Biomedical subjects

Daniel L Marks

Publications and source records attributed to Daniel L Marks.

9 recordsLinked to original sources

PubMed2003

Engineered microsphere contrast agents for optical coherence tomography.

Contrast agents are utilized in virtually every imaging modality to enhance diagnostic capabilities. We introduce a novel class of optical contrast agent, namely, encapsulating microspheres, that are based not on fluorescence but on scattering nanoparticles within the shell or core. The agents are suitable for reflection- or scattering-based techniques such as optical coherence tomography, light microscopy, and reflectance confocal microscopy. We characterize the optical properties of gold-, melanin-, and carbon-shelled contrast agents and demonstrate enhancement of optical coherence tomography imaging after intravenous injection of such an agent into a mouse.

Animals↗

PubMed2003

Fast-Fourier-domain delay line for in vivo optical coherence tomography with a polygonal scanner.

We demonstrate in vivo optical coherence tomography using a Fourier-domain optical delay line constructed with a commercially available polygonal scanner. The 20-faceted polygonal mirror array, capable of scanning at rates up to 15 kHz, is implemented at 4 kHz to acquire 500 x 500 pixel images at 8 frames/s with a signal-to-noise ratio of 80 dB. Features of this delay line include scalability to high repetition rates, 98.6% linearity in group delay over 2 mm, and bandwidth support exceeding 150 nm. Images are obtained in an animal model (Xenopus laevis), and limitations due to phase-delay nonlinearity and polygon asymmetry are discussed.

Animals↗

PubMed2003

Functional optical coherence tomography for detecting neural activity through scattering changes.

We have demonstrated functional optical coherence tomography (fOCT) for neural imaging by detecting scattering changes during the propagation of action potentials through neural tissue. OCT images of nerve fibers from the abdominal ganglion of the sea slug Aplysia californica were taken before, during, and after electrical stimulation. Images acquired during stimulation showed localized reversible increases in scattering compared with those acquired before stimulation. Motion-mode OCT images of nerve fibers showed transient scattering changes from spontaneous action potentials. These results demonstrate that OCT is sensitive to the optical changes in electrically active nerve fibers.

Abdomen↗

PubMed2003

Autofocus algorithm for dispersion correction in optical coherence tomography.

Practical clinical optical coherence tomography (OCT) systems require automatic tools for identifying and correcting flaws in OCT images. One type of flaw is the loss of image detail owing to the dispersion of the medium, which in most cases is unknown. We present an autofocus algorithm for estimating the delay line and material dispersion from OCT reflectance data, integrating a previously presented dispersion compensation algorithm to correct the data. The algorithm is based on minimizing the Renyi entropy of the corrected axial-scan image, which is a contrast-enhancement criterion. This autofocus algorithm can be used in conjunction with a high-speed, digital-signal-processor-based OCT acquisition system for rapid image correction.

Algorithms↗

PubMed2003

Projected index computed tomography.

Projected index computed tomography (PICT) is a new imaging technique that provides a computed reconstruction of the index of refraction of a sample. PICT makes use of data from standard optical coherence tomography images taken from several view angles to determine a mapping of the refractive indices of the sample. A rectilinear propagation model is assumed, so the data are understood to be related to the line integral of the refractive index in the beam paths. These data thus provide a set of angular projections of the sample. The spatial distribution of the index of the object may then be reconstructed by use of standard filtered backprojection techniques. The resultant PICT images are free of the spatial distortion that is inherent in standard optical cross-sectional images and correspond well to the manufactured dimensions of specific samples.

Image Processing, Computer-Assisted↗

PubMed2003

Digital algorithm for dispersion correction in optical coherence tomography for homogeneous and stratified media.

The resolution of optical coherence tomography (OCT) often suffers from blurring caused by material dispersion. We present a numerical algorithm for computationally correcting the effect of material dispersion on OCT reflectance data for homogeneous and stratified media. This is experimentally demonstrated by correcting the image of a polydimethyl siloxane microfludic structure and of glass slides. The algorithm can be implemented using the fast Fourier transform. With broad spectral bandwidths and highly dispersive media or thick objects, dispersion correction becomes increasingly important.

Journal Article↗

PubMed2003

The role of the melanocortin-3 receptor in cachexia.

Cachexia refers to a synergistic combination of a dramatic decrease in appetite and an increase in metabolism of fat and lean body mass. This combination is found in a number of chronic diseases and is an important determinant of mortality. In this paper, we provide evidence that in both acute and chronic disease models, blockade of the MC4-R results in a dramatic attenuation of cachexia. We have also demonstrated that blockade of the melanocortin-3 receptor (MC3-R) leads to enhanced disease-associated cachexia. Ultimately, this work may lead to investigation of drug therapy for this widespread medical problem.

Adipose Tissue↗

PubMed2003

Differential role of melanocortin receptor subtypes in cachexia.

Animals and humans respond to starvation with a complex neuroendocrine response that ultimately leads to an increase in appetite, a sparing of lean body mass (LBM) and burning of fat, and an overall decrease in basal metabolic rate. In contrast, cachexia is a pathological state of malnutrition associated with many infections and chronic diseases, wherein appetite is diminished concomitant with an increase in metabolic rate, and a relative wasting of LBM. In previous studies, we demonstrated that anorexia and weight loss in mouse cachexia models induced by lipopolysaccharide (LPS) administration and by tumor growth are ameliorated by central melanocortin-4 (MC4) receptor (MC4-R) blockade. In contrast to the results seen with MC4 blockade, melanocortin-3 (MC3) receptor knockout (MC3-RKO) mice show illness-induced anorexia and weight loss with LPS administration and with cytokine administration, and they have similar decreases in mobility. Both MC3-RKOs and MC4-RKOs have an intact corticosterone response and fever with LPS injection. In tumor models, we show that MC4-RKO mice resist the loss of LBM brought about by tumor growth, whereas MC3-RKO animals show enhanced tissue wasting. These data underscore the importance of central melanocortin signaling in weight homeostasis and demonstrate differential effects of MC3-R and MC4-R blockade on the development of cachexia.

Animals↗

PubMed2002

Digital refraction distortion correction with an astigmatic coherence sensor.

We demonstrate the sensing and correction of an isoplanatic refractive distortion (not lens aberrations), using the complete measurement of the partially coherent field in an aperture that the previously described astigmatic coherence sensor provides. Isoplanatic distortions, and in general distortions that do not cause energy loss, maintain the orthogonality of the coherent modes. We use the fact that a common distortion will occur to all coherent modes to separate the distortion from the source behind it, rather than requiring a reference source at a different wavelength. Digital deconvolution was performed on the full four-dimensional partially coherent field for simultaneously computing the distortion and the source intensity distribution.

Journal Article↗