical coherence tomography: a feasibility study

A need exists in respiratory medicine for a technology capable of identifying airway pathology on a micron scale. This study has demonstrated the feasibility of optical coherence tomography (OCT) for ultrahigh resolution imaging of the upper respiratory tract by in vitro studies of human tissue. OCT is a relatively new technique that can be used to noninvasively collect tomographic images of tissue microstructure with micron-scale resolution. OCT is analogous to ultrasound, measuring the intensity of infrared light rather than acoustical waves. Samples throughout the upper respiratory tract, from the epiglottis to the secondary bronchi, were imaged. The resulting images were compared with histopathology and verified the ability of OCT to delineate relevant structures such as the epithelium, mucosa, cartilage and its sublayers, and glands at a resolution higher than any clinical imaging technology. The ability of OCT to generate image resolution in the range close to that of histopathology in real time, as well as easy integration with small, relatively inexpensive endoscopes, low cost, and lack of a need for a transducing medium, supports the hypothesis that this optical technology could become a powerful modality in the diagnosis and management of a wide range of clinical respiratory pathology.

Optical Coherence Tomographic Imaging of Human Tissue at 1.55 μm and 1.81 μm Using Er- and Tm-Doped Fiber Sources

We demonstrate two short-coherence-length, rare-earth-doped fiber optical sources for performing optical coherence tomography (OCT) in human tissue. The first source is a stretched-pulse, mode-locked Er-doped fiber laser with a center wavelength of 1.55 μm, a power of 100 mW, and a bandwidth of 80 nm. The second is a Tm-doped silica fiber fluorescent source emitting up to 7 mW of power at 1.81 μm with a bandwidth of 80 nm. The OCT imaging depth of penetration in in vitro human aorta is compared using these sources and conventional 1.3-μm sources. © 1998 Society of Photo-Optical Instrumentation Engineers.

High-speed phase- and group-delay scanning with a grating-based phase control delay line

A rapid-scanning optical delay line that employs phase control has several advantages, including high speed, high duty cycle, phase- and group-delay independence, and group-velocity dispersion compensation, over existing optical delay methods for interferometric optical ranging applications. We discuss the grating-based phase-control delay line and its applications to interferometric optical ranging and measurement techniques such as optical coherence domain reflectometry and optical coherence tomography. The system performs optical ranging over an axial range of 3 mm with a scanning rate of 6m/s and a repetition rate of 2 kHz. The device is especially well suited for applications such as optical coherence tomography that require high-speed, repetitive, linear delay line scanning with a high duty cycle.

Optical frequency-domain reflectometry using rapid wavelength tuning of a Cr4+:forsterite laser

We present a cw chromium-doped forsterite laser that permits rapid wavelength tuning over a broad bandwidth and demonstrate the application of this source to frequency-domain ranging and optical tomography. The entire tuning range of 1200 to 1275 nm can be swept in less than 500 micros . This permits frequency-domain ranging to be performed with a scan rate of 2 kHz and an axial resolution of 15 microm .

Forward-imaging instruments for optical coherence tomography

We discuss the design and implementation of forward-imaging instruments for optical coherence tomography (OCT), which require the delivery, scanning, and collection of single-spatial-mode optical radiation. A hand-held surgical probe for use in open surgery can provide cross-sectional images of subsurface tissue before surgical incisions are made. A rigid laparoscope for minimally invasive surgical OCT imaging provides a simultaneous enface view of the area being imaged. OCT imaging is demonstrated on in vitro human specimens.