Oral biofilms play an important role in the development and the persistence of caries, gingivitis and periodontitis. The addition of antimicrobials to toothpastes and mouthwashes combined with biofilm mechanical disruption through dental cleaning devices is the most common way to control oral diseases. However, biofilms’ complicated structure increases their resistance to antiplaque and/or antimicrobials by limiting the diffusion of dentifrices into and inside the biofilm. Studies showed that fluid-dynamic activity generated by power toothbrushes can enhance mass transfer inside the remaining biofilm compared to simple diffusional transport. Microsprays have the advantage in that they are low volume but also have an air/water interface which facilitates biofilm removal. The role of the hydrodynamics in the enhancement of dentifrices inside the biofilm has become a topic of interest since it has been shown that mechanical perturbation caused by fluid-dynamic activity can significantly weaken biofilm structure. Here we showed that high-velocity microsprays enhance microparticles penetration and Chlorhexidine and Cetylpyridinium chloride antimicrobial activity inside Streptococcus mutans dental biofilms through the generation of hydrodynamic deformations. Using highspeed camera videography, we documented S. mutans biofilm extremely transient fluid behavior andthe generation of ripple-like structures at the biofilm/fluid interface when exposed to water microsprays. Mathematical modelling demonstrated that ripples were Kelvin Helmotz Instabilities suggesting the development of fluid-like turbulent mixing in biofilms. Shear stresses generated at the biofilm/burst interface might have enhanced beads and antimicrobials delivery inside the remaining biofilm by combining forced advection into the biofilm matrix with the mixing of the biofilm itself. This project provided further insight into the mechanical behaviour of biofilms as complex liquids and how high-shear fluid-biofilm interaction can be induced to modulate biofilm survival and tolerance.

Acquisition and reconstruction of data in three-dimensional positron emission tomography (3D PET) was introduced in 1990 almost 20 years after the first PET scanners were developed. 3D PET offers a significant sensitivity improvement over conventional, sliceoriented 2D PET, but at the cost of a three-fold increase in acceptance of scattered events. In addition, processing time is increased and new methods for applying corrections such as for photon attenuation, calibration, and detector/geometry normalisation are required. 3D PET raised concerns that the high quantitative accuracy that was possible with 2D PET (with its moderate sensitivity) would not be matched in 3D, primarily because of the greatly increased scattered photon component in the measured data. The aim of this thesis was to develop methods that enable quantitatively accurate measurements with 3D PET. A technique to correct for scattered photons prior to reconstruction has been developed, implemented and assessed. A device for normalising the data for detector efficiency and the geometry of the cylindrical detector system has been developed, and the factors affecting reconstruction investigated. A new approach to calibration of the reconstructed data to produce images of activity concentration which is independent of scatter has been implemented. Finally, the techniques have been applied to data from brain scans of human subjects. Evaluation of images reconstructed from 3D PET demonstrates that the methodology developed in this work produces data accurate to within 10% of the true activity concentration in an object with reasonably homogeneous density. 3D PET is shown to be as accurate as 2D PET, but with a sensitivity advantage that improves signal-to-noise by approximately a factor of three in the human brain and slightly less in other regions of the body.

The genetical and environmental control of a number of characters of agronomic importance in barley has been investigated by triple test cross (TTC) and linear model-fitting analyses. Additive and dominance genetic effects are observed for height, maturity, yield and yield component characters with the exception of tiller number, which was found to be almost totally controlled by environmental factors. Epistasis was found to be important only for grain number, although it was observed in other characters (notably neck length). Other analyses on the same data set have also been made to detect the presence of genotype x environment interactions and linkage in the above characteristics, together with phenotypic and genotypic correlations between them. Only height at harvest and 1000 grain weight appeared to have potential for early generation selection. As a result, efficient prediction of the potential of a particular cross for the majority of characters of agronomic importance is shown to be necessary and important. The second part of this work involved testing the effectiveness of cross-prediction methods based on the results of the genetical analyses. It is shown that the potential of a cross to produce superior inbred lines can be successfully predicted from TTC and model-fitting analyses. The prediction methods have been shown to be successful in identifying the cross from a number of crosses with the greatest potential in respect of both single characters and pairs of characters. It is shown that yield itself can be studied successfully in this way, as well as those characters with high heritability such as height and 1000 grain weight. Furthermore it is shown that an estimate of the additive genetic variance necessary for making the predictions, of similar accuracy to that obtained from the TTC, can be Je<-ive<J_ from an F family analysis, thus saving considerably on the experimental work necessary. The predictions are compared with the results of a normal selection programme practised on the same material to illustrate the need for early generation prediction and delay of selection until later generations to prevent loss of potentially useful lines in early generations.

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