Radionuclides, a complex tool with immense potential, can propel modern medical imaging forward while simultaneously posing grave risks. It is imperative to acknowledge the dual nature of radionuclides and find a balance. Thus, a thorough understanding and prudent utilization of radionuclides are essential. Surprisingly, there is a dearth of comprehensive studies exploring advanced strategies, particularly those involving optical sensors, for the quantification and visualization of radionuclides. Given this, we focus on the biological application and radiation detection of medical radionuclides in vivo. In this review, we start with the fundamental classification of ionizing radiation and delve into the intricate interactions between various types of ionizing radiation and matter, encompassing particle beam radiation and electromagnetic wave radiation. Additionally, we provide a comprehensive overview of progressive methods for identifying and quantifying radionuclides. Our analysis extends to the burgeoning research domains, including the clinical translation of radiopharmacy and evaluations based on diverse radionuclides. Furthermore, we shed light on the existing challenges faced by advanced optical sensors in detecting radionuclides and outline potential avenues for future research. By addressing these challenges and emphasizing innovative approaches, we aspire to pave the way for the responsible utilization of radionuclides, ultimately benefiting humanity as a whole.