Identification of the pedunculopontine nucleus and surrounding white matter tracts on 7T diffusion tensor imaging, combined with histological validation

Abstract

Background The pedunculopontine nucleus (PPN) has been studied as a possible target for deep brain stimulation (DBS) for Parkinson’s disease (PD). However, identifying the PPN can be challenging as the PPN is poorly visualized on conventional or even high-resolution MR scans. From histological studies it is known that the PPN is surrounded by major white matter tracts, which could function as possible anatomical landmarks. Methods This study aimed to localize the PPN using 7T magnetic resonance (MR) imaging and diffusion tensor imaging (DTI) of its white matter borders in one post-mortem brain. Histological validation of the same specimen was performed. The PPN was segmented in both spaces, after which the two masks were compared using the Dice Similarity Index (DSI). The DSI compared the similarity of two samples on an inter-individual level and validated the MR findings. The error in distance between the center of the two 3D segmentations was measured by use of the Euclidean distance. Results The PPN can be found in between the superior cerebellar peduncle and the medial lemniscus on both the FA-maps of the DTI images and the histological sections. The histological transverse sections showed to be superior to recognize the PPN (DSI: 1.0). The DTI images have a DSI of 0.82. The overlap-masks of both spaces showed a DSI of 0.32, whereas the concatenation-masks of both spaces showed a remarkable overlap, a DSI of 0.94. Euclidean distance of the overlap- and concatenation-mask in the two spaces showed to be 1.29 mm and 1.59 mm, respectively. Conclusion This study supports previous findings that the PPN can be identified using FA-maps of DTI images. For possible clinical application in DBS localization, in vivo validation of the findings of our study is needed.

A high-resolution interactive atlas of the human brainstem using magnetic resonance imaging

“…Regarding ex vivo imaging of the human brain and brainstem, previous work has been restricted to specific nuclei or anatomical subregions of the specimens ( Aggarwal et al, 2013 ; Edlow et al, 2012 ; Henssen et al, 2019 ), had limited angular resolution ( Aggarwal et al, 2013 ), or looked exclusively at white matter ( McNab et al, 2009 ). Nonetheless, these previous ex vivo studies certainly rendered images more detailed than those of conventional in vivo MRI, and we aim to build on these prior achievements.…”

Tractography patterns of pedunculopontine nucleus deep brain stimulation

“…It is not clearly visible on typical MRI scans, and combined with its distance from the bicommissural plane and large non-linear inter-individual variation in brain stem anatomy, a consensus on targeting strategy remains to be reached (Hamani et al 2016 ). Principle diffusion directions (Aravamuthan et al 2007 ) and FA contrast have been proposed as useful for locating the PPN (Henssen et al 2019 ), and we are grateful for the development and anatomic validation of an FA-based probabilistic atlas of the mesopontine tegmentum (Bianciardi et al 2018 ). Nonetheless, recognising both the difficulty and uncertainties in targeting the PPN region, and the limitations in applying atlases, diffusion tractography offers an objective, ‘clinical outcome grounded’ approach to locating the optimum target in this region of the brainstem, in an individual patient.…”