Jennifer K. Barton

Nanoshell-Enabled Photonics-Based Imaging and Therapy of Cancer

Metal nanoshells are a novel type of composite spherical nanoparticle consisting of a dielectric core covered by a thin metallic shell which is typically gold. Nanoshells possess highly favorable optical and chemical properties for biomedical imaging and therapeutic applications. By varying the relative the dimensions of the core and the shell, the optical resonance of these nanoparticles can be precisely and systematically varied over a broad region ranging from the near-UV to the mid-infrared. This range includes the near-infrared (NIR) wavelength region where tissue transmissivity peaks. In addition to spectral tunability, nanoshells offer other advantages over conventional organic dyes including improved optical properties and reduced susceptibility to chemical/thermal denaturation. Furthermore, the same conjugation protocols used to bind biomolecules to gold colloid are easily modified for nanoshells. In this article, we first review the synthesis of gold nanoshells and illustrate how the core/shell ratio and overall size of a nanoshell influences its scattering and absorption properties. We then describe several examples of nanoshell-based diagnostic and therapeutic approaches including the development of nanoshell bioconjugates for molecular imaging, the use of scattering nanoshells as contrast agents for optical coherence tomography (OCT), and the use of absorbing nanoshells in NIR thermal therapy of tumors.

IN VIVO BIDIRECTIONAL COLOR DOPPLER FLOW IMAGING OF PICOLITER BLOOD VOLUMES USING OPTICAL COHERENCE TOMOGRAPHY

We describe a novel optical system for bidirectional color Doppler imaging of flow in biological tissues with micrometer-scale resolution and demonstrate its use for in vivo imaging of blood flow in an animal model. Our technique, color Doppler optical coherence tomography (CDOCT), performs spatially localized optical Doppler velocimetry by use of scanning low-coherence interferometry. CDOCT is an extension of optical coherence tomography (OCT), employing coherent signal-acquisition electronics and joint time-frequency analysis algorithms to perform flow imaging simultaneous with conventional OCT imaging. Cross-sectional maps of blood flow velocity with <50-microm spatial resolution and <0.6-mm/s velocity precision were obtained through intact skin in living hamster subdermal tissue. This technology has several potential medical applications.

Use of an agent to reduce scattering in skin.

A method to increase light transport deeply into target areas of tissue would enhance both therapeutic and diagnostic laser applications. The effects of a hyperosmotic agent on the scattering properties of rat and hamster skin were investigated.

Recruitment of tendon crimp with applied tensile strain

The tensile stress-strain behavior of ligaments and tendons begins with a toe region that is believed to result from the straightening of crimped collagen fibrils. The in situ mechanical function is mostly confined to this toe region and changes in crimp morphology are believed to be associated with pathological conditions. A relatively new imaging technique, optical coherence tomography (OCT), provides a comparatively inexpensive method for nondestructive investigation of tissue ultrastructure with resolution on the order of 15 microm and the potential for use in a clinical setting. The objectives of this work were to assess the utility of OCT for visualizing crimp period, and to use OCT to determine how crimp period changed as a function of applied tensile strain in rat tail tendon fascicles. Fascicles from rat tail tendons were subjected to 0.5 percent strain increments up to 5 percent and imaged at each increment using OCT. A comparison between OCT images and optical microscopy images taken between crossed polarizing lenses showed a visual correspondence between features indicative of crimp pattern. Crimp pattern always disappeared completely before 3 percent axial strain was reached. Average crimp period increased as strain increased, but both elongation and shortening occurred within single crimp periods during the application of increasing strain to the fascicle.

Real-time in vivo color Doppler optical coherence tomography

Color Doppler optical coherence tomography (CDOCT) is a functional extension of optical coherence tomography (OCT) that can image flow in turbid media. We have developed a CDOCT system capable of imaging flow in real time. Doppler processing of the analog signal is accomplished in hardware in the time domain using a novel autocorrelation technique. This Doppler processing method is compatible with a high speed OCT system capable of imaging in real time. Using this system, we demonstrate cross-sectional imaging of bidirectional flow with CDOCT at four frames per second in a tissue-simulating phantom consisting of intralipid solution flowing in glass capillaries. As a demonstration of real-time imaging of blood flow in vivo we imaged pulsatible blood flow in a rat femoral artery at eight frames per second. Issues of velocity sensitivity, imaging speed, and range of velocity measurement are discussed, as well as potential applications of real-time CDOCT.

Texture analysis of optical coherence tomography images: feasibility for tissue classification

Optical coherence tomography (OCT) acquires cross-sectional images of tissue by measuring back-reflected light. Images from in vivo OCT systems typically have a resolution of 10 to 15 mm, and are thus best suited for visualizing structures in the range of tens to hundreds of microns, such as tissue layers or glands. Many normal and abnormal tissues lack visible structures in this size range, so it may appear that OCT is unsuitable for identification of these tissues. However, examination of structure-poor OCT images reveals that they frequently display a characteristic texture that is due to speckle. We evaluated the application of statistical and spectral texture analysis techniques for differentiating tissue types based on the structural and speckle content in OCT images. Excellent correct classification rates were obtained when images had slight visual differences (mouse skin and fat, correct classification rates of 98.5 and 97.3%, respectively), and reasonable rates were obtained with nearly identical-appearing images (normal versus abnormal mouse lung, correct classification rates of 64.0 and 88.6%, respectively). This study shows that texture analysis of OCT images may be capable of differentiating tissue types without reliance on visible structures.

Miniature endoscope for simultaneous optical coherence tomography and laser-induced fluorescence measurement

We have designed a multimodality system that combines optical coherence tomography (OCT) and laser-induced fluorescence (LIF) in a 2.0-mm-diameter endoscopic package. OCT provides approximately 18-microm resolution cross-sectional structural information over a 6-mm field. LIF spectra are collected sequentially at submillimeter resolution across the same field and provide histochemical information about the tissue. We present the use of a rod prism to reduce the asymmetry in the OCT beam caused by a cylindrical window. The endoscope has been applied to investigate mouse colon cancer in vivo.

Investigating pulsed dye laser-blood vessel interaction with color Doppler optical coherence tomography

A non-invasive method of imaging laser irradiated blood vessels has been achieved using Color Doppler Optical Coherence Tomography (CDOCT). This method may increase understanding of the mechanisms behind treatment of vascular disorders. The CDOCT system used a 1280 nm center wavelength superluminescent diode. A 585 nm, 360 aes pulsed dye laser was used to irradiate hamster dorsal skin flap window preparations. Irradiation sites were imaged with CDOCT prior to, immediately after, and 24 hours after laser irradiation. The processed CDOCT signal provided an estimate of the blood flow velocity. An increase in the blood vessel backscattered signal was observed as blood or vessel walls were coagulated. A decrease in damaged blood vessel reflectivity occurred after twenty four hours.

Optical coherence tomography with plasmon resonant nanorods of gold

We explored plasmon resonant nanorods of gold as a contrast agent for optical coherence tomography (OCT). Nanorod suspensions were generated through wet chemical synthesis and characterized with spectrophotometry, transmission electron microscopy, and OCT. Polyacrylamide-based phantoms were generated with appropriate scattering and anisotropy coefficients (30 cm(-1) and 0.89, respectively) to image distribution of the contrast agent in an environment similar to that of tissue. The observed signal was dependent on whether the plasmon resonance peak overlapped the source bandwidth of the OCT, confirming the resonant character of enhancement. Gold nanorods with plasmon resonance wavelengths overlapping the OCT source yielded a signal-to-background ratio of 4.5 dB, relative to the tissue phantom. Strategies for OCT imaging with nanorods are discussed.

Photothermal coagulation of blood vessels: A comparison of high-speed optical coherence tomography and numerical modelling

Optical-thermal models that can accurately predict temperature rise and damage in blood vessels and surrounding tissue may be used to improve the treatment of vascular disorders. Verification of these models has been hampered by the lack of time- and depth-resolved experimental data. In this preliminary study, an optical coherence tomography system operating at 4-30 frames per second was used to visualize laser irradiation of cutaneous (hamster dorsal skin flap) blood vessels. An argon laser was utilized with the following parameters: pulse duration 0.1-2.0 s, spot size 0.1-1.0 mm, power 100-400 mW. Video microscopy images were obtained before and after irradiations, and optical-thermal modelling was performed on two irradiation cases. Time-resolved optical coherence tomography and still images were compared with predictions of temperature rise and damage using Monte Carlo and finite difference techniques. In general, predicted damage agreed with the actual blood vessel and surrounding tissue coagulation seen in images. However, limitations of current optical-thermal models were identified, such as the inability to model the dynamic changes in blood vessel diameter that were seen in the optical coherence tomography images.