J. Stuart Nelson

Phase-resolved optical coherence tomography and optical doppler tomography for imaging fluid flow in tissue with fast scanning speed and high velocity sensitivity

We have developed a novel phase-resolved optical coherence tomography (OCT) and optical Doppler tomography (ODT) system that uses phase information derived from a Hilbert transformation to image blood flow in human skin with fast scanning speed and high velocity sensitivity. Using the phase change between sequential scans to construct flow-velocity imaging, this technique decouples spatial resolution and velocity sensitivity in flow images and increases imaging speed by more than 2 orders of magnitude without compromising spatial resolution or velocity sensitivity. The minimum flow velocity that can be detected with an axial-line scanning speed of 400 Hz and an average phase change over eight sequential scans is as low as 10 microm/s, while a spatial resolution of 10 microm is maintained. Using this technique, we present what are to our knowledge the first phase-resolved OCT/ODT images of blood flow in human skin.

Two-dimensional birefringence imaging in biological tissue using phase and polarization sensitive optical coherence tomography

Polarization sensitive optical coherence tomography (PS-OCT) was used to obtain images of optical birefringence in biological tissues. Through simultaneous detection of two orthogonal polarization states of the signal formed by interference of light backscattered from the biological sample and a mirror in the reference arm of a Michelson interferometer, the optical phase delay between light propagating along the fast and slow axes of birefringence was measured. Simultaneous detection of both polarizations also permits reconstruction of the electro-magnetic wave backscattered from the sample. Inasmuch as any fibrous structure will influence the polarization of light, PS-OCT is a potentially powerful technique in the field of biomedical imaging. It allows rapid non-contact investigation of tissue structural properties through spatially resolved imaging of birefringence.

Internal temperature measurements in response to cryogen spray cooling of a skin phantom

Cryogen spray cooling (CSC) can protect the epidermis from non-specific thermal injury during laser treatment of port wine stains and other hypervascular cutaneous malformations. Knowledge of skin internal temperatures in response to CSC is essential for optimization of this technique. We used an epoxy resin compound to construct a kind phantom and measured its internal temperatures in response to cooling with different cryogens at various spurt durations, spraying distances, and ambient humidity levels. The measured temperature distributions during CSC were fitted by a mathematical model based on thermal diffusion theory. For spurt durations up to 100 ms, temperature reduction within the phantom remained confined to the upper 200 μm, and was affected by spraying distance. Depending on the cryogen used, temperature reductions up to 45°C could be measured 20 μm below the surface at the end of a 100 ms spurt. However, the cryogen film temperature on the epoxy resin surface was up to 35°C lower, indicating lack of perfect thermal contact at the cryogen film-phantom interface. Theoretical predictions were within 10% of measured temperatures. Ice formation occurred following termination of the spurt and was influenced by the ambient humidity level.

Influence of spraying distance and postcooling on cryogen spray cooling for dermatologic laser surgery

Cryogen spray cooling (CSC) is used to minimize the risk of epidermal damage in various laser dermatological procedures such as treatment of port wine stain birthmarks and hair removal. However, the spray characteristics and combination of CSC and heating (laser) to obtain optimal treatments have not yet been determined. The distance between the nozzle tip and the skin surface for commercial devices was apparently chosen based on the position at which the cryogen spray reached a minimum temperature, presumably with the expectation that such a minimum would correspond to maximal heat flux. We have systematically measured spray characteristics of various nozzles, such as mean droplet diameter, velocity, temperature, and heat transfer coefficient, as a function of distance from the nozzle tip. Among other interesting correlations between these spray characteristics, it is shown that, for nozzle-to-skin distances between 20 to 80 mm, variations in the heat transfer coefficient are larger than those in the spray temperature and, therefore, maximization of the heat flux should be better dictated by the distance at which the heat transfer coefficient is maximized rather than that at which the spray temperature is minimized. Also, the influence of droplet diameter appears to be more influential on the heat transfer coefficient value than that of droplet velocity. Based on spray characteristic correlations, different ranges for positioning the nozzles are recommended, depending on the clinical application. Also, a 2D finite-difference method has been developed to study the spatial and temporal thermal variations within the skin. Our results show that it is possible to decrease significantly the epidermal damage after laser irradiation provided the heat transfer coefficient is significantly increased. The influence of post-cooling has minimal effects for the cases studied.

Pulsed photothermal profiling of hypervascular lesions: some recent advances

Pulsed photothermal radiometry (PPTR) can be used for non- invasive depth profiling of port wine stain (PWS) birthmarks, aimed towards optimizing laser therapy on an individual patient basis. Reconstruction of laser-induced temperature profile from the experimentally obtained radiometric signal involves the skin absorption coefficient in the infrared detection band. In the commonly used 3 - 5 micrometer detection band (InSb), the absorption coefficient varies by two orders of magnitude, while assumed to be constant in the reconstruction algorithms used thus far. We discuss the problem of choosing the effective absorption coefficient value to be used under such conditions. Next, we show how to account explicitly for the strong spectral variation of the infrared absorption coefficient in the image reconstruction algorithm. Performance of such improved algorithm is compared to that of the unaugmented version in a numerical simulation of photothermal profiling. Finally, we analyze implementation of a bandpass filter which limits the detection band to 4.5 - 5 micrometer. This reduces the absorption coefficient variation to a level that permits the use of unaugmented algorithm. An experimental test of the latter approach for in vivo characterization of the depth of PWS lesion and epidermal thickness will be presented, including a novel technique that uses two laser excitation wavelengths in order to separate the epidermal and vascular components of the radiometric signal.

Angiogenic response in the chick chorioallantoic membrane model to laser-irradiated cartilage

Laser radiation can be used to reshape cartilage grafts via thermally mediated stress relaxation. While several studies have addressed the biophysical changes accompanying reshaping, cartilage viability following laser irradiation has not been extensively investigated. The objective of this study was to determine the extent of angioinvasion of irradiated cartilage explant placed onto the chick chorioallantoic membrane (CAM) model. Angioinvasion of the tissue matrix does not occur in viable cartilage tissue, whereas denatured tissue is readily vasculairzed and/or resorbed in vivo. Porcine septal cartilage specimens were removed from freshly sacrificed animals and divided into three protocols (n=10 each group) consisting of an untreated control, cartilage boiled in saline solution for one hour, and a laser irradiated group (Nd:YAG, λ=1.32 μm, 30.8 W/cm2, irradiation time = 10 sec). Following laser irradiation, tissue specimens were washed in antibiotic solution sand cut into small cubes (~1.5 mm3). The cartilage specimens were placed onto the surface of twenty CAMs, six of which, survived the entire 14 days incubation period. After incubation, the membranes and specimens were fixed in situ with formaldehyde, an then photographed using a dissection microscope. Cartilage specimens were prepared for histologic evaluation and stained with hematoxylin and eosin. Examination with a dissecting microscope showed no obvious vascular invasion of the cartilage or loss of gross tissue integrity in both the control and laser treated groups. In contrast, boiled specimens appeared to be partially or completely resorbed by the surrounding CAM vascular network. These gross findings were also confirmed by histological examination. In summary, our preliminary studies suggest that cartilage specimens treated using the present laser parameters remain resistant to angioinvasion or metabolism by the CAM, whereas boiled tissue undergoes resorption. Clinically, uncontrolled heating may result in total resorption of cartilage with catastrophic sequelae such as infection, necrosis, and total graft resorption. This study underscores the importance of preserving cartilage viability during laser surgical procedures relying on a photothermal mechanism.

Development of an Optical Tissue Clearing Laser Probe System

Although low-level laser therapy (LLLT) has been a valuable therapeutic technology in the clinic, its efficacy may be reduced in deep tissue layers due to strong light scattering which limits the photon density. In order to enhance the photon density in deep tissue layers, this study developed an optical tissue clearing (OTC) laser probe (OTCLP) system which can utilize four different OTC methods: 1) tissue temperature control from 40 to

Histology of porcine nasal cartilage grafts following Nd:YAG (1320 nm) laser mediated reshaping : Effects of sequential irradiation

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Burn depth determination by high-speed fiber-based polarization-sensitive optical coherence tomography at 1.3 μm

; 2) laser pulse frequency from 5 to 30 Hz; 3) glycerol injection at a local region; and 4) a combination of the aforementioned three methods. The efficacy of the OTC methods was evaluated and compared by investigating laser beam profiles in ex-vivo porcine skin samples. Results demonstrated that total (peak) intensity at full width at half maximum of laser beam profile when compared to control data was increased: 1) 1.21(1.39)-fold at

Optical Doppler tomography for noninvasive imaging of in-vivo blood flow

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