Dosimetric analysis of graphitic carbon nitride quantum dots exposed to a gamma radiation for a low-dose applications.

In this study, graphitic carbon nitride quantum dots (g-C3N4QDs) were synthesized using different solvents, characterized, and then exposed to a gamma-ray source (137Cs) at irradiation doses of 0.1, 1.48, 2.05 and 3.25 Gy. The intensities of the emission bands progressively were attenuated as the received dose of gamma radiation increased. The changes were quantified with the help of a non-linear fit model. The material showed promising use as a dosimeter for low-dose radiation applications.

Shielding behavior of artisanal bricks against ionizing photons.

In the work reported in this article, were determined the shielding capabilities of three artisanal bricks used massively in the construction industry in Mexico. The linear attenuation coefficients for photons between 1 keV and 100 GeV are reported; and the half-value layers for energies used in the medical field, show that the three typical artisanal bricks have good shielding capabilities for photons below 50 keV. We compared the effective atomic numbers of one of our bricks against two widely used materials in the construction industry, and our results suggest that the greater the effective atomic number, the less material attenuation capacity. A comparison of the half-value layer of one of our bricks against the half-value layers of two clay bricks with different percentages of fly ash particles published in the literature, suggests that in the region between 0.001 and 2.8 MeV, all the three bricks have practically the same attenuation capacity and that from 2.8 MeV to 100 GeV the clay bricks with different percentages of fly ash particles, need less material to show the same attenuation capacity than our artisanal bricks. Energy Dispersed X-Ray Fluorescence suggests that regardless of the number of constituent elements in a sample, a critical mass per atom is required to have a positive impact on density; and as a consequence, in the capacity of attenuation of the materials. Normalized half-value layers suggest on the other hand, that the uncooked bricks have better shielding capabilities than cooked.

Study of X-ray scattered in a phantom as function of its maximum energy during bimodal PET/CT imaging.

In a simultaneous Bimodal System with Positron Emission Tomography and Computed Tomography, the X-rays from a collimated X-ray tube are scattered over an anatomical region on the patient being scanning and cause a contamination effect on the signal received by the detectors distributed in the shape of a ring of the PET system. To study this phenomenon, each of the components of a bimodal system was modeled and simulated by the Monte Carlo method in Geant4 an X-ray beam produced in a RTW tube MCBM 65B with typical fluences used in medical diagnosis of small animals hitting a cylindrical phantom with a diameter greater than Field of View (FOV) in the tomographic center. The number of ionization events that occur in each of the phoswich detectors of the PET were obtained, the spatial distribution of the scattered X-rays was studied according to three maximum energies of the spectrum and calculation was made to find a filter that was located at the input of the phoswich detector that attenuates the X-rays by 98% and that would allow transmitting the gamma rays of annihilation also by 98%. For this, simulations were carried out using various filter materials, finding that copper is an excellent candidate and were found an optimum thicknesses between 0.5 mm and 1.3 mm, according to the average energy of the X-rays used in the different exploration techniques.

Optimization by Monte Carlo method of photon fluence from the X-ray beam spectrum in a bimodal tomographic system.

A bimodal tomographic system with a RTW MCBM 65B-50Mo X-ray tube and a XPAD3s semiconductor camera that contains 8 bars, each one with 67,200 hybrid pixels are modeled in GEANT4 simulation code. Several conical X-ray spectra were simulated, particularly a spectrum with a peak energy of 17.4 keV used in tomography on small animals. Three phantoms located in the tomographic center were added to the simulation to evaluate the image quality and its magnification based on the simulation of different photon fluences and the rotation effect of the tomographic system with an average angular velocity of 360o per minute. The images were recorded and analyzed in 2D through ROOT software toolkit in virtual XPAD3 detector. The quantitative method 20-80% of the maximum intensity of radiation was used for obtain the contouring of the phantoms, this method is used in radiotherapy and radiodiagnosis imaging. For this purpose, the images were taken to DICOM format in order to estimate the optical density of the contours and to evaluate the optimum and minimum photon fluence to be used in the tomographic system in order to reduce the absorbed doses in the individuals. This study allowed to determine the optimal fluence to validate it with realistic fluences used in the tomographic prototype ClearPET /XPAD-CT and to make an intercomparison with the absorbed doses measured with detectors located in the tomographic center.

Assessment of the effectiveness of attenuation of Pb aprons by using TLD dosimetry and Monte Carlo calculations.

We developed an experimental set-up by using a continuous emission X-ray (Pantak DXT-3000) and three types of Pb aprons, with thicknesses of 0.25, 0.5, and 0.75mm, coated with Mylar fiber on their surface. Aprons were placed at a distance of 2.5m from the focus. Aluminum filtration was performed at the beam output to reproduce the qualities of narrow beams, N40 (Eeffective =33keV), N80 (Eeffective =65keV), and N100 (Eeffective =83keV), according to the ISO standard 4037 (1-3). Each apron was fixed with 10 thermoluminescent dosimeters (TLDs) over its surface, five dosimeters before and five dosimeters after irradiation with X-rays. Dosimeter readings were noted, and the attenuation coefficients for each effective energy were calculated. To confirm the method of effective energy of ISO-4037 and evaluate the effectiveness of aprons according to the energy range required for different medical practices, a Monte Carlo simulation using GEANT4 code was performed. Thus, the fluence and the absorbed dose in each of the dosimeters were determined, and then the coefficients of linear attenuation were calculated and compared with the experimental data and with those reported by the National Institute of Standards and Technology. Results were consistent between theoretical calculations and experimental measures. This work will serve to make assessments for other personalized radiation protectors made of Pb.

Concrete enclosure for shielding a neutron source.

In the aim to design a shielding for a 0.185 TBq (239)PuBe isotopic neutron source several Monte Carlo calculations were carried out using MCNP5 code. First, a point-like source was modeled in vacuum and the neutron spectrum and ambient dose equivalent were calculated at several distances ranging from 5 cm up to 150 cm, these calculations were repeated modeling a real source, including air, and a 1×1×1 m(3) enclosure with 5, 15, 20, 25, 30, 50 and 80 cm-thick Portland type concrete walls. At all the points located inside the enclosure neutron spectra from 10(-8) up to 0.5 MeV were the same regardless the distance from the source showing the room-return effect in the enclosure, for energies larger than 0.5 MeV neutron spectra are diminished as the distance increases. Outside the enclosure it was noticed that neutron spectra becomes "softer" as the concrete thickness increases due to reduction of mean neutron energy. With the ambient dose values the attenuation curve in terms of concrete thickness was calculated.

NSDUAZ unfolding package for neutron spectrometry and dosimetry with Bonner spheres.

NSDUAZ (Neutron Spectrometry and Dosimetry from the Universidad Autónoma de Zacatecas) is a user friendly neutron unfolding package for Bonner sphere spectrometer with (6)LiI(Eu) developed under LabView(®) environment. Unfolding is carried out using a recursive iterative procedure with the SPUNIT algorithm, where the starting spectrum is obtained from a library initial guess spectra to start the iterations. The NSDUAZ performance was evaluated using (252)Cf, (252)Cf/D(2)O, (241)AmBe neutron sources and the neutrons outside the radial beam port of a TRIGA Mark III nuclear reactor running to 10 W.

Neutron and photon spectra in LINACs.

A Monte Carlo calculation, using the MCNPX code, was carried out in order to estimate the photon and neutron spectra in two locations of two linacs operating at 15 and 18 MV. Detailed models of both linac heads were used in the calculations. Spectra were estimated below the flattening filter and at the isocenter. Neutron spectra show two components due to evaporation and knock-on neutrons. Lethargy spectra under the filter were compared to the spectra calculated from the function quoted by Tosi et al. that describes reasonably well neutron spectra beyond 1 MeV, though tends to underestimate the energy region between 10(-6) and 1 MeV. Neutron and the Bremsstrahlung spectra show the same features regardless of the linac voltage.

Niveles de dosis en radiología convencional / Radiation doses in conventional radiology

Se ha medido la dosis en la superficie de entrada de ojos, tiroides y gónadas en pacientes sometidos a estudio de radiografía de tele de tórax mediante dosimetría termoluminiscente. Se usaron 5 dosímetros en cada una de las exploraciones; encontrándose los siguientes rangos de dosis: 20 mGy a 350 mGy. En el caso de los pacientes pediátricos se propone incluir la información de las dosis recibidas en su cartilla de vacunación con el fin de hacer un seguimiento de los posibles efectos nocivos en su salud.

Doses have been measured at the entrance surface of eyes, thyroid and gonads in patients undergoing chest x-ray studies, using thermo-luminiscent dosimetry. Five dosimeters were used in each investigation, and doses ranged from 20 mGy to 350 mGy. In the case of pediatric patients, it is proposed that dose information from diagnostic examinations is included in the vaccination chart in order to correlate with findings at follow-up.

Monte Carlo simulation and measurements of clinical photon beams using LiF:Mg,Cu,P+PTFE.

Thermoluminescent response of LiF:Mg,Cu,P+PTFE under clinical photon irradiation was obtained. Thermoluminescent dosimeters (TLDs) were irradiated for determining entrance surface dose (ESD) in a solid water phantom when using standard clinical adult treatment protocols. A Monte Carlo simulation of photon interaction with matter was performed and absorbed dose determined. ESD calculated by MCNPX code was greater than those determined by direct measurements in phantom. The results obtained open the possibility for using this material as a TLDs in medical accelerators.

Neutron KERMA factors of human tissues

A program to calculate the neutron KERMA in human tissues has been developed. The program was developed in Mathcad and contains the neutron kerma factors of those elements that are present in different human tissues. Having the elemental composition of any human tissue the neutron kerma can be easily calculated. The program was tested using the elemental composition of tumor tissues such as sarcoma, melanoma, carcinoma and adenoid cystic. Neutron kerma for adipose and muscle tissue for normal adult was calculated. The results are in agreement with those published in literature. The neutron kerma for water was also calculated because in some dosimetric calculations water is used to describe normal and tumor tissues. From this comparison was found that at larger energies kerma factors are approximately the same, but energies less than 100 eV the differences are large.

Campos neutrónicos producidos por un acelerador lineal para radioterapia / Neutron fields produced by linear accelerator for radiotherapy

Mediante una serie de medidas y cálculos Monte Carlo se han determinado las características dosimétricas y los espectros de los fotoneutrones que se producen en torno a un acelerador lineal para radioterapia de 18 MV. Las medidas se realizaron con dosímetros termoluminiscentes TLD 600 y TLD 700 que se expusieron desnudos y emparedados con Cd, así como dentro de una esfera de parafina y dentro de esferas Bonner.

Measurements and Monte Carlo calculations has been utilized to determine the dosimetric features as well as the neutron spectra of photoneutrons produced around an 18 MV linear accelerator for radiotherapy. Measurementes were carried out with bare and Cd covered thermolumiscent dosimeters, TLD600 and TLD700, as well as inside a paraffine moderator. TLD pairs were also utilized as thermal neutrons inside a Bonner sphere spectrometer.

On the firing maps of a general class of forced integrate and fire neurons.

Integrate and fire processes are fundamental mechanisms causing excitable and oscillatory behavior. Van der Pol [Philos. Mag. (7) 2 (11) (1926) 978] studied oscillations caused by these processes, which he called 'relaxation oscillations' and pointed out their relevance, not only to engineering, but also to the understanding of biological phenomena [Acta Med. Scand. Suppl. CVIII (108) (1940) 76], like cardiac rhythms and arrhythmias. The complex behavior of externally stimulated integrate and fire oscillators has motivated the study of simplified models whose dynamics are determined by iterations of 'firing circle maps' that can be studied in terms of Poincaré's rotation theory [Chaos 1 (1991) 20; Chaos 1 (1991) 13; SIAM J. Appl. Math. 41 (3) (1981) 503]. In order to apply this theory to understand the responses and bifurcation patterns of forced systems, it is fundamental to determine the regions in parameter space where the different regularity properties (e.g., continuity and injectivity) of the firing maps are satisfied. Methods for carrying out this regularity analysis for linear systems, have been devised and the response of integrate and fire neurons (with linear accumulation) to a cyclic input has been analyzed [SIAM J. Appl. Math. 41 (3) (1981) 503]. In this paper we are concerned with the most general class of forced integrate and fire systems, modelled by one first-order differential equation. Using qualitative analysis we prove theorems on which we base a new method of regularity analysis of the firing map, that, contrasting with methods previously reported in the literature, does not requires analytic knowledge of the solutions of the differential equation and therefore it is also applicable to non-linear integrate and fire systems. To illustrate this new methodology, we apply it to determine the regularity regions of a non-linear example whose firing maps undergo bifurcations that were unknown for the previously studied linear systems.

Cultural change, hybridity and male homosexuality in Mexico.

PIP: This paper analyzes how contemporary perceptions of male homosexuality are being shaped in Mexico. Ethnographic analysis included four short case studies from 64 mostly middle class individuals for two years in Guadalajara City. Mexican sexual culture is often portrayed traditionally as grounded in values inherent in machismo and influenced by Catholicism. There is a contrast between these traditional interpretations of roles and sexual identities in Mexico and the identities that are being adopted by many contemporary Mexican homosexual men. The homosexual men were categorizable in terms of 1) those who dominated in the sexual relationship and who were capable of maintaining a nonstigmatized identity as regular men, 2) those who assumed a feminine role and were penetrated and who were stigmatized for their effeminate demeanor, and 3) a minority of men who assumed both roles and who were termed "anally active and passive". The study revealed that middle-class homosexuals established networks in which individuals, supported by their friends, acquired the strength to effect personal changes along with other larger cultural changes. Thus, individual actions are beginning to have a collective effect on the society at large.^ieng

Outcome of pediatric intensive care at six centers in Mexico and Ecuador.

OBJECTIVE: To improve understanding of the causes of morbidity and mortality among critically ill children in the countries studied. DESIGN: Survey of hospital records between 1992 and 1994. SETTING: Six pediatric intensive care units (ICUs) (four ICUs in Mexico City and two ICUs in Ecuador). PATIENTS: Consecutive patients (n = 1,061) admitted to the units studied. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: The mortality rate for low-risk patients (pediatric Risk of Mortality [PRISM] score of < or = 10, n = 701) was more than four times the rate predicted by the PRISM score (8.1% vs. 1.8%, p < .001), with an additional 11.3% of this group incurring major morbidity. The mortality rate for moderate-risk patients (PRISM scores of 11 to 20, n = 232) was more than twice predicted (28% vs. 12%, p < .001). For low-risk patients, death was significantly associated with tracheal intubation, central venous cannulation, pneumonia, age of < 2 months, use of more than two antibiotics, and nonsurgical diagnosis (after controlling for PRISM score). Central venous cannulation and tracheal intubation in the lower-risk groups were performed more commonly in units in Mexico than in one comparison unit in the United States (p < .001). CONCLUSIONS: For six pediatric ICUs in Mexico and Ecuador, mortality was significantly higher than predicted among lower-risk patients. Tracheal intubation, central catheters, pneumonia, sepsis, and nonsurgical status were associated with poor outcome for low-risk groups. We speculate that reducing the use of invasive central catheters and endotracheal intubation for lower-risk patients, coupled with improved infection control, could lower mortality rates in the population studied.

AIDS and condom-related knowledge, beliefs, and behaviors in Mexican migrant laborers.

PIP: A survey of 87 Mexican migrants (55 males and 32 females) who have lived and worked in the US since 1982 assessed AIDS and condom-related knowledge, beliefs, and sexual practices. Although respondents were highly knowledgeable about major modes of HIV transmission, one-third to one-half believed that HIV could be contracted from mosquito bites, public bathrooms, kissing, and the HIV test. Only 15% knew someone with AIDS. Ever-use of condoms was reported by 70.9% of men and 41.9% of women. Of the 68 subjects who had been sexually active in the year preceding the survey, 20 reported two or more partners. Among sexually active respondents, 16.2% always used condoms with their primary partner, while 43.0% used condoms consistently with occasional partners. 48.9% of men and 57.1% of women never used condoms with their primary partner; with casual partners, these rates were 30.8% and 44.4%, respectively. Worry about contracting AIDS, self-rated on a scale from 1 (very often) to 4 (never), averaged 2.84, with higher worry scores among those 18-31 years of age and with multiple partners. Respondents did not anticipate negative consequences of condom use (e.g., reduced sexual pleasure), but females expressed concern that carrying condoms would cause them to be viewed as promiscuous.^ieng

Leiomyosarcoma of the pulmonary veins with extension to the left atrium.

A leiomyosarcoma of the right pulmonary vein in a 43-year-old woman extended to the right atrial wall and compromised the posterior mitral leaflet. Successful surgical treatment was accomplished with a right pneumonectomy, partial resection of the left atrial wall, and mitral valve replacement under cardiopulmonary bypass. Six months later a mediastinal recurrence with extension to the left hemithorax was treated with resection and postoperative radiotherapy.

[Clinical usefulness of pulse oximetry in asthmatic children]. / Utilidad clínica de la oximetría de pulso en niños asmáticos.

Abnormalities in exchange and tendency to acidosis and hypoxemia are present in acute asthma. Oxygen arterial saturation (pulse oximetry) is a simple and useful method to evaluate oxygenation conditions. Pulse oximetry and spirometry before and after albuterol nebulization (150 mcg/kg/dose) were performed in 35 subjects. 7 to 14 years, with acute asthma, 68% were mild and the rest moderate. Oxygen saturation before nebulization was less than 90% in 30 children and increased in 27 of them (90%). Spirometry before nebulization showed FEV1 20% predictive values in 18 children and increased in 10 of them. Seven were hospitalized because of worse symptoms. 4 of them showed metabolic acidosis by the presence of lactate. We conclude that pulse oximetry is a useful and simple method for an objective evaluation of acute asthma and its complications, but it is not predictive for a therapeutic decision.

Another crack in the mirror: the politics of AIDS prevention in Mexico.

Beginning in the mid-1980s, nongovernmental groups and the government in Mexico launched efforts to combat AIDS by addressing the population openly and directly. The content and tone of AIDS prevention messages in Mexico have been strongly influenced by models developed in the United States, even when the social conditions of Mexico are strikingly different. However, the local organization of AIDS prevention efforts in Mexico has differed sharply from efforts in large U.S. cities in terms of the distribution of funds and the role played by civil society in the creation of programs. Based on the premise that social and political forces may strongly influence the success, or failure, of health education interventions, this article analyzes the emergence of AIDS prevention in the Mexican context. The epidemiological characteristics of AIDS in Mexico, the governmental response to the epidemic, the role of nongovernmental groups, the availability of funds, the opposition of the political right, and the adoption of models imported from the United States are all considered to be important factors that shape current efforts. The article highlights possible directions for future development of AIDS prevention in Mexico.