Theodora Leventouri

Investigation of MWCNT Reinforcement on the Strain Hardening Behavior of Ultrahigh Molecular Weight Polyethylene

We have investigated strain hardening behavior of ultrahigh molecular weight polyethylene (UHMWPE) reinforced with 2.0 wt% loading of multiwalled carbon nanotubes (MWCNTs). A solution spinning process was used to produce neat and MWCNT-reinforced filaments of UHMWPE. Tensile tests of filaments showed 62% and 114% improvement in strength and modulus, respectively. Strain hardening tests on filaments revealed spectacular contribution by MWCNTs in enhancing strength and modulus by more than one order of magnitude. SEM micrographs showed sufficient coating of nanotube surface with the polymer that promoted interface adhesion. This intimate interfacial interaction enforced alignment of nanotubes during repeated loading-unloading sequences and allowed effective load transfer to nanotubes. Close interaction between UHMWPE and nanotubes was further evidenced by Raman spectral distribution as a positive shift in the D-band suggesting compressive stress on nanotubes by lateral compression of polymer. Nanotubes thus deformed induced the desired strain hardening ability in the UHMWPE filament. Differential scanning calorimetry (DSC) tests indicated around 15% increase in crystallinity after strain hardening—which together with nanotube alignment resulted in such dramatic improvement in properties.

A new method for measuring the degree of preferred orientation in bulk textured YBa2Cu3Ox

Abstract A new technique for estimating the preferred orientation in bulk textured YBa 2 Cu 3 O x and related structures is developed. Conventional X-ray diffraction patterns, as measured from the bulk samples, and the March function in a program for refinement of the structure are used in order to generate a calibration curve for measuring the degree of preferred orientation in such samples without getting into the complication of pole-figure analysis. It is found that the values of the March coefficient G for a series of purposefully textured bulk samples range from 0.62 to 0.17.

The effect of partially oriented precursors on the melt-textured growth of YBa2Cu3O6+x

Abstract In order to accelerate the melt-textured growth technique, we introduce the use of partially oriented YBa2Cu3O6+x superconducting material as precursor for the melt-texture growth processing, instead of the randomly oriented precursors that are presently in use. We study the effect of this change of the starting material on the microstructure and the surface and bulk preferred orientation of the crystallites as a function of the peritectic solidification time, using scanning electron microscopy (SEM) and x-ray diffraction. The crystallites of these partially oriented precursors serve as “seeds” for the growth of large crystallites (∼300μ on the average) in highly textured samples, whereas the cooling rate around the peritectic temperature is increased by a factor of 2.5 compared to other melt textured growth processes that are reported. While these studies are on small pellets or bars, they demonstrate a principle that can be incorporated in commercial processing.

Investigations of a GPU-based levy-firefly algorithm for constrained optimization of radiation therapy treatment planning

Abstract Intensity modulated radiation therapy (IMRT) affords the potential to decrease radiation therapy associated toxicity by creating highly conformal dose distribution to tumor. Inverse optimization of IMRT treatment plans is often a time intensive task due to the large scale solution space, and the indubitably complexity of the task. Furthermore, the incorporation of conflicting dose constraints in the treatment plan, usually introduces an additional degree of intricacy. Metaheuristic algorithms have been proposed in the past for global optimization in IMRT treatment planning. However one disadvantage of the aforementioned methods is their extensive computational cost. One way to ameliorate their performance deficiency is to parallelize the application. In the current study we propose a GPU-based levy-firefly algorithm (LFA) for constrained optimization of IMRT treatment planning. The evaluation of our method was realized for two treatment cases: a prostate and a head and neck (H&N) cancer IMRT plans. The studies indicated an ascendable increase of the speedup factor as a function of the number of pencil beams with a maximum of ~11, whereas the performance of the algorithm was decreasing as a function of the population of the swarm particles. In addition, from our simulation results we concluded that 200 fireflies were sufficient for the algorithm to converge in less than 80 iterations. Finally, we demonstrated the effect of penalizing factors on constraining the maximum dose at the organs at risk (OAR) by impeding the dose coverage of the tumor target. The impetus behind our study was to elucidate the performance and generic attributes of the proposed algorithm, as well as the potential of its applicability for IMRT optimization problems.

Dosimetric and radiobiological comparison of CyberKnife M6TM InCise multileaf collimator over IRISTM variable collimator in prostate stereotactic body radiation therapy

The impetus behind our study was to establish a quantitative comparison between the IRIS collimator and the InCise multileaf collimator (MLC) (Accuray Inc. Synnyvale, CA) for prostate stereotactic body radiation therapy (SBRT). Treatment plans for ten prostate cancer patients were performed on MultiPlan™ 5.1.2 treatment planning system utilizing MLC and IRIS for 36.25 Gy in five fractions. To reduce the magnitude of variations between cases, the planning tumor volume (PTV) was defined and outlined for treating prostate gland only, assuming no seminal vesicle or ex-capsule involvement. Evaluation indices of each plan include PTV coverage, conformity index (CI), Paddicks new CI, homogeneity index, and gradient index. Organ at risk (OAR) dose sparing was analyzed by the bladder wall Dmaxand V37Gy, rectum Dmaxand V36Gy. The radiobiological response was evaluated by tumor control probability and normal tissue complication probability based on equivalent uniform dose. The dose delivery efficiency was evaluated on the basis of planned monitor units (MUs) and the reported treatment time per fraction. Statistical significance was tested using the Wilcoxon signed rank test. The studies indicated that CyberKnife M6™ IRIS and InCise™ MLC produce equivalent SBRT prostate treatment plans in terms of dosimetry, radiobiology, and OAR sparing, except that the MLC plans offer improvement of the dose fall-off gradient by 29% over IRIS. The main advantage of replacing the IRIS collimator with MLC is the improved efficiency, determined from the reduction of MUs by 42%, and a 36% faster delivery time.

A study of mechanical behavior and morphology of carbon nanotube reinforced UHMWPE/Nylon 6 hybrid polymer nanocomposite fiber

We report a phenomenal increase in strength, modulus, and fracture strain of ultra high molecular weight polyethylene (UHMWPE) fiber by 103 %, 219 %, and 108 %, respectively through hybridizing this fiber with Nylon 6 as a minor phase and simultaneously reinforcing it with single-walled carbon nanotubes (SWCNTs). Loading of Nylon 6 and SWCNTs into UHMWPE was 20.0 wt% and 2.0 wt%, respectively. Hybridized fibers were processed using a solution spinning method coupled with melt mixing and extrusion. We claim that the enhancement in strain-to-failure of the nanocomposites is due to induced plasticity in the hybridized Nylon 6-UHMWPE polymers. The enhancement in strength and stiffness in the nanocomposites is attributed to the load sharing of the SWCNTs during deformation. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) studies showed that changes in percent crystallinity, rate of crystallization, crystallite size, alignment of nanotubes, sliding of polymer interfaces and strong adhesion of CNT/polymer blends were responsible for such enhancements.

Raman and IR studies of the effect of Fe substitution in hydroxyapatites and deuterated hydroxyapatite

Abstract We have studied synthetic Fe-substituted hydroxyapatite Ca5-xFex(PO4)3OH and the corresponding deuterated samples with varying Fe concentrations x (0 ≤ x ≤ 0.3) by Raman and IR spectroscopy at room temperature. In the IR spectra, substitution of deuterons for protons affects the OH internal mode in a way consistent with the mass difference of the substituting ions, as well as a mode attributed to vibrations of the Ca3-(OH) unit. In the Raman spectra, the frequency of all modes is not noticeably affected by the Fe substitution. Raman bands show increased width and substantial reduction in intensity with increasing amount of Fe, presumably related to disorder introduced by the substitution. We find that the disorder is smaller in the hydroxyapatites compared to the deuterated ones.

A computational study on different penalty approaches for constrained optimization in radiation therapy treatment planning with a simulated annealing algorithm

In intensity modulated radiation therapy (IMRT) a treatment plan is a high dimensionality optimization problem with the goal to give the prescribed radiation dose to the Planning Target Volume (PTV) while sparing critical organs. A clinically acceptable plan is usually generated by a numerical optimization process in pursuit of attaining the above mentioned goal. Incorporation of dose volume constraints (DVCs) for the OARs introduce an additional degree of impediment to the optimization task. Heuristic algorithms have been ascertained in the past as a powerful tool for various problems in radiation therapy, such as beam angle optimization (BAO) and IMRT treatment planning. Simulated Annealing (SA) algorithm has the capability to find global minima for bound-constrained optimization problems. However, its performance depends on the penalty method which is utilized for the constraints. In the current study we investigate the performance of three penalty methods for IMRT treatment planning for five prostate cases with dose volume constraints. In addition, sensitivity analysis was performed in order to study the effect of the penalty variables on the treatment plan. Finally the merits and demerits of each method have been discussed.

SU-E-T-37: A GPU-Based Pencil Beam Algorithm for Dose Calculations in Proton Radiation Therapy

Purpose: Recent developments in radiation therapy have been focused on applications of charged particles, especially protons. Over the years several dose calculation methods have been proposed in proton therapy. A common characteristic of all these methods is their extensive computational burden. In the current study we present for the first time, to our best knowledge, a GPU-based PBA for proton dose calculations in Matlab. Methods: In the current study we employed an analytical expression for the protons depth dose distribution. The central-axis term is taken from the broad-beam central-axis depth dose in water modified by an inverse square correction while the distribution of the off-axis term was considered Gaussian. The serial code was implemented in MATLAB and was launched on a desktop with a quad core Intel Xeon X5550 at 2.67GHz with 8 GB of RAM. For the parallelization on the GPU, the parallel computing toolbox was employed and the code was launched on a GTX 770 with Kepler architecture. The performance comparison was established on the speedup factors. Results: The performance of the GPU code was evaluated for three different energies: low (50 MeV), medium (100 MeV) and high (150 MeV). Four square fields were selected for each energy, and the dose calculations were performed with both the serial and parallel codes for a homogeneous water phantom with size 300×300×300 mm3. The resolution of the PBs was set to 1.0 mm. The maximum speedup of ∼127 was achieved for the highest energy and the largest field size. Conclusion: A GPU-based PB algorithm for proton dose calculations in Matlab was presented. A maximum speedup of ∼127 was achieved. Future directions of the current work include extension of our method for dose calculation in heterogeneous phantoms.

A computational tool for patient specific dosimetry and radiobiological modeling of selective internal radiation therapy with 90Y microspheres

In recent years we have witnessed tremendous progress in selective internal radiation therapy. In clinical practice, quite often, radionuclide therapy is planned using simple models based on standard activity values or activity administered per unit body weight or surface area in spite of the admission that radiation-dose methods provide more accurate dosimetric results. To address that issue, the authors developed a Matlab-based computational software, named Patient Specific Yttrium-90 Dosimetry Toolkit (PSYDT). PSYDT was designed for patient specific voxel-based dosimetric calculations and radiobiological modeling of selective internal radiation therapy with (90)Y microspheres. The developed toolkit is composed of three dimensional dose calculations for both bremsstrahlung and beta emissions. Subsequently, radiobiological modeling is performed on a per-voxel basis and cumulative dose volume histograms (DVHs) are generated. In this report we describe the functionality and visualization features of PSYDT.