Comparative Analysis of Cross-Sectional Anatomy, Computed Tomography and Magnetic Resonance of the NormalSix-Banded Armadillo (Euphractus sexcintus) Head

1. Introduction

Euphractus sexcinctus, commonly known as the six-banded armadillo, is a member of the Chlamyphoridae family [1]. It is one of the largest armadillo species, reaching approximately 40 centimetres. This terrestrial and solitary mammal inhabits open areas, such as plains or savannahs, as well as forests and jungles. This animal can be found predominantly in the eastern regions of South America, particularly in countries like Argentina, Brazil, Uruguay, Paraguay, Bolivia and certain areas of Suriname [2,3].

The six-banded armadillo is characterized by its dermal plates that cover most of its back, including the head and tail. The carapace comprises six to eight flexible bands and shows a light colouration that varies from yellow to reddish brown [2]. The triangular, pointed-shaped and flat head of this animal features a distinctive mosaic-like structure, and it has up to twenty-five teeth, which are practical for his omnivorous diet that includes invertebrates to carrion and plants [2]. Both its front and hind legs have five toes, used to dig inverted U-shaped burrows and small tunnels [2]. The gestation period for this species lasts about two months, during which they typically have one to three babies. These young armadillos quadruple their weight within the first month and reach adulthood by nine months [1,2,3].

The six-banded armadillo is considered a Least Concern (LC) by the IUCN. However, it faces several risks, primarily due to extensive hunting for local use, including as a protein source and for handicraft and medicinal purposes [3]. With the increasing presence of exotic companion animals and the need to preserve wildlife, biologists, veterinarians and conservationists must understand the complexities of these animals’ anatomy, physiology and lifestyle to be able to diagnose faster and more accurately [4].

The significant anatomical differences among terrestrial mammals, along with the increasing interest in exotic pets as new companion animals, have created challenges for veterinary clinicians in interpreting diagnostic imaging studies. Diagnostic imaging techniques are crucial in enhancing clinical practice and facilitating data collection. Thanks to technological advancements, obtaining anatomical and diagnostic information has become quicker and more efficient, making these tools invaluable in medical practice. Traditional methods like radiology and ultrasound [4], along with modern imaging technologies such as CT and MRI [5,6,7,8,9,10,11], offer superior image resolution, rapid image acquisition, and an improved ability to differentiate overlapping structures. These techniques provide detailed anatomical and functional insights, with an increased capacity to distinguish between bone and soft tissue structures.

Several notable studies have explored the anatomy, physiology, and pathology of these species, particularly focusing on the nine-banded armadillo [12,13,14,15,16,17,18,19,20]. Research has examined various aspects, including the anatomy and functional morphology of xenarthrous vertebrae, spinal development, the skeleton and skull structures, the ear, the nasal cavity and paranasal sinuses, the morphology of the laryngeal cartilages, salivary glands, masticatory adaptations, and the visual cortex [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43]. Additionally, other studies have addressed pathological conditions, such as fractures [22] and osteoderm lesions [23]. However, to date, only a few papers have investigated the head in these animals [18,19,24,25,38]. Therefore, this research aimed to describe the normal central nervous system (CNS) and its associated structures in the six-banded armadillo using cross-sectional anatomy, CT and MRI. The use of CT, MRI, and detailed anatomical sections can provide pivotal information for enhancing anatomical understanding in education and clinical practice. This approach not only enriches teaching but also significantly improves patient care.

2. Materials and Methods

2.1. Animals

Three adult six-banded armadillo carcasses (two males and one female) from the species Euphractus sexcintus were collected from the “Rancho Texas Lanzarote Park” zoological park (Lanzarote, Canary Islands, Spain). These animals succumbed to natural causes, sparking an opportunity for valuable research.

2.2. CT Technique

To obtain detailed CT images, we employed a 16-slice helical CT scanner (Toshiba Astelion, Canon Medical System, Tokyo, Japan) at the Veterinary Hospital of Las Palmas de Gran Canaria University. The armadillos were carefully positioned in ventral recumbency, symmetrically on the CT table to ensure optimal imaging. We captured sequential CT images with a 1 mm slice thickness using a standard clinical protocol (100 kVp, 80 mA, 512 × 512 acquisition matrix, 1809 × 858 field of view, a spiral pitch factor of 0.94, and a gantry rotation time of 1.5 seconds). To enhance the visibility of the head, three CT windows with varying widths and levels were applied: a bone window (WW = 1500; WL = 300), a lung window (WW = 1400; WL = −500), and a soft tissue window (WW = 350; WL = 40) focusing on the bone window images for clarity. Furthermore, dorsal and sagittal multiplanar reconstructed (MPR) images were generated to better visualize the armadillo’s intricate anatomical head structures. All CT images were subsequently uploaded to an advanced image viewer (OsiriX MD, Apple, Cupertino, CA, USA) for further manipulation and analysis, ensuring a comprehensive evaluation of these unique specimens.

2.3. MRI Technique

A magnetic resonance imaging (MRI) study was conducted on the three six-banded armadillos using a 1.5-Tesla magnet (Toshiba, Vantage Elan, Japan) while the animals were positioned in ventral recumbency. A standard MRI protocol was followed to acquire spin-echo (SE) T1-weighted and T2-weighted images in transverse, sagittal and dorsal planes. For SE T1-weighted transverse images, the parameters were: echo time (TE) of 10 ms, repetition time (TR) of 800 ms, acquisition matrix size of 536 × 384, and a slice thickness of 4.5 mm with 4 mm spacing between slices. The SE T2-weighted transverse images were obtained with a TE of 120 ms, TR of 10,541 ms, acquisition matrix size of 624 × 448, and a slice thickness of 3 mm with 3 mm interslice spacing. The SE T2-weighted sagittal images had a TE of 120 ms, TR of 7529 ms, acquisition matrix size of 512 × 804, and a slice thickness of 2.8 mm with 2 mm interslice spacing. For SE T2-weighted dorsal images, the parameters were: TE of 120 ms, TR of 8282 ms, acquisition matrix size of 468 × 512, and a slice thickness of 3.4 mm with 3 mm interslice spacing. A medical imaging viewer (OsiriX MD, Geneva, Switzerland) was used to analyze the acquired images.

2.4. Anatomical Sections

After the scanning procedure, one of the three six-banded armadillos was placed in dorsal recumbency within expanded polystyrene containers and rapidly frozen at −80 °C for 72 hours. Then, serial sections, each half a centimetre thick, were cut using an electric band saw to produce sequential transverse anatomical cross-sections. The slices were immediately cleaned with water, numbered, and photographed on both sides.

2.5. Anatomic Evaluation

The anatomical cross-sections that most closely matched the CT and MR images were chosen to aid in identifying key structures of the six-banded armadillo’s head. Additionally, consultation with textbooks and relevant references on the anatomy of armadillos and other mammals was required. [21,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40].

3. Results

Eight representative transverse sections of the armadillo head were selected (Figure 1). Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9 comprise four images: A) anatomic cross-section. B) CT bone window, C) T1-weighted, and D) T2-weighted MR images. These images are presented in a rostrocaudal progression from the olfactory bulb (Figure 2) to the brainstem levels (Figure 9). In addition, Figure 10 contains three sagittal images: a sagittal CT bone window image (B), a sagittal MR T1W image (C), and a parasagittal MR T2W image (D). Finally, Figure 11 comprises a bone-window CT dorsal (B) and a dorsal MRI in T2W (D) images at the level of the tympanic cavity.

3.1. Anatomical Sections

Relevant formations of the central nervous system were identified by anatomical cross-sections. These structures were named in compliance with the International Committee on Veterinary Gross Anatomical Nomenclature. Hence, we could observe the cerebrum divided into a left and right telencephalic hemisphere by a deep longitudinal fissure (Figures 2A–8A). Most cranial sections showed an excellent depiction of the two large olfactory bulbs found on the inferior side of the cerebral hemispheres (Figure 2A). Here, we also distinguished enlarged olfactory recesses bounded medially by the perpendicular plate of the ethmoidal bone (Figures 2A and 3A). The following sections helped distinguish the connexion of both brain hemispheres by fibres of the white matter forming the corpus callosum (Figures 5A and 6A). Moreover, these sections allowed the identification of the lateral ventricles in each hemisphere (Figures 5A–8A).

Additionally, several structures of the diencephalon could be depicted, including the interthalamic adhesion crossing the third ventricle between the two thalami, the optic nerve, the pituitary gland and the lateral and medial geniculate bodies (Figures 5A, 7A and 8A), which were limited laterally by the internal capsule. The ventral part of this capsule and the piriform lobes were essential remarks to identify the amygdala and the globus pallidus (Figure 6A). On the most caudal sections, identifiable formations of the mesencephalon, such as the rostral colliculus, the mesencephalic aqueduct and the cerebral peduncles were distinct (Figure 8A). From the metencephalon, we could identify the cerebellar hemispheres, the vermis, the nodule covering part of the fourth ventricle and prominent ventral structures, including the middle cerebellar peduncles and the pons (Figure 9A).

Concerning the bony formations, different bones comprising the neurocranium were identified, including the frontal, temporal (with its squamous, petrous, and tympanic components), the zygomatic arch, the sphenoid, the maxilla, the mandible and the occipital bones (Figures 2A–9A). In addition, these transverse sections facilitated the visualization of structures of the oral cavity including the tongue, the palatine plexus and the soft palate (Figures 2A–8A), and other important components of the auditory system, such as the external ear with the acoustic meatus and the auricular cartilage (Figure 9A). Besides, the tympanic cavity and bulla were also depicted in these sections (Figures 7A–9A). Furthermore, we distinguished relevant muscles related to masticatory function, such as the temporalis, the medial and lateral pterygoid muscles, and the masseter muscles (Figures 4A–8A).

3.2. Computed Tomography Study

Concerning the nervous system, the CT images facilitated the distinction of relevant formations, such as the olfactory bulbs, which appeared symmetrical bilateral prominences with intermediate attenuation (Figures 2B, 10B, 11B). However, we could not identify the olfactory recesses with this imaging technique due to its low resolution for soft tissues. Despite this limitation, we successfully identified the longitudinal cerebral fissure separating the two telencephalic hemispheres (Figures 2B and 3B).

Additionally, we defined other nervous structures with similar attenuation, including the piriform lobes which were recognized thanks to their globose shape and ventrolateral position (Figure 6B). Moreover, these sections depicted significant parts of the diencephalon, such as the pituitary gland, which was observed resting in the pituitary fossa (Figure 8B). More caudally, the dorsal, transverse and sagittal CT images identified the cerebellar vermis and the adjacent cerebellar hemispheres (Figures 9B–11B).

Regarding the different bony structures, the CT images revealed pivotal skull bones showing high CT attenuation. Therefore, we distinguished the frontal, zygomatic process, the vomer, occipital, basioccipital, sphenoid, temporal and maxillary bones (Figures 2B–10B). Moreover, we identified different parts of the mandible, including the ramus and the mandibular condyle, the temporomandibular joint, and some components of the larynx and the hyoid apparatus (Figures 2B–11B). It is important to emphasize that the most notable bony structure of this animal, due to its species exclusivity, corresponds to the osteoderms, located across the entire dorsal portion of the head as a row of ossified geometric plates (Figures 2B–11B)

These CT images were also helpful in depicting essential elements of the auditory system, such as the external ear and the tympanic bullae recognizable on both sides of the midline of the skull, laterally followed by the external acoustic meatus (Figures 8B, 9B and 11B). Besides, other anatomical structures with intraluminal content, such as the nasopharyngeal and the oral cavities, were displayed with this technique, showing a vacuum effect (Figures 2B–10B).

In the ventral parts of the CT images, several muscles were recognized with the corresponding attenuation, such as the omohyoid and sternohyoid muscles, the medial and lateral pterygoid muscles, the masseter muscle, and the temporal muscles (Figures 2B–8B).

3.3. Magnetic Resonance Imaging (MRI)

No consistent anatomical differences were observed in the studied six-banded armadillos, and the anatomical sections satisfactorily matched with the structures shown in the MRI images. Furthermore, it is remarkable that compared to CT, the MRI technique portrayed a better view of the CNS structures. Therefore, the use of this technique allowed the identification of the olfactory bulbs (Figures 2C,D, 10C,D and 11D) as triangular, rostral structures with a moderate to hyperintense uniform signal intensity, divided by the longitudinal cerebral fissure (Figures 2C,D, 3C,D, 4C,D, 5D, 6D, 7C,D and 8D). Their identification was crucial to the olfactory recess observation (Figures 2D, 3D, 4D, 10D and 11D), which appears as hyperintense in the T2W images, although they were not visible in the T1W images. These more rostral images also facilitated the identification of the dorsal sagittal sinus (Figures 2C,D, 3C,D, 4C,D, 5C,D, 6C,D, 7C,D and 8C,D), represented as a moderate intense and hyperintense in T1W and T2W images, respectively. Furthermore, the telencephalic hemispheres were observed in both T1W and T2W, with moderate and regular intensity signals (Figures 3C,D, 4C,D, 5C,D, 6C,D, 7C,D, 8C,D, 9C,D, 10C,D and 11D).

Moreover, the transverse, sagittal and dorsal images depicted relevant components of the ventricular system, including the lateral ventricles, the dorsal and ventral part of the third ventricle, the mesencephalic aqueduct and the fourth ventricle displaying moderate intense and hyperintense signals in T1W and T2W images, respectively (Figures 5C,D, 6C,D, 7C,D, 8C,D, 9D, 10C,D and 11D). The lateral ventricles facilitated the observation of the caudate nucleus, shown with a moderate intensity and an oval shape (Figures 5C,D, 10D and 11D) and the hippocampus, a large bean-shaped structure with a moderate intensity signal in both T1W and T2W images (Figures 6C,D, 7C,D, 8C,D, 10D and 11D). In addition, the two parts of the third ventricle enclosed the interthalamic adhesion, limited laterally by the right and left sides of the thalamus (Figures 6D, 7D, and 10C,D). Pivotal white matter identifiable structures were the corpus callosum, the fornix and the internal capsule, with a lower intensity than other structures in the CNS, but only visible in T2W (Figures 5D, 6D, 7D, 8D, 10D and 11D). A slight hypointense layer of white matter was essential to demarcate the amygdala, showing a moderate intensity signal (Figure 6C,D).

On the more caudal transverse and sagittal MR images, we distinguished the colliculus and the cerebral peduncles with adequate resolution (Figures 8D and 10C,D). Close to these peduncles, we could visualize the genicular bodies, showing intermediate signals (Figures 7C,D, 8D and 10D). Other relevant formations of the nervous system, including the vermis and hemispheres of the cerebellum, as well as the middle cerebellar peduncles were displayed in the T1 and T2W images (Figures 9C,D, 10C,D and 11D).

MRI images were also pivotal to appreciate diverse eyeball components, such as the vitreous humour and lens, and the extraocular muscles (Figures 2C,D and 3C,D), displaying moderate or high intensity signals. Additionally, we accurately pinpointed different muscles responsible for the chewing movement of the mandible and the temporomandibular joint, including the lateral and medial pterygoid muscles, the masseter and temporal muscles, which presented moderate intensity signals in T1 and T2W transverse images (Figures 4C,D–-8C,D).

4. Discussion

Modern diagnostic imaging techniques have greatly enhanced anatomical knowledge and pathology detection in veterinary medicine [5,7,22,23]. Compared to traditional methods such as radiology and ultrasounds, advanced imaging techniques provide superior resolution of anatomical structures, more precise definitions of lesion extent and characteristics, faster image acquisition, and eliminating superimposition. These advantages have revolutionized research, veterinary practice, and education [6,7,8,9,10,11,12,13,14,17]. However, the use of these technologies in exotic animal medicine remains limited due to their high cost, availability, and logistical challenges in obtaining images from certain species. While CT and MRI studies of the CNS have been conducted in both exotic and domestic species, including dogs, horses, iguanas, porcupines, guinea pigs, rabbits, and members of the Dasypodidae family such as the nine-banded armadillo [10,25,26,27,28,29,30,31], there is a lack of detailed anatomical descriptions of these images for the normal six-banded armadillo. Therefore, this study provided a comprehensive anatomical analysis of the six-banded armadillo’s CNS and associated structures through the combination of anatomical cross-section and CT and MRI images.

In this investigation, transverse anatomical sections were quite helpful in adequately identifying the main formations of the six-banded armadillo head. Hence, these images facilitated the identification of pivotal structures of its encephalon and formations belonging to the oral cavity and the auditory system. However, its eyes were not discernible because of its small size and position. Despite this limitation, they were clearly defined in the CT and MR images.

Concerning CT images, the sagittal and dorsal sections provide clear images of the triangular, pointed-shaped and flat head of this animal. Interestingly, these CT images were also pivotal in visualizing the notable development of the tympanic portion of the temporal bone, which could be an important finding to explain its auditory capacities. In addition, other formations of the armadillo head were also defined. Thus, the CT bone window was especially useful in identifying various bony structures of the head, such as the osteoderms, the zygomatic arch, the frontal, the squamous and petrous parts of the temporal, the occipital bone, the maxilla and the mandible. In contrast, it exhibited a low capacity for identifying soft tissue like muscles or nervous tissue due to the minor density differences in CT images. Similar findings have been displayed in studies performed on other exotic species, including the rhinoceros iguana, the guinea pig, the six-banded armadillo, rabbits and the crested porcupine [24,28,30,32].

In this study, T1 and T2W MR images displayed a better definition of the nervous system and its associated structures than those obtained by CT. Moreover, T2W was crucial to visualize the great development of the rhinencephalic formations, including the olfactory bulbs and recesses, as well as the lateral and intermediate olfactory tracts, which has been related to a well-developed sense of smell by improving olfactory airflow and residence time of odorants within the olfactory regions [45,46]. These findings have been identified in the seven-banded armadillo [47] and to a lesser degree in a variety of animals, such as carnivores [48], rodents [49], and ungulates [50]. Other pivotal nervous formations were also pinpointed, including the hippocampus, caudate nucleus, the caudal colliculus and the amygdala. Regarding this structure, subjective image analysis and objective measurements identified a large amygdala. Studies performed on Dasypus hybridus have also identified a relevant development of its amygdala [47], mainly referring to the cortical and medial nuclei of the amygdaloid complex. They suggest that this finding could be associated with the notable development of the rhinencephalon, which has been demonstrated in other animals [47].

It is also crucial to note that our study, which focuses on the nervous system, is limited by including only three armadillo specimens. However, we consider that is possible to use this information as an initial reference for clinicians, biologist and researchers. Furthermore, to gain a more comprehensive understanding of these findings and to expand upon the information presented in studies like this, additional research involving a greater number of specimens and a deeper understanding of the species’ natural history is necessary.

5. Conclusions

The use of anatomical cross-sections combined with modern diagnostic techniques played a crucial role in accurately identifying the main structures that make up the armadillo head. Hence, this study revealed specific morphological characteristics of the armadillo’s head. The findings are valuable for assessing various pathological conditions, including skull fractures, malformations, and tumours. Additionally, these techniques can enhance veterinary anatomy education by providing a clear view of structures without the interference of overlapping parts, making it easier to visualize the extent of different types of lesions.