Scattering and ACTH regulation.

• Main Author: Smith, Vaughan Allan
• Language: en
• Published: University of Canterbury. Electrical Engineering 2011
• Online Access: http://hdl.handle.net/10092/5709

The work reported in this thesis is concerned with two completely separate topics - scattering and adrenocorticotropic hormone (ACTH) regulation. The accuracies of two approximate scattering formulations, the Born approximation and geometrical optics, are investigated and compared. The distortion associated with multiple scattering in Born inversion is shown to be severe in situations where the refractive indices of object components are only slightly (<5%) different from the background. Second and higher order multiple scattering is found to contribute negligible distortion to the reconstruction. Backpropagation is explicitly formulated and implemented for receivers in the far-field, but Fourier interpolation is found to be a preferable method for performing Born inversion in the simulation situation. Shift-and-add, a form of frequency diversity processing which has been previously proposed to reduce distortion in Born inversion, is comprehensively tested. Although the anticipated reduction in distortion is not achieved, some improvement is demonstrated. Furthermore, this work shows the need for iterative solutions to realistic inverse scattering problems. A new solution to the two-dimensional bent-ray computed tomography problem is developed. It is based on a novel ray description which does not require the ray paths to be known. The refractive index distribution is therefore found by solving linear equations, rather than through the conventional process of ray tracing. Reconstructions of circularly symmetric refractive index distributions demonstrate the feasibility of this approach and encourage further development. The information available from a recently developed method for sampling pituitary effluent (PES) in the horse is examined. The current sampling interval of 30s is found to be appropriate for measuring ACTH and arginine vasopressin (AVP), but not necessarily corticotropin-releasing hormone (CRH). Blood flow variations are found to have essentially negligible effect on measurement of hormone concentrations at the corticotropes. A recently proposed model of ACTH regulation, derived from static cell culture results, is tested on PES data and found to be inappropriate for the in vivo situation. The relationships between AVP, CRH, cortisol and ACTH are examined using cross-correlation, pulse analysis and models of ACTH regulation. The pulse analysis indicates that the delay between AVP or CRH and ACTH is 0-308. Results from all three techniques indicate that AVP is the important physiological ACTH secretagogue in the horse. Results from the modeling analysis suggest that the relative potency of AVP and CRH depends on the cortisol concentration, and that when this is low, CRH is the dominant secretagogue. Modeling results also reveal that the principal effects of cortisol on ACTH secretion occur in the pituitary, and not in the brain as is currently believed. In addition, they show that in some situations AVP, CRR and cortisol are sufficient to explain almost all ACTH secretion, although the pulse analysis indicates that additional secretagogues do have physiological effects. Furthermore, this work demonstrates the need for rapid sampling and makes some suggestions regarding important mechanisms operating in the corticotrope in vivo.