Establishment of an MRI ‐Based Diagnostic Approach to Identify Critical Pulpal Inflammation Zones in a Rat Caries‐Derived Pulpitis Model

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AIM: To establish a magnetic resonance imaging (MRI)-based diagnostic approach for identifying 'critical pulpal inflammation' zones in a rat model of caries-derived pulpitis. METHODOLOGY: Caries-derived pulpitis was induced in the molars of Sprague-Dawley rats using the caries-derived pulpitis model. T2-weighted images by 7 Tesla MRI and micro-computed tomography (CT) images were taken weekly to sequentially monitor the caries progression and pulp conditions in the same individuals. Caries lesions were classified into moderate, severe and exposed groups with reference to the micro-CT images. Sound teeth without caries were used as controls. Histopathological observations were performed to confirm the pulp inflammation. Pulpotomy was performed to remove the low magnetic resonance (MR) signal area suspected to be a 'critical pulpal inflammation' zone, which is considered unlikely to be preserved. Six weeks after pulpotomy, T2-weighted and micro-CT images of the molars were obtained, and histopathological evaluation was performed to observe the outcome of the pulpotomy. RESULTS: High MR signal areas expanded with caries progression from the moderate to the severe group, indicating a spreading inflammatory response, which was confirmed histologically. The exposed group showed a significantly lower MR signal area compared with the others (one-way analysis of variance, Tukey's post hoc test, p < 0.05). Matrix Metalloproteinase 9 (MMP9), a candidate molecular biomarker of serious pulpal inflammation, and inflammatory cell infiltration were observed in the same area. After pulpotomy, the remaining pulp tissue showed tertiary dentine formation without pulp necrosis and absence of inflammatory markers and MMP9, suggesting successful removal of critical pulpal inflammation. CONCLUSIONS: This study suggests the potential of MRI as a novel non-invasive and quantitative diagnostic method for critical pulpal inflammation zones. Moreover, MMP9 was identified as a reliable biomarker of critical pulpal inflammation. Future studies will focus on validating the potential biomarker role of MMP9 and analysing low MR signal areas for other novel biomarkers of critical pulpal inflammation.

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