Experimental and theoretical system analysis for canine myocardial oxygen supply and demand and their determinants

Imperial Users onl

Self-motion and the perception of stationary objects

One of the ways we perceive shape is through seeing motion. Visual motion may be actively generated (for example, in locomotion), or passively observed. In the study of how we perceive 3D structure from motion (SfM), the non-moving, passive observer in an environment of moving rigid objects has been used as a substitute for an active observer moving in an environment of stationary objects; the 'rigidity hypothesis' has played a central role in computational and experimental studies of SfM. Here we demonstrate that this substitution is not fully adequate, because active observers perceive 3D structure differently from passive observers, despite experiencing the same visual stimulus: active observers' perception of 3D structure depends on extra-visual self-motion information. Moreover, the visual system, making use of the self-motion information treats objects that are stationary (in an allocentric, earth-fixed reference frame) differently from objects that are merely rigid. These results show that action plays a central role in depth perception, and argue for a revision of the rigidity hypothesis to incorporate the special case of stationary objects

Hidden Structures in Super Form Factors

PhD thesisMaximally supersymmetric Yang–Mills theory in four dimensions has remarkable features such as conformal symmetry at the quantum level, evidence of integrability and the existence of a well defined holographic dual. The associated perturbative S-matrix and the mysterious roots of its striking simplicity are part of an active area of research which has recently witnessed enormous progress in making many of its special features manifest. These successes have led to the question of whether such hidden structures are necessarily confined to the realm of the S-matrix or whether they can also illuminate other aspects of the theory. The first step towards the study of more “off-shell quantities” is represented by supersymmetric form factors. In the first part of the thesis, we propose formulas for any tree-level form factor of the stress-tensor multiplet, derived from twistor worldsheet models. These are the analogue of the ones introduced for amplitudes, both in the twistor and in the more recent ambitwistor formulation. Another important line of research originates from the AdS/CFT correspondence. In this context, amplitudes are shown to be T-dual to polygonal lightlike Wilson loops. From the point of view of form factors, the dual holographic picture is that of a periodic lightlike Wilson line. The existence of such a picture constitutes a strong indication of invariance under dual conformal transformations. In the second part of the thesis, we give a prescription for the definition of a canonical integrand for super form factors at one loop in terms of region variables in dual space. This allows us to derive recursion relations at loop level and to study the properties of the resulting expressions under the action of dual conformal generators. We show that the dual conformal anomaly for an arbitrary number of particles and generic helicities matches the expression known for the amplitude case

Optimising the assessment of cerebral autoregulation from black box models

Cerebral autoregulation (CA) mechanisms maintain blood flow approximately stable despite changes in arterial blood pressure. Mathematical models that characterise this system have been used extensively in the quantitative assessment of function/impairment of CA. Using spontaneous fluctuations in arterial blood pressure (ABP) as input and cerebral blood flow velocity (CBFV) as output, the autoregulatory mechanism can be modelled using linear and non-linear approaches, from which indexes can be extracted to provide an overall assessment of CA. Previous studies have considered a single – or at most a couple of measures, making it difficult to compare the performance of different CA parameters. We compare the performance of established autoregulatory parameters and propose novel measures. The key objective is to identify which model and index can best distinguish between normal and impaired CA. To this end 26 recordings of ABP and CBFV from normocapnia and hypercapnia (which temporarily impairs CA) in 13 healthy adults were analysed. In the absence of a ‘gold’ standard for the study of dynamic CA, lower inter- and intra-subject variability of the parameters in relation to the difference between normo- and hypercapnia were considered as criteria for identifying improved measures of CA. Significantly improved performance compared to some conventional approaches was achieved, with the simplest method emerging as probably the most promising for future studies

Interactive volumetric segmentation for textile micro-tomography data using wavelets and nonlocal means

This work addresses segmentation of volumetric images of woven carbon fiber textiles from micro-tomography data. We propose a semi-supervised algorithm to classify carbon fibers that requires sparse input as opposed to completely labeled images. The main contributions are: (a) design of effective discriminative classifiers, for three-dimensional textile samples, trained on wavelet features for segmentation; (b) coupling of previous step with nonlocal means as simple, efficient alternative to the Potts model; and (c) demonstration of reuse of classifier to diverse samples containing similar content. We evaluate our work by curating test sets of voxels in the absence of a complete ground truth mask. The algorithm obtains an average 0.95 F1 score on test sets and average F1 score of 0.93 on new samples. We conclude with discussion of failure cases and propose future directions toward analysis of spatiotemporal high-resolution micro-tomography images

Computation of Fiber Orientation in X-Ray Micro-Tomography Reconstructions

No abstract availabl

Revisiting the frequency domain: the multiple and partial coherence of cerebral blood flow velocity in the assessment of dynamic cerebral autoregulation

Despite advances in modelling dynamic autoregulation, only part of the variability of cerebral blood flow velocity (CBFV) in the low frequency range has been explained. We investigate whether a multivariate representation can be used for this purpose. Pseudorandom sequences were used to inflate thigh cuffs and to administer 5% CO2. Multiple and partial coherence were estimated, using arterial blood pressure (ABP), end-tidal CO2 (EtCO2) and resistance area product as input and CBFV as output variables. The inclusion of second and third input variables increased the amount of CBFV variability that can be accounted for (p  <  10−4 in both cases). Partial coherence estimates in the low frequency range (<0.07 Hz) were not influenced by the use of thigh cuffs, but CO2 administration had a statistically significant effect (p  <  10−4 in all cases). We conclude that the inclusion of additional inputs of a priori known physiological significance can help account for a greater amount of CBFV variability and may represent a viable alternative to more conventional non-linear modelling. The results of partial coherence analysis suggest that dynamic autoregulation and CO2 reactivity are likely to be the result of different physiological mechanisms

Aerothermal Characterization of Silicon Carbide-Based TPS in High Enthalpy Airflow

Inductively-coupled plasma generators provide an ideal environment to reproduce the aerothermal heating experienced by a spacecraft re-entering a planetary atmosphere. The flight boundary layer chemistry is duplicated around a TPS model, ensuring a similarity between the flight and ground stagnation-point heat flux. Experiments conducted in an induction plasmatron on silicon carbide-based thermal protection materials will be described. Several specimens are tested under a wide range of pressure and temperature conditions and investigated by means of infrared radiometry and optical emission spectroscopy. The plasma to which the materials are exposed is characterized in details by calorimetric and Pitot pressure measurements, and numerically rebuilt by means of a nonequilibrium boundary layer model. The presentation will focus on the thermophysical properties of the material and their dependency on the testing environment. In particular, we will discuss the oxidation features of silicon carbide which are detected both via emission spectroscopy and post-test reflectivity measurements