963 research outputs found
Performance of a Motion Tracking System During Cyberknife Robotic Radiosurgery
No full textCyberknife (Accuracy Inc., Ca) is a robotic radio-surgery system that includes a compact 6 MV linac delivering up to 800 cGy per minute, and an automate arm to aim at any part of the body from any angle. An essential tool is the guidance system based on x-ray imaging cameras located on supports around the patient. A Cyberknife system has been operational at the Vicenza (Italy) Hospital for years and is mainly employed for treating benign and malignant tumors, and Arterior-Venous Malformations. In radiation therapy, delivery of high doses to targets that move with respiration is challenging because of possible spatial inaccuracies. The purpose of this work was to estimate the accuracy of the prediction algorithm used to compensate for system latency in a real-time respiratory tracking system. We have analyzed respiratory signals of 30 patients who had lung or liver Cyberknife treatments. The “Synchrony” (Accuracy Inc.) motion tracking system we use is based on the correlation between the position of LED markers, detected in real time, and the position of internal markers, sampled through x-ray imaging. The position of the external LED signals, though read in real time, must be predicted to compensate for a few hundred ms time lag in the feedback loop that redirects the beam to the current target position. The respiratory signals were described employing their frequency power spectrum, as recently proposed by other authors.
Prediction errors above 1.5 mm, lasting for periods longer than 5 seconds were observed for irregular breathers. These episodes correlate to the presence of a bimodal distribution in the power spectral density, and of very low frequencies contribution. A more refined approach would include a personalized choice of the prediction algorithm based on the very first minutes of treatment. Patient training aimed at reducing breathing irregularities might also result in improved spatial accuracy
Total scatter factors of small beams: a multidetector and Monte Carlo study.
No full textThe scope of this study was to estimate total scatter factors (S(c,p)) of the three smallest collimators of the Cyberknife radiosurgery system (5-10 mm in diameter), combining experimental measurements and Monte Carlo simulation. Two microchambers, a diode, and a diamond detector were used to collect experimental data. The treatment head and the detectors were simulated by means of a Monte Carlo code in order to calculate correction factors for the detectors and to estimate total scatter factors by means of a consistency check between measurement and simulation. Results for the three collimators were: S(c,p) (5 mm) = 0.677 +/- 0.004, S(c,p) (7.5 mm) = 0.820 +/- 0.008, S(c,p) (10 mm) = 0.871 +/- 0.008, all relative to the 60 mm collimator at 80 cm source-to-detector distance. The method also allows the full width at half maximum of the electron beam to be estimated; estimations made with different collimators and different detectors were in excellent agreement and gave a value of 2.1 mm. Correction factors to be applied to the detectors for the measurement of S(c,p) were consistent with a prevalence of volume effect for the microchambers and the diamond and a prevalence of scattering from high-Z material for the diode detector. The proposed method is more sensitive to small variations of the electron beam diameter with respect to the conventional method used to commission Monte Carlo codes, i.e., by comparison with measured percentage depth doses (PDD) and beam profiles. This is especially important for small fields (less than 10 mm diameter), for which measurements of PDD and profiles are strongly affected by the type of detector used. Moreover, this method should allow S(c,p) of Cyberknife systems different from the unit under investigation to be estimated without the need for further Monte Carlo calculation, provided that one of the microchambers or the diode detector of the type used in this study are employed. The results for the diamond are applicable only to the specific detector that was investigated due to excessive variability in manufacturing
From Morphological to Functional Definition of Organs at Risk: The Role of fMRI in Radiosurgery
No full textA kinetic study of the quartz-cristobalite phase transition
No full textCristobalite is a common silica polymorph in ceramics, as it can crystallize in SiO2-rich systems during high temp. processes. Its occurrence in final traditional ceramic bodies remarkably affects their thermal expansion, thus playing an important role in the shrinkage upon cooling. The quartz-cristobalite transformation kinetics is investigated by in-situ isothermal X-ray powder diffraction expts. and then correlated to the av. particle size (〈d〉) of the starting quartz using a model here developed. An Avrami-like rate equation, i.e. α(t) = 1 - exp(- k × t)n, in which the n-term is assumed to account for the dependence on the av. particle size, has provided the best fitting of theor. to exptl. data, yielding activation energy values that range from 181 to 234 kJ mol-1, and exponential n-coeffs. from 0.9 to 1.5. Ex-situ observations have demonstrated that the formation of cristobalite from quartz after 50 min, 2, 4 and 6 h at 1200 and 1300 °C, exhibits a remarkable dependence on 〈d〉 of quartz, showing comparable behaviors in the case of 〈d〉 equal to 15.8 and 28.4 μm, but significant differences for 〈d〉 of 4.1 μm. The formation of cristobalite is boosted remarkably at temp. higher than 1200 °C, with an increase by wt. even of 500%, with respect to its content at lower temp. The method of sample prepn. (dry powder, wet powder and tablet of compressed dry powder) seems to influence the results only at temp. > 1200 °C and in the case of fine powder
Bold FMRI integration into radiosurgery treatment planning of cerebral vascular malformations
No full textInfluence of composition on some industrially relevant properties of traditional sanitary-ware glaze
No full textTwo series of glazes have been produced from different combinations of the same raw materials in the range of interest for sanitary-ware applications: they are designed to allow one to get insight into network-forming and network-modifying species. Fusibility tests and hot stage microscope observations show the influence of even low differences in the starting chemical compositions on the transformation temperatures. X-ray powder diffraction, wavelength dispersion spectrometry and scanning electron microscopy prove that: (i) zircon, the most abundant crystalline phase, is homogeneously distributed and decreases by a 3% from its starting value; (ii) the glass-phase of glaze has a quasi-uniform composition. X-ray synchrotron radiation micro-tomography shows that glaze porosity is 15% by volume, and voids are prevalently not interconnected and with size up to 50 μm. The linear thermal expansion of the glass phase of glaze ranges between 6 and 7 × 10 -6 °C -1, without apparent correlation with composition
Feldspar and firing cycle effects on the evolution of sanitary-ware vitreous body
No full textThis work is focused on the study of macroscopic and microscopic properties of traditional sanitary-ware vitreous bodies as a function of feldspar flux and firing time-temperature profile, using a fixed slip formulation (50 wt.% clay, 25 wt.% quartz and 25 wt.% feldspar). Two flux particle sizes (45 and 75 μm), three flux compositions (Na-based feldspar, K-based feldspar and a mix of them) and three firing cycles with the same soaking temperature (i.e. 1240 C) have been combined to evaluate their effects on the relevant industrial properties of water absorption and thermal expansion. The micro-scale observables, phase composition and micro-morphology, have also been investigated. Despite a general similarity exhibited by the ceramic samples, qualitative and quantitative differences in terms of feldspar dissociation temperature, phase-composition and densification trends have been observed. In particular, for a fixed firing cycle, the combination of the sodium based feldspar with the smallest flux particle size leads systematically to a water absorption value that is below the 0.5 target value and to a glass amount that approaches 70 wt.%. Thermal expansion coefficients below the quartz α-β transition are found in the 6.2-6.9×10-6 C-1 range; the highest values seem to be favoured by incorporation of potassium based
Effects of particle size distribution and starting phase composition in Na-feldspar/kaolinite system at high temperature
No full textMullite-glass Gibbs energy of formation (ΔGeff), micro-texture and phase composition evolution are investigated in the Na-feldspar (F) and kaolinite (K) system, over the 1240-1320 °C interval, as a function of the starting F/K ratio by weight and particle size distribution of F (), using scanning electron microscopy, X-ray diffraction and thermodynamic modeling. Electron microscopy images show that size and aspect ratio of primary and secondary mullite have their largest figures for
the smallest and, in general, monotonically increase upon firing temperature. ΔGeff has been modeled by α()×(F/K)2+ß()×F/K+γ()(α,ß and γ are linear functions of ). The parameters of such a function have been determined by fitting it to the experimental ΔGeffs, inferred from quantitative phase analysis of X-ray diffraction patterns. We have gathered that (i) F/K affects energetics and mullite content more markedly than does, and (ii) the mullite formation is
energetically favored by decreasing F/K and increasing
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