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1.
Health Place ; 69: 102560, 2021 05.
Artigo em Inglês | MEDLINE | ID: mdl-33756438

RESUMO

Studies investigating neighborhood walkability and physical activity in people with type 2 diabetes (T2D) mainly used self-report measures, and only few studies assessed the association with glycemic control. This study assessed the associations between objectively measured (i.e. GIS based) and subjectively measured (i.e. questionnaire-based) neighborhood walkability and changes in glycemic markers in people with T2D, and whether this association was mediated by device-measured physical activity (PA), in the Diabetes Care System Cohort (n = 1230). Neither objective or subjectively measured walkability was associated with glycemic control. In mediation analyses we observed no overall mediation by PA.


Assuntos
Diabetes Mellitus Tipo 2 , Planejamento Ambiental , Exercício Físico , Humanos , Características de Residência , Caminhada
2.
Phys Med Biol ; 63(12): 125008, 2018 06 11.
Artigo em Inglês | MEDLINE | ID: mdl-29786612

RESUMO

A generic formalism is proposed for reference dosimetry in the presence of a magnetic field. Besides the regular correction factors from the conventional reference dosimetry formalisms, two factors are used to take into account magnetic field effects: (1) a dose conversion factor to correct for the change in local dose distribution and (2) a correction of the reading of the dosimeter used for the reference dosimetry measurements. The formalism was applied to the Elekta MRI-Linac, for which the 1.5 T magnetic field is orthogonal to the 7 MV photon beam. For this setup at reference conditions it was shown that the dose decreases with increasing magnetic field strength. The reduction in local dose for a 1.5 T transverse field, compared to no field is 0.51% ± 0.03% at the reference point of 10 cm depth. The effect of the magnetic field on the reading of the dosimeter was measured for two waterproof ionization chambers types (PTW 30013 and IBA FC65-G) before and after multiple ramp-up and ramp-downs of the magnetic field. The chambers were aligned perpendicular and parallel to the magnetic field. The corrections of the readings of the perpendicularly aligned chambers were 0.967 ± 0.002 and 0.957 ± 0.002 for respectively the PTW and IBA ionization chambers. In the parallel alignment the corrections were small; 0.997 ± 0.001 and 1.002 ± 0.003 for the PTW and IBA chamber respectively. The change in reading due to the magnetic field can be measured by individual departments. The proposed formalism can be used to determine the correction factors needed to establish the absorbed dose in a magnetic field. It requires Monte Carlo simulations of the local dose and measurements of the response of the dosimeter. The formalism was successfully implemented for the MRI-Linac and is applicable for other field strengths and geometries.


Assuntos
Campos Magnéticos , Fótons , Dosímetros de Radiação/normas , Humanos , Método de Monte Carlo , Radiometria/normas , Padrões de Referência
3.
Phys Med Biol ; 63(8): 085015, 2018 04 19.
Artigo em Inglês | MEDLINE | ID: mdl-29521280

RESUMO

As a prerequisite for clinical treatments it was necessary to characterize the Elekta 1.5 T MRI-linac 7 MV FFF radiation beam. Following acceptance testing, beam characterization data were acquired with Semiflex 3D (PTW 31021), microDiamond (PTW 60019), and Farmer-type (PTW 30013 and IBA FC65-G) detectors in an Elekta 3D scanning water phantom and a PTW 1D water phantom. EBT3 Gafchromic film and ion chamber measurements in a buildup cap were also used. Special consideration was given to scan offsets, detector effective points of measurement and avoiding air gaps. Machine performance has been verified and the system satisfied the relevant beam requirements of IEC60976. Beam data were acquired for field sizes between 1 × 1 and 57 × 22 cm2. New techniques were developed to measure percentage depth dose (PDD) curves including the electron return effect at beam exit, which exhibits an electron-type practical range of 1.2 ± 0.1 cm. The Lorentz force acting on the secondary charged particles creates an asymmetry in the crossline profiles with an average shift of +0.24 cm. For a 10 × 10 cm2 beam, scatter from the cryostat contributes 1% of the dose at isocentre. This affects the relative output factors, scatter factors and beam profiles, both in-field and out-of-field. The average 20%-80% penumbral width measured for small fields with a microDiamond detector at 10 cm depth is 0.50 cm. MRI-linac penumbral widths are very similar to that of the Elekta Agility linac MLC, as is the near-surface dose PDD(0.2 cm) = 57%. The entrance surface dose is ∼36% of Dmax. Cryostat transmission is quantified for inclusion within the treatment planning system. As a result, the 1.5 T MRI-linac 7 MV FFF beam has been characterised for the first time and is suitable for clinical use. This was a key step towards the first clinical treatments with the MRI-linac, which were delivered at University Medical Center Utrecht in May 2017 (Raaymakers et al 2017 Phys. Med. Biol. 62 L41-50).


Assuntos
Imageamento por Ressonância Magnética/métodos , Aceleradores de Partículas , Imagens de Fantasmas , Radiometria/métodos , Elétrons , Humanos , Campos Magnéticos , Posicionamento do Paciente , Água
4.
Phys Med Biol ; 63(5): 05NT04, 2018 03 08.
Artigo em Inglês | MEDLINE | ID: mdl-29239857

RESUMO

Accurate small-field dosimetry is critical for a magnetic resonance linac (MRI-linac). The PTW 60019 microDiamond is close to an ideal detector for small field dosimetry due to its small physical size, high signal-to-noise ratio and approximate water equivalence. It is important to fully characterise the performance of the detector in a 1.5 T magnetic field prior to its use for MRI-linac commissioning and quality assurance. Standard techniques of detector testing have been implemented, or adapted where necessary to suit the capabilities of the MRI-linac. Detector warmup, constancy, dose linearity, dose rate linearity, field size dependence and leakage were within tolerance. Measurements with the detector were consistent with ion chamber measurements for medium sized fields. The effective point of measurement of the detector when used within a 1.5 T magnetic field was determined to be 0.80 ± 0.23 mm below the top surface of the device, consistent with the existing vendor recommendation and alignment mark at 1.0 mm. The angular dependence was assessed. Variations of up to 9.7% were observed, which are significantly greater than in a 0 T environment. Within the expected range of use, the maximum effect is approximately 0.6% which is within tolerance. However for large beams within a magnetic field, the divergence and consequent variation in angle of photon incidence means that the microDiamond would not be ideal for characterising the profiles and it would not be suitable for determining large-field beam parameters such as symmetry. It would also require a correction factor prior to use for patient-specific QA measurements where radiation is delivered from different gantry angles. The results of this study demonstrate that the PTW 60019 microDiamond detector is suitable for measuring small radiation fields within a 1.5 T magnetic field and thus is suitable for use in MRI-linac commissioning and quality assurance.


Assuntos
Imageamento por Ressonância Magnética/métodos , Aceleradores de Partículas/instrumentação , Imagens de Fantasmas , Radiometria/instrumentação , Humanos , Campos Magnéticos , Fótons , Radiometria/métodos , Água
5.
Phys Med Biol ; 62(23): L41-L50, 2017 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-29135471

RESUMO

The integration of 1.5 T MRI functionality with a radiotherapy linear accelerator (linac) has been pursued since 1999 by the UMC Utrecht in close collaboration with Elekta and Philips. The idea behind this integrated device is to offer unrivalled, online and real-time, soft-tissue visualization of the tumour and the surroundings for more precise radiation delivery. The proof of concept of this device was given in 2009 by demonstrating simultaneous irradiation and MR imaging on phantoms, since then the device has been further developed and commercialized by Elekta. The aim of this work is to demonstrate the clinical feasibility of online, high-precision, high-field MRI guidance of radiotherapy using the first clinical prototype MRI-Linac. Four patients with lumbar spine bone metastases were treated with a 3 or 5 beam step-and-shoot IMRT plan. The IMRT plan was created while the patient was on the treatment table and based on the online 1.5 T MR images; pre-treatment CT was deformably registered to the online MRI to obtain Hounsfield values. Bone metastases were chosen as the first site as these tumors can be clearly visualized on MRI and the surrounding spine bone can be detected on the integrated portal imager. This way the portal images served as an independent verification of the MRI based guidance to quantify the geometric precision of radiation delivery. Dosimetric accuracy was assessed post-treatment from phantom measurements with an ionization chamber and film. Absolute doses were found to be highly accurate, with deviations ranging from 0.0% to 1.7% in the isocenter. The geometrical, MRI based targeting as confirmed using portal images was better than 0.5 mm, ranging from 0.2 mm to 0.4 mm. In conclusion, high precision, high-field, 1.5 T MRI guided radiotherapy is clinically feasible.


Assuntos
Neoplasias Ósseas/radioterapia , Região Lombossacral/efeitos da radiação , Imageamento por Ressonância Magnética/instrumentação , Aceleradores de Partículas/instrumentação , Planejamento da Radioterapia Assistida por Computador/métodos , Radioterapia Guiada por Imagem/métodos , Neoplasias da Coluna Vertebral/radioterapia , Idoso , Neoplasias Ósseas/secundário , Humanos , Pessoa de Meia-Idade , Imagens de Fantasmas , Radiometria , Dosagem Radioterapêutica , Neoplasias da Coluna Vertebral/patologia
6.
Med Phys ; 43(7): 3961, 2016 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-27370114

RESUMO

PURPOSE: A protocol for reference dosimetry for the MR-linac is under development. The 1.5 T magnetic field changes the mean path length of electrons in an air-filled ionization chamber but has little effect on the electron trajectories in a surrounding phantom. It is therefore necessary to correct the response of an ionization chamber for the influence of the magnetic field. Solid phantoms are used for dosimetry measurements on the MR-linac, but air is present between the chamber wall and phantom insert. This study aimed to determine if this air influences the ion chamber measurements on the MR-linac. The absolute response of the chamber and reproducibility of dosimetry measurements were assessed on an MR-linac in solid and water phantoms. The sensitivity of the chamber response to the distribution of air around the chamber was also investigated. METHODS: Measurements were performed on an MR-linac and replicated on a conventional linac for five chambers. The response of three waterproof chambers was measured with air and with water between the chamber and the insert to measure the influence of the air volume on absolute chamber response. The distribution of air around the chamber was varied indirectly by rotating each chamber about the longitudinal chamber axis in a solid phantom and a water phantom (waterproof chambers only) and measuring the angular dependence of the chamber response, and varied directly by displacing the chamber in the phantom insert using a paper shim positioned at different orientations between the chamber casing and the insert. RESULTS: The responses of the three waterproof chambers measured on the MR-linac were 0.7%-1.2% higher with water than air in the chamber insert. The responses of the chambers on the conventional linac changed by less than 0.3% when air in the insert was replaced with water. The angular dependence of the chambers ranged from 0.6% to 1.9% in the solid phantom on the MR-linac but was less than 0.5% in water on the MR-linac and less than 0.3% in the solid phantom on the conventional linac. Inserting a shim around the chamber induced changes of the chamber response in a magnetic field of up to 2.2%, but the change in chamber response on the conventional linac was less than 0.3%. CONCLUSIONS: The interaction between the magnetic field and secondary electrons in the air around the chamber reduces the charge collected from 0.7% to 1.2%. The large angular dependence of ion chambers measured in the plastic phantom in a magnetic field appears to arise from a change of air distribution as the chamber is moved within the insert, rather than an intrinsic isotropy of the chamber sensitivity to radiation. It is recommended that reference dosimetry measurements on the MR-linac can be performed only in water, rather than in existing plastic phantoms.


Assuntos
Ar , Imageamento por Ressonância Magnética/instrumentação , Aceleradores de Partículas/instrumentação , Imagens de Fantasmas , Radiometria/instrumentação , Radiometria/métodos , Desenho de Equipamento , Imageamento por Ressonância Magnética/métodos , Radioterapia Guiada por Imagem/instrumentação , Radioterapia Guiada por Imagem/métodos , Água
7.
Phys Med Biol ; 61(3): N80-9, 2016 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-26767389

RESUMO

At the UMC Utrecht, a linear accelerator with integrated magnetic resonance imaging (MRI) has been developed, the MR-linac. Patient-specific quality assurance (QA) of treatment plans for MRI-based image guided radiotherapy requires QA equipment compatible with this 1.5 T magnetic field. The purpose of this study was to examine the performance characteristics of the ArcCHECK-MR in a transverse 1.5 T magnetic field. To this end, the short-term reproducibility, dose linearity, dose rate dependence, field size dependence, dose per pulse dependence and inter-diode dose response variation of the ArcCHECK-MR diode array were evaluated on a conventional linac and on the MR-linac. The ArcCHECK-MR diode array performed well for all tests on both linacs, no significant differences in performance characteristics were observed. Differences in the maximum dose deviations between both linacs were less than 1.5%. Therefore, we conclude that the ArcCHECK-MR can be used in a transverse 1.5 T magnetic field.


Assuntos
Imageamento por Ressonância Magnética/métodos , Aceleradores de Partículas/instrumentação , Campos Magnéticos , Aceleradores de Partículas/normas , Radiometria/métodos , Reprodutibilidade dos Testes
8.
Phys Med Biol ; 59(15): 4099-109, 2014 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-24989159

RESUMO

The MR-linac is a hybrid MRI radiotherapy system allowing dose delivery in a 1.5 T magnetic field. This paper presents the design and performance of a prototype MR-linac compatible scanning water phantom. Since a scanning water phantom requires dose detectors, the performance air-filled ionization chambers in the magnetic field was characterized. We have found that the linearity and reproducibility of an ionization chamber are unaffected by the magnetic field. Also, moving the ionization chambers in a magnetic field during irradiation does not affect the dose response. When scanning in-plane profiles, the change in irradiation orientation can influence the ionization chamber dose response by up to 0.4%. However this effect can be eliminated by rotating the ionization chamber by 90° before measuring the in-plane profile. The performance of the total scanning water phantom was validated at a clinical setup in a 0 T magnetic field. There was no significant difference between the dose profiles measured with a standard clinical scanning water phantom and the profiles measured with the MR-linac compatible scanning water phantom. The performance of the MR-linac scanning water phantom in the MR-linac was validated using Gafchromic EBT2 film. There was no significant difference in dose profiles between the MR-linac scanning water phantom and the radiochromic film. These results indicate that automated scanning water phantom measurements using ionization chamber detectors are possible in the MR-linac.


Assuntos
Imageamento por Ressonância Magnética/métodos , Radiometria/métodos , Radiocirurgia/métodos , Imagens de Fantasmas , Radiometria/instrumentação , Radiocirurgia/instrumentação , Água/química
9.
Phys Med Biol ; 59(7): 1845-55, 2014 Apr 07.
Artigo em Inglês | MEDLINE | ID: mdl-24625540

RESUMO

At the UMC Utrecht a prototype MR-linac has been installed. The system consists of an 8 MV Elekta linear accelerator and a 1.5 T Philips MRI system. This paper investigates the performance of the IC PROFILER™, a multi-axis ionization chamber array, in a 1.5 T magnetic field. The influence of the magnetic field on the IC PROFILER™ reproducibility, dose response linearity, pulse rate frequency dependence, power to electronics, panel orientation and ionization chamber shape were investigated. The linearity, reproducibility, pulse rate frequency dependence, panel orientation and ionization chamber shape are unaffected by the magnetic field. When the measurements results are normalized to the centre reference chamber, the measurements can commence unaltered. Orientation of the ionization chambers in the magnetic field is of importance, therefore caution must be taken when comparing or normalizing results from several different axes. IC PROFILER™ dose profiles were compared with film dose profiles obtained simultaneously in the MR-linac. Deviation between the film and the IC PROFILER™ data was caused by the noise in the film, indicating correct performance of the IC PROFILER™ in the transverse 1.5 T magnetic field.


Assuntos
Campos Magnéticos , Radiometria/instrumentação , Estudos de Viabilidade , Modelos Lineares , Aceleradores de Partículas , Reprodutibilidade dos Testes , Fatores de Tempo
10.
Phys Med Biol ; 58(17): 5945-57, 2013 Sep 07.
Artigo em Inglês | MEDLINE | ID: mdl-23938362

RESUMO

In the UMC Utrecht a prototype MR-linac has been installed. The system consists of a 6 MV Elekta (Crawley, UK) linear accelerator and a 1.5 T Philips (Best, The Netherlands) Achieva MRI system. This paper investigates the feasibility to correct the ionization chamber reading for the magnetic field within the dosimetry calibration method described by Almond et al (1999 Med. Phys. 26 1847-70). Firstly, the feasibility of using an ionization chamber in an MR-linac was assessed by investigating possible influences of the magnetic field on NE2571 Farmer-type ionization chamber characteristics: linearity, repeatability, orientation in the magnetic field; and AAPM TG51 correction factor for voltage polarity and ion recombination. We found that these AAPM correction factors for the NE2571 chamber were not influenced by the magnetic field. Secondly, the influence of the permanent 1.5 T magnetic field on the NE2571 chamber reading was quantified. The reading is influenced by the magnetic field; therefore, a correction factor has been added. For the standardized setup used in this paper, the NE2571 chamber reading increases by 4.9% (± 0.2%) due to the transverse 1.5 T magnetic field. Dosimetry measurements in an MR-linac are feasible, if a setup-specific magnetic field correction factor (P1.5 T) for the charge reading is introduced. For the setup investigated in this paper, the P1.5 T has a value of 0.953.


Assuntos
Imageamento por Ressonância Magnética/instrumentação , Aceleradores de Partículas , Radiometria/instrumentação , Radiometria/normas , Eletricidade , Modelos Lineares , Campos Magnéticos , Fótons , Padrões de Referência
11.
Phys Med Biol ; 56(19): N207-14, 2011 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-21934191

RESUMO

In this note, the feasibility of complementing our hybrid 1.5 T MRI linac (MRL) with a megavoltage (MV) portal imager is investigated. A standard aSi MV detector panel is added to the system and both qualitative and quantitative performances are determined. Simultaneous MR imaging and transmission imaging can be performed without mutual interference. The MV image quality is compromised by beam transmission and longer isocentre distance; still, the field edges and bony anatomy can be detected at very low dose levels of 0.4 cGy. MV imaging integrated with the MRL provides an independent and well-established position verification tool, a field edge check and a calibration for alignment of the coordinate systems of the MRI and the accelerator. The portal imager can also be a valuable means for benchmarking MRI-guided position verification protocols on a patient-specific basis in the introductory phase.


Assuntos
Osso e Ossos/diagnóstico por imagem , Aumento da Imagem/métodos , Imageamento por Ressonância Magnética/métodos , Osso e Ossos/patologia , Estudos de Viabilidade , Humanos , Aumento da Imagem/instrumentação , Limite de Detecção , Imageamento por Ressonância Magnética/instrumentação , Aceleradores de Partículas/instrumentação , Ossos Pélvicos/diagnóstico por imagem , Ossos Pélvicos/patologia , Pelve/diagnóstico por imagem , Pelve/patologia , Imagens de Fantasmas , Doses de Radiação , Radiografia
12.
Phys Med Biol ; 56(15): 4815-25, 2011 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-21753236

RESUMO

To boost the possibilities of image guidance in radiotherapy by providing images with superior soft-tissue contrast during treatment, we pursue diagnostic quality MRI functionality integrated with a linear accelerator. Large respiration-induced semi-periodic target excursions hamper treatment of cancer of the abdominal organs. Methods to compensate in real time for such motion are gating and tracking. These strategies are most effective in cases where anatomic motion can be visualized directly, which supports the use of an integrated MRI accelerator. We establish here an infrastructure needed to realize gated radiation delivery based on MR feedback and demonstrate its potential as a first step towards more advanced image guidance techniques. The position of a phantom subjected to one-dimensional periodic translation is tracked with the MR scanner. Real-time communication with the MR scanner and control of the radiation beam are established. Based on the time-resolved position of the phantom, gated radiation delivery to the phantom is realized. Dose distributions for dynamic delivery conditions with varying gating windows are recorded on gafchromic film. The similarity between dynamically and statically obtained dose profiles gradually increases as the gating window is decreased. With gating windows of 5 mm, we obtain sharp dose profiles. We validate our gating implementation by comparing measured dose profiles to theoretical profiles calculated using the knowledge of the imposed motion pattern. Excellent correspondence is observed. At the same time, we show that real-time on-line reconstruction of the accumulated dose can be performed using time-resolved target position information. This facilitates plan adaptation not only on a fraction-to-fraction scale but also during one fraction, which is especially valuable in highly accelerated treatment strategies. With the currently established framework and upcoming improvements to our prototype-integrated MRI accelerator, we will realize more intricate MRI-guided linear accelerator control in the near future.


Assuntos
Aceleração , Imageamento por Ressonância Magnética/métodos , Radioterapia Assistida por Computador/métodos , Humanos , Imageamento por Ressonância Magnética/instrumentação , Movimento , Radioterapia Assistida por Computador/instrumentação , Fatores de Tempo
13.
Phys Med Biol ; 54(18): N409-15, 2009 Sep 21.
Artigo em Inglês | MEDLINE | ID: mdl-19687566

RESUMO

In the UMC Utrecht a prototype MRI accelerator has been installed to investigate the feasibility of real-time, MRI guided radiotherapy. The system consists of a 6 MV Elekta (Crawley, UK) accelerator and a 1.5 T Philips (Best, The Netherlands) MRI system. The system is installed in a standard radiotherapy bunker. The bunker is at the corner of a block of six bunkers, so there are three neighbouring clinical Elekta accelerators. During ramping of the magnet, the magnetic fringe field in the two nearest bunkers was measured as a function of the magnetic field strength of the MRI magnet. At 8 m, a maximum increase of 1.5 G was measured, at 12 m, 0.6 G. This is up to three times the earth's magnetic field. The clinical accelerators are needed to be re-calibrated in order to operate in such an external magnetic field. The resulting radiation field flatness of the clinical accelerators was measured and was similar to the situation before ramping the magnet.


Assuntos
Artefatos , Interpretação de Imagem Assistida por Computador , Imageamento por Ressonância Magnética/instrumentação , Aceleradores de Partículas/instrumentação , Campos Eletromagnéticos , Desenho de Equipamento , Análise de Falha de Equipamento , Reprodutibilidade dos Testes , Sensibilidade e Especificidade
14.
Phys Med Biol ; 54(12): N229-37, 2009 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-19451689

RESUMO

At the UMC Utrecht, The Netherlands, we have constructed a prototype MRI accelerator. The prototype is a modified 6 MV Elekta (Crawley, UK) accelerator next to a modified 1.5 T Philips Achieva (Best, The Netherlands) MRI system. From the initial design onwards, modifications to both systems were aimed to yield simultaneous and unhampered operation of the MRI and the accelerator. Indeed, the simultaneous operation is shown by performing diagnostic quality 1.5 T MRI with the radiation beam on. No degradation of the performance of either system was found. The integrated 1.5 T MRI system and radiotherapy accelerator allow simultaneous irradiation and MR imaging. The full diagnostic imaging capacities of the MRI can be used; dedicated sequences for MRI-guided radiotherapy treatments will be developed. This proof of concept opens the door towards a clinical prototype to start testing MRI-guided radiation therapy (MRIgRT) in the clinic.


Assuntos
Imageamento por Ressonância Magnética/instrumentação , Aceleradores de Partículas/instrumentação , Radioterapia Assistida por Computador/instrumentação , Desenho de Equipamento , Análise de Falha de Equipamento , Projetos Piloto , Integração de Sistemas
15.
Phys Med Biol ; 54(10): 2993-3002, 2009 May 21.
Artigo em Inglês | MEDLINE | ID: mdl-19387100

RESUMO

The UMC Utrecht is constructing a 1.5 T MRI scanner integrated with a linear accelerator (Lagendijk et al 2008 Radiother. Oncol. 86 25-9). The goal of this device is to facilitate soft-tissue contrast based image-guided radiotherapy, in order to escalate the dose to the tumour while sparing surrounding normal tissues. Dosimetry for the MRI accelerator has to be performed in the presence of a magnetic field. This paper investigates the feasibility of using a Farmer NE2571 ionization chamber for absolute dosimetry. The impact of the mcagnetic field on the response of this ionization chamber has been measured and simulated using GEANT4 Monte Carlo simulations. Two orientations of the ionization chamber with respect to the incident beam and the magnetic field which are feasible in the MRI accelerator configuration are taken into account. Measurements are performed using a laboratory magnet ranging from 0 to 1.2 T. In the simulations a range from 0 to 2 T is used. For both orientations, the measurements and simulations agreed within the uncertainty of the measurements and simulations. In conclusion, the response of the ionization chamber as a function of the magnetic field is understood and can be simulated using GEANT4 Monte Carlo simulations.


Assuntos
Artefatos , Imageamento por Ressonância Magnética/instrumentação , Aceleradores de Partículas/instrumentação , Radiometria/instrumentação , Radioterapia Conformacional/instrumentação , Desenho Assistido por Computador , Desenho de Equipamento , Análise de Falha de Equipamento , Dosagem Radioterapêutica , Reprodutibilidade dos Testes , Sensibilidade e Especificidade
16.
J Med Eng Technol ; 29(1): 27-32, 2005.
Artigo em Inglês | MEDLINE | ID: mdl-15764379

RESUMO

At a radiotherapy department cancer patients are treated with high energy electron and photon beams. These beams are produced by a linear accelerator. A closed circuit television system is used to monitor patients during treatment. Although CCD cameras are rather resistant to stray radiation, they are damaged by the low flux of neutrons which are produced by the linac as a side effect. PVC can be used to reduce damage to CCD cameras induced by neutron radiation. A box with 6 cm thick walls will extend the life of the camera at least by a factor of two. A PVC neutron shield is inexpensive. PVC is easy to obtain and the box is simple to construct. A similar box made out of PE will not reduce neutron damage to a CCD camera. Although PE is a good medium to moderate faster neutrons, thereby reducing some of the bulk defects, it will not capture thermal neutrons which induce surface damage.


Assuntos
Monitorização Fisiológica/instrumentação , Nêutrons , Fotografação/instrumentação , Proteção Radiológica/instrumentação , Radioterapia/instrumentação , Gravação em Vídeo/instrumentação , Desenho de Equipamento , Análise de Falha de Equipamento , Estudos de Viabilidade , Monitorização Fisiológica/métodos , Fotografação/métodos , Doses de Radiação , Proteção Radiológica/métodos , Radiometria , Gravação em Vídeo/métodos
17.
J Med Eng Technol ; 25(4): 169-72, 2001.
Artigo em Inglês | MEDLINE | ID: mdl-11601444

RESUMO

In a medical linear accelerator a bending magnet is used to bend the electron beam produced by the accelerator tube, in the treatment direction. For each electron energy the strength of the magnetic field has to be set to a specific level. Changing the magnetic field strength is done by changing the electric current through the bending magnet. When electron energy and magnetic field strength are not matched, performance of the linac can be affected. As electron energy, magneticfield strength and electrical current through the bending magnet are related to each other, it is reasonable to assume that for each electron energy the correct bending current can be predetermined. This calculated bending current reduces the number of variable parameters used to set up a treatment beam. Predetermining a variable simplifies the tuning procedures. It also prevents a deviation of the electron beam energy being compensated by variation of the bending current. Preventing false machine settings can contribute to increase linac performance and reduce down time and cost of ownership.


Assuntos
Aceleradores de Partículas , Calibragem , Campos Eletromagnéticos , Elétrons , Humanos , Magnetismo , Método de Monte Carlo , Aceleradores de Partículas/instrumentação , Imagens de Fantasmas , Fótons
18.
Ann Neurol ; 47(2): 238-41, 2000 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-10665496

RESUMO

CP-122,288 is a highly potent inhibitor of neurogenic plasma extravasation in animal models at doses without vasoconstrictor effect. We evaluated the acute antimigraine efficacy of intravenous and oral CP-122,288 in two double-blind studies. In a crossover design, patients randomly received 31.25 microg of CP-122,288 intravenously, placebo, or both. In the oral study, patients received placebo or one of four doses of CP-122,288 between 3.125 and 312.5 microg, using a novel "up and down" design for randomization. Both studies were stopped prematurely when target efficacy could not be achieved. Responder rates were 29% for CP-122,288 versus 30% for placebo (difference, -1%; 95% CI, -24-22%; intravenous study) and an overall rate of 25% for CP-122,288 versus 0% for placebo (difference, 25%; 95% CI; 10-40%; oral study). CP-122,288 was not clinically effective at doses and plasma concentrations in excess of those required to inhibit neurogenic plasma extravasation in animals. Neurogenic plasma extravasation is unlikely to play a crucial role in the pathophysiology of migraine headache.


Assuntos
Transtornos de Enxaqueca/tratamento farmacológico , Inflamação Neurogênica/prevenção & controle , Pirrolidinas/administração & dosagem , Agonistas do Receptor de Serotonina/administração & dosagem , Sumatriptana/análogos & derivados , Administração Oral , Adolescente , Adulto , Idoso , Estudos Cross-Over , Método Duplo-Cego , Feminino , Humanos , Injeções Intravenosas , Masculino , Pessoa de Meia-Idade , Placebos , Pirrolidinas/uso terapêutico , Agonistas do Receptor de Serotonina/uso terapêutico , Sumatriptana/administração & dosagem , Sumatriptana/uso terapêutico , Falha de Tratamento
19.
Med Phys ; 26(12): 2589-96, 1999 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-10619244

RESUMO

The measured electron spectra of linear accelerators from several manufacturers differ in comparison to the spectral form and width. As part of our investigations of linac performance and stability, we analyzed the electron spectra of our linacs. After building a spectrometer the electron spectra were measured. The measured spectral widths were comparable with the results published in the literature. It appeared that the phase of the recycled radio pulse in combination with the limited bandwidth of the bending magnet are responsible for unusual (multipeak) electron spectra. The scatter filters only have a relatively small widening effect on the spectrum. There was no indication that multipeak or wide spectra are related to linac instabilities.


Assuntos
Elétrons , Aceleradores de Partículas , Radioterapia/métodos , Análise Espectral/instrumentação , Fatores de Tempo
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