Small field size dose-profile measurements using gel dosimeters,gafchromic films and micro-thermoluminescent dosimeters

The introduction of mini-multi-leaf collimators (MMLC) into radiotherapy has seen the use of smaller field sizes become increasingly important. Small field sizes that tightly conform to precise target regions are sought in radiotherapy to deliver doses with a high therapeutic ratio. MMLCs have made it possible to shrink field sizes in radiotherapy to below half a centimetre. The dosimetry of such fields with conventional dosimeters such as gas-ionisation chambers is not feasible due to limitations caused by the chambers relatively large size compared to the size of the collimated beam. In this work, the dose distribution of radiotherapy beams collimated to such small sizes were examined using polyacrylamide gels dosimeters, Gafchromic films and micro-thermoluminescence dosimeters (micro-TLDs). Dose penumbra widths obtained with gel dosimeters, Gafchormic film and micro-TLDs were generally in agreement with each other, although a wider FWHM of the field was measured with gel in comparison to film. An asymmetric dose distribution between the two axis profiles of a 3 × 3 mm collimated field was observed and can be attributed to an inherent asymmetry of the MMLC.     

基于蒙特卡罗方法的6 MV Truebeam剂量计算

使用蒙特卡罗方法快速准确地模拟6 MV Varian Truebeam医用电子直线加速器射野剂量特性,探究厂家提供相位空间源的可用性及模拟方法的准确性。以Varian公司提供的出束窗口位置处相位空间文件作为Beamnrc/EGSnrc输入源,模拟射野形成结构并计算10 cm10 cm射野下的均匀水体模中的剂量分布,将计算结果与相同条件下的实验测量数据进行比较。同一计算机模拟时,传统完整机头模拟方法需4~5 h,文中所述方法模拟时间可缩减至48 min,剂量计算结果与相同条件下所得实际测量数据可较好地吻合,两者的百分深度剂量差异低于3%,且建成区吻合良好;不同深度处80%等剂量线所包含的射野区域内百分离轴剂量比差异均低于3%,且半影区效果好。使用厂家提供相位空间文件作为EGSnrc入射源,能快速模拟治疗头,得到剂量计算结果与实际测量值的差异满足剂量计算精度要求。     

Measurement of multi-slice computed tomography dose profile with the Dose Magnifying Glass and the MOSkin radiation dosimeter

This study describes the application of two in-house developed dosimeters, the Dose Magnifying Glass (DMG) and the MOSkin dosimeter at the Centre for Medical Radiation Physics, University of Wollongong, Australia, for the measurement of CT dose profiles for a clinical diagnostic 16-slice MSCT scanner. Two scanner modes were used; axial mode and helical mode, and the effect of varying beam collimation and pitch was studied. With an increase in beam collimation in axial mode and an increase of CT pitch in helical mode, cumulative point dose at scanner isocentre decreased while FWHM increased. There was generally good agreement to within 3% between the acquired dose profiles obtained by the DMG and the film except at dose profile tails, where film over-responded by up to 30% due to its intrinsic depth dose dependence at low doses.     

重离子辐照对人肝癌HepG2细胞蛋白质组图谱的影响

分别制备经1 Gy C离子辐照和未经辐照人肝癌HepG2细胞的总蛋白样品, 采用固相pH梯度双向凝胶电泳技术进行蛋白分离, 用ImageMaster 2D双向电泳凝胶图像分析软件分析数字化图谱。 结果显示, 差异表达的蛋白质点为17个, 其中1个仅在未经C离子辐照的HepG2细胞中表达, 8个蛋白质点在辐照后的细胞中表达上调, 8个蛋白质点在辐照后的细胞中表达下调。 建立了重复性较好和分辨率较高的分离HepG2细胞总蛋白的双向电泳技术。 实验结果表明, C离子辐照HepG2细胞后其蛋白质组发生了改变。     

Tridimensional dosimetry for prostate IMRT treatments using MAGIC-f gel by MRI

Intensity-modulated radiation therapy is able to deliver complex dose distributions and a system able to determine tridimensional dose distribution during the quality assurance of the treatments would be of great interest. In this context polymeric gels can be a useful dosimeter. This work aims to apply this tridimensional dosimetry technique for two prostate IMRT treatments, using MAGIC-f as a gel dosimeter combined with the magnetic resonance imaging to determinate the dose distributions. Dose images obtained by the gel were compared with the treatment planning images and the gamma index was calculated as a quantitative comparison. In both plans a high pass rate was achieved in the gamma analyses (95.5% and 94.0% for plans 1 and 2 respectively) showing that they were approved in the tridimensional quality assurance.     

Characterization of ferric ions diffusion in Fricke gel dosimeters by using inverse problem techniques

Diffusion of ferric ions in ferrous sulfate (Fricke) gels represents one of the main drawbacks of some radiation detectors, such as Fricke gel dosimeters. In practice, this disadvantage can be overcome by prompt dosimeter analysis, and constraining strongly the time between irradiation and analysis, implementing special dedicated protocols aimed at minimizing signal blurring due to diffusion effects. This work presents a novel analytic modeling and numerical calculation approach of diffusion coefficients in Fricke gel radiation sensitive materials. Samples are optically analyzed by means of visible light transmission measurements by capturing images with a charge-coupled device camera provided with a monochromatic filter corresponding to the XO-infused Fricke solution absorbance peak. Dose distributions in Fricke gels are suitably delivered by assessing specific initial conditions further studied by periodical sample image acquisitions. Diffusion coefficient calculations were performed using a set of computational algorithms based on inverse problem formulation. Although 1D approaches to the diffusion equation might provide estimations of the diffusion coefficient, it should be calculated in the 2D framework due to the intrinsic bi-dimensional characteristics of Fricke gel layers here considered as radiation dosimeters. Thus a suitable 2D diffusion model capable of determining diffusion coefficients was developed by fitting the obtained algorithm numerical solutions with the corresponding experimental data. Comparisons were performed by introducing an appropriate functional in order to analyze both experimental and numerical values. Solutions to the second-order diffusion equation are calculated in the framework of a dedicated method that incorporates finite element method. Moreover, optimized solutions can be attained by gradient-type minimization algorithms. Knowledge about diffusion coefficient for a Fricke gel radiation detector is helpful in accounting for effects regarding elapsed time between dosimeter irradiation and further analysis. Hence, corrections might be included in standard dependence of optical density differences and actual, non-diffused, absorbed dose distributions. The obtained values for ferric ion diffusion coefficient are around 0.65 mm² h^?1, being in good agreement with previous works corresponding to similar Fricke gel dosimeter compositions. Therefore, more accurate 2D and 3D dose mapping might be attained, thus constituting valuable improvements in Fricke gel dosimetry, and parallely a high precision method of diffusion modeling and calculation has been developed.     

A novel dosimetry system for computed tomography using phototransistor

Computed tomography (CT) dosimetry normally uses an ionization chamber 100 mm long to estimate the computed tomography dose index (CTDI), however some reports have already indicated that small devices could replace the long ion chamber to improve quality assurance procedures in CT dosimetry. This paper presents a novel dosimetry system based in a commercial phototransistor evaluated for CT dosimetry. Three detector configurations were developed for this system: with a single, two and four devices. Dose profile measurements were obtained with them and their angular responses were evaluated. The results showed that the novel dosimetry system with the phototransistor could be an alternative for CT dosimetry. It allows to obtain the CT dose profile in details and also to estimate the CTDI in longer length than the 100 mm pencil chamber. The angular response showed that the one device detector configuration is the most adequate among the three configurations analyzed in this study.     

TLD measurements and Monte Carlo simulations for glandular dose and scatter fraction assessment in mammography: A comparative study

The main purpose of this study was to validate and compare Mean Glandular Dose (MGD) values obtained using Monte Carlo simulations with experimental values obtained from Entrance Surface Dose (ESD) and depth dose measurements performed in a Hospital mammography unit. ESD and depth dose were measured using ThermoLuminescent Dosimeters (TLDs), and a tissue equivalent mammography phantom recommended by the American College of Radiology (ACR). Measurements and Monte Carlo simulations were also compared with the MGD calculated using the Automatic Exposure Control (AEC) system of the mammographic unit. In the simulations the Doppler energy broadening effect was also taken into account. The simulated ESD are about 5%–10% higher than the measured ESD values. The deviation between the measured and simulated MGD values in the phantom is of about 15%. The MGD evaluated using the AEC system is smaller both with respect to the Monte Carlo simulation and experimental result by a factor of about 15% and 25% respectively. Moreover the BackScatter Factor (BSF) estimated by Monte Carlo simulations was used for the MGD calculation according to the Wu’s method. Finally the inclusion of the energy broadening effect on MGD calculation produces negligible variations on the simulated results.     

Effects of heat deproteinate bone and polynucleotides on bone regeneration: an experimental study on rat

This experimental study evaluated the effects of polynucleotides on bone regeneration on rats. Defects with a diameter of 2mm were prepared in the thickness of cortical bone of 32 rat tibiae and filled with different compounds: polynucleotide gel (PDRN), deproteinated porcine cortical bone (HDB) obtained by high temperature heating in the form of granules and a paste made of HDB granules and PDRN gel. Bone regeneration of the gaps was histologically analysed after a treatment time ranging from 1 to 12 weeks. Both PDRN and HDB stimulated bone growth and repair, but the paste prepared combining HDB granules and PDRN showed the best performance with faster filling, better osteconductive and biocompatible properties and easier handling. This study suggests that the paste prepared combining HDB and PDRN gel induces rapid bone regeneration in different clinical situations.     

Implementation of GaN based real-time source position monitoring in HDR brachytherapy

For High Dose Rate (HDR) brachytherapy Quality Assurance (QA), we propose a seed position verification method by using two GaN dosimetric probes integrated in an instrumented phantom. As dose rate measured from one probe is related to the seed distance, seed position inside catheter can be determined. The proposed two-probe method extends measureable range and improves accuracy in comparison with the use of one single probe. One instrumented phantom implementing the method was designed, fabricated and tested in clinical conditions. The measureable range of the two-probe method is 105 mm compared to 36 mm using single probe. The validity of the method was experimentally verified.     

X-ray and gamma-ray absorbed dose profiles in teeth: An EPR and modeling study

Dose profiles in teeth have been experimentally and theoretically studied for different energies and geometries of incident X- and gamma-rays. The experiments were conducted with teeth inside of an Alderson phantom using monodirectional radiation beams at selected energies; they revealed two effects: an apparent lack of dose attenuation between the buccal and the lingual sides of the teeth for energies higher than 120 keV and an attenuation between first and last tooth layers for low-energy beams in the range from 0.28 to 0.57. Monte Carlo simulations confirmed the experimental data and provided dose profiles for other energies and geometries. In particular, exposure in the rotational radiation field produces pronounced dose profiles only for energies lower than 60 keV. The usefulness of these data to estimate the average energy of accidental radiation field is discussed.     

Evaluation of the meridional longitudinal spherical aberration from corneal topography measurements

This paper shows how corneal topographic data can be used to determine the value of the longitudinal spherical aberration. We have obtained the corneal profiles and the values of the longitudinal spherical aberration for the rays propagating within the steepest and flattest meridional planes, by using a real raytracing algorithm. These corneal profiles have been also fitted to conicoids and the asphericity parameter has been calculated. We have found that the longitudinal spherical aberration follows a parabolic dependence for a circular region of 5 mm in diameter. This parabolic dependence has been fitted with a polynomial function. The data provided by commercial topographic systems can be used to obtain the longitudinal spherical aberration along the selected meridians.     

Auditory brain stem responses from human infants: pure-tone masking profiles for clicks and filtered clicks

The effects of simultaneous pure-tone maskers on ABR wave V latency and amplitude were examined in three-month-old infants as a means of delineating the frequency specificity of these responses in the immature auditory system. Masking profiles at two intensities (60 and 40 dBn HL) were obtained for click, as well as 4000- and 1000-Hz filtered-click stimuli. Infant profiles, obtained by measuring both latency and amplitude shifts as a result of the discrete-frequency maskers, were compared to adult data obtained under an identical masking paradigm. Both latency and amplitude analyses showed masking profiles for infants which reveal greater low-frequency contribution to responses than found in adult profiles. Additionally, the infant profiles reveal clear differences in the degree of high-frequency spread of masking when comparisons are made to the adult data.     

Signal profile measurements of single- and double-volume acquisitions with image-selected in vivo spectroscopy for 31P magnetic resonance spectroscopy

The volume-selection performance was studied for single- and double-volume-of-interest (VOI) acquisition with the volume-selection method image-selected in vivo spectroscopy for ³¹P magnetic resonance spectroscopy. High-resolution signal profiles were measured using a phantom simulating a brain. Inside the phantom there was a small, remotely controlled, movable signal source filled with ortho-phosphoric acid. Signal profiles of the VOI were measured in three perpendicular directions for 1VOI (single VOI) and 2VOI (double VOI) acquisition. The measured signal profiles for both acquisitions were very similar, but they showed a discrepancy with regard to the intended VOI (iVOI). The transition regions were on average 3.8 mm and the average full width at half maximum of the signal profile was 30 mm for an iVOI size of 30130130 (mm³). No displacement was observed in the signal profiles. To avoid overlapping signal profiles, the minimum separation between two iVOIs was found to be 10 mm in our magnetic resonance (MR) system. A substantial negative signal contribution from regions outside the iVOI was measured in the y-direction for 1VOI acquisition and one of the two VOIs in 2VOI acquisition. The other VOI in 2VOI acquisition exhibited only minor contamination. The measurements presented underline the importance of detailed knowledge on the volume selection performance in in vivo magnetic resonance spectroscopy.     

MCNP Dose Calculations in a CT Phantom for Therapeutic External Photon Beam

In this paper, we have addressed the problem of the radiation transport with the Monte Carlo N particle(MCNP) code. This is a general purpose Monte Carlo tool designed to transport neutron, photon and electron in three dimensional geometries. To examine the performance of MCNP5 code in the field of external radiotherapy, we performed the modeling of an Electron Density phantom (EDP) irradiated by photons from 60Co source. The model was used to calculate the Percent Depth Dose (PDD) at different depths in an EDP. One field size for PDD has been examined. A 60Co photons source placed at 80 cm source to surface distance (SSD). The results of calculations were compared to TPS data obtained at National Institute of Oncology of Rabat.     

Studies of gelation process investigated by fast field cycling relaxometry and dynamical rheology: the case of aqueous low methoxyl pectin solution

The Fast Field Cycling Relaxometry and the dynamical rheology were applied to study the gelation process in aqueous low methoxyl pectin solution in the presence of divalent cations from the calcium chloride. The model-free approach to the analysis of 1H NMRD data was used to separate the information on the static and dynamic behaviour of the system provided by the dispersion profiles of the systems tested. Mechanical spectroscopy was applied as a complementary method to probe the network formation. The structural changes were most pronounced for the concentration of calcium chloride between 10 and 15 mM (the static parameter beta was found to vary from 0.936 x 10(7) to 1.27 x 10(7) 1/s2) revealing the sol-gel transition on visual inspection. The presence of the gel network was confirmed by the rheological measurements. All mechanical spectra exhibited the gel like character, and the solid like response was more evident at higher concentrations of calcium cations. The mean correlation time increased with increasing salt concentration, but remained unchanged for the concentrations above 15 mM. The dynamical changes were most significant at the phase transition from the sol to the gel state.     

Preparation of arsenic‐containing white rice grains as calibration standards for X‐ray fluorescence analysis of total arsenic in cereals

A preparation method of arsenic‐containing white rice grains as calibration standards was developed for the X‐ray fluorescence (XRF) analysis of arsenic in rice grains. Calibration standards were prepared by adding 10 g of white rice grains (from Japan) to 100 ml methanol‐containing dimethylarsinic acid corresponding to 2–100 µg arsenic. The mixture was heated, dried at 150 °C, cooled to room temperature, and then stored in a silica gel desiccator. A total of 5.0 g of each calibration standard was packed into a polyethylene cup (32 mm internal diameter and 23 mm height) covered with a 6‐µm‐thick polypropylene film and then analyzed by wavelength‐dispersive XRF spectrometry. The calibration curve for arsenic obtained from the white rice grains containing arsenic showed good linearity over a concentration range of 0.21–5.00 mg kg^?1 arsenic. The limit of detection of arsenic was 0.080 mg kg^?1. To check the reliability of the XRF method, the concentrations of arsenic in six samples of grain cereals and two samples of flour were compared with those obtained by atomic absorption spectrometry after acid decomposition. The values obtained by both analytical methods showed good agreement. Copyright © 2016 John Wiley & Sons, Ltd.     

Experimental study of cold gas spraying through a mask. Part 1

The paper presents experimental data for production of a coating using cold gas dynamic spraying with a mask with transverse size in the range 0.3–1 mm and placed at different distances from the substrate. The coated samples were produced, and coating profiles were measured in the vicinity of the masked zone. The tests with depositing of aluminum powder and copper powder demonstrated that the distinct profile of masked zone is obtained for placing the mask at a distance below critical (depending of spray conditions). The most accurate boundary of the masked zone takes place at a minimal distance (depends on the coating thickness). Depending on the spraying conditions, the increase in the mask-substrate distance may result either in monotonic decline of the masked zone width or a slight increase for a certain range. Experimental data are generalized by normalizing with the transverse size of the mask (under other equal conditions).     

Photoacoustic Tomography of Biological Sample Using Phase-controlled Focus Algorithm

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Photoacoustic Tomography of Biological Sample Using Phase-controlled Focus Algorithm

Photoacoustic (PA) imaging is a new imaging modality, which converts pressure signals received by a scanning detector to a local distribution of electromagnetic absorption density. In this paper an experiment result of a photoacoustic tomography to depths of ～7 mm for a real tissue is presented, using a 532-nm pulse YAG laser. The time-resolved stress detection technique was used for PA signal detection with a high temporal resolution. A phase-controlled focus algorithm was used for image reconstruction. Images of different depth profiles in tissue were obtained. The depth resolution was 30 μm and could be up to 10 μm using a wide-band tranducer.