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When a gamma photon interacts with a target nucleus a nuclear reaction can be generated, producing as a consequence the expulsion of particles from the atomic nucleus, this process is called photodisintegration. For this work, are of interest nuclear reactions of photodisintegration in which neutrons are ejected due to the interaction of photons with atomic nuclei of different materials in a linear accelerator for medical use. In this paper, the kinetic energy of photoneutrons produced by interactions with atomic nuclei of 184W, 63Cu, 27Al and 12C, which are some of the materials that constitute the head of a medical linear accelerator, is calculated. Also, the nuclei present in the construction materials of the room and the maze of the accelerator, such as, 23Na, 40Ca and 28Si, as also in the human body, ²H, 14N and 16O, are considered. It derives an exact theoretical expression, which has a linear dependence of the energy of the produced neutrons relative to the incident photon energy. It is found that, in the majority of cases, just photons with energies above 10 MV contribute to the production of neutrons. The values calculated from the expression obtained in this work are in good agreement with those reported in the literature, that are obtained by other approaches.

Cuando un fotón gamma interactúa con un núcleo blanco una reacción nuclear puede ser generada, produciendo como consecuencia la expulsión de partículas del núcleo atómico, este proceso se denomina fotodesintegración. Para este trabajo, son de interés las reacciones nucleares de fotodesintegración en las que los neutrones son expulsados debido a la interacción de los fotones con los núcleos atómicos de diferentes materiales en un acelerador lineal para uso médico. En este trabajo, la energía cinética de fotoneutrones producidos por la interacción con los núcleos atómicos de 184 W, 63 Cu, 27 Al y 12 C, que son algunos de los materiales que constituyen el cabezal de un acelerador lineal médico, es calculada. Además, los núcleos presentes en los materiales de construcción de la sala y el laberinto del acelerador, como por ejemplo, 23Na, 40Ca y 28Si, como también en el cuerpo humano, ²H, 14N y 16O, son considerados. Se obtiene una expresión exacta teórica, la cual tiene una dependencia lineal de la energía de los neutrones producidos en relación a la energía del fotón incidente. Se ha encontrado que, en la mayoría de los casos, sólo los fotones con energías por encima de 10 MV contribuyen a la producción de neutrones. Los valores calculados a partir de la expresión obtenida en este trabajo están en buen acuerdo con los reportados en la literatura, los cuales se obtienen mediante otros métodos.

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A scanning system developed for planar magnetic surfaces composed of a moving line of three magnetoresistive ultrasensitive transducers, complemented by a signal conditioning circuit is presented. After the calibration of the sensors, it was used to determine magnetized surface images with different shapes to evaluate the sensitivity of the device, and the images are represented in gray levels on a scale from 0 to 255 intensities, to get a visual representation of the magnetic field strength. The device is shown to be sensitive enough to detect gradients homogeneities and discontinuities in the magnetic field maps and images of magnetic susceptibility.

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The development of a device for generating ac magnetic fields based on a resonant inverter is presented, which has been specially designed to carry out experiments of magnetic hyperthermia. By determining the electric current in the LC resonant circuit, a maximum intensity of magnetic field around of 15 mT is calculated, with a frequency around of 206 kHz. This ac magnetic field is able to heat powdered magnetic materials embedded in biological systems to be used in biomedical applications. Indeed, in order to evaluate the sensitivity of the device we also present the measurements of the specific absorption rate in phantoms performed with commercially prepared Fe(3)O(4) and distilled water at different concentrations.

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Gastric motility (GM) has been assessed using bio-impedance (BI) techniques in the time domain using short term recordings and analyzing mainly the median of the area under the fast Fourier transform (FFT) spectra. The BI technique has been applied to the gastric system mainly for gastric emptying (GE) studies. However, gastric motility evaluation using BI has not been fully implemented. In this study, we propose the use of the BI technique for the evaluation of gastric motility considering global features of the fast FFT spectra. The study was performed in eleven healthy subjects who were evaluated in fasting and postprandial conditions. The results indicate that the use of the median of the area under the FFT spectra is equivalent to the use of the main peak of the spectra to determine the changes in gastric motility from the fasting to the postprandial state. This demonstrates that BI is a valid technique for gastric motility evaluation in short time recordings.

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The aim of this study was to evaluate the half-time gastric emptying reproducibility measured by magnetogastrography (MGG). Seven healthy subjects were subjected to a magnetic pulse of 32 mT for 17 ms, seven times in 90 min. The procedure was repeated one and two weeks later. Assessments of the gastric emptying were carried out for each one of the measurements and statistical analyses of ANOVA and Bland-Altman were performed for every group of data. A mean of 21.7 +/- 3.3 min was measured for the half-time of gastric emptying for yoghurt. Reproducibility estimation was above 85%. Therefore, magnetogastrography seems to be a reliable technique to be implemented in routine clinical trials.

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Los gestágenos clásicamente han sido relacionado con el cáncer mamario. Por esto, cuando se requiere indicar un anticonceptivos hormonal en una paciente tratada por cáncer de mama, surge la opción del uso de anticonceptivos con gestágenos solos (POP). Algunos estudios sugieren un rol protector de los gestágenos en el cáncer de mama: 1) La progesterona inhibe la proliferación mamaria inducida por el estradiol. 2) Induce apoptosis celular, inhibiendo la síntesis de las proteínas antiapoptóticas, 3) Inhibe enzimas que aumentan la concentración de estrógenos en la mama. Contrariamente, otras investigaciones muestran una acción favorecedora del cáncer de mama: 1) La actividad mitótica en las mamas es máxima durante la fase lútea, período en que predomina la progesterona. 2) En cultivos celulares los gestágenos estimulan la proliferación de algunas líneas celulares de cáncer de mama. 3) El uso de anticonceptivos orales con progestágenos y la medroxiprogesterona de depósito aumentan en mujeres jóvenes el riesgo de cáncer mamario. 4) En el reemplazo hormonal postmenopáusico varios estudios sugieren que la medroxiprogesterona incrementa el riesgo oncogénico. Conclusión: No existen datos que permitan afirmar con certeza que el uso de POP no tiene riesgos en pacientes con cáncer de mama.

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