Anatomy of the Locus Coeruleus Defined by a Multimodal Approach.

Abstract

The locus coeruleus (LC), the main noradrenergic nucleus near dorsal brainstem entry zones, is crucial yet poorly understood. Its exact topography and surgical risks are not well-defined, and it is rarely seen on standard imaging. No previous dissections have specifically examined the LC, leaving nearby structures largely uncharacterized. Our goal was to describe the surgical boundaries and imaging markers of the LC to minimize iatrogenic brainstem injury and guide future neuromodulation procedures. Five brainstems were dissected using the Klingler technique under ×6 to ×40 magnification with stepwise photographic documentation. Ten healthy volunteers underwent neuromelanin-sensitive magnetic resonance imaging (MRI) on 1.5 T and 3 T scanners, with LC segmentation used for diffusion-tensor tractography. The LC was identified as a rostro-caudally oriented bluish-gray nucleus medial to the superior cerebellar peduncle, subjacent to the fourth ventricular floor, and lateral to the dorsal raphe and periaqueductal gray. The suprafacial triangle, infracollicular corridor, and superior fovea triangle were in close proximity, with minimal trajectory error potentially damaging the nucleus. MRI at 1.5 T and 3 T consistently visualized the LC as a discrete hyperintensity aligning with cadaveric findings, though 1.5 T offered lower spatial resolution. Tractography revealed projections to the midbrain, cerebellum, motor cortex, and spinal cord. This study combines anatomical dissection and neuromelanin MRI to map the LC and its relationship to brainstem entry zones, providing imaging markers for surgical planning and a framework for future LC-targeted neuromodulation.