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is a non‐invasive approach that visualizes the CNS, especially the brain.

       One of the major goals of using structural MRI is to investigate the morphology, including the size and shape, of the anatomical structure of the brain.

       The functional MRI methods investigate the brain signals associated with brain functions, including BOLD fMRI and perfusion MRI.

       The BOLD fMRI detects the changes in the proportion of deoxygenated and oxygenated haemoglobin in the brain. This metabolic event is indirectly associated with neural activity.

1.3 How Does Neuroimaging Contribute to Clinical Practice?

      1.3.1 Introduction

      How can neuroimaging contribute to dental practice? Intuitively, it is hard to imagine that the knowledge of ‘brain activation’ would contribute anything for dentists to complete a Class II cavity restoration. However, the functional perspective of the brain–stomatognathic connection (Figure 1.1) highlights the association between the brain, mental functions and oral functions. Therefore, understanding the brain is the key to understanding the individual variation in oral functions and feeding and oral healthcare behaviour. In the following sections, we elaborate this association by examples of dental neuroimaging studies. Firstly, we discuss the contribution of neuroimaging to oral neuroscience. Secondly, we discuss the contribution of neuroimaging to the clinical disciplines of dentistry.

      1.3.2 Links Between Neuroimaging and Key Issues of Oral Neuroscience

As a new discipline of neuroscience, how does neuroimaging help investigate these key issues of oral neuroscience? Table 1.2 shows that neuroimaging research has been engaged with all the key issues in oral neuroscience (Iwata and Sessle 2019). For example, in terms of the pathway of pain processing, neuroimaging research extended our understanding of the circuitry from the spinal mechanism to brain mechanisms, showing cortical and subcortical activation associated with acute and chronic orofacial pain (Brügger et al. 2012; Gustin et al. 2011) and complicated mechanisms of pain modulation (Desouza et al. 2013; Gustin et al. 2012; Moayedi et al. 2012; Younger et al. 2010). Importantly, because neuroimaging research is conducted in human subjects, psychosocial factors, such as emotional and attentional factors related to pain, can be investigated (Weissman‐Fogel et al. 2011; Abrahamsen et al. 2010; Youssef et al. 2014). Neuroimaging also helps to reveal the brain mechanism of swallowing and chewing (Lowell et al. 2012; Quintero et al. 2013). Notably, by directly assessing the dental patients who received treatment, translation between clinical practice (e.g. installation of denture prosthesis) and neuroscience can be achieved (Kimoto et al. 2011; Luraschi et al. 2013). Also, the perceptual processing of oral functions, which relies on self‐reports from human subjects, including tactile and gustatory senses, can be studied using neuroimaging (Nakamura et al. 2011; Trulsson et al. 2010). As an in vivo imaging method, neuroimaging has become the crucial method for studying the perceptual and psychosocial aspects of oral functions, which are difficult to approach with animal models.

      1.3.3 Links Between Neuroimaging and Clinical Disciplines of Dentistry

      Clinically, neuroimaging research may help dental professions to understand the association between mental functions and dental treatment. For example, dentists may need to know the sensory processing of a patient who is chewing with an implant‐supported denture. If a dental implant alters the sensory feedback from occlusion, one should expect to identify changes in cortical activation at the sensory area when subjects are chewing. Neuroimaging would be a suitable tool for the study related to the treatment outcomes of dental practice.

      1.3.3.1 Prosthodontics

      Restoration of both structural deficits and functional impairment is key to successful prosthodontic treatment. For patients with tooth loss, replacing the missing teeth with a dental prosthesis will restore structural deficits. Moreover, patients should adapt to the prosthesis and improve chewing function. Recent neuroimaging findings have shed light on the mechanisms of adaptation of dental prostheses (Table 1.3). For example, longitudinal research revealed that adaptation of a new denture was associated with not only the improvement of masticatory performance but also changes in brain activation in the somatosensory cortex (Luraschi et al. 2013). Consistently, tactile stimulation on dental implants was associated with brain activation of the somatosensory areas (Habre‐Hallage et al. 2012). The findings suggest that changes in sensory feedback play a key role in improving oral functions by prostheses. Moreover, in partially edentulous patients, reduced occlusion was associated with reduced activation of the prefrontal cortex, and such reduced activation can be modulated by installing a denture (Kamiya et al. 2016). In patients with maxillary dental implants, tactile stimuli induced brain activation not only in the somatosensory cortex but also in the prefrontal cortex (Habre‐Hallage et al. 2012). Changes in somatosensory and prefrontal activation were also identified in another longitudinal fMRI study that dentated patients were chewing a piece of gum when wearing a plate to cover their palate. The change in brain activation was associated with the recovery of masticatory performance, which was initially impaired (by the palatal plate) and later restored (Inamochi et al. 2017). These novel findings suggest that beyond sensory processing, the attentional and cognitive processing related to wearing a denture, as evidenced by the changes in the prefrontal cortex, may play a vital role in the adaptation of prosthodontic treatment. Thus, the neuroimaging findings provide new clues for prosthodontic treatment by highlighting the patient’s adaptation to dental devices.

      1.3.3.2 Periodontics

      One of the primary roles of the periodontium is to support sensory feedback via the periodontal ligament. Neuroimaging would help clarify the neural pathway of sensory processing of periodontal stimuli (Kaneko et al. 2017; Kishimoto et al. 2019; Ono et al. 2015). Moreover, neuroimaging may contribute to elucidating the association between periodontal health and other systemic conditions (Table 1.3). A recently hotly debated issue is the association between dementia and neuroinflammation as well as neurotoxicity, which may relate to periodontal health (Tonsekar et al. 2017). To explore this brain–stomatognathic connection, researchers have directly assessed the association between periodontal health and the Aβ plaques of the brain, a critical feature of Alzheimer's dementia (Kamer et al. 2015). In this study, the pathological feature of Aβ plaque was assessed using PET, and the association between brain pathology and periodontal health (e.g. clinical attachment loss) can be quantified (Kamer et al. 2015). The association between oral health and other pathological brain features, such as lacunar infarction, can also be investigated using neuroimaging methods (Taguchi et al. 2013). Neuroimaging is a valuable tool for investigating the sensory pathway of periodontal inputs and the association between periodontal health and systemic conditions.

      1.3.3.3 Orthodontics

      Just like prosthodontic treatment, the success of orthodontic treatment is associated with patients' adaptation to the oral appliance. Again, the neuroimaging findings revealed that the use of the oral appliance is associated with an extended area of brain activation, not just confined to the somatosensory cortex (Horinuki et al. 2015; Ozdiler et al. 2019) (Table 1.3). In rats, experimental tooth movement was associated with changes in brain activity of the secondary somatosensory cortex and the insula (Horinuki et al. 2015). Critically, the animal model revealed that during tooth movement, brain activity change was also associated with inflammation, as identified by the expression of the inflammatory factors and macrophage infiltration in the periodontal tissue (Horinuki et al. 2015). The findings have demonstrated the strength of combined neuroimaging and histological approaches, which help

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