Mark E. Brezinski

Optical coherence tomography for optical biopsy : Properties and demonstration of vascular pathology

BACKGROUND Optical coherence tomography (OCT) is an recently developed medical diagnostic technology that uses back-reflected infrared light to perform in situ micron scale tomographic imaging. In this work, we investigate the ability of OCT to perform micron scale tomographic imaging of the internal microstructure of in vitro atherosclerotic plaques. METHODS AND RESULTS Aorta and relevant nonvascular tissue were obtained at autopsy. Two-dimensional cross-sectional imaging of the exposed surface of the arterial segments was performed in vitro with OCT. A 1300-nm wavelength, superluminescent diode light source was used that allows an axial spatial resolution of 20 microns. The signal-to-noise ratio was 109 dB. Images were displayed in gray scale or false color, Imaging was performed over 1.5 mm into heavily calcified tissue, and a high contrast was noted between lipid- and water-based constituents, making OCT attractive for intracoronary imaging. The 20-microns axial resolution of OCT allowed small structural details such as the width of intimal caps and the presence of fissures to be determined. The extent of lipid collections, which had a low backscattering intensity, also were well documented. CONCLUSIONS OCT represents a promising new technology for imaging vascular microstructure with a level of resolution not previously achieved with the use of other imaging modalities. It does not required direct contact with the vessel wall and can be performed with a catheter integrated with a relatively inexpensive optical fiber. The high contrast among tissue constituents, high resolution, and ability to penetrate heavily calcified tissue make OCT an attractive new imaging technology for intracoronary diagnostics.

Determination of the refractive index of highly scattering human tissue by optical coherence tomography.

We describe two new techniques, based on optical coherence tomography (OCT), for determining the refractive index of highly scattering human tissue. We obtained refractive indices of in vitro human tissue, using OCT to measure the physical and optical path lengths of the sample. We obtained measurements of the refractive index of in vitro and in vivo human tissue, using OCT to track the focal length shift that results from translating the focus along the optic axis within the tissue. The refractive indices of human skin, adipose, and muscle were measured and compared with previously published estimates.

Single mode fiber-optic catheter/endoscope for optical coherence tomography

Summary form only given. In order to apply OCT for imaging of internal organ systems, a flexible, small diameter, catheter/endoscope, which is capable of delivering, focusing, scanning, and collecting a single-spatial-mode optical beam, must be constructed. In this summary, we describe the design and performance of a prototype single-mode fiber-optic scanning OCT catheter with a diameter of 1 mm. OCT imaging may be performed at 1.3-micron wavelengths using either a superluminescent laser diode source or a Kerr-lens mode-locked Cr:forsterite laser, which provides high powers for high-speed imaging. This device is an enabling technology for OCT and will permit micron scale, cross-sectional medical diagnostic imaging in tissues such as the vascular system, the gastrointestinal tract, the urinary tract, and the respiratory tract.

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 μm 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 μm, 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.

HIGH-RESOLUTION OPTICAL COHERENCE TOMOGRAPHIC IMAGING USING A MODE-LOCKED TI:AL2O3 LASER SOURCE

A Kerr-lens mode-locked Ti:Al(2)O(3) oscillator, optimized for minimal coherence length, is demonstrated as a high-power source for high-resolution optical coherence tomographic imaging. Dispersion compensation and heterodyne noise rejection are demonstrated to yield in situ images of biological tissues with 3.7-mum resolution and 93-dB dynamic range.

Assessing atherosclerotic plaque morphology: comparison of optical coherence tomography and high frequency intravascular ultrasound.

BACKGROUND: OCT can image plaque microstructure at a level of resolution not previously demonstrated with other imaging techniques because it uses infrared light rather than acoustic waves. OBJECTIVES: To compare optical coherence tomography (OCT) and intravascular ultrasound (IVUS) imaging of in vitro atherosclerotic plaques. METHODS: Segments of abdominal aorta were obtained immediately before postmortem examination. Images of 20 sites from five patients were acquired with OCT (operating at an optical wavelength of 1300 nm which was delivered to the sample through an optical fibre) and a 30 MHz ultrasonic transducer. After imaging, the microstructure of the tissue was assessed by routine histological processing. RESULTS: OCT yielded superior structural information in all plaques examined. The mean (SEM) axial resolution of OCT and IVUS imaging was 16 (1) and 110 (7), respectively, as determined by the point spread function from a mirror. Furthermore, the dynamic range of OCT was 109 dB compared with 43 dB for IVUS imaging. CONCLUSIONS: OCT represents a promising new technology for intracoronary imaging because of its high resolution, broad dynamic range, and ability to be delivered through intravascular catheters.

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.

Optical coherence tomography: high-resolution imaging in nontransparent tissue

Optical coherence tomography (OCT) is a method of high-resolution imaging originally developed for the transparent tissue of the eye. Recently, the technology has been advanced toward the difficult challenge of imaging in nontransparent tissue. In the paper, three topics are addressed. First, the principles behind OCT imaging are discussed. Second, the difficulties associated with OCT imaging in nontransparent tissue are outlined. Finally, the feasibility of OCT for medical imaging is discussed. Specifically, OCT demonstrates its greatest potential in situations where conventional biopsy is either dangerous or ineffective.

Imaging of coronary artery microstructure (in vitro) with optical coherence tomography

OCT achieves high-resolution and image differentiation of vascular tissues to a degree that has not been previously possible with any method except excisional biopsy. Thus, OCT represents a promising new diagnostic technology for intracoronary imaging, which could permit the in vivo evaluation of critical vascular pathology.

Optical coherence tomographic elastography technique for measuring deformation and strain of atherosclerotic tissues

Objectives: To evaluate optical coherence tomographic elastography as a method for assessing the elastic properties of atherosclerotic plaque and the parameters that influence interpretation. Methods: Phantoms and aorta were examined in vitro to quantify speckle modulation and measure the displacement and strain maps. A correlation method was used as a speckle tracking technique for measuring axial and lateral displacement vectors and calculation of strain maps. The influence of correlation kernel size on accuracy of the method was evaluated. Results: In terms of a percentage error between calculated and measured displacements, the best results for phantoms were obtained with a 41 × 41 kernel (1.88% error). For both phantom and aorta images, it was found that, with the increasing size of cross correlation kernel, the axial and lateral displacement maps are less noisy and the displacement vectors are more clearly defined. However, the large kernels tend to average out the differences in displacements of small particles in phantoms and decrease the ability of speckle tracking to make microstructural assessments. Therefore, it is important to select kernel size carefully, based on the image features. Conclusions: Optical tomographic elastography can be used to assess the microstructural properties of atherosclerotic tissue at micrometre scale resolution, but preselected analysis criteria must be understood in a critical interpretation of the results.