X-ray fluorescence computed tomography as molecular imaging for preclinical research

X-ray fluorescence computed tomography (XFCT) is a measurement technique that combines X-ray fluorescence analysis, which is a highly sensitive trace element analysis method, with the information processing composed of CT image reconstruction. This paper overviews three typical XFCT imaging methods from the viewpoint of measurement technique. (1) In the pencil beam imaging system, the X-rays monochromatized with a monochromator are shaped into pencil beams through a slit, and excite a labeled substance. The generated X-ray fluorescence is collected with a semiconductor solid state detector (SSD). (2) In the sheet beam system, incident beams are sent so as to cover a subject. The X-ray fluorescence emitted from a labeled substance is collected by a multi-element detector array installed in parallel to the incident beam direction. (3) In the pinhole system, incident beams are sent so as to cover a subject. Among the X-ray fluorescence generated from the labeled elements, only what have passed through a plurality of pinholes is captured by a two-dimensional detector. The development of XFCT so far is heavily dependent on the excellent characteristics of synchrotron X-rays. However, in the future, the development of light sources and detectors will make people free from the restrictions that have to rely on large-scale synchrotron facilities. XFCT is expected to become a method that can be used at the laboratory level. (A.O.)