395 research outputs found
Dual-Gated Volumetric Modulated Arc Therapy
BACKGROUND: Gated Volumetric Modulated Arc Therapy (VMAT) is an emerging radiation therapy modality for treatment of tumors affected by respiratory motion. However, gating significantly prolongs the treatment time, as delivery is only activated during a single respiratory phase. To enhance the efficiency of gated VMAT delivery, a novel dual-gated VMAT (DG-VMAT) technique, in which delivery is executed at both exhale and inhale phases in a given arc rotation, is developed and experimentally evaluated.
METHODS: Arc delivery at two phases is realized by sequentially interleaving control points consisting of MUs, MLC sequences, and angles of VMAT plans generated at the exhale and inhale phases. Dual-gated delivery is initiated when a respiration gating signal enters the exhale window; when the exhale delivery concludes, the beam turns off and the gantry rolls back to the starting position for the inhale window. The process is then repeated until both inhale and exhale arcs are fully delivered. DG-VMAT plan delivery accuracy was assessed using a pinpoint chamber and diode array phantom undergoing programmed motion.
RESULTS: DG-VMAT delivery was experimentally implemented through custom XML scripting in Varian's TrueBeam™ STx Developer Mode. Relative to single gated delivery at exhale, the treatment time was improved by 95.5% for a sinusoidal breathing pattern. The pinpoint chamber dose measurement agreed with the calculated dose within 0.7%. For the DG-VMAT delivery, 97.5% of the diode array measurements passed the 3%/3 mm gamma criterion.
CONCLUSIONS: The feasibility of DG-VMAT delivery scheme has been experimentally demonstrated for the first time. By leveraging the stability and natural pauses that occur at end-inspiration and end-exhalation, DG-VMAT provides a practical method for enhancing gated delivery efficiency by up to a factor of two
Feasibility-Seeking and Superiorization Algorithms Applied to Inverse Treatment Planning in Radiation Therapy
We apply the recently proposed superiorization methodology (SM) to the
inverse planning problem in radiation therapy. The inverse planning problem is
represented here as a constrained minimization problem of the total variation
(TV) of the intensity vector over a large system of linear two-sided
inequalities. The SM can be viewed conceptually as lying between
feasibility-seeking for the constraints and full-fledged constrained
minimization of the objective function subject to these constraints. It is
based on the discovery that many feasibility-seeking algorithms (of the
projection methods variety) are perturbation-resilient, and can be proactively
steered toward a feasible solution of the constraints with a reduced, thus
superiorized, but not necessarily minimal, objective function value.Comment: Contemporary Mathematics, accepted for publicatio
Diffeomorphic density registration
In this book chapter we study the Riemannian Geometry of the density
registration problem: Given two densities (not necessarily probability
densities) defined on a smooth finite dimensional manifold find a
diffeomorphism which transforms one to the other. This problem is motivated by
the medical imaging application of tracking organ motion due to respiration in
Thoracic CT imaging where the fundamental physical property of conservation of
mass naturally leads to modeling CT attenuation as a density. We will study the
intimate link between the Riemannian metrics on the space of diffeomorphisms
and those on the space of densities. We finally develop novel computationally
efficient algorithms and demonstrate there applicability for registering RCCT
thoracic imaging.Comment: 23 pages, 6 Figures, Chapter for a Book on Medical Image Analysi
Retrieval of Precise Radial Velocities from Near-Infrared High Resolution Spectra of Low Mass Stars
Given that low-mass stars have intrinsically low luminosities at optical
wavelengths and a propensity for stellar activity, it is advantageous for
radial velocity (RV) surveys of these objects to use near-infrared (NIR)
wavelengths. In this work we describe and test a novel RV extraction pipeline
dedicated to retrieving RVs from low mass stars using NIR spectra taken by the
CSHELL spectrograph at the NASA Infrared Telescope Facility, where a methane
isotopologue gas cell is used for wavelength calibration. The pipeline
minimizes the residuals between the observations and a spectral model composed
of templates for the target star, the gas cell, and atmospheric telluric
absorption; models of the line spread function, continuum curvature, and
sinusoidal fringing; and a parameterization of the wavelength solution. The
stellar template is derived iteratively from the science observations
themselves without a need for separate observations dedicated to retrieving it.
Despite limitations from CSHELL's narrow wavelength range and instrumental
systematics, we are able to (1) obtain an RV precision of 35 m/s for the RV
standard star GJ 15 A over a time baseline of 817 days, reaching the photon
noise limit for our attained SNR, (2) achieve ~3 m/s RV precision for the M
giant SV Peg over a baseline of several days and confirm its long-term RV trend
due to stellar pulsations, as well as obtain nightly noise floors of ~2 - 6
m/s, and (3) show that our data are consistent with the known masses, periods,
and orbital eccentricities of the two most massive planets orbiting GJ 876.
Future applications of our pipeline to RV surveys using the next generation of
NIR spectrographs, such as iSHELL, will enable the potential detection of
Super-Earths and Mini-Neptunes in the habitable zones of M dwarfs.Comment: 64 pages, 28 figures, 5 tables. Accepted for publication in PAS
Some histochemical observations on the telencephalon of the bullfrog, Rana catesbeiana shaw
The histochemically determined distribution of acetylcholinesterase, monoamine oxidase and succinate dehydrogenase in the telencephalon of the bullfrog supports the classically recognized divisions of the pallium and subpallium. Analysis also corroborates the following gernerally recognized anuran-amniotic homologies: anuran medial pallium to amniotic medial cortex anuran septal nuclei to amniotic septal nuclei, anuran striatum to amniotic corpus striatum. On topographical and histochemical criteria the ventrocaudal and basomedial portions of the anuran telencephalon are considered possible homologues to the mammalian amygdala. It is suggested that two divisions can be recognized: a pars lateralis which may be homologous to the mammalian cortico-medial group. And a pars medialis which may be homologous to the mammalian baso-lateral group. Further analysis suggests, particularly when viewed in the light of recent experimental anatomical studies, that the anuran lateral pallium consists of a pars dorsalis and a pars ventralis. The pars dorsalis may be the reptilian homologue of the dorsal cortex and the pars ventralis may be the field homologue of both the reptilian piriform cortex and the dorsal ventricular ridge.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/49994/1/901570403_ftp.pd
First radial velocity results from the MINiature Exoplanet Radial Velocity Array (MINERVA)
The MINiature Exoplanet Radial Velocity Array (MINERVA) is a dedicated
observatory of four 0.7m robotic telescopes fiber-fed to a KiwiSpec
spectrograph. The MINERVA mission is to discover super-Earths in the habitable
zones of nearby stars. This can be accomplished with MINERVA's unique
combination of high precision and high cadence over long time periods. In this
work, we detail changes to the MINERVA facility that have occurred since our
previous paper. We then describe MINERVA's robotic control software, the
process by which we perform 1D spectral extraction, and our forward modeling
Doppler pipeline. In the process of improving our forward modeling procedure,
we found that our spectrograph's intrinsic instrumental profile is stable for
at least nine months. Because of that, we characterized our instrumental
profile with a time-independent, cubic spline function based on the profile in
the cross dispersion direction, with which we achieved a radial velocity
precision similar to using a conventional "sum-of-Gaussians" instrumental
profile: 1.8 m s over 1.5 months on the RV standard star HD 122064.
Therefore, we conclude that the instrumental profile need not be perfectly
accurate as long as it is stable. In addition, we observed 51 Peg and our
results are consistent with the literature, confirming our spectrograph and
Doppler pipeline are producing accurate and precise radial velocities.Comment: 22 pages, 9 figures, submitted to PASP, Peer-Reviewed and Accepte
La matriz extracelular: morfología, función y biotensegridad (parte I)
La matriz extracelular (MEC) representa una red tridimensional que engloba todos los órganos, tejidos y células del organismo. Constituye un filtro biofísico de protección, nutrición e inervación celular y el terreno para la respuesta inmune, angiogénesis, fibrosis y regeneración tisular. Y representa el medio de transmisión de fuerzas mecánicas a la membrana basal, que a través de las integrinas soporta el sistema de tensegridad y activa los mecanismos epigenéticos celulares. La alteración de la MEC supone la pérdida de su función de filtro eficaz, nutrición, eliminación, denervación celular, pérdida de la capacidad de regeneración y cicatrización y alteración de la transmisión mecánica o mecanotransducción. También la pérdida del sustrato para una correcta respuesta inmune ante agentes infecciosos, tumorales y tóxicos.
Los tumores son tejidos funcionales conectados y dependientes del microambiente. El microambiente tumoral, constituido por la MEC, células del estroma y la propia respuesta inmune, son determinantes de la morfología y clasificación tumoral, agresividad clínica, pronóstico y respuesta al tratamiento del tumor. Tanto en condiciones fisiológicas como patológicas, la comunicación recíproca entre células del estroma y el parénquima dirige la expresión génica. La capacidad oncogénica del estroma procede tanto de los fibroblastos asociados al tumor como de la celularidad de la respuesta inmune y la alteración de la tensegridad por la MEC. La transición epitelio-mesenquimal es el cambio que transforma una célula normal o «benigna» en «maligna». El citoesqueleto pseudomesenquimal otorga las propiedades de migración, invasión y diseminación. Y viceversa, el fenotipo maligno es reversible a través de la corrección de las claves que facilita el microambiente tumoral.Extracellular matrix (ECM) is a three-dimensional network that envelopes all the organs, tissues and cells of the body. A biophysical filter that provides protection, nutrition and cell innervation, it is the site for immune response, angiogenesis, fibrosis and tissue regeneration. It is also the transport medium for mechanical forces to the basal membrane through integrins that support the tensegrity system, activating cellular epigenetic mechanisms. The disruption of the ECM leads to a functional loss of nutrition, elimination, cell innervation, regenerative capacity and wound healing as well as alterations in mechanical transduction. This loss also disrupts the immune response to pathogens, tumour cells and toxins.
Tumours are functionally connected tissues which depend on the microenvironment. This tumour microenvironment, made up of ECM, stromal cells and the immune response, determines the morphology and tumour histopathological classification, clinical behaviour, prognosis and immune response to the tumour. Both in physiological and pathological conditions, reciprocity in the communication between stromal and parenchymal cells determine gene expression. The oncogenic capacity of the stroma depends on tumour associated fibroblasts, immune system cellularity and disruption of tensegrity by ECM. Epithelial-mesenchymal transition is the change that transforms a normal or benign cell into a malignant cell. The «pseudo-mensenchymal» cytoskeleton is responsible for migration, invasion and dissemination, and vice-versa, the malignant phenotype is reversible through the correction of the microenvironmental factors that favour tumour growth.Noguera Salva, Rosa, [email protected]
Mean Diffusional Kurtosis in Patients with Glioma: Initial Results with a Fast Imaging Method in a Clinical Setting
ABSTRACT BACKGROUND AND PURPOSE: Diffusional kurtosis imaging is an MR imaging technique that provides microstructural information in biologic systems. Its application in clinical studies, however, is hampered by long acquisition and postprocessing times. We evaluated a new and fast (2 minutes 46 seconds) diffusional kurtosis imaging method with regard to glioma grading, compared it with conventional diffusional kurtosis imaging, and compared the diagnostic accuracy of fast mean kurtosis (MKЈ) to that of the widely used mean diffusivity
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