Evaluation of the brachial plexus and spinal cord with diffusion tensor imaging and synthetic MRI in obstetric brachial plexus palsy: DTI of brachial plexus and spinal cord in OBPP.

Abstract

Purpose: Obstetric brachial plexus palsy (OBPP) is conventionally staged as a peripheral nerve injury, yet conventional magnetic resonance imaging (MRI) poorly reflects microstructural neural and spinal cord changes. We evaluate whether magnetic resonance image compilation (MAGIC), a form of synthetic MRI, shows strong agreement with conventional T2-weighted short-tau inversion recovery (T2-STIR) for postganglionic injury detection, compare diffusion tensor imaging (DTI) metrics between injured and adjacent intact segments of the brachial plexus and spinal cord, and explore whether these metrics predict recovery at 2-year follow-up in infants with OBPP.

Methods: In this single-center cohort with prospective enrollment and ambispective outcome assessment, 40 infants (aged 3-30 weeks) with unilateral OBPP underwent 3-T MRI during physiological sleep, comprising conventional T2-STIR, synthetic MRI (MAGIC T2-STIR), and DTI. Two blinded neuroradiologists compared synthetic and conventional sequences for postganglionic lesion detection (weighted κ). The DTI-derived fractional anisotropy (FA) and apparent diffusion coefficient (ADC) were measured at the neural foramen (dorsal root ganglion level) and spinal cord at every injured segment and at the nearest intact segments above and below; per infant, the minimum injured value and the mean intact value were compared and related to Narakas type and 2-year recovery.

Results: Synthetic MRI showed strong agreement with conventional T2-STIR (κ = 0.84-1.00); the intraobserver repeatability of DTI measurements was good to excellent [intraclass correlation coefficient (ICC) = 0.842-0.999), except for intact spinal cord ADC (ICC = 0.662). Compared with intact reference segments, injured segments showed lower FA and higher ADC at both the neural foramen (FA = 0.236 ± 0.092 vs. 0.433 ± 0.130; ADC = 2,116 ×10^-6 vs. 1,191.25 ×10^-6 mm²/s) and the spinal cord (FA = 0.321 ± 0.107 vs. 0.448 ± 0.143; ADC = 2,253.5 ×10^-6 vs. 1,191 ×10^-6 mm²/s) (all P < 0.001). Neither FA nor ADC differed by Narakas type or predicted 2-year recovery.

Conclusion: Synthetic MRI showed strong agreement with conventional T2-STIR for the detection of postganglionic OBPP and provided six contrasts from a single acquisition of similar duration; DTI abnormalities at injured segments extended to the spinal cord, suggesting that microstructural change in OBPP may not be confined to the peripheral nerve.

Clinical significance: Segmental DTI abnormality within the spinal cord raises the possibility that OBPP injury extends centrally; confirmation would require higher-resolution acquisition, technical validation against partial-volume and cerebrospinal fluid effects, and larger longitudinal cohorts before any implication for injury classification.