As detailed evaluation of the internal and periodontal structures of the cheek teeth is important in deciding on treatment when pulpar or apical infection occurs , CT and MR images have been acquired in the present study, aiming to highlight the best imaging modality for each structure. The overview of the two techniques on the same head allows a direct comparison of the potential of both. There are several studies published in equine medicine describing the qualities of CT or MRI for the diagnosis of head pathologies without comparison of both techniques [1,2,3,4,5,6] and with lower MRI field strength of 0.5 to 1.5 T [17, 25, 36].
The image quality of MRI in the present study was comparable to that of CT, but was better for bony tissues in CT and for soft tissues in MRI. The present results correspond with the findings of Gerlach et al.  and Kaminsky et al. , who proved the excellent quality of MRI in portraying the dental pulp, PDL, bone-marrow, gingiva, facial soft tissue, sinus mucosa, infraorbital nerve and vessels, owing to a high water content and hydrogen atoms. There was no detectable MRI signal from hard dental tissues, cortical bone, lamina dura and the IOC, whereas these structures could be visualised with good detail resolution by CT acquisition. The best MR image quality in the present study could be achieved with T2w scans for an anatomical overview and with PDw or STIR scans for more detailed images; this is in line with the findings of Kraft and Gavin , where T2w, STIR and PDw scans appeared superior to T1w scans in all images evaluated. This is related to the thinner slice thickness of the 3D T1w sequences, which are more often affected by artefacts compared to the transversal and dorsal orientated sequences of the STIR, PD and T2 weighted images with thicker slices.
Compared to low-field tomography, 3.0 T MRI has a higher signal-to-noise ratio, leading to better resolution. Consequently, sequences can be taken with a lower recording time and equal image quality compared to low-field MRI. The time for general anaesthesia for patients can be shortened and anaesthesia risk is reduced [38, 39]. The time required for imaging examination in the present study differed greatly between MRI and CT and between the different MRI scans. The CT was around 18 times faster than the entire MRI examination, due to the different imaging techniques and various MRI scans. The long MRI examination times were chosen to acquire images of excellent quality in the current study. The number of image alignments (transverse, dorsally orientated), resolution, or FOV might be decreased for clinical use to reduce scan time.
Due to the differences in the alignment of teeth , the MR images obtained could include artefacts, because image alignment was chosen for the entire skull and not for each tooth. Finally, some pulps and PDLs appeared blurry only due to image angulation (T2w, PDw, STIR) in the current study, that might be mistaken for pathology . The alignment for each tooth would take more time and anaesthetic risk in clinical patients would increase due to longer examination times . T1w images with 3D datasets offer an exception: through MPR, this technique provides the possibility of producing image planes in alignment with the teeth depicted. The time for image acquisition in T1w sequences is prolonged in high-field MRI, as T1 relaxation time is longer . Therefore, a short acquisition time for T1w images, as was applied in the current study, is always accompanied by worse image quality compared to the T2w scans.
In contrast to MRI, CT provides the possibility of different angulations through subsequent MPR and, thus, the potential to evaluate the dental and periodontal changes in alignment of every single tooth. Nevertheless, CT also shows limitations, as it is less useful in identifying early changes in the pulp  due to its inability to visualise soft tissues as excellently as T2w or PDw images.
Selected cheek teeth
As described in the literature, dental pathologies such as apical infections, dental fractures and infundibular caries occur predominantly in certain maxillary cheek teeth . The upper 08 s and 09 s show clinical signs most frequently [44,45,46] and, therefore, they were chosen for examination in the current study.
Selected age groups
The planning of surgical procedures in cases of dental and associated pathologies requires accurate anatomical knowledge and imaging of the cheek teeth and adjacent structures, which are illustrated in this study. Selected equine groups of different ages were chosen in our study as dental changes can be influenced by morphological, functional and mechanical changes, which occur with increasing age : Equine cheek teeth show second dentition and real longitudinal growth up to the horse’s age of five years (Group A in the current study); afterwards, the tooth length decreases as teeth are pulled out of the alveoli in the oral direction and the cheek teeth abrasion continues (Group B in our study: 6–15 years) [48, 49]. In older horses (Group C from an age of 15 years onwards), cheek teeth have an exposition to higher dental forces . All these aspects might influence changes of dental structures (e.g. pulpar size) or positional relations between dental and periodontal structures, which were examined in the current study.
Regarding the preselected horse groups (resulting in preselected dental age groups), the present evaluations should be treated with considerable caution: due to the small size of the study population, age distribution and image interpretation might not comply with the entire equine population.
Common pulp chamber
It can be assumed that CPCs are more common in younger teeth , which is in line with the current results, as CPCs were displayed in all teeth evaluated of age group A with a mean dental age of 2.25 years. Dacre et al.  located a CPC in teeth with a mean dental age of 2 years (via microscopic examination), whereas Kopke et al.  stated the maxillary CPC to be evident in teeth with a mean dental age of 4 years (in high resolution micro-CT scans). While CT  and MRI studies  observed a CPC in cheek teeth up to 6 years and with micro-CT even up the dental age of 9 years  in the present study, the oldest teeth showing a CPC had a dental age of 3.75 years. The difference in results could be caused by the different imaging techniques, imaging settings or measuring techniques. More precise results could probably be achieved by micro-CT scans and histological investigations.
Anatomical examinations of the endodontic system by Baker  and Dacre et al. , as well as MRI  and CT  imaging studies revealed a general pattern of five pulp horns in the maxillary 08 s and 09 s. These findings comply with the results of the current study, where five main pulp horns (P1-P5) were visible in 33 cheek teeth in the CT and MR images. Three out of all 36 cheek teeth evaluated showed only four pulps.
Contrary to the histological findings of Shaw et al. , which displayed an increase in pulpar sizes between 3.5- and 7-year-old teeth, the MRI measurements in the current study showed a continual reduction in size with age. This is consistent with MRI  and CT  studies demonstrating pulpar reduction with age. The reasons for the pulpar size decrease can be found in age-related physiological pulp modifications. Young equine pulp tissue consists of odontoblasts, connective tissue, nerves, vessels and different cell types. Attachment of secondary dentine is detected and the vital cell number decreases with age . As secondary dentine contains fewer protons, the pulp appears smaller in older equine cheek teeth in the MR images. In the CT images, pulpar tissue does not appears as well delineated as in the MRI scans. Therefore, pulpar existence, dimension and pathologies might not be detected as well with CT imaging as in the MRI scans. Finally, both imaging modalities, CT and MRI, seem to be inferior to histological examinations, which could be the reason for different pulpar dimensions found by Shaw et al.  and in imaging studies such as the current one.
In the present study, three pulps could not be measured in the MR images: one pulp was missing completely in each of the 09 s affected (Age group B: n = 1; Age group C: n = 2). All three teeth with one missing pulp in the MRI scans showed a higher attenuated or a gas spot-filled pulp in the CT scans. Missing pulps have been described by Gasse et al. , who carried out research on pulpar changes of the 07 s and 09 s in horses of different ages. Due to the expansion and proliferation of secondary dentine, the number of vital pulps is reduced in horses aged between 15 and 23 years. Fewer hydrogen atoms might result in less pulp detection in MRI scans and secondary dentine might be the explanation for higher attenuated pulps in the CT images of the current study.
Reasons for the absence of pulp in the MRI scans can also be found in pathogenic mechanisms that become visible comparing both imaging techniques. Whilst the MRI showed no causes for the absence of the pulp, CT revealed indications for infundibular and pulpar gas spots that can be interpreted as infundibular caries in the 09 s affecting the pulp horn. While Veraa et al.  and Bühler et al.  argued that infundibular changes often appear as a singular dental change in CT without significant relationship with pulpitis, Dacre et al.  describe infundibular changes which can cause and result in pulpitis, collapsing into the adjacent pulp. Inflammatory cells proliferate as a consequence of the pulpitis, which leads to varied pulp capillary blood flow, arteriovenous anastomoses and ischemic necrosis of the pulp . Finally, this initiates the reduction of pulp size through the production of tertiary dentine , decreasing the pulpar visibility in MRI.
Other processes, such as dental trauma  or haematogenous pulp infection , can result in secondary pulpitis. In addition to pulp stones, histologically referred to as intra-pulpar calcified structures without tubular configuration , all these mechanisms can cause decreased vascularity of the pulp itself , pulpar infection and destruction, and its inability to be visualised in MR images.
It is described in other studies that the pulps sometimes underwent an occlusal insult, resulting in occlusal necrosis and the production of repairing tertiary dentine, but more apically the horns were vital . This is the reason that all dorsally orientated MRI section planes should be assessed and reviewed for vital pulp tissue.
As a pathologically decreased or missing pulp in MRI does not always allow any conclusions regarding the aetiopathogenesis , evaluation of the adjacent structures complemented by CT imaging of bony and hard dental structures and occlusal surfaces is important.
Although all the horses examined appeared to be clinically healthy in terms of their teeth, the missing pulps (MRI) or infundibular gas spots (CT) might be an indicator for the start of dental pathology.
Distance between dental alveoli and the maxillary sinus and the infraorbital canal
Precise anatomical knowledge is essential for the evaluation of dental ascending infections and planning of surgical procedures. If a tooth with apical infection is located within the boundaries of the sinus cavity and induces a sinusitis, treatment of the sinus affected may be indicated .
Various declarations referring to the contact between teeth and paranasal sinuses due to inter-individual skull anatomy and age-related variances exist in the literature. While Hillmann  revealed that only the last three cheek teeth are in contact with the sinus floor, Dyce et al.  mentioned that the last premolar tooth’s alveolus is also in contact in young horses. In the current study, 94% of Triadan 08 and 09 showed contact with the maxillary sinus floor. The results obtained correspond to the increased risk of inducing secondary sinusitis in apically infected 08 s and 09 s that is described in literature .
Both the one-year-old teeth of Triadan 08 which did not show any contact with the sinus floor had direct contact to the IOC. If the IOC is in close contact with an infected tooth, local bony necrosis could occur due to facilitated expansion of proteolytic enzymes . The distance between the alveoli of 08 s and 09 s and the IOC increased by an average of 1.9 mm per year with age. While a five-year-old tooth shows about 12 mm to the IOC, the distance is measured at 41 mm for a 20-year-old 08 or 09. Infraorbital nerve trauma, associated with neuritis and headshaking, is described as a complication of surgical tooth extraction and sinustomy . Knowledge of the IOC’s position, as outlined above, can be essential for the prevention of these complications.