Brain imaging in preterm infants at term equivalent age : Value and comparison of MRI and ultrasound

Sammanfattning: Cranial ultrasound (cUS) and magnetic resonance imaging (MRI) are the two most commonly used brain imaging techniques in preterm infants. cUS can be performed at the bedside and detects all major brain abnormalities (haemorrhages, infarctions, cysts, dilatation of the lateral ventricles) that are associated with severe neurodevelopmental disability. However, there are more subtle brain abnormalities that can be visualized with cUS, such as signs of brain atrophy and periventricular echodensities, for which the clinical relevance is less clear. Furthermore, there is no consensus how to quantify the lateral ventricular size on cUS, a procedure that is often necessary to monitor the progression of posthaemorrhagic ventricular dilatation or grade the extent of ventriculomegaly at term age. In comparison to MRI, cUs has been shown to be less sensitive for the detection of non-cystic white matter (WM) injury, the most common form of WM injury in preterm infants. Indeed, MRI at term age has been reported to be superior to cUS in detecting white matter (WM) abnormalities and predicting outcome in preterm infants. However, in that study cUS was performed during the first 6 weeks only and no late scan in parallel to MRI at term age was included. Furthermore MRI data from large population-based cohorts of extremely preterm infants are missing. Therefore, the aim of the present thesis was to study the relevance of subtle brain abnormalities on cUS (brain atrophy and periventricular echodensities) for neurodevelopmental outcome, to acquire and compare MRI and cUS data at term age in a population-based cohort of extremely preterm infants (< 27 weeks) and to determine which 2D linear cUS measurement of the lateral ventricular size is the best to estimate total ventricular volume using 3D MRI as the golden standard. Our main findings were, firstly, that sonographic signs of brain atrophy at term age, but not persistent periventricular echodensities, are related to neurodevelopmental outcome at 3 years of age, secondly that in the cohort of extremely preterm infants the incidence of brain abnormalities detected with MRI was unexpectedly low compared to previously published numbers from other regions, thirdly, that the 2D cUS measurements of the frontal horn and ventricular midbody correlate best to lateral ventricular volume, and finally that 40% of extremely preterm infants had a normal cUs at term age and that in these infants MRI at term age added no or only marginal clinically relevant information to the cUS result. In conclusion, our findings underline the important clinical role of cUs as a bedside imaging technique, but also highlight the potential of MRI as a complementary imaging tool in preterm infants.