Faculdade de Medicina

7 History and Philosophy of Science: Towards a New Epistemology Maria Burguete This chapter discusses a new concept of doing and thinking about chemistry while this scientific field is engaged with new phenomena. The new concept can give new answers to old problems related to ...

Abstract In the hard determinism of Newtonian physics all aspects of the universe are deterministic and therefore all future behavior in the universe is determined by its present state. Hard determinism is incompatible with the existence of free will, but not with the belief in the existence of ...

To obtain normative human cerebral data and evaluate the anatomic information in quantitative diffusion anisotropy magnetic resonance (MR) imaging. Quantitative diffusion anisotropy MR images were obtained in 13 healthy adults by using single-shot echo-planar MR imaging and a combination of tetrahedral and orthogonal gradient encoding (whole-brain coverage in about 1 minute). White matter (WM) anatomy was assessed at visual inspection, and values were measured in various brain regions. Different anisotropy measures, including total anisotropy (A sigma), were compared on the basis of information content, rotational invariance, and susceptibility to noise. Partial volume and noise effects were simulated. Anisotropy MR images depicted WM features not typically seen on conventional MR images (e.g., external capsule, thalamic substructures, basal ganglia, occipital WM, thickness of the internal capsule). Statistically significant anisotropy differences occurred across brain regions, which were reproducible within and across subjects. A sigma was highest in commissural WM and progressively lower in projection and association WM. This order paralleled that of known resistance to spread of vasogenic edema, which suggested that anisotropy may be sensitive to WM histologic structure. Gray matter (GM) A sigma data were consistent with zero anisotropy, and partial volume WM-GM effects were approximately linear. A sigma image quality could be effectively improved by means of averaging. Quantitative diffusion anisotropy images can be obtained rapidly and demonstrate subtle WM anatomy. Different histologic types of WM have significant and reproducible anisotropy differences.

Diffusion MRI allows the obtaining of an approximation of the water displacement's probability density function (PDF) and orientation distribution function (ODF). Examples of techniques used in obtaining these distributions being q-space imaging (QSI), and q-ball imaging (QBI), respectively. Shannon information quantifies the discriminative power of a symbol based on its probability. We quantified the information amount of a white matter fiber bundle being used to discriminate those fibers using specific diffusion MRI data treatment techniques. The equations developed are new and it is also described how they will help in future experimental calculations. An example of experimental ODF surfaces and ODF based white matter fiber tracking in living humans is also shown to highlight possible future advantages of Shannon information usage in describing crossing white matter fiber bundles.