Probabilistic diffusion tractography of the optic radiations and visual function in preterm infants at term equivalent age
1NICU, Institute of Pediatrics and Neonatology, Fondazione IRCCS Ospedale Maggiore Policlinico, Mangiagalli e Regina Elena, University of Milan, Italy, 2Robert Steiner MR, Unit, 3Department of Paediatrics, Imaging Sciences Department, MRC Clinical Sciences Centre, Imperial College London, Hammersmith Campus, London, UK, 4Paediatric Neurology Unit, Catholic University, Rome, 5E. Medea Scientific Institute, Conegliano Research Centre, Conegliano (TV), Italy, 6Department of Neonatology Hospital Vall d’Hebron, Barcelona, Spain and 7Hammersmith/St Mary's Comprehensive Biomedical Research Centre, DuCane Road, London, UK
Correspondence to: Dr Serena J. Counsell, Robert Steiner MR Unit, Imaging Sciences Department, Imperial College London, Hammersmith Hospital, DuCane Road, London, W12 0HS, UK E-mail: serena.counsell{at}imperial.ac.uk
Children born prematurely have a high incidence of visual disorders which cannot always be explained by focal retinal or brain lesions. The aim of this study was to test the hypothesis that visual function in preterm infants is related to the microstructural development of white matter in the optic radiations. We used diffusion tensor imaging (DTI) with probabilistic diffusion tractography to delineate the optic radiations at term equivalent age and compared the fractional anisotropy (FA) to a contemporaneous evaluation of visual function. Thirty-seven preterm infants (19 male) born at median (range) 28+4 (24+1–32+3) weeks gestational age, were examined at a post-menstrual age of 42 (39+6–43) weeks. MRI and DTI were acquired on a 3 Tesla MR system with DTI obtained in 15 non-collinear directions with a b value of 750 s/mm2. Tracts were generated from a seed mask placed in the white matter lateral to the lateral geniculate nucleus and mean FA values of these tracts were determined. Visual assessment was performed using a battery of nine items assessing different aspects of visual abilities. Ten infants had evidence of cerebral lesions on conventional MRI. Multiple regression analysis demonstrated that the visual assessment score was independently correlated with FA values, but not gestational age at birth, post-menstrual age at scan or the presence of lesions on conventional MRI. The occurrence of mild retinopathy of prematurity did not affect the FA measures or visual scores. We then performed a secondary analysis using tract-based spatial statistics to determine whether global brain white matter development was related to visual function and found that only FA in the optic radiations was correlated with visual assessment score. Our results suggest that in preterm infants at term equivalent age visual function is directly related to the development of white matter in the optic radiations.
Key Words: optic radiation; preterm; infant; brain; tractography
Abbreviations: DTI, diffusion tensor imaging; FA, fractional anisotropy; GA, gestational age; PMA, post-menstrual age; PVL, periventricular leucomalacia; ROI, region of interest; ROP, retinopathy of prematurity; SNR, sound-to-noise ratio; TBSS, tract-based spatial statistics
Received September 6, 2007. Revised December 4, 2007. Accepted December 12, 2007.