The most critical radiograph for the accurate diagnosis and treatment planning of root canal therapy is the Periapical radiograph.

- A periapical radiograph provides a detailed image of the entire tooth structure, including the crown, root, and surrounding bone.

This type of radiograph is essential because it allows clinicians to:
- Visualize the root apex and the surrounding periapical tissues, which is crucial for detecting any periapical pathology such as abscesses or cysts.
- Assess the length, shape, and number of root canals, aiding in proper cleaning, shaping, and obturation during root canal therapy.
- Monitor post-treatment healing by comparing pre- and post-operative images.

In contrast, other radiographs serve different purposes:
- Panoramic radiographs provide a broad overview of the jaws but lack the detail necessary for root canal treatment planning.
- Occlusal radiographs are typically used for locating impacted teeth or assessing maxillary and mandibular bone but do not provide detailed root and periapical views.
- Bitewing radiographs are primarily used to detect interproximal caries and bone levels, not root canal anatomy.

Therefore, the periapical radiograph remains the gold standard for diagnosing and planning endodontic treatment.

Reference: Orstavik D., Pitt Ford T.R., Hargreaves K.M., "Problem Solving in Endodontics", 2nd Edition, Chapter 3: Radiographic Assessment, Pages 45-60.