Fuga de radiação de unidades de radiologia pediátrica

Em radiologia pediátrica, é necessário que uma pessoa segure os pacientes durante a exposição, já gue os pacientes são crianças e recém-nascidos. Portanto, torna-se importante uma apropriada determinação e minimização da radiação de fuga das unidades de radiodiagnóstico. Fez-se medidas de radiação de fugas em cinco unidades de raios X, sendo quatro unidades produzidas pela mesma companhia. Três das quatro unidades produzidas pela mesma companhia apresentaram uma contribuição anormal da radiação de fuga. Discute-se a não-adequação dos atuais cabeçotes para radiologia pediátrica e apresenta-se sugestões para um novo enfoque em radiação pediátrica.In pediatric radiology, it is necessary that a person stay with the patients, as they are children and newly bom, during radiation exposure. Therefore, the proper evaluation and minimization of radiation leakage from radiodiagnostic units becomes important. Measurements of leakage radiation were carried out in five X-ray units, where four of which are manufactured by the same company. Three of the four units produced by the same company, showed an abnormal contribution of the leakage radiation. The inadequacy of the presently available housing tubes for pediatric radiology is discussed. Suggestions regarding new approach in pediatric radiology are presented

A thin target approach for portal imaging in medical accelerators

A new thin-target method (patent pending) is described for portal imaging with low-energy (tens of keV) photons from a medical linear accelerator operating in a special mode. Low-energy photons are usually produced in the accelerator target, but are absorbed by the target and flattening filter, both made of medium- or high- Z materials such as Cu or W. Since the main contributor to absorption of the low-energy photons is self-absorption by the thick target through the photoelectric effect, it is proposed to lower the thickness of the portal imaging target to the minimum required to get the maximum low-energy photon fluence on the exit side of the target, and to lower the atomic number of the target so that predominantly photoelectric absorption is reduced. To determine the minimum thickness of the target, EGS4 Monte Carlo calculations were performed. As a result of these calculations, it was concluded that the maximum photon fluence for a 4 MeV electron beam is obtained with a 1.5 mm Cu target. This value is approximately five times less than the thickness of the Cu target routinely used for bremsstrahlung production in radiotherapeutic practice. Two sets of experiments were performed: the first with a 1.5 mm Cu target and the second with a 5 mm Al target (Cu mass equivalent) installed in the linear accelerator. Portal films were taken with a Rando anthropomorphic phantom. To emphasize the low-energy response of the new thin target we used a Kodak Min-R mammographic film and cassette combination, with a strong low-energy response. Because of its high sensitivity, only 1 cGy is required. The new portal images show a remarkable improvement in sharpness and contrast in anatomical detail compared with existing ones. It is also shown that further lowering of the target's atomic number (for example to C or Be) produces no significant improvement.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48963/2/m80816.pd

Inverse inequalities on non-quasi-uniform meshes and application to the mortar element method

We present a range of mesh-dependent inequalities for piecewise constant and continuous piecewise linear finite element functions u defined on locally refined shape-regular (but possibly non-quasi-uniform) meshes. These inequalities involve norms of the form ∥h α u∥ W s,p (Ω) for positive and negative s and α, where h is a function which reflects the local mesh diameter in an appropriate way. The only global parameter involved is N, the total number of degrees of freedom in the finite element space, and we avoid estimates involving either the global maximum or minimum mesh diameter. Our inequalities include new variants of inverse inequalities as well as trace and extension theorems. They can be used in several areas of finite element analysis to extend results – previously known only for quasi-uniform meshes – to the locally refined case. Here we describe applications to (i) the theory of nonlinear approximation and (ii) the stability of the mortar element method for locally refined meshes

Inverse Inequalities on Non-Quasiuniform Meshes and Application to the Mortar Element Method

We present a range of mesh-dependent inequalities for piecewise constant and continuous piecewise linear finite element functions u defined on locally refined shape-regular (but possibly non-quasiuniform) meshes. These inequalities involve norms of the form kh ff uk W s;p(\Omega\Gamma for positive and negative s and ff, where h is a function which reflects the local mesh diameter in an appropriate way. The only global parameter involved is N , the total number of degrees of freedom in the finite element space, and we avoid estimates involving either the global maximum or minimum mesh diameter. Our inequalities include new variants of inverse inequalities as well as trace and extension theorems. They can be used in several areas of finite element analysis to extend results - previously known only for quasiuniform meshes - to the locally refined case. Here we describe applications to: (i) the theory of nonlinear approximation and (ii) the stability of the mortar element method for locally refined meshes

Inverse inequalities for non-quasiuniform meshes and applications to the mortar element method

Abstract. We present a range of mesh-dependent inequalities for piecewise constant and continuous piecewise linear finite element functions u defined on locally refined shape-regular (but possibly non-quasi-uniform) meshes. These inequalities involve norms of the form �h α u � W s,p (Ω) for positive and negative s and α, where h is a function which reflects the local mesh diameter in an appropriate way. The only global parameter involved is N, the total number of degrees of freedom in the finite element space, and we avoid estimates involving either the global maximum or minimum mesh diameter. Our inequalities include new variants of inverse inequalities as well as trace and extension theorems. They can be used in several areas of finite element analysis to extend results—previously known only for quasi-uniform meshes—to the locally refined case. Here we describe applications to (i) the theory of nonlinear approximation and (ii) the stability of the mortar element method for locally refined meshes. 1