Introduction. The masticatory muscles are the main components of the masticatory system in dogs, ensuring normal jaw function and feed grinding. Dysfunction of the masticatory muscles, caused by trauma, congenital pathologies, or inflammatory processes, significantly reduces quality of life of an animal. One of the most widespread pathologies is masticatory muscle myositis, an immune-mediated disease characterised by inflammation and subsequent atrophy of the masticatory muscles [1]. It is most often diagnosed in large breed dogs, such as German Shepherds, Labradors, Boxers, and Golden Retrievers, mainly at the age of 2 and 5 years [2]. According to the existing studies, the prevalence of this pathology equals up to 0.5% of all visits to veterinary clinics of dogs with head diseases [3]. Cases of neoplasms, trauma, fibrous changes, and abscesses affecting the head muscles are also common.

Traditional diagnostic methods such as radiography and ultrasound scanning are not capable for detailed visualization of the structure and topography of the masticatory muscles, whereas computed tomography (CT) scanning makes it possible to assess the shape, volume, and density of muscles in high-resolution, as well as detect the pathological foci of inflammation, necrosis, myositis, and neoplasms, even at the early stages [4]. Furthermore, the anatomical features of the masticatory muscles vary depending on the breed and individual parameters of an animal, and this must be taken into account when planning surgeries or diagnostic procedures such as biopsy. For this purpose, CT scanning is the most appropriate technique indeed, provided that the resulting images are correctly interpreted [5]. However, currently, CT is not widely used for the diagnosis of masticatory muscle pathologies in veterinary patients. The aim of the present study is to perform computed tomography (CT) scanning of the masticatory muscles in dogs and to conduct a comparative analysis of the topography and morphology of the masticatory muscles using the CT images and macroscopic anatomy sample sections to clarify the correspondence of CT images to the actual anatomical structure and to evaluate the diagnostic capacity of this technique. The target audience of the study are the veterinary specialists seeking to improve their skills of using CT scans for diagnosing the inflammatory, traumatic and neoplastic processes in the head of animals.

Materials and Methods. The study was conducted at the “VitaVet” Veterinary Clinic and in the anatomical theatre of Don State Technical University (Rostov-on-Don) from November 2023 to March 2024. The objects of the study were medium-sized dogs (two mixed-breed dogs and one corgi), aged 2 to 5 years, weighing 15 to 30 kg, two males and one female. Masticatory muscle pathologies were not clinically confirmed in the animals participating in the study.

A GE Revolution ACT computed tomography machine (General Electric, China) was used. The following parameters were applied:

— power: 120 kV at 100–120 mA,

— distance from the tube to the center of the object of examination: 75 cm,

— scanning pitch: 0.625 mm,

— number of scans: 761–1522.

Scans were processed using Inobitec DICOM Viewer software (Inobitec LLC, Russia).

Contrast enhancement was performed using the Omnipaque agent 350 mg iodine/ml (100 ml, at the proportion of 3 ml/kg of animal mass, administered at the rate of 2 ml/sec. A region of interest (ROI) enhanced with contrast agent was set around the diaphragm. Three-phase scanning was performed: arterial, venous, and delayed. The scanning delay in the arterial phase was no more than 3 s; in the venous phase, 15 s immediately after the arterial phase; and in the delayed phase - 180 s after the end of the venous phase.

Following CT scanning, the animals were euthanized and frozen at –20°C for 7 days in a continuously temperature-monitored freezer (Biryusa, JSC, Russia). This temperature regime was chosen in compliance with N.I. Pirogov’s “ice anatomy” technique, which enables preservation of anatomical structures in their natural state and minimizes soft tissue deformation during cutting.

A circular table saw (Festool, Germany) that ensures getting strictly axial sections was used to obtain the sample sections for macroscopic anatomy analysis. Each section was numbered from head to tail. The appropriate sample sections were used for comparative analysis against the axial and sagittal CT projections of these structures; the remaining sections were frozen for further study.

Results. Based on the analysis of computed tomography scans and macroscopic anatomy sample sections, the anatomical and morphological properties of the following muscles in dogs were described in detail:

1. The masseter muscle (musculus masseter) (Fig. 1a): is characterised by a powerful, pronounced multipennate structure and complex innervation (the masseteric nerve - nervus massetericus). The muscle allows to elevate and move forward the mandible. CT revealed clear distinction between deep (pars profunda) and superficial (pars superficialis) parts of the muscle, as well as their relationships with the surrounding structures, which is particularly important while assessing trauma and tumour processes [6]. Comparison of axial CT projections to the macroscopic anatomy sections revealed good matching of silhouettes with the general topography: the silhouettes and positions of body parts matched, as confirmed by photo evidences. The only systematic discrepancy was in a slight decrease in the thickness of some areas of the sample sections, probably caused by shrinkage of soft tissues during freezing and by cutting technique features.

2. The temporalis muscle (musculus temporalis) (Fig. 1b): its fan shape ensures precise control of jaw movements, including jaw relation and lateral movement. CT scans enable detailed imaging of muscle fiber directions (the anterior fibers run vertically upward, the middle fibers run diagonally downward, and the posterior fibers run horizontally downward), which is critical for diagnosing fibrosis and inflammatory processes [7][8][9]. Innervation is ensured by the mandibular division of the trigeminal nerve, in particular, the muscle is supplied by the deep temporal nerves. Comparison of CT scans to macroscopic anatomy sections revealed that position and shape of the temporalis muscle on the scans generally correspond to the anatomical material, whereas on CT images, the boundaries between muscle deep parts were better distinguished. Freezing and cutting technique in some cases led to local deformation of the marginal parts of muscle in the sections, which explains the minor local differences in thickness.

3. The pterygoid muscles (musculus pterygoideus): are located on the opposite side of the masseter muscle, include two parts:

— the medial pterygoid muscle (m. pterygoideus medialis) (Fig. 1c): is attached along the free edge of the pterygoid fossa of the mandible from the facial vascular notch (incisura vasorum) to the angle of the mandible. The direction of the muscle fibers is caudoventral. The medial part of the pterygoid muscle is involved in elevating and protruding the mandible [10][11][12]. Macroscopic anatomy examination of the section confirmed the anatomical place of attachment and the direction of fibers (caudoventral). Unlike more superficial muscles, the medial pterygoid muscle was sometimes less contrast on CT scans due to its proximity to osseous structures, however the spatial relationships with the surrounding tissues (mandibular bone, vascular and nerve bundles) were reliably visualised. For clinical practice, this means that CT scans adequately depict the topography of the medial pterygoid muscle and can be used when planning interventions in the pterygoid fossa area;

— lateral pterygoid muscle (m. pterygoideus lateralis) (Fig. 2): is a thick muscle, attached along the posterior edge of the mandible. Its fibers are directed caudally. The lateral muscle is responsible for protruding the mandible. CT scans reveal high variability in the structure of these muscles across the breeds, which must be taken into account when planning surgical interventions [13][14][15]. In the frame of our comparative analysis it was revealed that general topography of the pterygoid muscles on the CT projections and on the macroscopic anatomy sections matched; however, on CT scans, the thin fascial layers were seen less contrastly, and the boundary between the muscle parts, particularly in the lateral muscle, was quite complicated. On the sample sections these muscles appear to be more pronounced relative to the bone markers, which is partly explained by the compression/displacement effect during cutting.

4. The digastric muscle (musculus digastricus) (Fig. 3): is a thin fusiform muscle consisting of a single belly. Innervation is ensured by divisions of the trigeminal (nervus trigeminus) and facial (nervus facialis) nerves. It is responsible for jaw lowering and lateral movement [6][10][11]. On our CT scans, the digastric muscle visualized as a thin, fusiform structure; on the axial projections it was possible to differentiate it from the surrounding tissues with application of sufficient resolution. Comparison against the macroscopic anatomy sample sections confirmed the position and orientation of the muscle. However, the areas of increased density were more easily detected on the CT scans, which in the anatomical material correlated with the areas of fibrosis or localized altered density.

Discussion and Conclusion. Comparative analisis of axial and sagittal CT scans against macroscopic anatomy sample sections conducted using our study material revealed a high degree of correspondence between the topographic markers and silhouettes of most masticatory muscles. The main differences can be explained not by CT imaging errors, but by technical factors (freezing, tissue shrinkage, and cutting mechanics). The data obtained confirm CT scanning reliability in reflecting layer-by-layer structure of the masticatory muscles and its its preference among imaging techniques, since traditional diagnostic methods for detecting pathologies in the pterygoid and masseter muscles, such as clinical examination (palpation) and radiography, are difficult to perform and do not provide a complete picture of the condition of these muscles [13][16][17].

Computed tomography scanning is a powerful and future-oriented tool for diagnosing masticatory muscle pathologies in dogs, which can:

– visualize minor changes in muscle fibers, including microscopic tears that cannot be detected by means of radiography; as well as hemorrhages (hematomas), foreign bodies stuck in the masticatory muscles, myositis, abscesses (accumulations of pus in muscle tissue), etc. [18][19][20];

– identify the early stages of inflammatory processes and neoplasms, differentiating them by nature, thanks to the use of contrast agents: the contrast agent, which is administered before CT scanning, is distributed from the blood channels to the soft tissues, and neoplasms become more clearly visualized, even in the early stages. The percentage of contrast washout helps to determine benign or malignant nature of the tumour [21];

– provide a better understanding of the topographic anatomy of muscles due to the use of 3D models constructed from CT data, which is particularly important when planning surgeries [15][16][17];

in case the needs for histological or cytological material collection, enables easy puncture due to understanding location of the suspected tumour in the masticatory muscles [22][23][24].

Thus, CT diagnostics not only fosters early detection of pathologies and determination of the number, scale, location, and intensity of morphological changes reflecting the degree of muscle involvement in the pathological process, but also enables planning highly efficient treatment.

However, a number of limitations should be borne in mind when considering the obtained results. Firstly, the study was conducted in a small number of animals (n=3), which limits the possibility of statistical data analysis and application of the results to all dog breeds. Secondly, the objects of the study were mainly middle-aged and medium-sized animals, therefore the anatomical differences between miniature and large breeds could not be fully followed. Thirdly, the macroscopic anatomy sample sections were taken from the cadavers of animals whose pathologies were not clinically confirmed, therefore, assessment of the spectrum of in vivo changes associated with specific diseases could not be made. The limitations of the technique itself should also be noted: despite its high spatial resolution, CT scanning has limitations with regard to assessing soft tissue structures, especially in the absence of contrast enhancement. Differentiation between inflammatory and neoplastic changes in some cases requires additional Magnetic Resonance Imaging (MRI) or histological analysis. The subjective factor of professional qualification of a specialist interpreting the CT scans also plays a significant role.

Therefore, the prospects for further research are related to expansion of the sample size to include the animals of different breeds and ages, as well as to incorporate the clinical cases of myositis, tumours and post-traumatic changes in the masticatory muscles. In the future, we believe it would be advisable to conduct comparative CT and MRI studies, as well as develop the standards for assessing condition of the masticatory muscles in dogs by computed tomography scans, taking into account dog breed and age. This will improve the accuracy of diagnostics and efficiency of treatment.