While the ability of peri-substitution to stabilise species that are typically short-lived is well documented, its capacity to promote kinetically disfavoured reactivity is less well known. In this work, we examine the ability of the rigid scaffold to enhance reactions that would typically require the addition of an external catalyst, particularly coupling reactions. The spontaneous dehydrocoupling of a primary arsine to give the first arsanylidene–s⁴–phosphorane was recently published by the Kilian group, but its mechanism was unknown. Based on experimental observations, a stepwise ionic mechanism is proposed, and indirect evidence for this mechanism is obtained by chemically mimicking the proposed pathway through hydride abstraction and deprotonation. In the process, a new route to the previously published phosphanylidene–s⁴–phosphorane is found, and the first examples of arsino-phosphonium and phosphino-phosphonium salts with secondary arsine/phosphine moieties are obtained. A series of peri-substituted phosphine-borane adducts were found to undergo a spontaneous P–B dehydrocoupling reaction, occurring at room temperature and in the absence of a catalyst. This is the first example of this reaction occurring under such mild conditions, and highlights the ability of peri- substitution to substantially lower the kinetic barrier to certain reactions. A thermally induced “dealkacoupling” reaction, eliminating a C–H coupled product with formation of a P–P/P–As bond, has been observed in peri-substituted phosphines and arsines. This reaction appears to be unique in the context of the literature, and was surprisingly facile, occurring under relatively mild conditions and essentially quantitatively with 100% diastereoselectivity in the products. Kinetic studies on a representative reaction reveal it to have a low activation barrier (Δ[super]‡ H° = +57 ± 7 kJ mol⁻¹), and suggest the mechanism is radical in nature. The reaction likely relies on peri-substitution to stabilise the radical intermediates of the reaction. This concept was then extended to C-heteroatom couplings. A selection of peri-substituted substrates were found to undergo P–P coupling, with apparent elimination of C–O and C–S coupled products. GC analysis revealed that C–S coupling does indeed occur with these systems, although there are competing side reactions. A variety of arsenic and antimony coupling precursors were also synthesised and characterised.

The root meristem of Allium sativum has been described from longitudinal and transverse sections of the root apex. No evidence for the presence of a quiescent centre can be gained from a Norper-kappe analysis of cell patterns. The presence at the pole of the stake of a quiescent centre comprising 30-50 cells has been demonstrated by autoradiographs of roots fed with tritiated thymidine. Rates of mitosis in different regions of the root meristem have been measured by three independent methods. A) metaphase accumulation b) continuous labelling c) pulse labelling The results of the three methods are consistent and show that the rate of mitosis in the quiescent centre is of the order of ten times that in other parts of the meristem. The duration of the different phases of the mitotic cycle have been measured for various regions of the meristem by a pulse labelling for various regions of the meristem by a pulse labelling technique. It has been found that the time spent in mitosis and the later stages of interphase is approximately similar in all regions of the meristem. The extent of the mitotic cycle in quiescent centre cells is due to the length of time spent in G1 the period of interphase prior to DNA synthesis. Quiescent centre cells contain the 2G amount of DNA during approximately 90% of their cell cycle. It has been found that during recovery from colohicine treatment the mitotic index in the quiescent centre rises while it falls in other regions of the meristem. The cells forming the primordium from which the root regenerates after colochicine treatment are derivatives of quiescent centre cells. Using the number of cells carrying micronuclei as an estimate of radiation damage the radiosensitivity of different regions of the root meristem of Allium Sativium have been investigated after various doses of X-rays. It has been found that the cap initials are the most radiosenstive and quiescent centre cells by far the least sensitive. The duration of G1 is shorter in the cap initials than elsewhere in the meristem while it is longest in the quiescent centre. Cells from the two regions of the stele are approximately equally sensitive, the average values of G1 in these two regions are the same. Rates of mitosis in the different regions of the meristem have been measured by metaphase accumulation at various times after irradiation with 250 rads of X-rays. It the quiescent centre three or four days after irradiation the rate of mitosis is greater than in the other regions of the root being ten times that found in the quiescent centre of unirradiated control roots. Average rates of micosis in the rest of the meristem are considerably reduced.

Extracellular polymeric substances (EPS) are produced by epipelic diatoms as a function of their locomotive mechanism and also protect cells from desiccation and heavy metal toxicity. EPS are carbohydrate-rich and form an important carbon source for heterotrophic microorganisms. In addition, the polymeric structure of EPS results in sediment inter-grain binding, thereby increasing the resistance of sediments to erosion forces. Despite the importance of generic carbohydrates (measured as an index of EPS), there is little information on their spatial distribution or factors influencing their abundance. In this study, a fine-scale sectioning technique was developed and provided the first depth profiles of sediment carbohydrates at a scale relevant to microphytobenthos. The operational separation of sediment carbohydrates into bulk and colloidal fractions was examined and fractions were shown to vary in spatial and temporal characteristics. Colloidal carbohydrates were concentrated in the surface 200 mum layer of intertidal mud flats and therefore influenced sediment interface processes such as diffusion and sediment erosion. Colloidal carbohydrates were positively correlated to sediment chlorophyll a concentrations and also varied with tidal height and with sediment topography. During the emersion period, colloidal carbohydrate concentrations increase significantly, however, bulk carbohydrate concentrations remain unchanged. Biochemical analysis of the carbohydrate fractions showed marked, but not statistically proven, differences in the proportion of monosaccharides present and suggested these operational fractions may arise from different sources and are subject to varying turnover rates. Bulk sediment carbohydrates increase in concentration with sediment depth in the upper millimetres of intertidal sediments and were positively correlated to increases in sediment density. The increase in density with depth reflects post- depositional compaction and has important implications for models which predict sediment erosion based on sediment density. These findings are discussed in relation to contemporary thinking on sediment processes.

As ribonucleases catalisam a clivagem do RNA e são componentes ubíquos das células, desde procariotos até eucariotos. A família T2 é a classe de RNases mais amplamente distribuída entre os organismos vivos. A ampla distribuição desta família sugere que ela deve ter um papel funcional muito importante na biologia celular destes organismos. Em Solanaceae, há basicamente dois grupos destas proteínas, um representado pelas S-RNases, as quais estão envolvidas com a rejeição do pólen na autoincompatibilidade gametofítica e um outro grupo mais diverso que é denominado de S-like RNases, com funções muito diversificadas. As S-RNases se apresentam na forma de um gene multialélico, altamente polimórfico, que está contido no lócus S. O lócus S é composto por uma combinação de proteínas SLF (S-locus F-box), responsáveis pela determinação do fator polínico, e uma S-RNase produzida apenas no pistilo, de forma que estes genes estão fortemente ligados formando o haplótipo S. Esses produtos gênicos interagem possibilitando a rejeição do auto-pólen num fenômeno denominado distinção colaborativa do pólen não próprio. No Capítulo II, foram utilizados métodos filogenéticos para determinar os principais agrupamentos de alelos na genealogia de S-RNases do gênero Solanum. A topologia da árvore não mostrou sinal filogenético para espécies, contudo, isso era esperado uma vez que a diversificação destes alelos ocorreu anteriormente à diversificação das espécies, gerando um fenômeno denominado de polimorfismos trans-específicos. Além disso, foram realizadas análises de seleção positiva, as quais encontraram um alto número de resíduos com altas probabilidades, indicando que estão sob pressão de seleção positiva darwiniana. Com o intuito de compreender a diversidade estrutural destes alelos, foram construídos modelos teóricos com base em modelagem por homologia dos principais clados da filogenia, já que estas sequências apresentam um elevado grau de polimorfismo. Os resultados mostraram grande variação estrutural na região hipervariável destas sequências, enquanto que regiões conservadas não apresentaram grandes mudanças estruturais e com estruturas secundárias características. No Capítulo III, foram realizadas diversas análises com o objetivo de compreender a diversificação estrutural e funcional de RNases T2 na família Solanaceae. As análises filogenéticas mostraram a formação de três principais grupos, sendo um de S-RNases, e os outros dois de S-like RNases. Com relação ao Clado 2, podemos inferir que houveram ao menos dois eventos de duplicação gênica. Além disso, também foram utilizados os métodos de NSsites e branch-site para inferência de seleção positiva como uma forma de identificar possíveis sinais de diversificação molecular. Muitos resíduos parecem estar sob seleção em ambos os métodos, embora um número maior fosse encontrado no NSsites (41), com estes resíduos localizando-se em regiões mais flexíveis da proteína, enquanto que os aqueles selecionados de acordo com o brach-site (8) estavam situados em posições mais rígidas da estrutura. Em súmula, os resultados encontrados nos capítulos que compreendem esta tese demonstram que análises teóricas podem contribuir efetivamente de inúmeras maneiras com o intuito de desenvolver uma melhor compreensão dos fenômenos biológicos relacionados com a evolução molecular de famílias multigênicas, além de também contribuir no entendimento dos processos de diversificação de genes multialélicos, utilizando como modelo de estudo a família gênica RNase T2. / The ribonucleases catalyze the cleavage of RNA and are ubiquitous components of cells, from prokaryotes to eukaryotes. The T2 family is the most widely category of RNases distributed among living organisms. The wide distribution of this family suggests that it should play an important functional role in cell biology of these organisms. Basically, there are two groups of these proteins in Solanaceae, one represented by SRNases, which are involved in the rejection of pollen in gametophytic self-incompatibility and a more diverse group, which is termed S-like RNases with very diverse functions. SRNases are a highly polymorphic and multiallelic gene contained in the S locus. The S locus consists of a combination of SLF proteins (F-box S-locus), responsible for the pollen factor, and one S-RNase expressed only in the pistil, and those genes are tightly linked as an S-haplotype. Products of these genes interact enabling the rejection of self-pollen in a phenomenon called collaborative non-self recognition. In Chapter II, we used phylogenetic methods to determine the main clusters of alleles in the genealogy of SRNase in the Solanum genus. The topology of the tree showed no phylogenetic sign to species delimitation, but it was expected since the diversification of these alleles occurred previously the diversification of the species, generating a phenomenon called trans-specific polymorphism. In addition, analyzes of positive selection were carried out, and it resulted in a significant number of residues with high probability, indicating they are under Darwinian positive selection. In order to understand the structural diversity of alleles, theoretical models were constructed based on homology modeling of the major clades found in the phylogeny, since it has a high degree of polymorphism in these sequences. The results showed that major structural variations are located in the hypervariable regions of these sequences, while conserved regions performed without major structural changes and showed stable secondary structures. In Chapter III, we used several analyzes to understand the structural and functional diversification of RNase T2 in the Solanaceae family. Phylogenetic analyses showed the clustering of three main groups, one with just SRNases and the two others composed by S-like RNases. Regarding to Clade 2, we could infer that at least two gene duplication events have occurred. In addition, we used two methods for inference of positive selection, NSsites and branch-site, as a mean to identify possible signals of molecular diversification. Many residues seem to be under selection in both methods, although a higher number was found in NSsites (41), and these residues were located in more flexible regions of the protein, while those selected according to the brach-site (8) were located at more rigid positions of the structure. In summary, the results found in the chapters of this thesis show that theoretical analyses could effectively contribute in many ways in order to develop a better understanding of biological phenomena related to the molecular evolution of gene families. Also, it contributes to the understanding of the processes related to multiallelic genes diversification, using gene family RNase T2 as a model.

As ribonucleases catalisam a clivagem do RNA e são componentes ubíquos das células, desde procariotos até eucariotos. A família T2 é a classe de RNases mais amplamente distribuída entre os organismos vivos. A ampla distribuição desta família sugere que ela deve ter um papel funcional muito importante na biologia celular destes organismos. Em Solanaceae, há basicamente dois grupos destas proteínas, um representado pelas S-RNases, as quais estão envolvidas com a rejeição do pólen na autoincompatibilidade gametofítica e um outro grupo mais diverso que é denominado de S-like RNases, com funções muito diversificadas. As S-RNases se apresentam na forma de um gene multialélico, altamente polimórfico, que está contido no lócus S. O lócus S é composto por uma combinação de proteínas SLF (S-locus F-box), responsáveis pela determinação do fator polínico, e uma S-RNase produzida apenas no pistilo, de forma que estes genes estão fortemente ligados formando o haplótipo S. Esses produtos gênicos interagem possibilitando a rejeição do auto-pólen num fenômeno denominado distinção colaborativa do pólen não próprio. No Capítulo II, foram utilizados métodos filogenéticos para determinar os principais agrupamentos de alelos na genealogia de S-RNases do gênero Solanum. A topologia da árvore não mostrou sinal filogenético para espécies, contudo, isso era esperado uma vez que a diversificação destes alelos ocorreu anteriormente à diversificação das espécies, gerando um fenômeno denominado de polimorfismos trans-específicos. Além disso, foram realizadas análises de seleção positiva, as quais encontraram um alto número de resíduos com altas probabilidades, indicando que estão sob pressão de seleção positiva darwiniana. Com o intuito de compreender a diversidade estrutural destes alelos, foram construídos modelos teóricos com base em modelagem por homologia dos principais clados da filogenia, já que estas sequências apresentam um elevado grau de polimorfismo. Os resultados mostraram grande variação estrutural na região hipervariável destas sequências, enquanto que regiões conservadas não apresentaram grandes mudanças estruturais e com estruturas secundárias características. No Capítulo III, foram realizadas diversas análises com o objetivo de compreender a diversificação estrutural e funcional de RNases T2 na família Solanaceae. As análises filogenéticas mostraram a formação de três principais grupos, sendo um de S-RNases, e os outros dois de S-like RNases. Com relação ao Clado 2, podemos inferir que houveram ao menos dois eventos de duplicação gênica. Além disso, também foram utilizados os métodos de NSsites e branch-site para inferência de seleção positiva como uma forma de identificar possíveis sinais de diversificação molecular. Muitos resíduos parecem estar sob seleção em ambos os métodos, embora um número maior fosse encontrado no NSsites (41), com estes resíduos localizando-se em regiões mais flexíveis da proteína, enquanto que os aqueles selecionados de acordo com o brach-site (8) estavam situados em posições mais rígidas da estrutura. Em súmula, os resultados encontrados nos capítulos que compreendem esta tese demonstram que análises teóricas podem contribuir efetivamente de inúmeras maneiras com o intuito de desenvolver uma melhor compreensão dos fenômenos biológicos relacionados com a evolução molecular de famílias multigênicas, além de também contribuir no entendimento dos processos de diversificação de genes multialélicos, utilizando como modelo de estudo a família gênica RNase T2. / The ribonucleases catalyze the cleavage of RNA and are ubiquitous components of cells, from prokaryotes to eukaryotes. The T2 family is the most widely category of RNases distributed among living organisms. The wide distribution of this family suggests that it should play an important functional role in cell biology of these organisms. Basically, there are two groups of these proteins in Solanaceae, one represented by SRNases, which are involved in the rejection of pollen in gametophytic self-incompatibility and a more diverse group, which is termed S-like RNases with very diverse functions. SRNases are a highly polymorphic and multiallelic gene contained in the S locus. The S locus consists of a combination of SLF proteins (F-box S-locus), responsible for the pollen factor, and one S-RNase expressed only in the pistil, and those genes are tightly linked as an S-haplotype. Products of these genes interact enabling the rejection of self-pollen in a phenomenon called collaborative non-self recognition. In Chapter II, we used phylogenetic methods to determine the main clusters of alleles in the genealogy of SRNase in the Solanum genus. The topology of the tree showed no phylogenetic sign to species delimitation, but it was expected since the diversification of these alleles occurred previously the diversification of the species, generating a phenomenon called trans-specific polymorphism. In addition, analyzes of positive selection were carried out, and it resulted in a significant number of residues with high probability, indicating they are under Darwinian positive selection. In order to understand the structural diversity of alleles, theoretical models were constructed based on homology modeling of the major clades found in the phylogeny, since it has a high degree of polymorphism in these sequences. The results showed that major structural variations are located in the hypervariable regions of these sequences, while conserved regions performed without major structural changes and showed stable secondary structures. In Chapter III, we used several analyzes to understand the structural and functional diversification of RNase T2 in the Solanaceae family. Phylogenetic analyses showed the clustering of three main groups, one with just SRNases and the two others composed by S-like RNases. Regarding to Clade 2, we could infer that at least two gene duplication events have occurred. In addition, we used two methods for inference of positive selection, NSsites and branch-site, as a mean to identify possible signals of molecular diversification. Many residues seem to be under selection in both methods, although a higher number was found in NSsites (41), and these residues were located in more flexible regions of the protein, while those selected according to the brach-site (8) were located at more rigid positions of the structure. In summary, the results found in the chapters of this thesis show that theoretical analyses could effectively contribute in many ways in order to develop a better understanding of biological phenomena related to the molecular evolution of gene families. Also, it contributes to the understanding of the processes related to multiallelic genes diversification, using gene family RNase T2 as a model.

As ribonucleases catalisam a clivagem do RNA e são componentes ubíquos das células, desde procariotos até eucariotos. A família T2 é a classe de RNases mais amplamente distribuída entre os organismos vivos. A ampla distribuição desta família sugere que ela deve ter um papel funcional muito importante na biologia celular destes organismos. Em Solanaceae, há basicamente dois grupos destas proteínas, um representado pelas S-RNases, as quais estão envolvidas com a rejeição do pólen na autoincompatibilidade gametofítica e um outro grupo mais diverso que é denominado de S-like RNases, com funções muito diversificadas. As S-RNases se apresentam na forma de um gene multialélico, altamente polimórfico, que está contido no lócus S. O lócus S é composto por uma combinação de proteínas SLF (S-locus F-box), responsáveis pela determinação do fator polínico, e uma S-RNase produzida apenas no pistilo, de forma que estes genes estão fortemente ligados formando o haplótipo S. Esses produtos gênicos interagem possibilitando a rejeição do auto-pólen num fenômeno denominado distinção colaborativa do pólen não próprio. No Capítulo II, foram utilizados métodos filogenéticos para determinar os principais agrupamentos de alelos na genealogia de S-RNases do gênero Solanum. A topologia da árvore não mostrou sinal filogenético para espécies, contudo, isso era esperado uma vez que a diversificação destes alelos ocorreu anteriormente à diversificação das espécies, gerando um fenômeno denominado de polimorfismos trans-específicos. Além disso, foram realizadas análises de seleção positiva, as quais encontraram um alto número de resíduos com altas probabilidades, indicando que estão sob pressão de seleção positiva darwiniana. Com o intuito de compreender a diversidade estrutural destes alelos, foram construídos modelos teóricos com base em modelagem por homologia dos principais clados da filogenia, já que estas sequências apresentam um elevado grau de polimorfismo. Os resultados mostraram grande variação estrutural na região hipervariável destas sequências, enquanto que regiões conservadas não apresentaram grandes mudanças estruturais e com estruturas secundárias características. No Capítulo III, foram realizadas diversas análises com o objetivo de compreender a diversificação estrutural e funcional de RNases T2 na família Solanaceae. As análises filogenéticas mostraram a formação de três principais grupos, sendo um de S-RNases, e os outros dois de S-like RNases. Com relação ao Clado 2, podemos inferir que houveram ao menos dois eventos de duplicação gênica. Além disso, também foram utilizados os métodos de NSsites e branch-site para inferência de seleção positiva como uma forma de identificar possíveis sinais de diversificação molecular. Muitos resíduos parecem estar sob seleção em ambos os métodos, embora um número maior fosse encontrado no NSsites (41), com estes resíduos localizando-se em regiões mais flexíveis da proteína, enquanto que os aqueles selecionados de acordo com o brach-site (8) estavam situados em posições mais rígidas da estrutura. Em súmula, os resultados encontrados nos capítulos que compreendem esta tese demonstram que análises teóricas podem contribuir efetivamente de inúmeras maneiras com o intuito de desenvolver uma melhor compreensão dos fenômenos biológicos relacionados com a evolução molecular de famílias multigênicas, além de também contribuir no entendimento dos processos de diversificação de genes multialélicos, utilizando como modelo de estudo a família gênica RNase T2. / The ribonucleases catalyze the cleavage of RNA and are ubiquitous components of cells, from prokaryotes to eukaryotes. The T2 family is the most widely category of RNases distributed among living organisms. The wide distribution of this family suggests that it should play an important functional role in cell biology of these organisms. Basically, there are two groups of these proteins in Solanaceae, one represented by SRNases, which are involved in the rejection of pollen in gametophytic self-incompatibility and a more diverse group, which is termed S-like RNases with very diverse functions. SRNases are a highly polymorphic and multiallelic gene contained in the S locus. The S locus consists of a combination of SLF proteins (F-box S-locus), responsible for the pollen factor, and one S-RNase expressed only in the pistil, and those genes are tightly linked as an S-haplotype. Products of these genes interact enabling the rejection of self-pollen in a phenomenon called collaborative non-self recognition. In Chapter II, we used phylogenetic methods to determine the main clusters of alleles in the genealogy of SRNase in the Solanum genus. The topology of the tree showed no phylogenetic sign to species delimitation, but it was expected since the diversification of these alleles occurred previously the diversification of the species, generating a phenomenon called trans-specific polymorphism. In addition, analyzes of positive selection were carried out, and it resulted in a significant number of residues with high probability, indicating they are under Darwinian positive selection. In order to understand the structural diversity of alleles, theoretical models were constructed based on homology modeling of the major clades found in the phylogeny, since it has a high degree of polymorphism in these sequences. The results showed that major structural variations are located in the hypervariable regions of these sequences, while conserved regions performed without major structural changes and showed stable secondary structures. In Chapter III, we used several analyzes to understand the structural and functional diversification of RNase T2 in the Solanaceae family. Phylogenetic analyses showed the clustering of three main groups, one with just SRNases and the two others composed by S-like RNases. Regarding to Clade 2, we could infer that at least two gene duplication events have occurred. In addition, we used two methods for inference of positive selection, NSsites and branch-site, as a mean to identify possible signals of molecular diversification. Many residues seem to be under selection in both methods, although a higher number was found in NSsites (41), and these residues were located in more flexible regions of the protein, while those selected according to the brach-site (8) were located at more rigid positions of the structure. In summary, the results found in the chapters of this thesis show that theoretical analyses could effectively contribute in many ways in order to develop a better understanding of biological phenomena related to the molecular evolution of gene families. Also, it contributes to the understanding of the processes related to multiallelic genes diversification, using gene family RNase T2 as a model.

Thesis (Ph. D.)--University of Texas at Austin, 1998. / Vita. Includes bibliographical references (leaves 128-138). Available also in a digital version from Dissertation Abstracts.

A 9.4 T magnetic resonance imaging (MRI) system was used to evaluate the response of a human tumour xenograft model to radiation therapy. The apparent diffusion coefficient (ADC) and the transverse relaxation time (T2) of human glioblastoma multiforme (GBM) tumour xenografts in NIH-iii nude mice were measured before, and at multiple points after, treatment of the tumours with 200 kVp x-rays. The response was characterized as a function of a number of variables of interest in the clinical treatment of cancer with external beam radiation therapy. Mean tumour ADC and T2 responses after single fractions of radiation were investigated, with measurements being made until 14 days after treatment. Single fraction doses ranged from 50 cGy to 800 cGy. Fractionated treatments were used to deliver 800 cGy in two or three fractions with fraction spacings of 24 or 72 hours. The role of hypoxia on ADC and T2 response was investigated by using an externally-applied, suture-based ligature to induce a state of reduced oxygenation in tumours during treatment, after which ADC and T2 were measured using serial MRI. Finally, tumours were dissected in order to provide insight into possible pathophysiological mechanisms explaining the observed responses. Tissue sections were prepared and reviewed by a pathologist. This work adds to the body of literature describing tumour ADC and T2 response to anticancer therapy, and adds to the understanding of ADC and T2 response to radiation therapy in particular. This works supports that of others suggesting the use of ADC and T2 as potential biomarkers for tumour response to treatment. / Medical Physics

Spinal cord injury (SCI) leads to axonal damage and limits the ability of the brain to communicate with the rest of the body. Several bioengineered approaches have been developed for the recovery of SCI. Among these techniques, degradable guidance tubes have shown promising results. However, design of nerve guide tubes requires several design considerations and has been a significant challenge. To assess the efficacy of a prototypical implanted nerve guide tubes, it is essential to perform continuous monitoring. In this respect, magnetic resonance imaging (MRI) is one of the most reliable imaging techniques as it offers the ability to achieve extraordinary high temporal and spatial resolution in addition to its non-invasive features. In spite of the excellent image quality of non-enhanced MRI various types of contrast agents have been developed to further enhance the contrast and allow improved visualization. The MRI contrast agents principally work by shortening the T1 or T2 relaxation times of protons located nearby. The presented study was intended to evaluate the in vitro degradation of the nerve guide tubes made of poly (lactic-co-glycolic acid) (PLGA). PLGA tubes incorporated with different concentrations of superparamagnetic iron oxide (SPIO) were scanned by MRI 3T on weekly basis during the degradation period. Spin-echo (SE) sequence with various echo times (TEs) ranged from 13.3 to 314.4 msec was applied. T2 mapping was computed using in-house algorithm developed in Matlab. Least square fit was used to find the slope of the decay curve by plotting log intensity on the y-axis and echo time on the x-axis. The average T2 values were calculated. Mass loss and water uptake of the degrading tubes were also measured weekly. Moreover, the micro-structural changes of the tubes were investigated using the scanning electron microscope (SEM). The MRI results showed that the concentration of SPIO affects the signal intensity of the T2 weighted images reducing the T2 relaxation time value. Accordingly, a linear correlation between SPIO concentration and T2 relaxation time was found. At the beginning of degradation, the SPIO nanoparticles were trapped within the polymeric network. Therefore, water penetration was the predominant factor affecting the T2 relaxation times. At week 5, a significant mass loss was observed. From this stage onwards, the trapped SPIO were released from the polymeric network increasing T2 relaxation time dramatically. According to SEM images, the size of the pores in PLGA guide tubes was increased with the degradation. Approaching the end of degradation, shrinkage of the tubes was observed and the degraded nerve guide tubes were shown to be collapsed. Similar shape variation was observed in T2 weighted MR images. In summary, this study provided an approach to non-invasive monitoring of degradation behavior of nerve guide tubes using contrast enhancement. The developed technique is of great importance since it opened an insight to non-invasive monitoring of tissue engineered scaffolds for in vivo studies.

T2 relaxation imaging can be used to measure signal from water trapped between myelin bilayers; the ratio of myelin water signal to total water is termed the myelin water fraction (MWF). First, results from multi-component T2 relaxation and diffusion tensor imaging (DTI) were compared for 19 multiple sclerosis (MS) subjects at 1.5 T to better understand how each measure is affected by pathology. In particular, it was determined that the detection of a long-T2 signal within an MS lesion may be indicative of a different underlying pathology than is present in lesions without long-T2 signal. Next, the single-slice T2 relaxation measurement was implemented, refined, and validated at 3.0 T. Scan parameters were varied for phantoms and in-vivo brain, and changes in multi-exponential fit residuals and T2 distribution-derived parameters such as MWF were monitored to determine which scan parameters minimized artifacts. Measurements were compared between 1.5 T and 3.0 T for 10 healthy volunteers. MWF maps were qualitatively similar between field strengths. MWFs were significantly higher at 3.0 T than at 1.5 T, but with a strong correlation between measurements at the different field strengths. Due to long acquisition times, multi-component T2 relaxation has thus far been clinically infeasible. The next study aimed to validate a new 3D multi-component T2 relaxation imaging technique against the 2D single-slice technique most commonly used. Ten healthy volunteers were scanned with both the 2D single-slice and 3D techniques. MWF maps were qualitatively similar between scans. MWF values were highly correlated between the acquisition methods. Although MWF values were generally lower using the 3D technique, they were only significantly so for peripheral brain structures, likely due to increased sensitivity of slab-selective refocusing pulses used for the 3D approach. The 3D T2 relaxation sequence was then applied to the study of MS to take advantage of the increased brain coverage. Thirteen MS subjects and 11 controls underwent T2 relaxation and DTI examinations to produce histograms of MWF and several DTI-derived metrics. MS MWF histograms differed considerably from those of controls, and differences in MS MWF histograms did not mirror differences in DTI histograms relative to matched controls.