oa Editorial [Hybrid Imaging (Guest Editors: Birger Hesse and Abass Alavi)]

Over the past three decades, we have witnessed significant advances in medical imaging, which have improved the assessment of a multitude of disorders with higher accuracy and more precise spatial resolution. The introduction of Computed Tomography (CT) in 1973 opened a new era in structural imaging, which has continued up to now with further improvements in the practical applications of this powerful modality. CT has been of great value in a lot of disorders, particularly in a large number of surgical procedures, which are planned after acquisition of CT images demonstrating the anatomic lesions before operation. Soon after CT was introduced, Magnetic Resonance Imaging (MRI) became a main focus of interest because of its ability to investigate soft tissue abnormalities, particularly in the central nervous and musculoskeletal systems. For most neurological diseases, MRI has become the study of choice for detection and accurate characterization of the underlying disorder. Likewise, many musculoskeletal abnormalities lend themselves well to MR imaging because of its very high spatial and contrast resolutions in detecting lesions in this organ system. The practice of orthopedic surgery has also been substantially impacted upon by the capabilities that are provided by this very powerful modality. In spite of the enormous contributions made by these two very powerful structural imaging techniques, practitioners of medicine have realized that changes noted by CT and MRI are related to the appearance of the, often later, structural manifestations of disease, and may be rather insensitive to early pathological abnormalities or early changes in already known pathological lesions. By now, it has become clear that alterations in cellular metabolism at the molecular level are often early indicators of disease activity, sometimes even asymptomatic, and therefore detecting such changes may provide evidence for abnormalities not detectable by current anatomical imaging techniques. The changes on the molecular level may never translate to structural abnormalities or may be delayed for weeks or months after the disease process is initiated. Likewise, changes following treatment may not be apparent on these images for an extended period of time after therapeutic interventions are implemented. Since some treatments may not be effective and should be terminated and replaced with alternative therapies, such delays may adversely impact the optimal management of many serious diseases including cancer, cardiac diseases, and central nervous and orthopedic disorders. Positron Emission Tomography (PET), which utilizes positron emitting radionuclides labeled with biologically important analogues, has overcome many of the deficiencies that are enumerated above. This very powerful modality provides fairly high resolution images that are comparable to those of modern tomographic structural imaging techniques. In addition, this modality provides images with high contrast compared to the background. This allows detection of the disease in early stages, which leads to the introduction of rapid interventions soon after the disease has been discovered. Furthermore, PET is the only modality that allows accurate quantification of disease activity with high precision and reproducibility. The introduction of 18FfluoroDeoxyGlucose (FDG) by investigators at PENN started a new era in medical imaging, which has expanded into multiple domains over the past three decades. While the original intent for introducing this compound was to investigate neuropsychiatric disorders, it soon became apparent that FDG was an outstanding marker for assessing disease activity in cancer. Today FDG-PET is routinely employed for managing a large number of malignancies at diagnosis, staging, monitoring response to treatment, and detection of recurrence. However, FDG is a nonspecific tracer for detecting cancer and is also taken up by the inflammatory cells in various settings. This has led to the utility of this compound in assessing infection, inflammation, atherosclerosis, thrombosis and muscle disease. At the same time the obvious need of more specific tracers has resulted in a rapidly growing list of new, more specific PET compounds. In recent years, PET and CT instruments have been assembled as a unit and in fact a majority of studies performed around the globe employ this combination for an effective utilization of both modalities. This approach allows comparing images from both instruments side by side or fused together for precise localization of molecular abnormalities at different anatomical sites as visualized by CT scan. It has been clearly demonstrated that hybrid imaging with either SPECT or PET combined with CT hardware provides important information beyond that achievable with either technique alone. In addition, hybrid imaging reduces the number of equivocal results based on one imaging technique without the benefit of the other. For decades, physicians have employed sideby- side visual interpretation of images generated by different modalities obtained at different times. However, along with the introduction of combined hybrid imaging instruments, major efforts have been made in developing software that allows fusion images of various organs for research and clinical purposes particularly. This has been of great value in organs such as the brain where anatomic landmarks allow co-registration of structural and functional imaging successfully. However, the role of such approaches may disappear as hybrid imaging with dedicated instruments becomes widely available in the future......