We propose to establish a special detector-training network aiming to extend the application of cutting-edge technologies developed in High Energy Physics (HEP) to other domains, such as medical imaging. A large number of detector techniques, developed for and used in particle physics experiments, has been transferred successfully to other fields. Most prominent are their applications in medicine, X-ray imaging, materials research, astrophysics and others. These technological spin-offs are of highest value for society and will be a focal point in this Network. Not only for positron emission tomography (PET), the Network will likewise respond to the need to develop new ultra-fast photo-detection for the future generation of linear colliders with unprecedented beam intensities. The Multi-Site Network, to be coordinated by CERN, would be an interdisciplinary, multi-national initiative whose main goal is to train young researchers in the domain of novel high-speed photo-detection instruments in HEP calorimetry and also to make use of photon time of flight (TOF) in PET, thus enabling us for the first time to considerably enhance image quality with instant benefits to patients and medical institutions. This multi-site ITN comprises 11 full partners, 7 in academia and 4 in the private sector, known world-wide in the fields of frontier detector technologies, photo detection techniques and novel scintillator production methods, as well as in the domain of semiconductor technology, medical instrumentation, image reconstruction and fusion, and surgical tracking and navigation. As such, the Network combines the necessary competence, expertise and infrastructure for top-level training of young researchers. It is foreseen to hire 18 Early Stage Researchers (ESRs) plus 4 Experienced Researchers (ERs) and structure the training project into five principle research-oriented work packages, and three work package for management, training and outreach.