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Biomedical subjects

Vasan Venugopalan

Publications and source records attributed to Vasan Venugopalan.

8 recordsLinked to original sources
PubMed2005

Free electron laser ablation of articular and fibro-cartilage at 2.79, 2.9, 6.1, and 6.45 microm: mass removal studies.

BACKGROUND AND OBJECTIVE: The wavelength and tissue-composition dependence of cartilage ablation was examined using selected mid-infrared laser wavelengths. STUDY DESIGN/MATERIALS AND METHODS: The mass removal produced by pulsed laser ablation of articular and fibro-cartilage (meniscus) were measured. The wavelengths examined were 2.79, 2.9, 6.1, and 6.45 microm and provided by a free electron laser (FEL) emitting 4 microsecond macropulses consisting of 1-2 picoseconds duration micropulses delivered at 350 picosecond intervals. The measurement of tissue mass removal was conducted using a microbalance during laser ablation. RESULTS: These studies demonstrated that for articular cartilage the highest mass removal was achieved at lambda = 6.1 microm followed by, in order, lambda = 2.79, 2.9, and 6.45 microm. In comparison, the maximum mass removal for fibro-cartilage was achieved using lambda = 6.1 microm radiation with no statistically significant differences in mass removal provided by the other wavelengths. In evaluation of the comparative influence of each wavelength on tissue type, there was no difference in ablation efficiency between articular and fibro-cartilage at lambda = 6.1 microm. However, the ablation efficiency of articular cartilage was higher than that of fibro-cartilage at both lambda = 2.79 and 2.9 microm. By contrast, lambda = 6.45 microm radiation ablated fibro-cartilage more efficiently than articular cartilage at radiant exposures greater than 12 J/cm2. CONCLUSIONS: The mass removal of articular and fibro-cartilage produced by FEL ablation at selected mid-IR wavelengths was measured as a function of incident radiant exposure. The ablation efficiency was found to depend on both wavelength and tissue type. The 6.1 microm wavelength was found to provide the highest ablation efficiency for both articular and fibro-cartilage.

Animals↗
PubMed2004

Use of the delta-P1 approximation for recovery of optical absorption, scattering, and asymmetry coefficients in turbid media.

We introduce a robust method to recover optical absorption, reduced scattering, and single-scattering asymmetry coefficients (microa, micro's, g1) of infinite turbid media over a range of (micro's/microa) spanning 3 orders of magnitude. This is accomplished through the spatially resolved measurement of irradiance at source-detector separations spanning 0.25-8 transport mean free paths (l*). These measurements are rapidly processed by a multistaged nonlinear optimization algorithm in which the measured irradiances are compared with predictions given by the delta-P1 variant of the diffusion approximation to the Boltzmann transport equation. The ability of the delta-P1 model to accurately describe radiative transport within media of arbitrary albedo and on spatial scales comparable to l* is the key element enabling the separation of g1 from micro's.

Algorithms↗
PubMed2004

Sampling tissue volumes using frequency-domain photon migration.

We demonstrate the use of Monte Carlo simulations to generate photon scattering density functions (PSDFs) that represent the tissue volume sampled by steady-state and frequency-domain photon migration. We use these results to illustrate how scaling laws can be developed to determine the mean sampling depth of the multiply scattered photons detected by photon migration methods that remain valid outside the bounds of the standard diffusion approximation, i.e., at small source-detector separations and in media where the optical absorption is significant relative to scattering. Using both the PSDF computation and the newly formulated scaling laws, we focus on a comprehensive description of the effects of source modulation frequency, optical absorption, and source-detector separation on the depth of the sampled tissue volume as well as the sensitivity of frequency-domain photon migration measurements to the presence of a localized absorption heterogeneity.

Biophysical Phenomena↗
PubMed2003

Optoacoustic tomography using time-resolved interferometric detection of surface displacement.

We introduce a minimally invasive technique for optoacoustic imaging of turbid media using optical interferometric detection of surface displacement produced by thermoelastic stress transients. The technique exploits endogenous or exogenous optical contrast of heterogeneous tissues and the low attenuation of stress wave propagation to localize and image subsurface absorbers in optically turbid media. We present a system that utilizes a time-resolved high-resolution interferometer capable of angstrom-level displacement resolution and nanosecond temporal resolution to detect subsurface blood vessels within model tissue phantoms and a human forearm in vivo.

Absorption↗
PubMed2003

Optoacoustic determination of optical attenuation depth using interferometric detection.

We use a modified Mach-Zehnder interferometer to measure surface displacement resulting from the thermoelastic response of a target to the absorption of a short laser pulse with axial and temporal resolutions of 0.1 nm and 3 ns, respectively. These measurements are used in conjunction with a solution to the thermoelastic wave equation and a nonlinear optimization algorithm to extract optical attenuation depth. We demonstrate the ability to determine the effective optical attenuation depth of homogeneous targets with either diffuse or specular reflecting surfaces with a precision of <or=4% for attenuation depths spanning 0.1 to 2 mm.

Absorption↗
PubMed2002

Role of laser-induced plasma formation in pulsed cellular microsurgery and micromanipulation.

We investigate experimentally the physical processes underlying pulsed cellular microsurgery and micromanipulation using nanosecond 532- and 1064-nm laser pulses focused at high numerical aperture. We find that the laser parameters employed for many microirradiation techniques are congruent with those leading to optical breakdown in water. We determine the size and shape of the laser-induced plasma, pressure of the emitted shock wave, and size and energy of the cavitation bubble formed by the expanding plasma. We discuss implications of the results for biophysical microirradiation procedures.

Cells↗
PubMedDate not supplied

Radiative transport in the delta-P1 approximation: accuracy of fluence rate and optical penetration depth predictions in turbid semi-infinite media.

Using the delta-P(1) approximation to the Boltzmann transport equation we develop analytic solutions for the fluence rate produced by planar (1-D) and Gaussian beam (2-D) irradiation of a homogeneous, turbid, semi-infinite medium. To assess the performance of these solutions we compare the predictions for the fluence rate and two metrics of the optical penetration depth with Monte Carlo simulations. We provide results under both refractive-index matched and mismatched conditions for optical properties where the ratio of reduced scattering to absorption lies in the range 0< or =(micro(s')/micro(a))< or =10(4). For planar irradiation, the delta-P(1) approximation provides fluence rate profiles accurate to +/-16% for depths up to six transport mean free paths (l*) over the full range of optical properties. Metrics for optical penetration depth are predicted with an accuracy of +/-4%. For Gaussian irradiation using beam radii r(0) > or =3 l*, the accuracy of the fluence rate predictions is no worse than in the planar irradiation case. For smaller beam radii, the predictions degrade significantly. Specifically for media with (micro(s')/micro(a))=1 irradiated with a beam radius of r(0)=l*, the error in the fluence rate approaches 100%. Nevertheless, the accuracy of the optical penetration depth predictions remains excellent for Gaussian beam irradiation, and degrades to only +/-20% for r(0)=l*. These results show that for a given set of optical properties (micro(s')/micro(a)), the optical penetration depth decreases with a reduction in the beam diameter. Graphs are provided to indicate the optical and geometrical conditions under which one must replace the delta-P(1) results for planar irradiation with those for Gaussian beam irradiation to maintain accurate dosimetry predictions.

Computer Simulation↗