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ssion tomography (PET) and single photon emission computed tomography (SPECT) systems are used to image accumulation and distribution of radiopharmaceuticals to provide physiological information for diagnostic and therapeutic purposes. However, these images often lack sufficient anatomical detail, a fact that has triggered the deve- lopment of a new technology termed hybrid imaging. Hybrid imaging is not anymore a specialty for research purposes only. It is more and more coming into clinical routine and in the last few years, multimodality devices have started replacing single moda- lity imaging devices in the clinical workflow. Hybrid imaging is a term to describe the combination of X-ray computed tomography (CT) systems and magnetic resonance imaging (MR) with nuclear medicine imaging devices (PET and SPECT systems) in order to provide the technology for acquiring images of anatomy and function in a registe- red format during a single imaging session with the patient positioned on a common imaging table. The first part of the presentation will be a short overview of the devices and technologies used for hybrid imaging in human medicine. There are two primary advantages to this technology. First, the x-ray transmission images acquired with CT can be used to perform attenuation correction of the PET and SPECT emission data. In addition, the MR and CT anatomical images can be fused with the PET and SPECT functional images to provide precise anatomical localization of radiopharmaceuticals. Technology and problems common to SPECT/CT and PET/CT are respiratory motion, quantification, radiation dose considerations, CT-based attenuation correction, contrast CT or metallic parts and motion between or during studies. Advances in SPECT and PET include the introduction of a new PET and SPECT scintillators and processing software. The advances in SPECT and PET are directly tied to improving the quality and accuracy of the acquired projections either through the better instrumentation or better correc- tion algorithms. NaI(Tl) scintillator is the most used for SPECT applications, but also CsI(Tl), CsI(Na) and LaBr are starting to be used. Advantages of new scintillators are less 3 dead edges and better spatial resolution. The new scintillators have comparable effec- tive atomic number and densities and it was used in Small FOV cameras are covered by an array of several pixelated scintillators (2-3mm). The fast PET scintillator lutetium oxyorthosilicate (LSO), improved the count rate capability of PET scanners. The incre- ased light output resulted in improved spatial resolution from 6.5 mm to 4 mm and energy resolution allowing the scatter rejection (energy) threshold to be raised to 435 keV. The fast decay of the scintillation light decreases the system deadtime and allows the coincidence window to be shortened to around 4ns. The LSO time resolution of a few hundred picoseconds allows TOF info. New reconstruction algorithms for impro- 40