Proton density MR imaging: A simple and powerful sequence to visualise anatomical structures in Functional Neurosurgery

Introduction: Direct visualisation of subcortical structures has become increasingly important in functional neurosurgery, improving targeting accuracy and enabling reliable postoperative verification. Proton density–weighted (PDW) MRI is widely accessible, rapid, and provides exceptional grey–white matter contrast, yet its full utility for functional neurosurgical targeting has not been comprehensively reviewed. This study summarises the available evidence and illustrates the use of PDW imaging to identify key anatomical targets relevant to deep brain stimulation (DBS), radiofrequency ablation (RFA), and focused ultrasound (FUS). Methods: A systematic review of the literature following PRISMA guidelines was performed using predefined terms related to PDW imaging and functional neurosurgery. In-vivo studies describing PDW visualisation of basal ganglia nuclei, thalamic subnuclei, white-matter pathways, and brainstem targets were included. To complement the literature, representative PDW images from the authors’ clinical practice were analysed and correlated with established human anatomy. These images were used to demonstrate relevant radiological anatomy and postoperative confirmation of electrode placement or lesion location across a range of functional neurosurgical procedures. Results: Thirteen studies comprising 326 subjects met the inclusion criteria. Published work described PDW visualisation of the ventral intermediate nucleus (Vim), globus pallidus, habenula, mammillothalamic tract, centromedian nucleus, and pedunculopontine nucleus (PPN). Additional important structures in functional neurosurgery that are visible on PDW images, and used by the authors in clinical practice, but not yet mentioned in the literature, include the pallidothalamic tract, leading to the ventral oral nuclei; anterior limb of the internal capsule; fornix; bed nucleus of the stria terminalis; nucleus accumbens and anterior cingulum. Conclusion: PDW MRI is a powerful and underutilised technique for direct, anatomy-based targeting in functional neurosurgery. Its robustness, accessibility, and high-quality structural contrast support accurate stereotactic planning and objective postoperative verification. Wider adoption of PDW imaging may improve surgical precision, reduce reliance on indirect targeting methods, and expand the range of structures amenable to image-guided functional neurosurgical intervention.