High b-value diffusion tensor imaging of the neonatal brain at 3T

BACKGROUND AND PURPOSE: Diffusion-weighted MR imaging studies of the adult brain have shown that contrast between lesions and normal tissue is increased at high b-values. We designed a prospective study to test the hypothesis that diffusion tensor imaging (DTI) obtained at high b-values increases image contrast and lesion conspicuity in the neonatal brain.MATERIALS AND METHODS: We studied 17 neonates, median (range) age of 10 (2–96) days, who were undergoing MR imaging for clinical indications. DTI was performed on a Philips 3T Intera system with b-values of 350, 700, 1500, and 3000 s/mm². Image contrast and lesion conspicuity at each b-value were visually assessed. In addition, regions of interest were positioned in the central white matter at the level of the centrum semiovale, frontal and occipital white matter, splenium of the corpus callosum, posterior limb of the internal capsule, and the thalamus. Apparent diffusion coefficient (ADC) and fractional anisotropy (FA) values for these regions were calculated.RESULTS: Isotropic diffusion image contrast and lesion-to-normal-tissue contrast increased with increasing b-value. ADC values decreased with increasing b-value in all regions studied; however, there was no change in FA with increasing b-value.CONCLUSIONS: Diffusion image contrast increased at high b-values may be useful in identifying lesions in the neonatal brain.

Diffusion-weighted MR imaging is increasingly being used to investigate neonatal cerebral pathologic lesions. Previous studies have shown that diffusion-weighted imaging (DWI) is able to demonstrate lesions that are not always discernible on conventional MR imaging, and the usefulness of this imaging technique to assess infarction^1-3 and metabolic disorders^4,5 in the neonatal brain is established. In addition to the qualitative assessment of injury, diffusion tensor imaging (DTI) provides directionally invariant measurements of mean diffusivity and diffusion anisotropy. These objective measurements provide information regarding water molecular mobility, which reflect tissue microstructure and thereby provide insight into mechanisms of brain development and disease processes.Diffusion-weighted MR imaging with b-values of more than 2000 s/mm² has been performed in animal studies,^6,7 in the adult brain,^8-14 and in infants.^8,15 These studies suggest that diffusion contrast characteristics are altered at higher b-values. In addition, adult studies of cerebral infarction¹³ and white matter disease¹⁴ have shown increased lesion conspicuity at higher b-values, and so it is possible that high b-value DTI in neonates may also improve lesion conspicuity.To our knowledge, the only studies investigating the effects of high b-value diffusion imaging in the neonatal brain were done on infants whose results on conventional MR imaging was considered normal and did not examine the full diffusion tensor.^8,15 In this prospective study, we tested the hypothesis that diffusion imaging at high b-values enhances contrast between lesions and normal tissue in neonates and thereby increases lesion conspicuity. Furthermore, we acquired diffusion data in 6 noncollinear directions of sensitization, enabling us to examine the effects of high b-values on diffusion anisotropy.The aims of this study were 1) to assess isotropic DWI contrast and lesion conspicuity at b-values between 350 and 3000s/mm² and 2) to assess whether apparent diffusion coefficient (ADC) or fractional anisotropy (FA) in the neonatal brain change with increasing b-value.