What must be open for a bitewing image to be diagnostic?

For a bitewing image to be considered diagnostic, the contact points between adjacent teeth must be open. This allows for accurate assessment of interproximal areas where decay can often occur and where superimposed structures might obscure relevant details. Ensuring these contact points are visible is critical for evaluating the health of the teeth and the supporting structures. When the contact points are open in a bitewing radiograph, it means there is clear visualization of the tooth surfaces that come into contact, which is essential for diagnosing issues like caries or periodontal disease.

The other options do not necessarily contribute directly to the diagnostic quality of a bitewing image. While the occlusal surface shows the relationship of the biting surfaces, it does not ascertain interproximal contact health. The midline of the arch might guide positioning but is not a requirement for diagnostic clarity, and the roots of the teeth are often not visible or relevant in bitewing images, which focus on the crowns and interproximal areas. Thus, the opening of contact points is paramount for a diagnostic interpretation in bitewing radiography.