All periapical radiographs were collected from the Hospital of Stomatology, Chongqing Medical University, Chongqing, China, between January and December 2019. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the Research Ethics Committee of College of Stomatology, Chongqing Medical University, Chongqing, China [CQHS-REC-2020 (LSNo.158)]. Verbal informed consent was obtained from all patients. The inclusion criteria were: (I) permanent teeth diagnosed as deep caries or pulpitis; (II) accessibility of clinical electronic medical record; (III) accessibility of periapical radiographs; and (IV) periapical radiographs that radiographic penetration depth of caries was more than 3/4 of the dentine thickness. The exclusion criteria were: (I) teeth with a full restorative crown; (II) teeth with abnormal anatomical structures and position; (III) unqualified periapical radiographs with noise, dimness, misshaping, and darkness; (IV) missing important clinical parameters; and (V) teeth with multiple carious lesions. All periapical radiographs were cropped to show only one tooth per image and optimal position. The clinical diagnosis of deep caries or pulpitis was further conducted by a panel of three endodontists with more than ten years clinical experience according to clinical guidelines (20). A total of 844 periapical radiographs were enrolled, and 717 (85%) were randomly selected to train the CNN models, 127 (15%) were used to test the CNN models.

Image digital pre-processing strategies were conducted before training the CNN algorithm: (I) the maxillary teeth were rotated to the direction of the mandibular teeth; (II) the images were uniformly resized to 256×256 pixels and the pixel values were normalized to 0–1 range; (III) a global histogram equalization algorithm of the original periapical radiographs was used to increase the contrast and enhance the clarity of subtle image details. Also, the following data augmentation strategies were used: random rotation (±15°), random panning (±5% in height and width), and random inversion (horizontal and vertical).

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