Localization strategies for robotic endoscopic capsules: a review

Introduction: Nowadays, mass screening campaigns for colorectal cancer diagnosis in the early and curable stage is essential yet limited due to many reasons, for example, invasiveness, fear of pain, and embarrassment for patients. Indeed, mass screening programs, allowing precancerous lesion detection, are the most cost-effective way to reduce mortality and potentially eradicate colorectal cancer threat. Accurate localization represents a key element in capsule endoscopy, that is, estimation of position and orientation of endoscopic capsule devices enables enhancement of technological and medical features, such as reliable closed-loop control of active-locomotion capsules, accurate lesions localization, retargeting of pathologies, and follow-up. Areas covered: This contribution provides an exhaustive and critical review of localization strategies, both internal and external, implemented so far for endoscopic capsules. Starting from basic theoretical principles, it describes the most promising methodologies, for example, magnetic and electromagneticbased ones, but also other significant techniques, such as, ultrasound-based localization strategies. Expert commentary: Authors believe that the integration of external and internal localization methodologies with a multimodality approach can increase the overall accuracy and reliability of the endoscopic device pose estimation both for establishing an optimal control during the endoluminal procedure within a deformable environment and for autonomously identifying position of internal pathologies for retargeting and follow-up.