Computed Tomography is a well established technology which is extensively used in day to day medicine. The main developments since the first introduction of CT, about 30 years ago, were focused on better resolution and faster scans. While these dimensions are still being improved, there is a lot of focus on two new directions: lower dose and spectral CT. The lower dose trend   strives to provide same or better image quality at much lower X-ray radiation doses, minimizing the radiation induced risk. The spectral CT development enables separating materials that otherwise are not separable with conventional CT. The focus of this talk is an overview on spectral CT technologies, their basic physics, and the various approaches to enable it.

The advantage of spectral CT over conventional one is in the ability to separate between the two X-ray attenuation mechanisms – scatter and photoelectric (PE) absorption. The overall attenuation is impacted by the attenuation coefficient per unit mass and the by the density. Therefore conventional CT cannot separate between highly absorbing material at low density and a dense material with lower attenuation. For example Iodine contrast agent, injected to the vein, and bone or calcifications in human body. Iodine is often used in CT scans to better view blood vessels. In conventional CT both Iodine and Ca appear as highly attenuating regions, as compared to soft tissues. Using the CT image to separate Iodine and bone is essentially impossible, though some software applications of bones removal do exist. Such separation is possible by using dual energy CT scan, since the fraction of scatter and PE in Iodine and Ca are different. Further material separation, especially for contrast agent materials with k-edge within the CT spectrum, can be achieved by multi energy scans.

Currently there are three different approached for dual energy CT, and active research and development of multi energy CT. These methods enable new medical applications and strengthen the quantitative CT trend. Dual energy CT is achieved by the following methods: (1) dual source and dual detectors arrays; (2) fast kVp switching tube; (3) dual layer detection technology. Energy separation beyond dual energy is mainly based on photon counting technologies that in theory can enable arbitrary number of energy bins. These methods and some of the multi energy results will be presented.