Objectives: This study was aimed at determining the specific signs of close relationship between impacted mandibular third molar (IMTM) root and inferior alveolar nerve (IAN) canal as visualised on digital panoramic radiograph or orthopantomograph (OPG) which should indicate as well as contraindicate the need for cone beam computed tomography (CBCT) imaging based on hierarchical model of Fryback and Thornbury. Materials and Methods: The sample consisted of 120 individuals (200 IMTM) who had undergone OPG as well as CBCT imaging as preoperative radiographic evaluation before surgical extraction. On panoramic radiographs; Rood and Sheehab's radiographic signs of IMTM root and IAN canal, impacted third molar position based on Winter's classification and depth of impaction based on Pell and Gregory's classification were evaluated. On CBCT; presence/absence of corticalization and the status of the buccal and lingual cortices (thinning/perforation) were evaluated. These findings of OPG were compared to CBCT findings to determine the better modality based on hierarchical model of Fryback and Thornbury. Results: Statistically significant association was found between radiographic signs of Rood and Sheehab such as 'no relation' and 'superimposition' on OPG and presence of corticalization between IMTM root and IAN canal on CBCT. Statistically significant association was also found between 'mesioangular' and 'vertical' positions of Winter's classification as well as 'class 1A' of Pell and Gregory's classification of IMTM on OPG and presence of corticalization on CBCT. CBCT was found to be having lesser variance and greater F value as compared to OPG for evaluation of IMTM. Conclusions: On OPG; Rood and Sheehab's radiographic signs such as 'no relation' and 'superimposition', Winter's 'mesioangular' and 'vertical' and Pell and Gregory's 'Class 1A' are not indicative for CBCT examination before surgical extraction is attempted. On OPG; Rood and Sheehab's radiographic signs such as interruption of white line, darkening of root, darkening of canal, deflection of root, narrowing of canal as well as diversion of canal are all frequently associated with absence of corticalization between IMTM root and IAN canal and hence require CBCT examination before surgical extraction is attempted so that post-operative neurological complications can be minimised. Also, Winter's horizontal, distoangular and others as well Pell and Gregory's classes 1B,1C,2A,2B,2C,3A,3B,3C are indicative for CBCT examination before surgical extraction is attempted. Fryback and Thornbury model-based comparison proves that CBCT is a better radiographic modality as compared to OPG for evaluation of IMTM relation with IAN canal.