Material Identification Method with the Aim of Medical Imaging

In this section, we would like to consider how atomic number information can be obtained from the information obtained from the X-ray equipment. In the example shown in Fig. 4.1a, it is a concept of a general radiography system, and Fig. 4.1b is that of a computed tomography (CT) system. In general, radiography, the amount of X-ray attenuation is determined by μ, which depends on the atom, and the thickness t of the object. The measurable physical quantities are the incident X-ray intensity I0 and the penetrating X-ray intensity I, and they are in the relation of μt = ln(I0/I). The μt trends for each element shown in Fig. 4.1a is the value when t is a constant (t = 1 cm). Because this physical quantity of μt is strongly affected by the density of the object, they are roughly divided into gases and solids. Looking at it in more detail, we can see that solid materials also have various densities. Density is not a physical quantity that strongly depends only on the atomic number. As a result, information about the atomic number Z cannot be clearly extracted from the measured quantity of μt. On the other hand, for CT examinations, because the detector can be rotated 360° to obtain information related to different projection angles, a two-dimensional μ map can be obtained as an X-ray image. This analysis includes the process of image reconstruction, and the actual CT value is defined as a relative value related to the μ of water. Because CT has a wide range of medical applications and is an essential modality in modern medicine, a great deal of research has been done on this topic. However, we should note that this relative μ value does not completely reflect material information. As shown in Fig. 4.1b, when μ values for different atomic numbers are plotted against energy, the effect of the density of the material has a strong influence; therefore, it is not possible to extract atomic number information contained in μ.