This paper presents a methodology for three-dimensional (3D) computer modelling of the breast, using a combination of 3D geometrical primitives and voxel matrices that can be further subjected to simulated x-ray imaging, to produce synthetic mammograms. The breast phantom is a composite model of the breast and includes the breast surface, the duct system and terminal ductal lobular units. Cooper's ligaments, the pectoral muscle, the 3D mammographic background and breast abnormalities. A second analytical x-ray matter interaction modelling module is used to generate synthetic images from monoenergetic fan beams. Mammographic images of various synthesized breast models differing in size, shape and composition were produced. A preliminary qualitative assessment performed by three radiologists and a quantitative evaluation study using fractal and grey-level histogram analysis were conducted. A comparative study of extracted features with published data has also been performed. The evaluation results indicated good correlation of characteristics between synthetic and actual radiographs. Applications foreseen are not only in the area of breast imaging experimentation but also in education and training.

In 13CO2 breath tests, based on 13C:12C ratio measurements, the appearance of 13C in exhaled CO2 was monitored after the administration of a 13C-labelled compound. Independently of the substrate used, the existence of a bicarbonate pool into which the CO2 produced enters before being exhaled, imposes a delay on the appearance of changes in the 13C:12C ratio. To estimate the nature and magnitude of this delay, we applied a two-compartment model to describe the kinetics of the body bicarbonate pool and we evaluated the 13C:12C ratio of CO2 entering that pool from the measured 13C:12C ratio in the exhaled CO2 after an oral intake of "naturally labelled" 13C-glucose. Our results demonstrated that discrepancies between total and exogenous glucose oxidation in relation to the peak occurrence time, as well as the absolute quantities, could be adequately explained by the interference of the bicarbonate stores.

This paper presents a mammography simulator and demonstrates its applicability in feasibility studies in dual-energy (DE) subtraction mammography. This mammography simulator is an evolution of a previously presented x-ray imaging simulation system, which has been extended with new functionalities that are specific for DE simulations. The new features include incident exposure and dose calculations, the implementation of a DE subtraction algorithm as well as amendments to the detector and source modelling. The system was then verified by simulating experiments and comparing their results against published data. The simulator was used to carry out a feasibility study of the applicability of DE techniques in mammography, and more precisely to examine whether this modality could result in better visualization and detection of microcalcifications. Investigations were carried out using a 3D breast software phantom of average thickness, monoenergetic and polyenergetic beam spectra and various detector configurations. Dual-shot techniques were simulated. Results showed the advantage of using monoenergetic in comparison with polyenergetic beams. Optimization studies with monochromatic sources were carried out to obtain the optimal low and high incident energies, based on the assessment of the figure of merit of the simulated microcalcifications in the subtracted images. The results of the simulation study with the optimal energies demonstrated that the use of the DE technique can improve visualization and increase detectability, allowing identification of microcalcifications of sizes as small as 200 microm. The quantitative results are also verified by means of a visual inspection of the synthetic images.

The extent to which an oral load of glucose is absorbed from the gut and oxidized during prolonged exercise is a matter of controversy. Four healthy volunteers, 18-28 yr, were submitted on 4 different days to a 105-min treadmill exercise at 22, 39, 51, and 64% of their individual VO2max. After 15 min adaptation to exercise, they received orally 100 g naturally labeled [13C]glucose. Oxidation of the exogenous glucose was followed by 13CO2 measurements in the expired air; total carbohydrate and lipid oxidation were evaluated by indirect calorimetry. Between 22 and 51% VO2 max, total carbohydrate, lipid oxidation, and exogenous glucose oxidation were linearly correlated with the relative work load (r = 0.81; P less than 0.01). Between 51 and 64% VO2 max, exogenous glucose oxidation and lipid oxidation tended to level off, whereas endogenous carbohydrate oxidation was markedly enhanced. The lesser contribution of exogenous glucose during the most intense exercise might be due to a decrease in the oxidation in the muscles or to a lesser availability of this exogenous glucose.

A new method of optimized efficiency for the retrospective reconstruction of tomograms is presented. The method has been developed for use with isocentric fluoroscopic units and is capable of performing digital tomosynthesis of anatomical planes of user selected orientation and distance from the isocenter. Optimization of efficiency has been achieved by segmenting the reconstruction process into discrete transformations that are specific to groups of pixels, rather than performing pixel by pixel operations. These involve a number of projections of the acquired image matrices as well as parallel translations and summing. Application of this method has resulted in a significant reduction of computing time. The proposed algorithm has been experimentally tested on a radiotherapy simulator unit with the use of a phantom and the obtained results are reported and discussed.