Search

Introduction: As preterm birth and survival rates continue to rise, circulatory failure remains a leading cause of mortality and morbidity. Structural and functional variations in circulatory physiology within the preterm population have been described, secondary to pre and post natal factors. This study applied magnetic resonance imaging techniques to assess cardiac function; analysed suitable techniques for indexing functional parameters by body size and applied a novel method of body composition quantification to investigate the impact of varying growth patterns on cardiac measures. Methods: Steady state free precession MRI imaging was used to analyse cardiac function in 78 preterm neonates. Analysis techniques were validated by comparison to phase contrast measures in 40 infants. Current methods of indexing for body size were compared. A modified DIXON whole body MRI scan utilising chemical shift was validated in phantoms and in vivo and successfully applied to 20 infants to assess adipose tissue content. Results: Validation of acquisition and analysis techniques demonstrated an acceptable level of accuracy. Functional measures indexed by body weight generally decreased with increasing corrected gestational age but patterns were altered when indexed by body surface area. Allometric transformation did not significantly improve correlation between size and function. Full body MDIXON scans demonstrated variations in body fat percentage of between 15 and 25%. Indexing cardiac function by lean body mass gave differing trends to total weight. Conclusions: Analysis of cardiac function in preterm neonates using MRI can be used to describe normative ranges and causes of variation for functional parameters. However, until an appropriate indexing technique for body size can be determined, the impact of pre and post natal factors cannot be fully understood. Analysis of body composition using MRI imaging may present a new indexing technique and allow us to investigate the effects of different growth patterns on cardiac function.

The incidence of premature birth is increasing in absolute number and as a proportion of all births around the world. Many pathologies seen in this cohort are related to abnormal blood supply. Fetal and premature cardiovascular systems differ greatly as to maintain adequate blood flow to the developing organs in the uterine and extra-uterine environments require very different circulations. Subsequently following preterm birth the immature cardiovascular system undergoes abrupt adaptations, often resulting in the prolonged patency of the fetal shunt, ductus arteriosus. The impact of a patent ductus arteriosus (PDA) is poorly understood. However it is thought that large ductal shunt volumes may result in congestive cardiac failure and systemic hypo-­‐perfusion. Cardiac MRI has contributed greatly to the understanding of many cardiovascular diseases and congenital defects in paediatric and adult patients. Translating these imaging techniques to assess the preterm cardiovascular system requires careful optimization due to their condition, size and significantly increased heart rate. The work presented in this thesis employs multiple functional CMR techniques to investigate the preterm cardiovascular system in the presence and absence of PDA and the resultant cardiac function. A novel technique utilizing PC MRI to quantify PDA shunt volume and its impact on flow distribution is presented. Despite large shunt volumes, systemic circulation remained within normal range, although slight reduction is detectable when assessed at group level. Subsequently the impact of PDA and associated increased work load on left ventricular dimensions and function was then investigated using SSFP imaging. Results indicated that cardiac function was maintained even in the presence of large shunt volumes. Finally 4D PC sequences were employed to evaluate pulse wave velocity and flow regime within the preterm aorta, demonstrating the feasibility of hemodynamic assessment in this cohort. The findings of these studies provide insight into the impact of PDA. The reliable measurement and assessment of preterm cardiovascular system provides the potential to improve the understanding of the development and effects of certain pathologies seen in this cohort.