Single-photon emission computed tomography (SPECT)/computed tomography (CT) and positron emission tomography (PET)/CT are crucial for improving the quality of diagnostic imaging in nuclear medicine. These hybrid imaging modalities can simultaneously provide both functional and anatomical information by combining nuclear medicine images (SPECT or PET) with anatomical images (CT). However, the use of CT in SPECT/CT and PET/CT comes with the challenge of increased radiation exposure to patients. Therefore, from the perspective of radiation protection, it is necessary to optimize not only the dose from radiopharmaceuticals but also the CT dose, depending on the clinical application. Recent advances, such as the introduction of low-dose CT techniques and adaptive filtering technologies, have enabled the reduction of radiation exposure while maintaining diagnostic quality. CT in SPECT/CT and PET/CT are indispensable tools for improving diagnostic accuracy, enhancing the precision of quantitative evaluations, and optimizing treatment planning. Appropriate dose management and technological advancements will continue to be critical issues in balancing diagnostic efficacy with patient safety. In this chapter, we described not only some especially important technical points of SPECT/CT and PET/CT, but also MIRD method as a fundamental estimation method of radiation exposure dose, CTDI, various correction methods within reconstruction methods, and the newest technologies for nuclear medicine imaging.