Review of medical imaging with emphasis on X-ray detectors

Medical imaging can be looked at from two different perspectives, the medical and the physical. The medical point of view is application-driven and involves finding the best way of tackling a medical problem through imaging, i.e. either to answer a diagnostic question, or to facilitate a therapy. For this purpose, industry offers a broad spectrum of radiographic, fluoroscopic, and angiographic equipment. The requirements depend on the medical problem: which organs have to be imaged, which details have to be made visible, how to deal with the problem of motion if any, and so forth. In radiography, for instance, large detector sizes of up to 43 cm x 43 cm and relatively high energies are needed to image a whole chest. In mammography, pixel sizes between 25 and 70 mu m are favorable for good spatial resolution, which is essential for detecting microcalcifications. In cardiology, 30-60 images per second are required to follow the heart's motion. In computed tomography, marginal contrast differences down to one Hounsfield unit have to be resolved. In all cases, but especially in pediatrics, the required radiation dose must be kept as low as reasonably achievable. Moreover, three-dimensional(3D) reconstruction of image data allows much better orientation in the body, permitting a more accurate diagnosis, precise treatment planning, and image-guided therapy. Additional functional information from different modalities is very helpful, information such as perfusion, flow rate, diffusion, oxygen concentration, metabolism, and receptor affinity for specific molecules. To visualize, functional and anatomical information are fused into one combined image. The physical point of view is technology-driven. A choice of different energies from the electromagnetic spectrum is available for imaging; not only X-rays in the range of 10-150 keV, but also gamma rays, which are used in nuclear medicine, X-rays in the MeV range, which are used in portal imaging to monitor radiation therapy, visible and near infrared light (1-3 eV) for retina inspection and mamma transillumination, and even Terahertz waves (0.5-20meV) are under discussion. Feasibility is determined by the existing radiation sources, the materials available for absorbing and converting the radiation used, the microelectronic circuits for integrating or counting readout, and the computing power required to process and, where applicable, reconstruct data in real-time. Furthermore, other physical effects can be utilized such as the phase information a wave front receives when passing through an object. Some new developments will be discussed, e.g. energy-resolving methods for distinguishing different tissues in the patient, quanta-counting detection, phase contrast imaging, CCDs for very high spatial resolution, fast volume CT scanners, and organic semiconductors for a new generation of detection devices. Admittedly, apart from imaging performance, economic factors also have to be taken into account. (c) 2006 Elsevier B.V. All rights reserved.