Purpose or Case Report: To explore tract-specific developmental changes with deterministic tractography in a population of typically developing fetuses.
Methods & Materials: The study was IRB approved and HIPAA compliant. Healthy women with normal second trimester prenatal ultrasound were recruited prospectively. Subjects were imaged at 3T. Structural images were acquired using single-shotT2 images, which were processed using a validated slice-to-volume (SVR) reconstruction to generate isotropic images. DTI comprised 2–8 scans of the fetal head in orthogonal planes with 1-2 b=0s/mm2 images, and 12 diffusion-sensitized images at b=500s/mm2. DTI was processed using motion-tracked SVR algorithm. Deterministic tractography was performed with Trackvis with a FACT algorithm. ROIs were placed on the motion corrected T2 images to delineate the following tracts: forceps major, forceps minor, and bilateral inferior fronto-occipital fasciculi, inferior longitudinal fasciculi, cingula, uncinate fasciculi and corticospinal tracts. Fetal motion was estimated based on differences in position between individual slices of the diffusion acquisition. Descriptive statistics were used to summarize the population. A multiple regression analysis was used to evaluate the association with gestational age (GA) with volume, FA and ADC of each tract. Sex and motion were included in the regression.
Results: Tractography was successful for all tracts in 40/59 subjects (66%) (23 male and 17 female). The GA ranged from 29.3-38.1 weeks (w), with a mean of 33w (SD 3w). We found a trend of volume and FA increase, and ADC decrease in association with increasing GA in all tracts. This trend was statistically significant for most but not all the tracts and was independent of sex and motion. Volume increase with GA was fastest in the forceps minor (.23ml/w, P .004) and right UF (.068ml/w, P .001). The greatest rate of FA increase was seen in forceps major (.0076293/w, P .005). The right ILF had the highest rate of decrease in ADC (-.00000232 mms2/w, P <.001).
Conclusions: MT-SVR DTI enables high success rate of fetal tractography. Results show a complex process of age-related and tract-specific changes in volume, FA, and ADC that lays the foundation for the developmental landscape of the brain at birth and for the developmental trajectories that follow in the first year of life.