Diffusion tensor imaging (DTI) is a popular MRI technique that is extensively used in brain research to describe the orientation of white-matter fibers (1). It was first introduced in 1994 (2). DTI measures the impact of tissue architecture on the diffusion-weighted signal by examining the motion of water molecules. The procedure of fiber tracking, known as tractography, allows for the virtual dissection and three-dimensional representation of white-matter tracts (3).

The distribution of action potentials is modulated by white matter, which functions as a relay system and coordinates communication among various brain regions. White matter tracts in the brain can be categorized into three types: association, commissural, and projection fibers. Association fibers connect different regions within the same cerebral hemisphere and can be classified as short association fibers, which link adjacent gyri, and long association fibers, which establish connections between more distant areas. One of the important functions of association tracts is linking perceptual and memory centers in the brain. Commissural fibers are axons that connect the two hemispheres of the brain. These fibers form tracts such as the corpus callosum, the anterior commissure, and the posterior commissure. Commissural tracts facilitate communication between the left and right sides of the cerebrum and play a role in both homologous and heterotopic associations. Projection fibers encompass both efferent and afferent fibers that connect the cortex with the lower regions of the brain and the spinal cord. They serve as conduits for information transmission between the cerebrum and the rest of the body (4).

DTI tractography is extensively used in the field of brain research, as a means of measuring white-matter integrity. It has considerable potential in terms of both diagnosis and prognosis for a number of brain pathologies, including brain tumors, neurodegenerative diseases, and stroke (5–15). With the growing availability of high field-strength MRI (1.5 and 3 tesla) in veterinary facilities (16–19), this technique is being increasingly used to study white-matter anatomy (16, 20–22) and structural connectivity (23–26) in domestic mammals such as dogs (16, 21, 23) cats (17, 24–26), ferrets (24), sheep (20, 27–29) and horses (19, 22) but also in other animal species such as reptiles (30), birds, cetaceans (31) and rodents (32).

MRI scanners designed for human medicine can also be used to examine large animal brains. Large animals can spontaneously develop a range of typically human brain diseases, not least Alzheimer’s disease (33, 34), along with Parkinson’s disease (35, 36), lysosomal storage diseases (37, 38), and gliomas (39), so these imaging data could be extremely useful both in experimental research and in veterinary medicine. Especially, in experimental research, the use of large animal models, in addition to murine models might be useful to use innovative therapies in human due to their large and convoluted brain, closer to human than murine and the spontaneous development of the disease.

Nevertheless, DTI tractography cannot be validated in large animals without a thorough knowledge of their white-matter tract morphology. A recent study provided detailed anatomical descriptions of the main association, commissural, and projection fibers in dogs, cats, and horses using a fiber dissection technique (40). However, although MRI of the one-humped camel [Camelus dromedarius (41)] brain under a 1 or 1.5 T field has been shown to be possible (42–46), the spatial anatomy of white-matter tracts in this species, which can develop poorly understood neurodegenerative diseases such as prion disease (47), Dubduba syndrome (48), and poliencephalomacia (49, 50), has not yet been complemented by DTI tractography. Researchers could use the latter to pinpoint damage in specific white-matter tracts. This damage could then be correlated with clinical symptoms (47–50). Furthermore, the use of DTI tractography in healthy individuals could enhance current knowledge about the camel brain’s anatomy and functioning, based on mapping of its structural connectivity.

The present study was intended to use DTI tractography to characterize the one-humped camel brain fiber bundles. In this context the aim of this study was to investigate the reconstruction of the most representative tracts that connect one part of the brain to another within the white matter, namely the cingulum (belonging to association fibers), corpus callosum (belonging to commissural fibers), and internal capsule (belonging to projection fibers). To this end, a fiber dissection technique and T2-weighted images were used in conjunction with 3 T MRI and the fiber tracking results were compared to the description of these tracts in the literature.

First, it is important to note that no published data are available on DTI tractography of normal white-matter tracts in the one-humped camel. The present study yielded T2-weighted images of normal camel brain in 3 T with good resolution for the first time. It was essential to perform T2-weighted images, in order to understand the normal anatomy of white-matter fibers in the DT images. The anatomical description of the three fiber subgroups of association, commissural and projection fibers we established in the present study was closely correlated with descriptions found in dissection and anatomical textbooks. When we compared our results with those of other studies where DTI tractography and fiber dissection were performed to examine the brains of other species, we found both similarities and differences.

Concerning similarities, the corpus callosum in the one-humped camel consists of a flat bundle of fibers spanning part of the longitudinal fissure. This result is similar to that described in many large animals, especially in horses, large and small ruminants (20, 27, 40, 60). The internal capsule anatomy described in this study is similar to that of other large animals. The internal capsule carries sensory information from the thalamus to the cerebral cortex, allowing for the processing and interpretation of sensory inputs. It also carries motor information from the cerebral cortex to the spinal cord, enabling voluntary movements and coordinated motor responses. Given the unique adaptations of one-humped camels to desert environments, the internal capsule in these animals likely plays a role in supporting their specialized behaviors and physiological needs (61). For example, the internal capsule may contribute to the one-humped camel’s ability to navigate challenging terrains, since the internal capsule works as a relay station for the body’s motor fonction (61).

Differences concerning our DTI tractography findings relate to the cingulum. Our fiber dissection results show a curved band of neural tissue located in the cerebral cortex, encircling the corpus callosum with parietal and occipital radiations also described by Pascalau et al. in 2016. For instance, we failed to observe in DTI tractography theses parietal and occipital radiations. Moreover, only a small part of the cingulum attached to the corpus callosum could be reconstructed.

DTI tractography has a major advantage over post mortem anatomical techniques such as fiber dissection, as it allows for noninvasive dissection, yielding a three-dimensional representation of several white-matter tracts from a single imaging dataset (1, 3, 62). Due to its reliance on water molecule diffusion, DTI tractography provides an indirect assessment of axonal pathways. However, it struggles to accurately distinguish between crossing and branching patterns within a single voxel. As a result, tractography algorithms may produce erroneous connections or prematurely terminate tracked fibers, leading to anatomical inconsistencies between MRI-based reconstructions and dissection (53). As can be seen in our results, the current study is no exception. In our study, we acknowledge that due to the limitations mentioned earlier, such as difficulties in accurately characterizing crossing and branching patterns, certain white matter tracts were likely reconstructed incorrectly and exhibited anatomical inaccuracies. Specifically, we anticipate that the inferior fronto-occipital fasciculus, uncinate fasciculus, superior and inferior longitudinal fasciculi (associative fibers), as well as the rostral and caudal commissures (commissural fibers), and the corona radiata (projection fibers), may have been affected by these challenges and consequently misrepresented in our findings. The poor bad quality of the brain specimens can also explain the difficulty of some tracks reconstruction. This also explains the artefacts or halos present on T2-weighted images (Figure 1) and b0 images (Figures 2, 3). Moreover, even though manual ROI delineation can generate biases, we were unable to use automated segmentation, as there are currently no digital atlases of the one-humped camel brain. To minimize the biases of manual ROI delineation, we used T2-weighted images.

Given that the brain specimen used in this study was extracted from the skull and underwent formalin fixation, it is important to consider the potential impact of fixation-induced tissue changes and magnetic susceptibility artifacts at the tissue-air interface on the quality of MRI data. Postmortem DTI acquisitions require fixation to prevent autolysis, which can degrade the structural characteristics observed in in vivo conditions. Although the fixation process generally reduces overall water diffusion compared to in vivo conditions, its specific effect on diffusion anisotropy is not yet fully understood. Some studies indicate a decrease in anisotropy in fixed brains (63, 64), while others suggest that the fixative preserves relevant tissue microstructure (65, 66). To avoid these artifacts as far as possible, we immersed the brain in a bag filled with saline solution immediately prior to acquisition, as saline solution is isosignal with cerebrospinal fluid (51).

We chose to perform DTI reconstruction because it is the most popular and extensively used (and thus the most reproducible) fiber exploration technique (1). Future studies, however, should compared DTI with others algorithms on fiber tracking. For example, Q-ball imaging (QBI) reconstruction, in contrast to DTI reconstruction, takes multiple fiber orientations into account, rather than simply yielding an ellipsis. It could therefore prove particularly useful when multiple fibers cross within a single voxel (67).

We demonstrated that DTI tractography is possible in the ex vivo one-humped camel brain, allowing for the reconstruction of white-matter tracts. Scanning time (1 h) was compatible with anesthesia, and the T2-weighted and DTI parameters used in this study can be referenced for subsequent studies of the camel head. Finally, theses acquisition parameters could be easily adapted for in vivo studies, and it would be possible to reduce acquisition time, which is a limiting factor for the MRI examination of one-humped camels because anesthesia in large animals requires a lot of attention and is very often risky for the animal. Another limiting factor for in vivo studies is the requirement for high field MRI systems that can adequately support the weight of the animal being scanned. However, there are numerous MRI systems available that are capable of scanning the head of large animals in vivo, such as horses. Therefore, it appears feasible to use these existing MRI resources for scanning the head of a one-humped camel as well.

Furthermore, ex vivo imaging protocols necessitate the use of larger b-values (3,000 in our study). This is done to counterbalance the significant reduction in the apparent diffusion coefficient observed ex vivo and thereby maintain diffusion contrast. Additionally, the T2 relaxation time is substantially reduced in ex vivo conditions, which limits the available window for data acquisition. Consequently, stronger gradient magnitudes are required to minimize echo times and preserve the signal-to-noise ratio in ex vivo imaging (68, 69). In the present formaldehyde-fixed post mortem study, we had to run five series of acquisitions (5 × 12 min) on average in order to achieve a sufficient signal, but this would not be necessary for in vivo studies, given that the signal would naturally be stronger. Fewer iterations of each direction would be acquired, thereby possibly reducing acquisition time. However, only one sequence was tested in our study, so it would be necessary to compare this one with any other potential sequences.

The results of the present study show the reconstruction of the most representative tracts in the one-humped camel brain using DTI data acquired post mortem. Cingulum, corpus callosum and internal capsule could be reconstructed. Further DTI studies are needed to improve and validate these findings and to produce a tractography atlas for future neuroimaging research in the one-humped camel.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The animal studies were approved by the Comité d’Ethique Science et Santé Animales – Toulouse – France. The studies were conducted in accordance with the local legislation and institutional requirements was obtained from the owners for the participation of their animals in this study.

BC and GM drafted the manuscript. NP, AA, RP, AM, KE, SK, BB, and CM helped to establish the experimental procedures. GA and GM jointly supervised this work. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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