X-ray phase-contrast tomography opens new era for cardiovascular research

X-rays have been used to reveal hidden inner structures without destroying the object of their penetrative power since Wilhelm Röntgen discovered X-rays in 1895. X-ray imaging is now indispensable in several fields, including medicine and industry. On the other hand, biological soft tissues, which mainly comprise light elements (hydrogen, carbon, nitrogen, and oxygen), are almost transparent to hard X-rays. Thus, absorption-based imaging typically produces poor-image contrast when used on soft tissues. To overcome this difficulty, contrast media or staining is occasionally used. However, such treatments are not always possible. In addition, there is a risk that the treatments will cause structural changes. Developments in the use of X-ray phase imaging, including phase tomography, have been attracting attention since the mid-1990s.1,2 In general, the complex refractive index of X-rays is represented as n =1—δ + iβ. The phase factor δ is approximately 103 times higher than the absorption factor β for light elements in the hard X-ray region.2 This means that phase contrast imaging using δ has a much higher sensitivity towards soft tissue than simple absorption contrast imaging which is based on differences in β. High sensitivity in phase contrast imaging is suitable for visualizing small density differences, which often represent important structural information in biological soft tissue. Several trials have been held using synchrotron radiation (SR) sources and laboratory sources. To date, the progress of X-ray phase imaging has been supported by developments in X-ray sources, such as third-generation SR sources, optical elements, and image detectors.