Hybrid PET/CT and SPECT/CT Imaging

1.1 Definition of nuclear medicine, molecular imaging and hybrid imaging Nuclear Medicine uses radioactive probes commonly referred to as tracers for the diagnosis and treatment of diseases. Monitoring the metabolic fate of nanomolar amounts of radiolabeled substances by tracking their photon emission with SPECT (Single Photon Emission Tomography) and PET (Positron Emission Tomography) was the first widely practiced branch of Molecular Imaging (MI). MI is the visualization, characterization and measurement of biological processes at the molecular and cellular levels in humans and other living systems (Mankoff, 2007). Among the different techniques summarized in MI, tracer imaging in Nuclear Medicine has the highest molecular sensitivity, tracing substances in the 10E-3 to 10E-5 mol/l range. Conventional imaging with x-ray was primarily used as a snapshot of anatomy and depicts tissue by its physical characteristics (e.g., X-ray density). Contrast agents in CT are used to increase the visibility of vessels and organ surfaces. Functional information is at best limited to that of perfusion and permeability. This is one of the most significant differences between Radiology and Nuclear Medicine. The advantage of functional imaging is that of increasing sensitivity because metabolic changes precede anatomical changes and can be detected long before structural changes appear. Functional imaging, however, has a low specificity in distinctly different pathologies (e.g., degenerative, inflammatory or malignant bone lesions) if they are visualized by unspecific common properties as hyperaemia, increased regional tracer permeability and osteoplastic metabolism. To overcome this and to increase specificity, highly specific probes (e.g., receptor imaging) have been developed. With minimal uptake outside the targeted tissues, specific probes offer little information about the surrounding tissues and consequently do not provide the topographical information needed by surgeons or therapy planning systems in radio oncology. The advantage of anatomical imaging by CT is its high anatomical resolution and usually good topographical information. On the other hand it is a poor predictor of the functional consequences of a finding (e.g., borderline stenosis in coronary CT angiography) and consequently a poor predictor of prognosis. Structural data do not necessarily correlate with the metabolic status of disease and they have a limited diagnostic sensitivity in cases with abnormal anatomy (e.g., scar versus residual tumour). Anatomic tumour response metrics (WHO criteria, Response Evaluation Criteria in Solid Tumours (RECIST)) are insufficient to predict therapy response, particularly in assessing the activity of newer cancer therapies that stabilize disease and may