Poster: ECR 2016 / C-2103 / Influence of X-ray energy spectrum in the post-reconstruction method for beam hardening correction in polyenergetic CT reconstruction by: B. Yang, H. Geng, W. W. Lam, W. K. R. Wong, K. Y. Cheung, S. K. Yu; Hong Kong/HK
Purpose Tomotherapy delivers an intensity‐modulated radiation therapy ( IMRT ) treatment by the synchronization of gantry rotation, multileaf collimator ( MLC ), and couch movement. This dynamic nature makes the quality assurance ( QA ) important and challenging. The purpose of this study is to develop some methodologies using an Arc CHECK for accurate QA measurements of the gantry angle and speed, MLC synchronization and leaf open time, couch translation per gantry rotation, couch speed and uniformity, and constancy of longitudinal beam profile for a Tomotherapy unit. Methods Four test plans recommended by AAPM Task Group 148 ( TG 148) and the manufacturer were chosen for this study. Helical and static star shot tests are used for checking the leaves opened at the expected gantry angles. Another helical test is to verify the couch traveled the expected distance per gantry rotation. The final test is for checking the couch speed constancy with a static gantry. Arc CHECK can record the detector signal every 50 ms as a movie file, and has a virtual inclinometer for gantry angle measurement. These features made the measurement of gantry angle and speed, MLC synchronization and leaf open time, and longitudinal beam profile possible. A shaping parameter was defined for facilitating the location of the beam center during the plan delivery, which was thereafter used to calculate the couch translation per gantry rotation and couch speed. The full width at half maximum ( FWHM ) was calculated for each measured longitudinal beam profile and then used to evaluate the couch speed uniformity. Furthermore, a mean longitudinal profile was obtained for constancy check of field width. The machine trajectory log data were also collected for comparison. Inhouse programs were developed in MATLAB to process both the Arc CHECK and machine log data. Results The deviation of our measurement results from the log data for gantry angle was calculated to be less than 0.4°. The percentage differences between measured and planned leaf open time were found to be within 0.5% in all the tests. Our results showed mean values of MLC synchronization of 0.982, 0.983, and 0.995 at static gantry angle 0°, 45°, and 135°, respectively. The mean value of measured couch translation and couch speed by Arc CHECK had less than 0.1% deviation from the planned values. The variation in the value of FWHM suggested the couch speed uniformity was better than 1%. The mean of measured longitudinal profiles was suitable for constancy check of field width. Conclusion Precise and efficient methods for measuring the gantry angle and speed, leaf open time, couch translation per gantry rotation, couch speed and uniformity, and constancy of longitudinal beam profile of Tomotherapy using Arc CHECK have been developed and proven to be accurate compared with machine log data. Estimation of the Tomotherapy binary MLC leaf open time is proven to be precise enough to verify the leaf open time as small as 277.8 ms. Our method also makes the observation and quantification of the synchronization of leaves possible.
Abstract The aim of current work was to present a novel evaluation procedure implemented for checking the constancy of beam path accuracy of a CyberKnife system based on ArcCHECK. A tailor‐made Styrofoam with four implanted fiducial markers was adopted to enable the fiducial tracking during beam deliveries. A simple two‐field plan and an isocentric plan were created for determining the density override of Arc CHECK in MultiPlan and the constancy of beam path accuracy respectively. Correlation curves for all diodes involved in the study were obtained by analyzing the dose distributions calculated by MultiPlan after introducing position shifts in anteroposterior, superoinferior, and left–right directions. The ability of detecting systematic position error was also evaluated by changing the position of alignment center intentionally. The one standard deviation ( SD ) result for reproducibility test showed the RMS of 0.054 mm and the maximum of 0.263 mm, which was comparable to the machine self‐test result. The mean of absolute value of position errors in the constancy test was measured to 0.091 mm with a SD of 0.035 mm, while the root‐mean‐square was 0.127 mm with a SD of 0.034 mm. All introduced systematic position errors range from 0.3 to 2 mm were detected successfully. Efficient method for evaluating the constancy of beam path accuracy of CyberKnife has been developed and proven to be sensitive enough for detecting a systematic drift of robotic manipulator. Once the workflow is streamlined, our proposed method will be an effective and easy quality assurance procedure for medical physicists.
When skin is occluded by continuous wearing of incontinence pads, it becomes over-hydrated, making the skin more susceptible to mechanical damage and bacterial attack than normal skin. This project focused on understanding the impact of skin occlusion on (i) the hydration of the stratum comeum (SC) - the outermost layer of the skin, and (ii) friction between the skin and nonwoven materials. A methodology for measuring the excess water in over-hydrated skin using evaporimetry was developed, validated and used to compare the performances of five commonly used evaporimetry devices, and to investigate their strengths and limitations. All measurements were made on the volar forearm skin of one young female subject. Good reproducibility was found for each of the five devices, but some significant differences were found between measurements made with different devices. Some possible causes for these discrepancies were investigated with partial success: correction factors obtained from various calibration procedures were applied and reduced differences to some extent, but significant difference still remained. It was concluded that the methodology developed could be used with confidence to compare readings made with the same device, but it would be unwise to trust the absolute values obtained until the reasons for differences between devices have been more fully explained. A new approach for measuring the water distribution within the SC - Opto-Thermal Transient Emission Radiometry (OTTER) - was adopted in this work and a methodology was developed for measuring the saturation profile within over-hydrated SC. The relationship between the SC saturation at the surface (measured using OTTER) and the water vapour flux from over-hydrated SC (measured using evaporimetry) was investigated using the volar forearms of three young female subjects. As expected, strong correlation was found during desorption, with a dog-leg in the plot at about 36% saturation from two subjects, which was consistent with the transition between loosely and tightly bound water reported by Berardesca (1997). Two methods for measuring friction between nonwoven materials and the over-hydrated or normal skin were developed and validated, on the volar forearms of five young female subjects. Coefficients of friction were measured with the two methods and compared. Good reproducibility and remarkably good agreement was found between the two methods, even though one of the methods (curved friction) assumed the arm to be a rigid cylinder and the nonwoven material to be inextensible. Additional theoretical work (by Cottenden (2007)) and experimental work (by Karavokiros (2007)) was conducted to explain and extend the findings. The theoretical analysis showed that the equation describing friction around a cylinder is valid for any convex prism, and the experimental work supported the solution very well. It was concluded that the curved method developed was simple to run and produced results with good reproducibility.
Although evaporimetry (the measurement of water vapour flux density from the skin) has often been used to study the impact on skin hydration of using products such as baby diapers and incontinence pads, it is difficult to interpret results and to compare data from different studies because of the diversity of unvalidated methodologies used. The aim of this work was to develop a robust methodology for measuring the excess water in over-hydrated skin and test it on volar forearm and hip skin which had been occluded with saline soaked patches. Three repeat measurements were made on the volar forearm and the hip of five young (31-44 years) and six older (67-85 years) women and moderately good within-subject repeatability was found for both skin sites for both subject groups. Measurements taken from the hip were significantly higher (P = 0.001) than those from the arm and had larger coefficients of variation (3.5-22.1%) compared to arms (3.0-14.0%). There were no significant differences between young and older skin, implying that women for future studies could be recruited without regard to age. This is the first time that a robust evaporimetric methodology for quantifying excess water in over-hydrated skin has been described and validated, and it will form a solid basis for future work.
An analytical mathematical model for friction between a fabric strip and the volar forearm has been developed and validated experimentally. The model generalizes the common assumption of a cylindrical arm to any convex prism, and makes predictions for pressure and tension based on Amontons' law. This includes a relationship between the coefficient of static friction ( μ) and forces on either end of a fabric strip in contact with part of the surface of the arm and perpendicular to its axis. Coefficients of friction were determined from experiments between arm phantoms of circular and elliptical cross-section (made from Plaster of Paris covered in Neoprene) and a nonwoven fabric. As predicted by the model, all values of μ calculated from experimental results agreed within ±8 per cent, and showed very little systematic variation with the deadweight, geometry, or arc of contact used. With an appropriate choice of coordinates the relationship predicted by this model for forces on either end of a fabric strip reduces to the prediction from the common model for circular arms. This helps to explain the surprisingly accurate values of μ obtained by applying the cylindrical model to experimental data on real arms.
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