Radiofrequency electrical conductivity reveals a distinct dimension of early human brain development.

Quantitative MRI has characterised early human brain development primarily through measures of water mobility, leaving other biophysical properties of maturing tissue unexplored. Here we show that radiofrequency electrical conductivity, reflecting ionic composition, membrane density and extracellular geometry in addition to water content, can be extracted retrospectively and at scale from the phase of conventional MRI acquisitions, revealing a distinct dimension of early brain tissue maturation and state. Applying electrical property tomography to 888 neonatal MRI sessions (784 subjects, 26-45 weeks post-menstrual age) alongside infant and childhood data, we find that whole-brain conductivity declines steeply during the perinatal period and remains independently associated with age after adjustment for mean diffusivity and ventricular volume, confirming that it represents a distinct developmental signal that is not reducible to tissue water content alone. Preterm birth was associated with elevated conductivity at term-equivalent age, particularly in deep grey matter. In neonates with hypoxic-ischaemic encephalopathy (n = 25), conductivity was elevated while mean diffusivity was reduced within the same tissue compartments, a dissociation indicating that these measures capture mechanistically distinct aspects of the tissue response to injury. These findings establish radiofrequency electrical properties as a new biophysical window on perinatal brain maturation and its perturbation by injury, extending quantitative MRI beyond measures of water mobility.