Modelling of post-irradiation events in polymer gel dosimeters

The nuclear magnetic resonance (NMR) spin-spin relaxation time (T-2) is related to the radiation-dependent concentration of polymer formed in polymer gel dosimeters manufactured from monomers in an aqueous gelatin matrix. Changes in T-2 with time post-irradiation have been reported in the literature but their nature is not fully understood. We investigated those changes with time after irradiation using FT-Raman spectroscopy and the precise determination of T-2 at high magnetic field in a polymer gel dosimeter, A model of fast exchange of magnetization taking into account ongoing gelation and strengthening of the gelatin matrix as well as the polymerization of the monomers with time is presented. Published data on the changes of T-2 in gelatin gels as a function of post-manufacture time are used and fitted closely by the model presented. The same set of parameters characterizing the variations of T-2 in gelatin gels and the increasing concentration of polymer determined from Fr-Raman spectroscopy are used successfully in the modelling of irradiated polymer gel dosimeters. Minimal variations in T-2 in an irradiated PAG dosimeter are observed after 13 h.

Ultrasonic absorption in polymer gel dosimeters

Ultrasonic absorption in polymer gel dosimeters was investigated. An ultrasonic interferometer was used to study the frequency (f) dependence of the absorption coefficient (alpha) in a polyacrylamide gel dosimeter (PAG) in the frequency range 5-20 MHz. The frequency dependence of ultrasonic absorption deviated from that of an ideal viscous fluid. The presence of relaxation mechanisms was evidenced by the frequency dependence of alpha/f(2) and the dispersion in ultrasonic velocity. It was concluded that absorption in polymer gel dosimeters is due to a number of relaxation processes which may include polymer-solvent interactions as well as relaxation due to motion of polymer side groups. The dependence of ultrasonic absorption on absorbed dose and formulation was also investigated in polymer gel dosimeters as a function of pH and chemical composition. Changes in dosimeter pH and chemical composition resulted in a variation in ultrasonic dose response curves. The observed dependence on pH was considered to be due to pH induced modifications in the radiation yield while changes in chemical composition resulted in differences in polymerisation kinetics. (C) 2003 Elsevier B.V. All rights reserved.

MRI study of radiation effect on Fricke gel solutions

The quality control optimization of medical processes that use ionizing radiation in the treatment of diseases like cancer is a key element for patient safety and success of treatment. The major medical application of radiation is radiotherapy, i.e. the delivery of dose levels to well-defined target tissues of a patient with the purpose of eliminating a disease. The need of an accurate tumour-edge definition with the purpose of preserving healthy surrounding tissue demands rigorous radiation treatment planning. Dosimetric methods are used for dose distribution mapping region of interest to assure that the prescribed dose and the irradiated region are correct. The Fricke gel (FXG) is the main dosimeter that supplies visualization of the three-dimensional (3D) dose distribution. In this work the dosimetric characteristics of the modified Fricke dosimeter produced at the Radiation Metrology Centre of the Institute of Energetic and Nuclear Research (IPEN) such as gel concentration dose response dependence, xylenol orange addition influence, dose response between 5 and 50Gy and signal stability were evaluated by magnetic resonance imaging (MRI). Using the same gel solution, breast simulators (phantoms) were shaped and absorbed dose distributions were imaged by MRI at the Nuclear Resonance Laboratory of the Physics Institute of Sao Paulo University. (C) 2007 Elsevier Ltd. All rights reserved.

An evaluation of dosimetric characteristics of MAGIC gel modified by adding formaldehyde (MAGIC-f)

Purpose: To evaluate the dosimetric characteristics of a new formulation of MAGIC gel, called MAGIC-f, which contains the addition of 3.3% formaldehyde, resulting in a gel with increased thermal stability. Methods: MAGIC-f gel was prepared and stored in hermetically sealed plastic containers. After irradiation, magnetic resonance images (MRI) were acquired to evaluate dose and dose distribution. Dosimetric characterization was performed by means of depth dose measurements, dose response sensitivity and linearity, temporal stability, energy and dose rate dependence, dose integration using sequential beams, temperature influence during MRI acquisition and dose distribution integrity. Results: MAGIC-f depth dose measurements are compatible with the dosimetric table data within +/- 4% uncertainty. The dosimeter's R-2 response varies linearly with dose at least from 0 to 6 Gy. The time-course of the sensitivity of the dosimeter following irradiation, indicated stabilization after 2 weeks. The dosimeter's response to irradiation was altered by 6% when increasing the energy from cobalt beams to 10 MV beams. The dose rate dependence of this new formulation of gel dosimeter is small: less than 2.5% for a variation from 200 to 500 cGy/min, and the dependence with the fractionation scheme is about 50% smaller than for standard MAGIC gel, The dependence on scanning temperature was also verified, and the integrity of the dose distribution was confirmed for a period of 90 days. Conclusions: The results demonstrate the applicability of this new dosimeter in tridimensional dose distribution measurements. (C) 2012 Elsevier Ltd. All rights reserved.

C-13-NMR, H-1-NMR, and FT-Raman study of radiation-induced modifications in radiation dosimetry polymer gels

H-1- and C-13-NMR spectroscopy and FT-Raman spectroscopy are used to investigate the properties of a polymer gel dosimeter post-irradiation. The polymer gel (PACT) is composed of acrylamide, N,N'-methylene-bisacrylamide, gelatin, and water. The formation of a polyacrylamide network within the gelatin matrix follows a dose dependence nonlinearly correlated to the disappearance of the double bonds from the dissolved monomers within the absorbed dose range of 0-50 Gy. The signal from the gelatin remains constant with irradiation. We show that the NMR spin-spin relaxation times (T-2) of PAGs irradiated to up to 50 Gy measured in a NMR spectrometer and a clinical magnetic resonance imaging scanner can be modeled using the spectroscopic intensity of the growing polymer network. More specifically, we show that the nonlinear T-2 dependence against dose can be understood in terms of the fraction of protons in three different proton pools. (C) 2000 John Wiley & Sons, Inc.

The relationship between radiation-induced chemical processes and transverse relaxation times in polymer gel dosimeters

The effects of ionizing radiation in different compositions of polymer gel dosimeters are investigated using FT-Raman spectroscopy and NMR T-2 relaxation times. The dosimeters are manufactured from different concentrations of comonomers (acrylamide and N,N'-methylene-bis-acrylamide) dispersed in different concentrations of an aqueous gelatin matrix. Results are analysed using a model of fast exchange of magnetization between three proton pools. The fraction of protons in each pool is determined using the known chemical composition of the dosimeter and FT-Raman spectroscopy. Based on these results, the physical and chemical processes in interplay in the dosimeters are examined in view of their effect on the changes in T-2 The precipitation of growing macroradicals and the scavenging of free radicals by gelatin are used to explain the rate of polymerization. The model describes the changes in T-2 as a function of the absorbed dose up to 50 Gy for the different compositions. This is expected to aid the theoretical design of new, more efficient dosimeters, since it was demonstrated that the optimum dosimeter (i.e, with the lowest dose resolution) must have a range of relaxation times which match the range of T-2 values which can be determined with the lowest uncertainty using an MRI scanner.

Investigation of ultrasonic properties of PAG and MAGIC polymer gel dosimeters

Ultrasonic speed of propagation and attenuation were investigated as a function of absorbed radiation dose in PAG and MAGIC polymer gel dosimeters. Both PAG and MAGIC gel dosimeters displayed a dependence of ultrasonic parameters on absorbed dose with attenuation displaying significant changes in the dose range investigated. The ultrasonic attenuation dose sensitivity at 4 MHz in MAGIC gels was determined to be 4.7 +/- 0.3 dB m(-1) Gy(-1) and for PAG 3.9 +/- 0.3 dB m(-1) Gy(-1). Ultrasonic speed dose sensitivities were 0.178 +/- 0.006 m s(-1) Gy(-1) for MAGIC gel and -0.44 +/- 0.02 m s(-1) Gy(-1) for PAG. Density and compressional elastic modulus were investigated to explain the different sensitivities of ultrasonic speed to radiation for PAG and MAGIC gels. The different sensitivities were found to be due to differences in the compressional elastic modulus as a function of dose for the two formulations. To understand the physical phenomena underlying the increase in ultrasonic attenuation with dose, the viscoelastic properties of the gels were studied. Results suggest that at ultrasonic frequencies, attenuation in polymer gel dosimeters is primarily due to volume viscosity. It is concluded that ultrasonic attenuation significantly increases with absorbed dose. Also, the ultrasonic speed in polymer gel dosimeters is affected by changes in dosimeter elastic modulus that are likely to be a result of polymerization. It is suggested that ultrasound is a sufficiently sensitive technique for polymer gel dosimetry.

Evaluation of Polymer Gel Dosimeters for Measurements of Dose and LET in Proton Beams

This project assessed the effectiveness of polymer gel dosimeters as tools for measuring the dose deposited by and LET of a proton beam. A total of three BANG® dosimeter formulations were evaluated: BANG®-3-Pro-2 BANGkits™ for dose measurement and two BANG®-3 variants, the LET-Baseline and LET-Meter dosimeters, for LET measurement. All dosimeters were read out using an OCT scanner. The basic characteristics of the BANGkits™ were assessed in a series of photon and electron irradiations. The dose-response relationship was found to be sigmoidal with a threshold for response of approximately 15 cGy. The active region of the dosimeter, the volume in which dosimeter response is not inhibited by oxygen, was found to make up roughly one fourth of the total dosimeter volume. Delivering a dose across multiple fractions was found to yield a greater response than delivering the same dose in a single irradiation. The dosimeter was found to accurately measure a dose distribution produced by overlapping photon fields, yielding gamma pass rates of 95.4% and 93.1% from two planar gamma analyses. Proton irradiations were performed for measurements of proton dose and LET. Initial irradiations performed through the side of a dosimeter led to OCT artifacts. Gamma pass rates of 85.7% and 89.9% were observed in two planar gamma analyses. In irradiations performed through the base of a dosimeter, gel response was found to increase with height in the dosimeter, even in areas of constant dose. After a correction was applied, gamma pass rates of 94.6% and 99.3% were observed in two planar gamma analyses. Absolute dose measurements were substantially higher (33%-100%) than the delivered doses for proton irradiations. Issues encountered while calibrating the LET-Meter gel restricted analysis of the LET measurement data to the SOBP of a proton beam. LET-Meter overresponse was found to increase linearly with track-average LET across the LET range that could be investigated (1.5 keV/micron – 3.5 keV/micron).

Autologous Platelet Gel: Five Cases Illustrating Use On Sickle Cell Disease Ulcers.

Leg ulcers represent a particularly disabling complication in patients with sickle cell disease (SCD). Platelet gel (PG) is a novel therapeutic strategy used for accelerating wound healing of a wide range of tissues through the continuous release of platelet growth factors. Here, we describe the use of PG preparation according to Anitua's PRGF (preparations rich in growth factors) protocol for treating chronic nonhealing ulcers in patients with SCD. A positive response occurred in 3 patients with an area reduction of 85.7% to 100%, which occurred within 7 to 10 weeks, and a 35.2% and 20.5% of area reduction in 2 other patients, who however, had large ulcers. After calcium chloride addition, the platelet-rich plasmas demonstrated enhanced platelet-derived growth factors-BB (P < .001), transforming growth factor-β1 (P = .015), vascular endothelial growth factors (P = .03), and hepatocyte growth factors (nonsignificant) secretion. Furthermore, calcium chloride addition induced a significant decrease in platelet number (P = .0134) and there was no leukocyte detection in the PG product. These results demonstrate that PG treatment might impact the healing of leg ulcers in sickle cell disease, especially in patients with small ulcers.

Microbial Evaluation Of Traumatized Teeth Treated With Triple Antibiotic Paste Or Calcium Hydroxide With 2% Chlorhexidine Gel In Pulp Revascularization.

Revascularization outcome depends on microbial elimination because apical repair will not happen in the presence of infected tissues. This study evaluated the microbial composition of traumatized immature teeth and assessed their reduction during different stages of the revascularization procedures performed with 2 intracanal medicaments. Fifteen patients (7-17 years old) with immature teeth were submitted to the revascularization procedures; they were divided into 2 groups according to the intracanal medicament used: TAP group (n = 7), medicated with a triple antibiotic paste, and CHP group (n = 8), dressed with calcium hydroxide + 2% chlorhexidine gel. Samples were taken before any treatment (S1), after irrigation with 6% NaOCl (S2), after irrigation with 2% chlorhexidine (S3), after intracanal dressing (S4), and after 17% EDTA irrigation (S5). Cultivable bacteria recovered from the 5 stages were counted and identified by means of polymerase chain reaction assay (16S rRNA). Both groups had colony-forming unit counts significantly reduced after S2 (P < .05); however, no significant difference was found between the irrigants (S2 and S3, P = .99). No difference in bacteria counts was found between the intracanal medicaments used (P = .95). The most prevalent bacteria detected were Actinomyces naeslundii (66.67%), followed by Porphyromonas endodontalis, Parvimonas micra, and Fusobacterium nucleatum, which were detected in 33.34% of the root canals. An average of 2.13 species per canal was found, and no statistical correlation was observed between bacterial species and clinical/radiographic features. The microbial profile of infected immature teeth is similar to that of primarily infected permanent teeth. The greatest bacterial reduction was promoted by the irrigation solutions. The revascularization protocols that used the tested intracanal medicaments were efficient in reducing viable bacteria in necrotic immature teeth.

Recolonization Of Mutans Streptococci After Application Of Chlorhexidine Gel.

Streptococcus mutans is specifically suppressed by intensive treatment with chlorhexidine gel, but the time for recolonization and the effect on other oral bacteria are not totally clear. In this study, recolonization of mutans streptococci was evaluated in nine healthy adult volunteers, who were highly colonized with this microorganism. Stimulated saliva was collected before (baseline) and at 1, 7, 14, 21 and 28 days after application of 1% chlorhexidine gel on volunteers' teeth for two consecutive days. On each day, the gel was applied using disposable trays for 3 x 5 min with intervals of 5 min between each application. Saliva was plated on blood agar to determine total microorganisms (TM); on mitis salivarius agar to determine total streptococci (TS) and on mitis salivarius agar plus bacitracin to determine mutans streptococci (MS). Chlorhexidine was capable of reducing the counts of MS and the proportion of MS with regard to total microorganisms (%MS/TM) (p<0.05), but these values did not differ statistically from baseline (p>0.05) after 14 days for MS and 21 days for %MS/TM. The counts of TM and TS and the proportion of MS to total streptococci did not differ statistically from baseline (p>0.05) after chlorhexidine treatment. The results suggest that the effect of chlorhexidine gel treatment on suppression of mutans streptococci is limited to less than a month in highly colonized individuals.

Liposomal Lidocaine Gel For Topical Use At The Oral Mucosa: Characterization, In Vitro Assays And In Vivo Anesthetic Efficacy In Humans.

To characterize liposomal-lidocaine formulations for topical use on oral mucosa and to compare their in vitro permeation and in vivo anesthetic efficacy with commercially available lidocaine formulations. Large unilamellar liposomes (400 nm) containing lidocaine were prepared using phosphatidylcholine, cholesterol, and α-tocoferol (4:3:0.07, w:w:w) and were characterized in terms of membrane/water partition coefficient, encapsulation efficiency, size, polydispersity, zeta potential, and in vitro release. In vitro permeation across pig palatal mucosa and in vivo topical anesthetic efficacy on the palatal mucosa in healthy volunteers (double-blinded cross-over, placebo controlled study) were performed. The following formulations were tested: liposome-encapsulated 5% lidocaine (Liposome-Lido5); liposome-encapsulated 2.5% lidocaine (Liposome-Lido2.5); 5% lidocaine ointment (Xylocaina®), and eutectic mixture of lidocaine and prilocaine 2.5% (EMLA®). The Liposome-Lido5 and EMLA showed the best in vitro permeation parameters (flux and permeability coefficient) in comparison with Xylocaina and placebo groups, as well as the best in vivo topical anesthetic efficacy. We successfully developed and characterized a liposome encapsulated 5% lidocaine gel. It could be considered an option to other topical anesthetic agents for oral mucosa.

Síntese e estudo de ortossilicatos de zinco luminescentes com aplicação da técnica sol-gel

Usually, the concepts of the Sol-Gel technique are not applied in experimental chemistry courses. This work presents a feasible experiment for chemistry instruction, which involves the synthesis of luminescent materials - Zn2SiO4, with and without Mn2+ as a dopant - by the Sol-Gel technique. The obtained materials were analyzed by scanning electron microscopy, X-Ray diffraction, IR spectroscopy and luminescence measures by UV-vis spectroscopy. The results allow the students to confirm the luminescent properties of the zinc orthosilicate luminophores as well as the structural features expected from literature data.

CROMATOGRAFIA DE AFINIDADE COM CORANTE RED A versus TROCA IÔNICA-PERMEABILIDADE EM GEL: COMPARAÇÃO DA PRATICIDADE NA PURIFICAÇÃO DE ENTEROTOXINA ESTAFILOCÓCICA A

Culture supernatant of Staphylococcus aureus 722 in 3% triptone plus 1% yeast extract was used for EEA purification, proceeding comparison between dye ligand Red A affinity chromatography and classic chromatography. The capture of SEA with Amberlite CG-50 allowed rapid enterotoxin concentration from the culture supernatant. However, the ratio of 15 mg of the resin to a total of 150 mg of the toxin satured the resin, giving only 10 to 30% of SEA recuperation from the supernatant. The elution of concentrated material throught the Red A column resulted in a recovery of 60,87% of the toxin, and required 76 hours, indicating advantage on classic chromatography. Ion exchange column plus gel filtration recovered only 6,5 % of the SEA, and required 114 hours to conclude the procedure. The eletrophoresis of purified SEA indicated high grade of toxin obtained from Red A column, with 90 % of purity, compared to 60 % of classic column.