Nuclear Medicine Computed Tomography Physics

In nuclear medicine, computed tomography (CT) is used with positron emission tomography (PET) and single-photon emission computed tomography (SPECT) machines, which are combinedly called PET-CT and SPECT-CT, respectively. This dual-modality acquires coregistered images of structural and functional details in a single study with datasets complementary to each other and highlights the abnormal functional images with accurate localization of structures. CT imaging, when used in nuclear medicine, helps to improve the quantification of functional images by correcting for attenuation, scatter, and partial volume effects. The images from both modalities are fused using software, and standard uptake values are calculated. The concept of combining planar images of 2 modalities dates back to the 1960s, even before the invention of CT machines. However, software for fusion imaging started to develop in the late 1980s. The proper dual-modality fusion approach was pioneered at the beginning of the 1990s by Hasegawa et al at the University of California, who used a diagnostic CT scanner with a SPECT camera to make the first SPECT/CT device. However, the first commercial SPECT-CT was introduced in 1999 by General Electric (The Hawkeye) using a low-power x-ray source with a conventional SPECT camera.  A similar concept of combining PET with CT was suggested independently in 1991, and the first working prototype PET/CT scanner was developed in 1998, but the first commercial PET-CT was available in 2001. The development of SPECT/CT has improved the sensitivity and the confidence that many nuclear medicine investigations are interpreted. This modality has been useful in diagnosing hyperparathyroidism, thyroid disorders, sentinel node imaging, neuroendocrine imaging, bone disorders, brain imaging, infection imaging, and studies requiring patient-specific dosimetry for dose planning. The combined SPECT/CT is a game-changer in many aspects, initiating a paradigm shift in SPECT utilization into the quantitative realm in nuclear emission tomography. Similarly, the PET-CT has been a game-changer in managing oncology, cardiology, and neurology patients, along with many others. Head and neck cancers, thyroid cancers, lung cancers, breast cancers, esophageal cancers, colorectal cancers, melanoma, lymphoma, and solitary pulmonary nodules have shown better accuracy when diagnosed with PET-CT. The difference in accuracy between PET/CT and PET alone or CT alone for staging and restaging is statistically substantial and averages nearly 10% to 15% across all malignancies. Recent advancements within the last 5 years have further solidified the role of PET-CT and SPECT-CT in nuclear medicine, particularly with the integration of novel radiotracers and improved imaging technologies. For instance, a 2021 study highlighted the utility of F-FDG PET-CT in pediatric oncology, demonstrating its efficacy in staging and follow-up of lymphomas, brain cancers, and sarcomas while emphasizing strategies to minimize radiation exposure through low-dose CT protocols. Similarly, a 2023 investigation explored the use of SPECT-CT with 99mTc-labeled tracers for radiotherapy planning in lung cancer patients, showing significant reductions in radiation doses to functional lung tissue, thus enhancing treatment precision and patient outcomes. In prostate cancer, a 2020 study validated the superior diagnostic performance of Ga-PSMA PET-CT over traditional imaging for detecting metastases, offering improved sensitivity and specificity that guide more tailored therapeutic approaches. Additionally, a 2022 publication underscored the potential of PET-CT in assessing interstitial lung disease in systemic sclerosis patients, leveraging its ability to detect metabolic changes before structural damage becomes evident on CT alone. These recent findings illustrate how PET-CT and SPECT-CT continue to evolve, incorporating advanced radiopharmaceuticals and hybrid imaging techniques to push the boundaries of diagnostic accuracy and personalized medicine across diverse clinical applications.