SciCombinator

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Journal: Zeitschrift fur medizinische Physik

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The purpose of this study was to investigate the suitability of the microDiamond detector (mDD) type 60019 (PTW-Freiburg, Germany) for radial dose function measurements with High Dose Rate (HDR) 192Ir brachytherapy sources. An HDR 192Ir source model mHDR v2r (Nucletron BV, an Elekta company, The Netherlands) was placed at the centre of a MP3 water phantom (PTW-Freiburg, Germany) within a 4F needle. Three mDDs were employed to measure the radial dose function of the source by acquiring profiles along the source transverse axis. Meanwhile, the experimental setup was simulated using the Monte Carlo (MC) code MCNP6.1™ (Los Alamos National Laboratory, USA) to calculate phantom-size, absorbed-dose energy dependence and volume averaging correction factors. After applying the correction factors, the radial dose function gL® for the line source approximation was calculated as defined in the TG-43 formalism at radial distances from 0.5cm to 10cm and compared to the consensus gL® (ESTRO and AAPM). The percentage differences to the consensus gL® for all the three mDDs were from -2.3% to +1.4% for distances r≤5cm and -6.2% to +2.6% for larger distances. These results indicate the suitability of the mDD for HDR brachytherapy measurements when all required corrections are applied.

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Quantitative susceptibility mapping provides a measure for the local susceptibility within a voxel in magnetic resonance imaging (MRI). So far, theoretical and numerical studies focus on the assumption of a constant susceptibility inside each MR voxel. For blood vessel networks, however, susceptibility differences between blood and surrounding tissue occur on a much smaller length scale than the typical voxel size in routine MRI. In this work, the dependency of the quantitative susceptibility value on vessel size and voxel size is analyzed.

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The University Hospital of Düsseldorf, Germany (UKD) recently installed the Respiratory Gating for Scanners module (RGSC) (Varian Medical Systems, Palo Alto, USA). The aim of this article is to report on the commissioning and clinical implementation of the RGSC system. The steps encompassed the validation of the manufacturer’s specifications including functionality tests using a commercial and in-house developed breathing phantom, to establish calibration procedures, and clinical workflow analysis involving breath acquisition and patient data evaluation. In this context also the RGSC signal without motion was performed to assess the calibration procedure. Reproducibility test were conducted as well with breathing phantoms. Fifteen clinical breath curves were examined in order to assess the impact of treatment related uncertainties such as noises of the CT, patient positioning, movement of the CT table, unintended patient motion. Finally, different binning approaches were assessed and the effect on the CT reconstructions and methodic advantages were investigated. All technical specifications of the manufacturer were confirmed. A baseline drift of 1.83mm of the measured breath curve occurred during longitudinal movement of the CT table. This drift is smaller if the direction of table motion coincides precisely with the level of calibration. If the calibration is carried out on extensions for patient positioning we measured a baseline drift up to 6mm. It was found that especially for a combination of a ceiling mounted IR-camera and amplitude based 4D-CT reconstructions, precise calibration is prerequisite. The evaluations of patient breath curves and corresponding CT reconstructions revealed patient specific aspects and variations, respectively. Consequently patient selection criteria need to be established in parallel with the technical implementation and validation phase of respiratory gating.

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To investigate the capabilities of a modern pseudo-continuous arterial spin labeling (PCASL) technique for non-invasive assessment of the temporal and spatial distribution of the liver perfusion in healthy volunteers on a clinical MR system at 3T.

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Detailed information on the different methods used to compute charged-particle fluence spectra during a Monte Carlo (MC) calculation is scattered throughout the literature or discussed in internal reports. This work summarizes the most commonly used methods and introduces an alternative approach, makes comparisons between the different techniques, both from a theoretical ground and performing ad-hoc MC calculations, and discusses the advantages and constraints of each technique. It is concluded that methods based on the apportion of a track segment to the different energy bins of a linear or logarithmic grid are independent of the length of the track segment and the amount of energy loss between its extremes. This is the case for two of the methods presented, but not for a third one, the former group being considered to yield more accurate distributions in most cases. It is shown that the positron fluence contribution to the total restricted cema may amount up to several percent, and its omission lead to cema underestimates of that order. The influence of restricted radiative energy losses of electrons and positrons on fluence distribution and cema calculations are discussed on the grounds of the relative weight of restricted and unrestricted stopping powers, leading to expect a practically negligible influence on dosimetry calculations. The expectation is confirmed with MC calculations of a high-energy photon beam in gold, leading to the conclusion that restricted radiative energy losses can be disregarded for the most commonly used threshold energies for secondary charged particle production.

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Proton beams used for radiotherapy have potential for superior sparing of normal tissue, although range uncertainties are among the main limiting factors in the accuracy of dose delivery. The aim of this study was to benchmark an N-vinylpyrrolidone based polymer gel to perform three-dimensional measurement of geometric proton beam characteristics and especially to test its suitability as a range probe in combination with an anthropomorphic phantom. For single proton pencil beams as well as for 3×3cm2 mono-energy layers depth dose profiles, lateral dose distribution at different depths and proton range were evaluated in simple cubic gel phantoms at different energies from 75 to 115MeV and different dose levels. In addition, a 90MeV mono-energetic beam was delivered to an anthropomorphic 3D printed head phantom, which was filled with gel. Subsequently, all phantoms underwent magnetic resonance imaging using an axial pixel size of 0.68-0.98mm and with slice thicknesses of 2 or 3mm to derive a 3-dimensional distribution of the T2 relaxation time, which correlates with radiation dose. Indices describing lateral dose distribution and proton range were compared against predictions from a treatment planning system (TPS, for cubic and head phantoms) and Monte Carlo simulations (MC, for the head phantom) after manual rigid co-registration with the T2 relaxation time datasets. For all pencil beams, the FWHM agreement with TPS was better than 1mm or 7%. For the mono-energetic layer, the agreement with TPS in this respect was even better than 0.3mm in each case. With respect to range, results from gel measurements differed no more than 0.9mm (1.6%) from values predicted by TPS. In case of the anthropomorphic phantom, deviations with respect to a nominal range of about 61mm as well as in FWHM were slightly higher, namely within 1.0mm and 1.1mm respectively. Average deviations between gel and TPS/MC were similar (-0.3mm±0.4mm/-0.2±0.5mm). In conclusion, polymer gel dosimetry was found to be a valuable tool to determine geometric proton beam properties three-dimensionally and with high spatial resolution in simple cubic as well as in a more complex anthropomorphic phantom. Post registration range errors of the order of 1mm could be achieved. The additional registration uncertainty (95%) was 1mm.

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For selective internal radiation therapy (SIRT) the calculation of the 3D distribution of spheres based on individual blood flow properties is still an open and relevant research question. The purpose of this work is to develop and analyze a new treatment planning method for SIRT to calculate the absorbed dose distribution. For this intention, flow dynamics of the SIRT-spheres inside the blood vessels was simulated. The challenge is treatment planning solely using high-resolution imaging data available before treatment. The resolution required to reliably predict the sphere distribution and hence the dose was investigated. For this purpose, arteries of the liver were segmented from a contrast-enhanced angiographic CT. Due to the limited resolution of the given CT, smaller vessels were generated via a vessel model. A combined 1D/3D-flow simulation model was implemented to simulate the final 3D distribution of spheres and dose. Results were evaluated against experimental data from Y90-PET. Analysis showed that the resolution of the vessels within the angiographic CT of about 0.5mm should be improved to a limit of about 150μm to reach a reliable prediction.

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The characteristics of radiation detectors have to be assessed for dosimetry in the presence of magnetic fields, i.e. in conditions found in combined machines for magnetic resonance imaging and radiotherapy. While a lot of attention is directed toward correction factors for absolute dosimetry in magnetic fields, relative dose measurements are an equally important task to be performed. There is a need to experimentally analyze detector response differences in the build-up region in the presence of a transverse magnetic field.

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Recent work has shown that One Lung Flooding (OLF) enables acoustic access to central lung tumours which can be used for non-invasive ablation using therapeutic ultrasound (HIFU). Therefore acoustic properties of flooded lung as a saline-tissue compound was determined in earlier work, which revealed that atypical acoustical condition in lung exists. Their influence on the HIFU ablation process under aspects of clinical requirements has to be investigated before clinical introduction. For this study a MATLAB based ultrasound simulation tool and a customized bioheat solver were used to determine the temporal course of HIFU induced heating with the corresponding ablation zones. This work revealed that due to the low attenuation in flooded lung the heat induction and therefore the lesion size in lung tumours is enhanced. However, HIFU raster ablation schemes should only be used for benign tumours and the volumetric ablation scheme for malignancies. A minimum power density of 0.1Wcm-3 is required during volumetric ablation to radical ablate lung tumours. The simulations indicate that up to 3 T1 (∅ 3cm) tumours with a sufficient margin (>3mm) can be ablated during one flooding session. The ablation margin is dependent upon perfusion, intra-lobular temperature, as well as ablation temperature, and can be adjusted within range of 2-6mm depending on nodule size. The acoustic conditions in flooded lung are beneficial for thermal HIFU ablation in lung but require an individualized HIFU treatment planning.

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A new chemical exchange MRI method is proposed which allows for direct detection of exchanging solute protons with concurrent water background suppression.