Diffusion-weighted and functional magnetic resonance imaging of the brain in preterm and term-born adolescents

Magnetic resonance imaging (MRI) is widely used in clinical and research settings in the adolescent population. Technical development has allowed the use of fine-grained methods to assess both the structural and functional properties of the brain. However, the specific technical limitations and improvements are mostly studied in phantom or adult studies, which may have an impact on their reliability as research tools when studying the younger population. Very preterm (VPT) birth is associated with several neurodevelopmental impairments. The present MRI tools provide opportunities to study brain maturation in detail. This thesis is a part of the multidisciplinary longitudinal follow-up study on the development and functioning of very low birth weight infants from infancy to school age (PIPARI). The follow-up cohort consists of infants born VPT (birth weight ≤1500 g and/or gestational age <32 weeks) in Turku University Hospital in 2001–2006 and term-born controls born in 2001–2004 in the same hospital. This thesis includes only children born VPT in 2004–2006 and controls born between 2003–2004 due to an upgrade of the MRI scanner during the recruitment. In Study I, the diffusion-weighted imaging (DWI) metrics at term-equivalent age were compared to the motor outcome at 11 years of age in children born VPT. Study II assessed the effect of the susceptibility correction to the DWI metrics in a healthy adolescent population. In Study III, temporal fluctuation of the resting state brain functioning was compared between 13-year-old adolescents born VPT and at term. The main prematurity-related findings of this thesis were that the DWI metrics of the corpus callosum, left corona radiata and right optic radiation at term are associated with later motor outcome in children born VPT and that adolescents born VPT show a decrease in active time, fluidity and range in brain activation during rest. These findings may reflect the adjustments in brain microstructure and function caused by the VPT birth. Fine-grained MRI methods are reliable tools for studying the mechanisms behind the clinical phenotypes of adolescents when technical limitations and age-appropriate analysis adjustments are considered.Diffuusiopainotteisen ja toiminnallisen aivojen magneettikuvantamisen käyttö nuoruusiässä entisillä pikkukeskosilla ja täysiaikaisilla verrokeilla Magneettikuvaus (MRI) on laajassa kliinisessä ja tieteellisessä käytössä lapsia ja nuoria tutkittaessa. Tekninen kehitys mahdollistaa yhä hienojakoisempia aivojen tutkimuksia. MRI:n teknisiä korjauksia on tutkittu pääosin mallintamalla tai aikuisilla, mikä voi heikentää luotettavuutta alaikäisillä. Hyvin ennenaikaisesti syntyvillä lapsilla neurologisen kehityksen poikkeavuuksien riski on täysiaikaisena syntyviä suurempi. Poikkeavuudet voivat liittyä aivojen kehityksen muutoksiin, joita nykyisillä tekniikoilla voidaan tutkia aiempaa yksityiskohtaisemmin. Väitöskirja on osa PIPARI-tutkimusta (Pienipainoisten riskilasten käyttäytyminen ja toimintakyky imeväisiästä kouluikään). Seurantakohortti koostuu pikkukeskosina (syntymäpaino ≤1500 g ja/tai raskauden kesto