The radiopaque structure shown in the box is most likely:
The genial tubercles (also called the mental spine) are located on the lingual surface of the mandible slightly above the inferior border and in the midline. They are bony protuberances, more or less spine-shaped, that often are divided into a right and left prominence and a superior and inferior prominence. 
They attach the genioglossus muscles (at the superior tubercles) and the geniohyoid muscles (at the inferior tubercles) to the mandible. They are well visualized on mandibular occlusal radiographs as one or more small projections. Their appearance on periapical radiographs of the mandibular incisor region is variable; often they appear as a radiopaque mass (3 to 4 mm in diameter) in the midline below the incisor roots. Lingual foramen is a radiolucent structure seen in the midline.
Ref: White and Pharoah, pg-144,145
Lingual foramen
Genial tubercle
PCD
None of the above