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Reduced fractional anisotropy on diffusion tensor MR imaging following hypoxic-ischaemic encephalopathy

Presented at the Neonatal Society 2004 Autumn Meeting (programme).

Ward P1,2, Counsell S2, Allsop J1, Cowan F2, Edwards D1,2, Rutherford M2

1 Department of Paediatrics, MRC Clinical Sciences Centre, Imperial College Faculty of Medicine, Hammersmith Hospital, London W12 0NN
2 Imaging Sciences Department, MRC Clinical Sciences Centre, Imperial College Faculty of Medicine, Hammersmith Hospital, London, W12 0NN

Introduction: Apparent diffusion coefficients (ADC) measured by diffusion-weighted imaging (DWI) are reduced in severe white matter (WM), basal ganglia and thalamic (BGT) injury due to hypoxicischaemic encephalopathy (HIE). However ADC values may be normal or even high during the first week after delivery in moderate injury, which is still associated with significant later impairment. We hypothesized that fractional anisotropy (FA) measured by diffusion tensor imaging (DTI) may be abnormal in moderate perinatal brain injury.

Aim: To use DTI to measure ADC and FA in infants moderate and severe hypoxic-ischemic brain injury.

Methods: The study was approved by the Hammersmith Hospitals Research Ethics Committee. 24 infants with HIE and nine normal control infants were studied. All infants were born at >36 weeks gestational age and MR images obtained within three weeks of delivery (Philips Eclipse 1.5 T). Conventional T1 and T2 weighted imaging was used to categorise tissue abnormality into moderate or severe WM and /or BGT injury. DTI was acquired in twelve (six non-collinear) directions with: b value 710s/mm2, TR 6000ms, TE 100ms, FOV 240mm,matrix 100x100. Regions of interest within white matter and central grey matter were defined and average measurements for ADC and FA made using in-house software. Data were examined for normality and comparisons made using unpaired ttests or Mann Whitney were appropriate.

Results: During the first week FA values were decreased with both severe and moderate WM and BGT injury (p<0.02), while ADC values were reduced only in severe injury (p<0.02). ADC values pseudonormalised during the second week while FA values continued to decrease.

Discussion: FA is reduced in moderate brain injury after HIE. A low FA may reflect a break down in white matter organisation. Moderate BGT injury may result in atrophy but not overt infarction, it is possible that delayed apoptosis is more marked than immediate necrosis (1) and this may account for normal early ADC values. The accompanying low FA within moderate grey matter, which is associated with significant later impairment, may be a useful diagnostic sign in infants with normal ADC values.

Acknowledgements: MRC, The Academy of Medical Sciences, The Health Foundation

1. D’Arceuil H et al Stroke 2000

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