Tomografía multidetector-16 de cráneo en niños: estimación de dosis a órganos críticos por DTL

En este estudio se estimaron las dosis por radiación dispersa en los órganos críticos en pacientes pediátricos del Hospital Infantil de México “Federico Gómez” de la Secretaria de salud sometidos a estudios de tomografía multicorte-16 de cráneo, con la finalidad de mostrar la relación entre las dosis a órgano con las cantidades dosimétricas utilizadas en TAC, así como conocer las dosis a las cuales irradiamos a nuestros niños en tales órganos. Los protocolos de exploración en los que se determinó las dosis a órganos fueron tomografía simple, tomografía contrastada y reconstrucción tridimensional de cráneo. El tomógrafo utilizado fue un Siemens Somatom 16 plus multicorte y en la dosimetría se emplearon dosímetros termoluminiscentes de LiF:Mg,Cu,P+PTFE debido a sus características, tales como bajo desvanecimiento, equivalencia con el tejido, fácil manejo y lectura e independencia de su respuesta con la energía de la radiación. Las dosis absorbidas en órganos en el estudio simple en general fueron la mitad de las encontradas en el estudio contrastado, esto concuerda con el valor del Índice de Dosis en Tomografía en Volumen (CTDIvol ), DLP (producto dosis longitud) y dosis efectiva pues estos también fueron la mitad del valor en relación al valor promedio. En el caso de la tomografía de reconstrucción tridimensional de cráneo el CTDIvol fue prácticamente el mismo que en tomografía simple: no obstante las dosis a órganos difirieron significativamente y el DLP y la dosis efectiva del estudio reconstrucción tridimensional fueron mayor que el estudio simple de cráneo. En la comparación del valor del CTDIvol de la tomografía de reconstrucción tridimensional con el de la tomografía contrastada se obtuvo la mitad del valor para la primera, sin embargo las dosis a órgano no mostraron estar relacionados de la misma forma e incluso la dosis absorbida en el caso de la glándula tiroides fue superior por un factor de 8 con respecto al valor promedio del estudio de reconstrucción tridimensional. En el estudio de reconstrucción tridimensional de cráneo la dosis absorbida por radiación dispersa en glándula tiroides alcanzó una dosis en la que existe un exceso relativo del riesgo o muerte por cáncer, a pesar que el valor del CDTIvol, DLP y dosis efectiva en este estudio son acorde a las normas internacionales. La glándula tiroides en el estudio no es parte del área de interés diagnostico lo que hace necesario la aplicación de medidas para limitar su irradiación. El valor del CTDIvol no es un indicador la dosis absorbida a órganos críticos debida radiación dispersa y las cantidades dosimétricas no siempre guardan una relación con las dosis a órgano para un estudio de la misma región anatómica en diferentes protocolos de exploración. Esto hace necesario la determinación de las dosis a órgano críticos y su relación con el CTDIvol, DLP y dosis efectiva en de las diversas regiones anatómicas con sus respectivos protocolos de exploración tomográfica

Radiative Relaxation Quantum Yields for Synthetic Eumelanin

We report absolute values for the radiative relaxation quantum yield of synthetic eumelanin as a function of excitation energy. These values were determined by correcting for pump beam attenuation and emission re-absorption in both eumelanin samples and fluorescein standards over a large range of concentrations. Our results confirm that eumelanins are capable of dissipating >99.9% of absorbed UV and visible radiation through non-radiative means. Furthermore, we have found that the radiative quantum yield of synthetic eumelanin is excitation energy dependent. This observation is supported by corrected emission spectra, which also show a clear dependence of both peak position and peak width upon excitation energy. Our findings indicate that photoluminescence emission in eumelanins is derived from ensembles of small chemically distinct oligomeric units which can be selectively pumped. This hypothesis lends support to the theory that the basic structural unit of eumelanin is oligomeric rather than hetero-polymeric.Comment: Paper accepted for publication in Photochem. Photobiol. (March 2004). All figues included in pdf fil

Effects of Inflorescence Stem Structure and Cell Wall Components on the Mechanical Strength of Inflorescence Stem in Herbaceous Peony

Herbaceous peony (Paeonia lactiflora Pall.) is a traditional famous flower, but its poor inflorescence stem quality seriously constrains the development of the cut flower. Mechanical strength is an important characteristic of stems, which not only affects plant lodging, but also plays an important role in stem bend or break. In this paper, the mechanical strength, morphological indices and microstructure of P. lactiflora development inflorescence stems were measured and observed. The results showed that the mechanical strength of inflorescence stems gradually increased, and that the diameter of inflorescence stem was a direct indicator in estimating mechanical strength. Simultaneously, with the development of inflorescence stem, the number of vascular bundles increased, the vascular bundle was arranged more densely, the sclerenchyma cell wall thickened, and the proportion of vascular bundle and pith also increased. On this basis, cellulose and lignin contents were determined, PlCesA3, PlCesA6 and PlCCoAOMT were isolated and their expression patterns were examined including PlPAL. The results showed that cellulose was not strictly correlated with the mechanical strength of inflorescence stem, and lignin had a significant impact on it. In addition, PlCesA3 and PlCesA6 were not key members in cellulose synthesis of P. lactiflora and their functions were also different, but PlPAL and PlCCoAOMT regulated the lignin synthesis of P. lactiflora. These data indicated that PlPAL and PlCCoAOMT could be applied to improve the mechanical strength of P. lactiflora inflorescence stem in genetic engineering

Phenotypic plasticity in cell walls of maize brown midrib mutants is limited by lignin composition

The hydrophobic cell wall polymer lignin is deposited in specialized cells to make them impermeable to water and prevent cell collapse as negative pressure or gravitational force is exerted. The variation in lignin subunit composition that exists among different species, and among different tissues within the same species suggests that lignin subunit composition varies depending on its precise function. In order to gain a better understanding of the relationship between lignin subunit composition and the physico-chemical properties of lignified tissues, detailed analyses were performed of near-isogenic brown midrib2 (bm2), bm4, bm2-bm4, and bm1-bm2-bm4 mutants of maize. This investigation was motivated by the fact that the bm2-bm4 double mutant is substantially shorter, displays drought symptoms even when well watered, and will often not develop reproductive organs, whereas the phenotypes of the individual bm single mutants and double mutant combinations other than bm2-bm4 are only subtly different from the wild-type control. Detailed cell wall compositional analyses revealed midrib-specific reductions in Klason lignin content in the bm2, bm4, and bm2-bm4 mutants relative to the wild-type control, with reductions in both guaiacyl (G)- and syringyl (S)-residues. The cellulose content was not different, but the reduction in lignin content was compensated by an increase in hemicellulosic polysaccharides. Linear discriminant analysis performed on the compositional data indicated that the bm2 and bm4 mutations act independently of each other on common cell wall biosynthetic steps. After quantitative analysis of scanning electron micrographs of midrib sections, the variation in chemical composition of the cell walls was shown to be correlated with the thickness of the sclerenchyma cell walls, but not with xylem vessel surface area. The bm2-bm4 double mutant represents the limit of phenotypic plasticity in cell wall composition, as the bm1-bm2-bm4 and bm2-bm3-bm4 mutants did not develop into mature plants, unlike the triple mutants bm1-bm2-bm3 and bm1-bm3-bm4