Bulletin of the American Physical Society
74th Annual Meeting of the Southeastern Section
Volume 52, Number 13
Thursday–Saturday, November 8–10, 2007; Nashville, Tennessee
Session CB: Medical Physics |
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Chair: John Wikswo, Vanderbilt University Room: Scarritt-Bennett Center Laskey C |
Thursday, November 8, 2007 10:45AM - 10:57AM |
CB.00001: Simulations of Keratoconus Patient Vision with Optical Eye Modeling Bo Tan, Ying-Ling Chen, J.W.L. Lewis, Lei Shi, Ming Wang Keratoconus (KC) is an eye condition that involves progressive corneal thinning. Pushed by the intraocular pressure, the weakened cornea bulges outward and creates an irregular surface shape. The result is degraded vision that is difficult to correct with regular eye glasses or contact lens. In this study we use the optical lens design software, ZeMax, and patient data including cornea topography and refraction prescription to construct KC eye models. The variation of KC ``cone height'' on the cornea is used to simulate KC progression. The consequent patients' night vision and Snellen letter chart vision at 20 feet are simulated using these anatomically accurate 3-dimensional models. 100 million rays are traced for each image simulation. Animated results illustrate the change of KC visual acuity with the progression of disease. This simulation technique provides a comprehensive tool for medical training and patient consultation/education. [Preview Abstract] |
Thursday, November 8, 2007 10:57AM - 11:09AM |
CB.00002: Refractive Error Evaluation in Eccentric Photorefracation K. Baker, B. Tan, L. Shi, Y. Chen, J.W.L. Lewis Eccentric photorefraction (EPR) is a technique to measure eye refractive errors (RE) particularly in non-cooperative children. A camera and a decentered light source are used to illuminate eyes and photograph the reflex. Recently the National Eye Institute VIP study examined 10 methods to screen the crucial children eye disorders including RE. The 3 EPR based devices were found to have lower detection sensitivities. The deficiency is inherent in the current optical designs and a lack of knowledge of parameters that influence RE analysis. The new EPR experimental design includes multi-eccentric-meridian illuminations, pupil finding algorithm, and feedback loop data acquisition. NIR is used to prevent chromatic aberration. A novel integrated intensity analysis is developed to control multiple intraocular scattering/reflection. The RE is calculated from the 21 normalized related intensities of the 2-D array illuminations. Using computer eye modeling and simulations, error analysis is performed for monochromatic aberrations and eye orientation. Experimental data comparison is obtained from a physical model eye. [Preview Abstract] |
Thursday, November 8, 2007 11:09AM - 11:21AM |
CB.00003: Zernike Interpretation in Ocular Photorefraction Images Lei Shi, Ying-Ling Chen, Kevin Baker, J.W.L. Lewis, Bo Tan, Ming Wang Photorefraction (PR) is a common method used in public vision screening for near/far-sightedness and cross eyes. The eye is photographed with an illuminating source close to the camera. Diagnosis is given by the intensity distribution across the pupil reflex. In this study, an enhanced PR system is assembled and used to obtain monocular images from patients in Wang Vision Institute. Thirteen rapidly sequenced IR images are taken for each eye. A target-finding algorithm locates the pupil, and the scaled intensity distribution of the pupil is color-coded into 8 levels. The false-color maps show distinguished patterns between normal and abnormal eyes. Zernike analysis of the image provides quantitative measure of the 1$^{st}$, 2$^{nd}$, and high-order ocular aberrations. The results reveal that normal eyes are predominantly described by 1st order coefficients, while abnormal eyes exhibit a significant contribution from high-order terms. This study shows that PR can be extended to detect high-order aberration in addition to its traditional applications. [Preview Abstract] |
Thursday, November 8, 2007 11:21AM - 11:33AM |
CB.00004: Time series as a diagnostic tool for EKG Cahit Erkal, Aydin Cecen A preliminary analysis of heart rate variability (peak-to-peak intervals based on EKG) will be presented using the tools of nonlinear dynamics and chaos. We show that uncertainty determination of the most commonly used invariant-the correlation dimension- and a proper implementation of time series analysis tools are necessary to differentiate between the healthy and unhealthy state of the heart. We present an example analysis based on normal and atrial fibrillation EKGs and point of some pitfalls that may give rise to misleading conclusions. [Preview Abstract] |
Thursday, November 8, 2007 11:33AM - 11:45AM |
CB.00005: Activation Time of Cardiac Tissue In Response to a Linear Array of Spatial Alternating Bipolar Electrodes David Mashburn, John Wikswo Prevailing theories about the response of the heart to high field shocks predict that local regions of high resistivity distributed throughout the heart create multiple small virtual electrodes that hyperpolarize or depolarize tissue and lead to widespread activation. This resetting of bulk tissue is responsible for the successful functioning of cardiac defibrillators. By activating cardiac tissue with regular linear arrays of spatially alternating bipolar currents, we can simulate these potentials locally. We have studied the activation time due to distributed currents in both a 1D Beeler-Reuter model and on the surface of the whole heart, varying the strength of each source and the separation between them. By comparison with activation time data from actual field shock of a whole heart in a bath, we hope to better understand these transient virtual electrodes. Our work was done on rabbit RV using florescent optical imaging and our Phased Array Stimulator for driving the 16 current sources. Our model shows that for a total absolute current delivered to a region of tissue, the entire region activates faster if above-threshold sources are more distributed. [Preview Abstract] |
Thursday, November 8, 2007 11:45AM - 11:57AM |
CB.00006: Analytic solution for source distributions achieving a uniform dose William Atkinson Interstitial brachytherapy involves implanting many small radioactive sources into the tumor, the goal being delivering a uniform radiation dose to the target volume. We assumed a spherical tumor irradiated by a continuous distributed radiation source. Solution of the ensuing integral equation shows that the source density is very low near the center of the sphere, increases rapidly toward the surface, and becomes infinite at the surface. Integration of the source density over a given spherical sub-volume shows that only about 6{\%} of the total activity is contained in the core up to 50{\%} of the tumor radius, while about one-half of the activity has to be placed in the outer spherical shell having a thickness of one-tenth of the tumor radius. This situation is approximated in the high-dose-rate (HDR) treatment of the prostate using 192Ir. The results are in good agreement with the recommendations given in the traditional Patterson-Parker tables for radium and cesium treatment. [Preview Abstract] |
Thursday, November 8, 2007 11:57AM - 12:09PM |
CB.00007: Detection of Free Radicals in Vitamin E-doped Ultra-High Molecular Weight Polyethylene Benjamin Walters Free-radical-induced oxidation of ultra-high molecular weight polyethylene (UHMWPE) liners of the artificial hip- or knee-joint adversely affects the performance of the total joint. [1] To combat oxidation, vitamin E is added to UHMWPE as an antioxidant. [2] In this study, we use 10{\%} by wt. vitamin E (alpha-tocopherol) in UHMWPE powder. Free radicals are produced by irradiating test samples with x-rays and detected using an X-band electron spin resonance (ESR) spectrometer. Test samples for this study are: 1. vitamin E-UHMWPE mixed before irradiation; 2. non-irradiated vitamin E mixed with irradiated UHMWPE; 3. irradiated vitamin E; and 4. irradiated UHMWPE (control). ESR spectra are recorded as a function of time for more than two weeks. While control shows the presence of alkyl/allyl/polyenyl radicals, the vitamin E-mixed powder presents additional signals in the spectrum due to tocopheroxyl radicals. Analyses of the preliminary ESR data will be presented. References: [1] M.S. Jahan et al., Biomed. Mater. Res. 25 (1991) 1005. [2] N. Shibata et al., J. Biomed. Sci. Eng., 1 (2006) 107. [Preview Abstract] |
Thursday, November 8, 2007 12:09PM - 12:21PM |
CB.00008: Orientation and shape dependence of embryonic wound healing Holley Lynch, Xiaoyan Ma, M. Shane Hutson Wounds in embryonic epithelia heal without scarring. They do so via the combined action of two cytoskeletal structures: an actin-rich supracellular purse-string at the wound margin; and actin-based projections like filopodia. Neither structure is absolutely required for wound closure and their relative importance depends strongly on wound shape. To further investigate this dependence, we have followed the healing process in fruit fly embryos using confocal microscopy after precise laser incisions. The wound shape and rate of healing depend on the orientation of the incision. Cuts along the long axis of the embryo initially expand to greater areas and round up. Cuts along the short axis expand less and remain elliptical. These short-axis wounds heal more quickly and in a different manner. For such cuts, cellular projections tend to bridge across the ends of the wound. After such bridges are formed, the smaller holes (towards the ends of the wound) close quickly. On the other hand, for cuts along the long axis, the cellular projections tend to bridge across the middle of the wound -- often leaving two to three holes of similar size that then close independently at similar rates. [Preview Abstract] |
Thursday, November 8, 2007 12:21PM - 12:33PM |
CB.00009: Correlation between Recoil Velocity after Laser Ablation and Cell-edge Orientation Xiaoyan Ma, M. Shane Hutson In order to determine the interfacial tensions along cell-cell boundaries in living fruit fly (\textit{Drosophila}) embryos, we have developed a microsurgical method based on laser ablation and laser-scanning confocal microscopy. Following ablation of one cell edge, we follow the recoil dynamics (strain relaxation) of adjacent GFP-labeled cell edges (with time resolution down to 2 ms). The initial recoil velocities are consistently fit best by a double exponential decay. We observe that the initial recoil velocity shows a strong correlation with cell edge orientation. This correlation is particularly pronounced in late dorsal closure where both the recoil velocities and the distribution of cell edge orientations have sharp peaks near 30 and 150\r{ } with respect to the long (anterior-posterior) axis of the embryo. In early dorsal closure, the distribution of cell edge orientation has three much weaker peaks and the recoil velocities only show a weak maximum near 90\r{ }. [Preview Abstract] |
Thursday, November 8, 2007 12:33PM - 12:45PM |
CB.00010: Novel micromirrors to obtain three-dimensional images of cells Charles Wright, Erik Boczko, John Wikswo, Kevin Seale Confocal scanning laser microscopy and multiphoton microscopy provide 3D data from biological specimens, but with limited z-axis precision. We are developing a system which uses multiple microscale mirrors to obtain more accurate 3D data on living cells while using classical widefield microscopy. Etched silicon wells coated with aluminum have been used to obtain 3D images of pollen grains and protozoa. Because the sides of the well are angled, reflections along these planes provide information along the z-axis, and a back-projection algorithm can be used to reflect the data points to reconstruct a 3D image. We are currently optimizing the system to make measurements of the volume of an individual budding yeast cell as it progresses through the cell cycle. Due to the roughly prolate spheroidal shape of the yeast cell, data from this organism are suitable for fitting to a simple 3D surface, and integration provides the volume of the cell over time. The mirrored silicon wells can also be coupled to a microfluidic device to allow for measurements of 3D data for cells in traps. In addition, we are forming a mirrored well on the end of an aluminum rod which can then be positioned optimally above a cell, obviating the need for the cell to sit within the well. [Preview Abstract] |
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