SnapG: An Automated Tool for Myelin g-ratio Measurement with Batch Processing Capability

The g-ratio, calculated as the axon diameter divided by the total myelinated fiber diameter, is widely used to assess the degree of myelination in the central and peripheral nervous systems. Changes in g-ratios accompany demyelinating, hypomyelinating, and remyelinating conditions, and can also result from activity-dependent, adaptive myelination. Although manual tracing from electron micrographs has been the gold-standard to measure g-ratios, this process is laborious and limits throughput. To address this challenge, we developed SnapG, an automated and user-friendly software tool to streamline g-ratio analysis from electron micrographs. SnapG implements an end-to-end pipeline for axon and myelin segmentation on EM images. The tool integrates robust segmentation and post-processing steps to extract axon and fiber diameters and generates both per-image and cohort-level summary statistics. SnapG produces g-ratio distributions that agree well with expert manual annotations and established tools across imaging modalities. It reduces user intervention, supports large image and batch processing, and achieves faster processing times while delivering accurate, reproducible measurements. SnapGs automated reporting thereby facilitates high-throughput analyses and improves consistency across experiments. SnapG can accelerate studies of myelin biology and provides a practical alternative to existing tools.