A stereotaxic, population-averaged T1w ovine brain atlas including cerebral morphology and tissue volumes

Abstract

Standard stereotaxic reference systems play a key role in human brain studies. Stereotaxic coordinate systems have also been developed for experimental animals including non-human primates, dogs, and rodents. However, they are lacking for other species being relevant in experimental neuroscience including sheep. Here, we present a spatial, unbiased ovine brain template with tissue probability maps (TPM) that offer a detailed stereotaxic reference frame for anatomical features and localization of brain areas, thereby enabling inter-individual and cross-study comparability. Three-dimensional data sets from healthy adult Merino sheep (Ovis orientalis aries, 12 ewes and 26 neutered rams) were acquired on a 1.5 T Philips MRI using a T1w sequence. Data were averaged by linear and non-linear registration algorithms. Moreover, animals were subjected to detailed brain volume analysis including examinations with respect to body weight (BW), age, and sex. The created T1w brain template provides an appropriate population-averaged ovine brain anatomy in a spatial standard coordinate system. Additionally, TPM for gray (GM) and white (WM) matter as well as cerebrospinal fluid (CSF) classification enabled automatic prior-based tissue segmentation using statistical parametric mapping (SPM). Overall, a positive correlation of GM volume and BW explained about 15% of the variance of GM while a positive correlation between WM and age was found. Absolute tissue volume differences were not detected, indeed ewes showed significantly more GM per bodyweight as compared to neutered rams. The created framework including spatial brain template and TPM represent a useful tool for unbiased automatic image preprocessing and morphological characterization in sheep. Therefore, the reported results may serve as a starting point for further experimental and/or translational research aiming at in vivo analysis in this species.

Translational models for vascular cognitive impairment: a review including larger species

“…MRI protocols are now available for dogs [59], cats [60], non-human primates [61–63], pigs [64, 65] and sheep [66]. MRI (T1, T2, FLAIR) is advantageous for analysis of tissue volume and lesions [66], as well as for anatomical evaluation of particular brain areas [67]. Perfusion and diffusion-weighted sequences reveal cerebral blood flow (CBF) dynamics and vascular permeability [68].…”

“…Templates, automatic segmentation and labelling routines for larger species are essential for studies aiming at quantitative morphometric analysis of MRI and/or PET images. Automatic labelling and processing routines have been developed for rhesus and cynomolgus monkeys [61, 69, 70], sheep [67], pigs [71, 72], and dogs [73]; this enables efficient, observer-independent analysis of grey and white matter regions.…”

Computation of a high‐resolution MRI 3D stereotaxic atlas of the sheep brain

“…This also allowed us to avoid nonisotropic voxel image bias. Indeed, most of the images acquired with a nonisotropic voxel size often exhibited a better in‐plane resolution compared to the thickness resolution (Black, ; McLaren et al, ; Nitzsche et al, ). To be transformed into isotropic voxel size images, as required when building an atlas, images must often be resampled, either from a higher to a lower spatial resolution, or the reverse.…”

“…The atlas computed herein possesses a high spatial resolution compared to most atlases based on living nonhuman primates (Black et al, ; Watanabe et al, ; Frey et al, ). Although the number of animals used is in the upper part of the range often encountered in in vivo studies (Watanabe et al, ; Black et al, ; Kovacevic et al, ; Ma et al, ; Nitzsche et al, ), it is still possible to extend the number of subjects by adding images into the processing directory. Cortical gyri and inner brain structures were segmented into the standard coordinate space (Fig.…”

Straight trajectory planning for keyhole neurosurgery in sheep with automatic brain structures segmentation

“…The sheep atlas by Nitzsche B. et al [9] was used for the automatic segmentation procedure: this atlas provides high spatial resolution (isotropic voxel size 0.25mm), features a standard coordinate system according to Talairach space conventions and it is based on averaged T1w images of 14 Merino sheep (T1w incoherent gradient echo, 1.5T, recon matrix 512, slices 110, voxel size 0.39×0.39×1.0mm, Gyroscan Interea, Philips). This ensures the conformity with a proper coordinates system, overcomes the drawback arising from inter-individual variability and allows to distinguish white matter (WM), grey matter (GM) and cerebrospinal fluid (CSF) as relevant brain structures.…”